U.S. patent application number 13/394465 was filed with the patent office on 2012-09-13 for determining a plane of an anatomical body part.
This patent application is currently assigned to Brainlab AG. Invention is credited to Martin Haimerl, Sabine Kling, Mario Schubert, Melanie Wegner.
Application Number | 20120232802 13/394465 |
Document ID | / |
Family ID | 41258360 |
Filed Date | 2012-09-13 |
United States Patent
Application |
20120232802 |
Kind Code |
A1 |
Haimerl; Martin ; et
al. |
September 13, 2012 |
DETERMINING A PLANE OF AN ANATOMICAL BODY PART
Abstract
A data processing method for determining the position of a main
plane of an anatomical body part, comprising the steps of:
providing absolute auxiliary point data which describe the position
of at least one actual auxiliary point of the body part relative to
a marker device attached to the body part, the at least one actual
auxiliary point being outside the main plane; providing relative
point data which constrain the possible positions of the main plane
relative to the at least one actual auxiliary point; providing
absolute main point data which describe the position of one or two
actual main points of the body part relative to the marker device
attached to the body part, said one or two actual main points lying
in the main plane and/or calculating the position of at least one
virtual main point relative to the marker device, said at least one
virtual main point being in the main plane and being calculated
based on the absolute auxiliary point data and the relative point
data; calculating a position of the main plane relative to the
marker device, wherein the calculation uses the relative point data
and auxiliary point data as well as the provided absolute main
point data and/or the calculated position of the at least one
virtual main point.
Inventors: |
Haimerl; Martin; (Gilching,
DE) ; Schubert; Mario; (Poing, DE) ; Kling;
Sabine; (Taufkirchen, DE) ; Wegner; Melanie;
(Kirchseeon, DE) |
Assignee: |
Brainlab AG
|
Family ID: |
41258360 |
Appl. No.: |
13/394465 |
Filed: |
August 10, 2010 |
PCT Filed: |
August 10, 2010 |
PCT NO: |
PCT/EP2010/061624 |
371 Date: |
May 22, 2012 |
Current U.S.
Class: |
702/19 |
Current CPC
Class: |
A61B 5/103 20130101;
A61B 5/6878 20130101; A61B 5/4528 20130101; A61B 90/39 20160201;
A61B 34/10 20160201 |
Class at
Publication: |
702/19 |
International
Class: |
G06F 19/00 20110101
G06F019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 8, 2009 |
EP |
PCT/EP2009/061623 |
Claims
1. A data processing method for determining the position of a plane
of an anatomical body part called main plane, constituted to be
performed by a computer and comprising the steps of: providing data
called absolute auxiliary point data which describe the position of
at least one point of the body part, called actual auxiliary point,
relative to a marker device attached to the body part, the at least
one actual auxiliary point being outside the main plane; providing
data called relative point data which constrain the possible
positions of the main plane relative to the at least one actual
auxiliary point; providing data called absolute main point data
which describe the position of one or two points of the body part,
called actual main points, relative to the marker device attached
to the body part, said one or two actual main points lying in the
main plane and/or calculating the position of at least one point
called virtual main point relative to the marker device, said at
least one virtual main point being in the main plane and being
calculated based on the absolute auxiliary point data and the
relative point data; calculating a position of the main plane
relative to the marker device, wherein the calculation uses the
relative point data and auxiliary point data as well as the
provided absolute main point data and/or the calculated position of
the at least one virtual main point.
2. The data processing method according to claim 1, wherein the
step of calculating the position of the main plane includes the
step of determining at least one virtual auxiliary point and/or at
least one virtual main point on the basis of the relative point
data and the absolute main point data and/or absolute auxiliary
point data, and wherein the step of calculating the position of the
main plane also includes the step of calculating the position of
the main plane on the basis of the at least one virtual auxiliary
point and/or at least one virtual main point, wherein the at least
one virtual main point is not included in the absolute main point
data and the at least one virtual auxiliary point is not included
in the absolute auxiliary point data.
3. The data processing method according to claim 1, wherein the
relative point data describe at least one scalar value used for
describing a positional relationship between a particular auxiliary
point of the one or more actual auxiliary points and the virtual
main point and/or between the particular auxiliary point and the
virtual auxiliary point, the position of the virtual auxiliary
point being in particular symmetrical to the position of the
particular auxiliary point with respect to the main plane, and/or
between one or more of the actual auxiliary points and the main
plane.
4. The data processing method according to claim 1, further
comprising the steps of: providing data called body part data which
constrain the possible relative positions between landmarks of the
body part and/or between the landmarks and the main plane;
providing data called landmark data which respectively correlate at
least some of the actual and/or virtual main points and/or
auxiliary points with at least some of the landmarks of the body
part; and if more than one solution for calculating the position of
the main plane is possible, selecting one of the possible solutions
for the position of the main plane on the basis of the landmark
data and body part data.
5. The data processing method according to claim 1, wherein all of
the actual auxiliary points described by the absolute auxiliary
point data and used for calculating the position of the main plane
are outside the main plane but on the same side of the main
plane.
6. The data processing method according to claim 1, wherein if the
position of only one main point of the body part relative to the
marker device is available for the calculation, then the absolute
auxiliary point data describe the position of at least two
auxiliary points of the body part relative to the marker device,
the at least two auxiliary points being outside the main plane; and
if the positions of only two main points of the body part relative
to the marker device are available for the calculation, then the
absolute auxiliary point data describe the position of at least one
auxiliary point of the body part relative to the marker device, the
at least one auxiliary point being outside the main plane.
7. The data processing method according to claim 5, wherein the
relative point data describe at least one constraint for the
possible positions of the main plane relative to the at least one
auxiliary point, if there are two main points, and describe at
least two constraints for the possible positions of the main plane
relative to the at least one auxiliary point, if there is only one
main point.
8. A data processing method which includes the data processing
method of claim 1 and which is a method for determining the
position of the main plane of the anatomical body part and of
another plane called auxiliary plane of the anatomical body part,
wherein in order to determine the main plane, the data processing
method of any one of the preceding claims is performed, and in
order to determine the auxiliary plane, absolute auxiliary point
data describe the position of at least two of the auxiliary points
of the body part relative to the marker device, the at least two
auxiliary points being outside the main plane; further comprising
the step of providing relative auxiliary plane data which describe
a predetermined positional relationship between the auxiliary plane
and the main plane; wherein the position of the auxiliary plane is
determined by assuming that the at least two auxiliary points lie
in the auxiliary plane, and is determined on the basis of the
relative auxiliary plane data.
9. A data processing method which includes the data processing
method of claim 8 for determining the position of the main plane
and of the auxiliary plane and which is additionally a method for
determining the position of a standard plane which is different
from the auxiliary plane, further comprising the steps of:
providing relative standard plane data which describe the expected
positional relationship between the auxiliary plane and the
standard plane; and determining the position of the standard plane
on the basis of the position of the auxiliary plane and the
relative standard plane data.
10. The data processing method according to claim 1, wherein: one
of the landmarks represented by one of the auxiliary points is the
sinistral or dextral anterior superior iliac spine landmark; and/or
the main plane is the mid-sagittal plane; and/or the standard plane
is the anterior pelvic plane; and/or the standard plane data
describe the angle between the anterior pelvic plane and the
auxiliary plane; and/or the anterior pelvic plane and the auxiliary
plane intersect each other along a line connecting the sinistral
and the dextral anterior superior iliac spine landmarks of the body
part; and/or another auxiliary point represents a landmark defined
by the acetabulum or a part of the acetabulum or the fossa of the
acetabulum or another point of the acetabulum; and/or the relative
point data describe a constraint for an inter-fossa distance or an
inter-teardrop distance of the pelvis.
11. A data processing method for determining additional points
called additional auxiliary points which can be used for
determining the position of a plane of a body part, the plane being
called a main plane, in particular according to claim 1, comprising
the steps of: providing at least one auxiliary point; providing
relative auxiliary point data which describe at least one
constraint which limits the possible positions for an additional
auxiliary point, the additional auxiliary point being symmetrical
to one of the one or more auxiliary points with respect to the main
plane; and providing candidate point data in steps, wherein in each
step, the position of a new candidate point is described by the
candidate point data, wherein in the current step, the following
steps are performed: a) providing new data called new candidate
point data which describe a relative position of a new candidate
point with respect to the marker device, the new candidate point
being a new candidate for a position of the additional auxiliary
point; b) checking whether the positions of the new candidate point
comply with the at least one constraint described by the relative
auxiliary point data; c) repeating steps a) and b) if the position
of the new candidate point does not comply with the at least one
constraint, or accepting the new candidate point as an additional
auxiliary point if the position of the candidate point does comply
with the at least one constraint.
12. A program which, when it is running on a computer or is loaded
onto a computer, causes the computer to perform the method
according to claim 1; and/or a program storage medium on which the
program is stored; and/or a computer on which the program is
running or in the memory of which the program is loaded; and/or a
signal wave, in particular a digital signal wave, carrying
information which represents the program.
13. A navigation system for computer-assisted surgery, comprising:
the computer of claim 12; a detection device for detecting the
position of the main and auxiliary points and for generating
detection signals which represent the position of the main and
auxiliary points and for supplying the detection signals to the
computer of the preceding claim, the computer being designed to
determine the absolute point data and the relative point data on
the basis of the received detection signals in order to process the
absolute data in accordance with claim 1; and a user interface for
receiving data from the computer in order to provide information to
the user, the computer being designed to calculate the data in
accordance with the data processing method according to claim
1.
14. A determination method for determining the position of a main
plane of an anatomical body part, comprising the steps of:
generating detection signals by detecting a pointer, said pointer
contacting a body part or a tool, called main point detection tool,
attached to the body part; determining, on the basis of the
detection signals, the absolute main point data and absolute
auxiliary point data as mentioned in claim 1, and then performing
claim 1 on the basis of the determined absolute main point data and
absolute auxiliary point data.
15. The navigation system of claim 13 further comprising a tool,
called main point detection tool, for detecting at least one actual
main point, the plane point detection tool comprising: a proximal
side constituted to be fitted to a patient by contacting the
patient along a first direction of length-wise extension of the
tool, the proximal side is constituted to be able to be aligned
with a plane of an anatomical body part along the first direction
of length-wise extension, the direction in which the proximal side
faces and along which the tool is flexibly bendable for bringing
the tool into contact with the patient being a third direction of
extension of the tool; a distal side which comprises at least one
pointer insertion member comprising a tapered recess for inserting
the tip of a pointer, the at least one pointer insertion member
being prominently positioned in a first section of the distal side
which is, in the direction of length-wise extension, closer to a
first end of the plane point detection tool than to a second end;
wherein the distal side further comprises a second section having a
flat surface and being closer to the second end than to the first
end; both the proximal side and distal side extending in the first
direction and in a second direction of extension, the second
direction being traverse to the first direction, the tool being
flexibly bendable in the third direction while being rigid in the
second direction, the second direction being a direction of
width-wise extension of the tool, the width-wise extension of the
tool being smaller than the length-wise extension.
Description
[0001] The present invention relates to determining a plane of an
anatomical body part (for example a pelvis or head) which shall be
referred to in the following as a main plane. The "anatomical body
part" shall also be referred simply as the "body part" for short,
and the "anatomical pelvis" simply as the "pelvis". A main plane is
in particular a plane of an anatomical body part which main plane
is used in surgery (for example in brain or hip surgery) as a
reference for defining the positions of other body parts (for
example the acetabulum), in particular of other anatomical
structures. A main plane is in particular a plane which can be
defined by anatomical landmarks of the anatomical body part (for
example the pelvis). Planes which can be defined by these landmarks
and/or which are used for defining the position of the femur or
acetabulum are in particular the mid-sagittal plane (MSP) or the
anterior pelvic plane (APP).
[0002] The main plane and the auxiliary plane are in particular not
parallel and are in particular constituted to define the position
of a reference system, in particular coordinate system. Generally
speaking, at least two planes of the anatomical body part (for
example the pelvis) are defined in order to determine a reference
system (coordinate system) with respect to which the
above-mentioned position of another anatomical body part (e.g.
femur or the acetabulum) can be defined. One of these planes is the
aforementioned main plane (for example the MSP), the other plane
can be a plane referred to here as the auxiliary plane (for example
the spinae joint center plane or SJCP to be defined later) or can
be a plane referred to here as the standard plane (for example the
APP). Like the main plane, the auxiliary plane, and standard plane
are preferably defined by landmarks of the anatomical body part
(for example the pelvis).
[0003] It is advantageous to determine the position of the main
plane reliably and with a sufficient degree of accuracy to allow
the position of the other body parts, in particular anatomical
structures (for example the acetabulum) to be determined with a
sufficient degree of accuracy.
[0004] In accordance with a common prior-art method, the pelvis is
registered in a supine position in particular for hip surgeries and
then turned over to a lateral position. In the supine position,
landmarks are detected by means of a pointer in order to determine
the location of landmarks on the MSP and APP relative to a marker
device fixed to the pelvis.
[0005] Reference is also made to U.S. 2008/132783 A1. In accordance
with this patent application, points are determined in three
cardinal planes. Reference is also made to U.S. 2003/0153829, U.S.
2002/0077540 and WO 2005/084541.
[0006] The object of the invention is to allow a position, in
particular an orientation of a main plane of an anatomical body
part (for example a pelvis) to be determined on the basis of the
position of (main and auxiliary) points which in particular
represent landmarks. Preferably, all represented landmarks can be
detected (scanned) when the patient is in one position, in
particular the lateral position.
[0007] The above object is solved by the subject-matter of the
independent claims. The dependent claims are directed to
advantageous embodiments.
[0008] As far as herein the "position" of a plane is mentioned, the
term position can mean absolute location of the plane or just
orientation of the plane. In particular determination of the latter
is often sufficient in hip surgery since the orientation of the
plane relative to the orientation of the acetabulum is of main
interest for the surgeon.
[0009] One advantage of the invention is that it is not necessary
to reposition the anatomical body part (for example the pelvis) and
thus the patient on the table. Furthermore, the present invention
relies in particular on landmarks which are easily accessible for a
surgeon. In particular, points which are symmetrical to the
auxiliary points with respect to the main plane and remote from the
main plane (for example the ASIS point) can be detected but do not
have to be detected. These symmetrical and/or remote points can be
difficult to detect.
[0010] Advantageously, determining the main plane does not rely on
fluoroscopic images produced during surgery. Thus, the exposure of
the patient and operating theatre staff to radiation is
reduced.
[0011] Advantageously, the present invention can rely on position
data for landmarks which can be detected to a high degree of
accuracy and which are in particular not covered by soft tissue or
fat. These position data are in particular detected by means of a
pointer (see below).
[0012] A method in accordance with the invention is in particular a
data processing method or comprises the data processing method. The
data processing method is preferably performed using technical
means, in particular a computer. The computer in particular
comprises a processor and a memory in order to process the data, in
particular electronically. The calculating steps described are in
particular performed by a computer. Steps of determining or
calculating are in particular steps of determining data within the
framework of the technical data processing method, in particular
within the framework of a program. A computer is in particular any
kind of data processing device. A computer can be a device which is
generally thought of as such, for example desktop PCs or notebooks
or netbooks, etc., but a computer can be also any programmable
apparatus, such as a mobile phone or an embedded processor. In
particular, a computer can comprise a system (network) of
"sub-computers", wherein each sub-computer represents a computer in
its own right. A computer in particular comprises interfaces in
order to receive data and/or to perform an analog-to-digital
conversion.
[0013] Merely as a non-limiting example of an anatomical body part,
the term "anatomical pelvis" or "pelvis" is used in the following
instead of "anatomical body part".
[0014] The data processing method of the present invention is a
method for determining the position, in particular orientation of
the main plane (e.g. MSP) of the anatomical pelvis. The pelvis is
in particular the pelvis of a subject (patient) who is in
particular lying in a lateral position. The position, in particular
orientation of the main plane is in particular determined relative
to a marker device which is attached to the pelvis.
[0015] A marker device can for instance be a reference star or a
pointer or one or more (individual) markers in a predetermined
spatial relationship. A marker device comprises one, two, three or
more markers in a predetermined spatial relationship. This
predetermined spatial relationship is in particular known to a
navigation system, for example stored in a computer of the
navigation system.
[0016] The function of a marker is to be detected by a marker
detection device (for example, a camera or an ultrasound receiver),
such that its spatial position (i.e. its spatial location and/or
alignment) can be ascertained. The detection device is in
particular part of a navigation system. The markers can be active
markers. An active marker emits for example electromagnetic
radiation and/or waves, wherein said radiation can be in the
infrared, visible and/or ultraviolet spectral range. The marker can
also however be passive, i.e. can for example reflect
electromagnetic radiation from the infrared, visible and/or
ultraviolet spectral range. To this end, the marker can be provided
with a surface which has corresponding reflective properties. It is
also possible for a marker to reflect and/or emit electromagnetic
radiation and/or waves in the radio frequency range or at
ultrasound wavelengths. A marker preferably has a spherical and/or
spheroid shape and may therefore be referred to as a marker sphere;
markers can also, however, exhibit a cornered--for example,
cubic--shape.
[0017] A "reference star" refers to a device which a number of
markers, advantageously three markers, are attached to, wherein the
markers are (in particular detachably) attached to the reference
star such that they are stationary, thus providing a known (and
advantageously fixed) position of the markers relative to each
other. The position of the markers relative to each other can be
individually different for each reference star used within the
framework of a surgical navigation method, in order to enable the
corresponding reference star to be identified by a surgical
navigation system on the basis of the position of the markers
relative to each other. It is thus also then possible for the
objects (for example, instruments and/or parts of a body) to which
the reference star is attached to be identified and/or
differentiated from each other. In a surgical navigation method,
the reference star serves to attach a plurality of markers to an
object (for example, a bone or a medical instrument) in order to be
able to detect the position of the object (i.e. its spatial
location and/or alignment). Such a reference star in particular
comprises a way of being attached to the object (for example, a
clamp and/or a thread) and/or a holding element which ensures a
distance between the markers and the object (in particular in order
to assist the visibility of the markers to a marker detection
device) and/or marker holders which are mechanically connected to
the holding element and which the markers can be attached to.
[0018] The position of the main plane is in particular described in
a reference system (coordinate system) of a navigation system, in
particular a surgical navigation system (also referred to as a
computer-assisted navigation system or image-guided surgery
system).
[0019] A navigation system, in particular a surgical navigation
system, is understood to mean a system which may comprise: at least
one marker device; a transmitter which emits electromagnetic waves
and/or radiation and/or ultrasound waves; a receiver which receives
electromagnetic waves and/or radiation and/or ultrasound waves; and
an electronic data processing device which is connected to the
receiver and/or the transmitter, wherein the data processing device
(for example, a computer) comprises in particular a processor
(CPU), a working memory, advantageously an indicating device for
issuing an indication signal (for example, a visual indicating
device such as a monitor and/or an audio indicating device such as
a loudspeaker and/or tactile indicating device such as a vibrator)
and advantageously a permanent data memory, wherein the data
processing device processes navigation data forwarded to it by the
receiver and can advantageously output guidance information to a
user via the indicating device. The navigation data can be stored
in the permanent data memory and for example compared with data
which have been stored in said memory beforehand.
[0020] According to an embodiment of the invention, absolute main
point data are provided. These absolute main point data can
describe the position of one (actual) main point or the position of
two (actual) main points of the pelvis relative to the marker
device which is in particular attached to the pelvis or of more
(actual) main points. In particular, the absolute main point data
describe the position of only one main point of the pelvis or the
position of only two main points of the pelvis relative to the
marker device. The main points are points which lie in the main
plane and the position of which is in particular defined by
landmarks of the pelvis. Generally the main points can lie anywhere
on the main plane. According to another embodiment, no absolute
main point data are provided but instead virtual main points are
calculated based on the absolute auxiliary point data and the
relative point data. The virtual main points lie in the main plane
and can but have not to lie in the body part. These calculated
virtual main points can be used for the further determination, in
particular calculation in accordance with the described inventive
method in the same way as the actual main points described by the
absolute main pint data. According to a further embodiment both the
provided actual main points and the calculated virtual main points
are used for the further determination in accordance with the
invention.
[0021] A landmark is a defined position of an anatomical
characteristic of an anatomical body part which is always identical
or recurs with a high degree of similarity in the same anatomical
body part of multiple patients. Typical landmarks are for example
the anterior superior iliac spine (ASIS) points or the tips of the
dorsal process of a vertebra. The points (main points or auxiliary
points) can represent such landmarks. A landmark which lies on (in
particular on the surface of) a characteristic anatomical structure
of the body part can also represent said structure. The landmark
can represent the anatomical structure or only a point or part of
it. For instance, a landmark can also lie on the anatomic structure
which is in particular a prominent structure. An example of such an
anatomic structure is the posterior aspect of the iliac crest.
Other landmarks include a landmark defined by the rim of the
acetabulum, for instance by the center of the rim. Another example
is where a landmark represents the bottom or deepest point of an
acetabulum, which is derived from a multitude of detection points.
Thus, one landmark can in particular represent a multitude of
detection points. As mentioned above, a landmark can represent an
anatomical characteristic which is defined based on a
characteristic structure of the body part. Additionally, a landmark
can also represent an anatomical characteristic defined by a
relative movement of two body parts, such as the rotational center
of the femur when moved relative to the acetabulum.
[0022] A detection point is in particular a point on the surface of
the anatomical structure which is detected, for example by a
pointer.
[0023] Absolute auxiliary point data are also provided in
accordance with the invention. These absolute auxiliary point data
describe the position of at least one auxiliary point of the pelvis
relative to the marker device. Unlike the main points, an auxiliary
point lies outside the main plane. If there is more than one
auxiliary point, the auxiliary points lie in particular in the
so-called auxiliary plane (already mentioned above), as will be
explained in more detail below. The auxiliary points represent the
position of the anatomical characteristics and are in particular
(directly or indirectly) defined by landmarks. Both the main points
and the auxiliary points are in particular, but not necessarily,
points lying on the surface of the pelvis. As mentioned above, the
main points and auxiliary points can also be defined indirectly, as
will be explained in more detail below, for instance by the rim of
the acetabulum or by a rotation center or by landmarks which are
symmetrical relative to a plane (for example the main plane or
auxiliary plane), again to name but a few examples of indirect
definitions of auxiliary points or main points.
[0024] According to one advantageous embodiment of a determination
method in accordance with the invention, the position of the main
points and/or auxiliary points (which represent landmarks) can be
detected by a step of contacting the pelvis with the
above-mentioned pointer which is handled by a medical assistant and
brought into contact with the pelvis. This optional detection step
allows the absolute main point data and absolute auxiliary point
data to be provided and in particular does not form a part of the
claimed data processing method but can form a part of a general
determination method for determining the position of the main
plane. The step of detecting in particular does not encompass or
comprise an invasive step representing a substantive physical
intervention on the body which requires professional medical
expertise to be carried out and/or which entails substantial health
risk even when carried out with required professional care and
expertise. The step of detecting does in particular not comprise or
encompass a surgical step. The step of detecting does in particular
not comprise or encompass a step for treatment of a human or animal
body by surgery or therapy. The step of providing the absolute main
point data and absolute auxiliary point data includes in particular
a step of receiving the absolute main point data and absolute
auxiliary point data by the data processing method. The received
data have been in particular generated by the detection step. The
detection step is in particular not part of the data processing
method but is optionally part of the determination method in
accordance with the invention. As mentioned above, a multitude of
detected points (detection points) on the surface of the pelvis can
result in the position of just one landmark and therefore just one
main point and/or auxiliary point being detected. A multitude of
detection points are for example necessary in order to determine
the deepest point (fossa) in the acetabulum which is a landmark.
Another example is when a multitude of detection points are
necessary in order to define several positions of the femur
relative to the acetabulum. This allows the center of rotation of
the relative movement of the femur to be determined. This center of
rotation is then a landmark.
[0025] The data processing method of the present invention is
directed to data processing and in particular does not include
steps relating to contacting an anatomical structure.
[0026] Relative point data are also provided in accordance with the
invention. These relative point data constrain the possible
position of the main plane relative to the at least one auxiliary
point, in particular relative to only one of the one or more
auxiliary points and/or relative to only two of the auxiliary
points. The relative point data can in particular constrain the
possible positions between a particular auxiliary point of the one
or more auxiliary points and another point referred to as the
virtual auxiliary point, which is not included in the absolute
auxiliary point data. This virtual auxiliary point is in particular
a point which is symmetrical to the particular auxiliary point of
the one or more auxiliary points with regard to the main plane.
Thus, the possible positions of the main plane relative to the at
least one auxiliary point are in particular constrained by
constraining the possible positions between the at least one
auxiliary point and the virtual auxiliary point and/or between the
at least one auxiliary point and a virtual main point (see below).
An example of such a constraint is that the auxiliary point and its
corresponding (symmetrical) virtual point must have a certain
distance. A further example of the relative point data is that
these data describe an angle between two lines defined by at least
three points, at least one of these three points can be a main
point (virtual or actual main point). Thus also positional
relationships, in which (virtual or actual) main points are
involved, can represent constraints for the possible positions of
the main plane relative to the at least one auxiliary point. The
absolute main point data describe in particular the position of one
or two actual main points while they do not describe the position
of the virtual main points. The virtual main points are in
particular determined, in particular calculated by means of the
data processing method. The actual main points are in particular
received by the data processing method as mentioned above.
Correspondingly, the absolute auxiliary point data describe the
position of at least one actual auxiliary point while they do not
in particular describe the position of virtual auxiliary points.
The virtual auxiliary points are in particular determined, in
particular calculated by means of the data processing method.
[0027] The constraints described by the relative point data are in
particular represented by one or two or more scalar values. These
scalar values are in particular used for describing a positional
relationship between the main plane and a zero-dimensional or
one-dimensional geometrical object, such as a point or line, or
between two such geometrical objects. The constraints given by the
relative point data are in particular incomplete, i.e. do not allow
the exact position of the main plane relative to the at least one
auxiliary point to be calculated in a reference system (coordinate
system) based only on the position of the at least one auxiliary
point and the relative point data. Additional information is
necessary. This additional information is for instance the absolute
main point data and/or absolute auxiliary point data and/or the
pelvis data. The relative point data are in particular incomplete
in that at least the position of one main point and at least the
position of one auxiliary point are necessary in addition to the
relative point data, in order to calculate a position of the main
plane. Thus, the relative point data can directly constrain the
possible positions of the main plane relative to the at least one
auxiliary point by describing a positional relationship (for
example a distance or angle) between the at least one auxiliary
point and the main plane. The relative point data can also
indirectly constrain the positions of the main plane relative to
the at least one auxiliary point by describing a positional
relationship (for example a distance or angle) between at least one
auxiliary point and at least one virtual auxiliary point.
[0028] In particular, the relative point data can include but do
not need to include geometrical constraints which describe a
predetermined positional relationship between planes of the
anatomical pelvis, in particular between the auxiliary plane and
the main plane and/or between the main plane and the standard plane
and/or between the standard plane and the auxiliary plane. The
relative point data in particular do not describe that this
predetermined positional relationship is a perpendicular
relationship. Nevertheless, the constraints allow the number of
possible positions to be restricted. The relative point data
comprise in particular one scalar value, in particular only one
scalar value, if the absolute point data comprise two main points.
The relative point data also in particular comprise two scalar
values, in particular only two scalar values, if only one main
point is provided by the absolute point data. Examples of scalar
values are distances and angles.
[0029] The relative point data are in particular stored in a data
storage (e.g. RAM, ROM, or any database). The relative point data
can be generated, preferably before the surgery starts, in
particular outside the operating theatre. The relative point data
are in particular based on at least one x-ray image of the pelvis.
The relative point data are in particular only based on
two-dimensional x-ray images. In particular, the relative point
data describe constraints which can be derived from (for example
based on) a two-dimensional image, in particular only one
two-dimensional image (but also possibly two or three or more), for
example only one x-ray image. Thus, the relative point data in
particular describe positional relationships between geometrical
objects, wherein said relationships are present in two dimensions
(for example in an image plane). In this way, the data processing
method of the present invention can use data which are easily
obtained and in particular available before surgery. Thus, it is
not necessary to take additional x-ray images, in particular during
surgery. It is a particular aspect of the invention that data
available before surgery are used to reduce the workload and so aid
the surgeon when gathering data from the pelvis by means of a
pointer. Furthermore, uncertainties in the position of the
determined main plane due to a change in the location of the
patient (due to being turned over from the supine position to the
lateral position) can be avoided. A specific example of relative
point data is the (shortest) distance from an auxiliary point to
the main plane. Another example is where there are two auxiliary
points which define a line. The relative point data can then
describe the distance from one of the auxiliary points to the main
plane when following the line and/or can define the angle of the
line relative to the main plane. The relative point data may also
be based on anatomical knowledge, for instance based on generic or
statistical models of the pelvis. Based on these models, for
instance distances or angels are derived which are used as relative
point data. Furthermore, it is also possible to use, in particular
before surgery, a mechanical tool in order to measure the relative
point data. For instance, the distance between the two ASIS points
may be determined by using a mechanical tool and then the resulting
distance may be entered into the system as relative point data. The
relative point data can not just describe constraints for the
possible positions of the main plane relative to the at least one
actual auxiliary point but can in addition describe constraints for
the possible positions of the main plane relative to at least one
virtual auxiliary point.
[0030] As mentioned above, only a minimum number of auxiliary
points, preferably but not obligatory in combination with an actual
main point can be used as basis for the calculation of the position
of the main plane. Thus, there is a minimum of information needed
for calculating the position of the main plane. However, of course,
more than this minimum information can be used as a basis for the
calculation. In particular several actual main points or several
actual auxiliary points can represent the basis for the
calculation. In that case, a possible error in the calculation of
the position of the main plane may be reduced by using this
additional information in accordance with general known error
reduction methods in case of more information as necessary is
available.
[0031] The step of calculating the position of the main plane
relative to the marker device preferably includes a step of
determining a virtual auxiliary point and/or a virtual main point
as described above. The position of the main plane is then
determined on the basis of the determined virtual main point and/or
virtual auxiliary point. There is thus preferably an intermediate
step of determining at least one virtual auxiliary point and/or at
least one virtual main point (see below).
[0032] The majority and in particular preferably all of the actual
auxiliary points used for calculating the position of the main
plane are preferably outside the main plane but on the same side of
the main plane. This is particularly advantageous if the main plane
divides the pelvis into parts of at least approximately the same
size. This is for instance the case with the mid-sagittal plane
(MSP). In this way, it is possible to ensure that the auxiliary
points are easily accessible for the surgeon and that the data
provided for the data processing method are reliable data, since
they were easily and clearly accessible for the surgeon. Another
embodiment will be described below, in which the auxiliary points
are provided on both sides of the main plane but preferably close
to the main plane.
[0033] Preferably, the absolute auxiliary point data are used to
determine (calculate) at least one additional main point which lies
in the main plane and which is the above-mentioned virtual main
point. A virtual main point is not included in the absolute main
point data, but does lie in the main plane. A virtual main point is
in particular not based on the detection of a landmark by means of
a pointer. If there are in particular no actual main point or only
one or only two actual main points, then there is not enough
information available to determine the position of the main plane
on the basis of the absolute main point data. Therefore, in
accordance with one embodiment of the invention, a particular
auxiliary point is used to determine at least one virtual main
point. One or more or all of the one or more auxiliary points can
be a particular auxiliary point. For the calculation of the virtual
main points also already calculated virtual main points and/or
actual main points may be used.
[0034] The particular auxiliary point can also be used to determine
(calculate) at least one additional auxiliary point which lies
outside the main plane and which is the above-mentioned virtual
auxiliary point. A virtual auxiliary point is not included in the
absolute auxiliary point data. A virtual auxiliary point is in
particular not based on the detection of a landmark by means of a
pointer. The virtual auxiliary point can have a defined positional
relationship, in particular a symmetrical relationship with respect
to the main plane and preferably also with respect to the
particular auxiliary point. The virtual auxiliary point is for
example a point which is symmetrical to the particular auxiliary
point with respect to the main plane. As mentioned above, the
position of the main plane can then be determined on the basis of
the particular auxiliary point and the corresponding (symmetrical)
virtual auxiliary point. Thus, there is then enough information
available to determine the position of the main plane on the basis
of the virtual auxiliary point.
[0035] As mentioned above, the relative point data include at least
one scalar value. This at least one scalar value describes a
positional relationship between an auxiliary point and the main
plane (for instance, the (shortest) distance from the auxiliary
point to the main plane). The scalar value can also describe a
positional relationship between two auxiliary points, one of which
is a virtual auxiliary point. This virtual auxiliary point is in
particular a point at a position which is symmetrical to the other
(actual) auxiliary point with respect to the main plane. An example
of this is the sinistral and dextral anterior superior iliac spine
(ASIS) point. For instance, the dextral ASIS point is the actual
auxiliary point and the sinistral ASIS point is the virtual
auxiliary point. This situation applies in particular if the
patient is lying on their sinistral side.
[0036] The scalar value can in particular describe the distance
between two fossa points of the acetabulum (i.e. the deepest point
of the acetabulum). The relative point data describe this distance
according to an embodiment of the present invention. One of the two
fossa points is an actual auxiliary point and the other one is a
virtual auxiliary point. Thus, the determination method can
generate the absolute auxiliary point data e.g. by detecting the
position of one of the fossa points by means of a pointer. The
generated absolute auxiliary point data can then be received by the
data processing method. The distance between the two fossa points
which is called inter-fossa distance can be deduced from an x-ray
image. The inventors of the present invention have found that
deducing the distance from an x-ray image is not obligatory. The
inter-fossa distance within males and within females are rather
constant. Therefore, a particular distance for a male or for a
female can be assumed. This distance represents an example for
relative point data described by a scalar value. Typical values for
inter-fossa distance for males is 116 mm. Typical values for
inter-fossa distance for females is 125 mm. For Asian people the
values are a bit lower (114 mm inter-fossa for males and 122 mm for
females). In particular an inter-fossa distance of 116 mm +/-7 mm
for males and 125 mm +/-8 mm has been determined by the inventors.
In particular, an inter-teardrop-distance of 112 mm +/-6 mm for
males and an inter-teardrop-distance of 121 mm +/-8 mm for females
has been determined by the inventors. Thus, preferably, the
predetermined value for inter-fossa distance for males is set to be
higher than 109 mm and/or smaller 123 mm for males and/or set to be
higher than 118 mm and/or lower than 133 mm for females. In
particular, the predetermined value for an inter-teardrop-distance
is set to be higher than 106 mm and/or lower than 118 mm for males
and/or higher than 113 mm and/or lower than 129 mm for females.
[0037] Thus, according to an advantageous embodiment of the present
invention, the inter-fossa distance is a predetermined value which
is provided according to a step of the data processing method, in
particular received by the data processing method as relative point
data (e.g. from a database). In particular, this distance can be
stored in a database and can be received from the data processing
method by accessing the database. Besides the inter-fossa distance,
the inventors of the present invention have also found that the
inter-teardrop distance is rather constant. Therefore according to
another advantageous embodiment, the inter-teardrop distance is
used as relative point data. In particular, there is a fixed
relationship between inter-teardrop distance and inter-fossa
distance which can be used if the position of one of the fossa
points is described by the absolute auxiliary point data. In
particular, the teardrop distance is smaller than the inter-fossa
distance by a predefined difference which difference is in
particular larger than 1 or 2 mm and smaller than 3 or 4 mm. In
particular, the difference is about 2 to 3 mm. The inter-teardrop
distance is generally used by a physician. The physician determines
the inter-teardrop distance by measuring a distance in an x-ray
image of the patient. Preferably, the data processing method
includes a step of providing the difference between the
inter-teardrop distance and the inter-fossa distance so that one of
the two distances can be calculated if the other one is provided
(in particular received by the data processing method). For
instance the physician measures the inter-teardrop distance from a
x-ray image of the patient and inputs the distance into the data
processing method. Thus the inter-teardrop distance is received by
the data processing method and the inter-fossa distance can be
calculated from the inter-teardrop distance based on the
aforementioned known difference. If no measurement of the distance
is performed, according to an aspect of the invention, a
predetermined value for the inter-teardrop distance and/or
inter-fossa distance is used by the data processing method, in
particular typical value is used by the data processing method. In
other words, the data processing method includes the step of
providing a predetermined value for the inter-teardrop distance
and/or inter-fossa distance according to an embodiment.
[0038] Generally, according to an advantageous aspect of the
invention, properties, in particular distances and angles of the
anatomical body part (in particular the pelvis) are used for
defining the relative point data which properties are at least
approximately constant for particular types of human beings (in
particular for particular types of racial populations (e.g. Asian,
Caucasian or African) and/or for a particular type of gender (male
or female). Preferably those properties are used which are
describable by a scalar value (e.g. distance or angle) and which
are at least approximately constant, i.e. the variation of the
scalar value is low or zero. Low variation in the meaning of the
present invention is in particular if the standard deviation from a
mean scalar value is lower than 5%, 2% or 1% of the scalar value
which describes the property. More preferably, low variation means
that the variation of the scalar value results in a variation of
less than 2 degree for the position (orientation) of the acetabulum
and/or femural shaft. The position (in particular orientation) is
preferably described by an angle with respect to a plane, in
particular standard plane (see below). Preferably, a variation of
the scalar value is considered to be a low variation, if the
variation of the scalar value results in a variation of the angle
of less than 5 degrees, in particular less than 2 degrees, in
particular with respect to a target position (in particular target
orientation) which is in particular also described by an angle with
respect to a plane (in particular the standard plane). By referring
to general properties which are approximately constant at least for
different types of human beings, further analysis of the anatomical
body parts by means of analytical devices (like x-ray or CT or MRT)
can be avoided and costs and time can be spared. Furthermore,
radiation to the patient and to the medical team can be
reduced.
[0039] According to a further advantageous embodiment, the
aforementioned properties of the anatomical body part which are at
least approximately constant for particular types of human beings
are described to be a function of other properties of the human
being. For instance, the inter-fossa distance or the inter-teardrop
distance can be described to be a function of the length of the
femur or the body height. In this way, it can be possible to
determine the data used as relative point data more exactly and
more individually by varying an approximately constant anatomical
property (in particular described by a scalar value) in accordance
with the function.
[0040] Where the provision of an "auxiliary point" is mentioned
here, it is meant that an actual auxiliary point is provided unless
otherwise specified, i.e. an auxiliary point included in the
absolute auxiliary point data. In all other cases it may be both
(virtual and actual auxiliary point). Where the provision of a
"main point" is mentioned here, it is meant that an actual main
point is provided unless otherwise specified, i.e. a main point
included in the absolute main point data. In all other cases, it
may be both (virtual and actual main point). In particular, the
absolute main point data describe only those positions of main
points which are used to calculate the position of the main plane.
In particular, the absolute auxiliary point data describe only
those positions of auxiliary points which are used to calculate the
position of the main plane. As the actual main and/or auxiliary
points preferably do, the virtual main and/or auxiliary points in
particular (but not necessarily) represent landmarks. In
particular, as far as herein the determination (calculation) of a
position of a plane is concerned and a reference is made to main
points, if not otherwise specified, this means preferably but not
obligatory that not only actual main points can be used for the
determination (calculation) but also virtual main points can be
used (in addition or exclusively). The same applies if a positional
relationship between a main point and any other geometric object
(e.g. point or plane) is concerned.
[0041] In particular, as far as herein the determination
(calculation) of a plane is concerned and a reference is made to
auxiliary points, if not otherwise specified or clear from the
description, this means preferably but not obligatory that not only
actual auxiliary points can be used for the determination
(calculation) but also virtual auxiliary points. The same applies
if a positional relationship between an auxiliary point and another
geometric object (e.g. point or plane) is concerned.
[0042] Preferably, the position of the actual auxiliary point in
combination with the relative point data allows the position of the
virtual auxiliary point to be determined. If the virtual auxiliary
point is symmetrical to the main plane with respect to the actual
auxiliary point, determining the position of the virtual auxiliary
point allows the position of the main plane to be determined.
[0043] Also the virtual auxiliary points may be determined
indirectly based on the detection of points outside the body part.
For instance, the plane on which the patient is lying is determined
by using a pointer. Assuming further, the patient is lying in
lateral position, e.g. its sinistral ASIS point is in contact with
the plane. Furthermore, assuming, the relative point data describe
the distance between the sinistral and the dextral ASIS point, then
the determined location of the plane on which the patient is lying,
allows to calculate, based on the relative point data and the
position of the dextral ASIS point (which is provided by the
absolute auxiliary point data), the position of the virtual
sinistral ASIS point. This virtual auxiliary point may be used for
the calculation of the planes in the same manner as the
aforementioned actual auxiliary points.
[0044] The present invention also preferably uses anatomical
knowledge concerning the pelvis. For example, as mentioned above,
the relative point data can use this knowledge when referring to
statistical models of the pelvis in order to determine distances or
angles. For example, the above-mentioned inter-fossa or
inter-teardrop distance represent anatomical knowledge (found by
the inventors) based on which the relative point data are
provided.
[0045] Furthermore, the anatomical knowledge is preferably used to
provide body part data (also referred to as pelvis data). These
body part data (pelvis data) constrain the possible relative
(anatomical) position between landmarks of the pelvis and/or
between the landmarks and the main plane. For instance, body part
data (pelvis data) describe that one of the landmarks is more
anterior or more posterior or more distal or more proximal or more
cranial or more caudal than another landmark. The body part data
(pelvis data) can also constrain the possible relative (anatomical)
positions between (prominent) anatomical positions of (prominent)
anatomical structures such as a crest or an anatomical plane (for
example the mid-sagittal plane) or between such structures and
landmarks. Thus, the body part data can also be defined in the form
of inequality constraints.
[0046] In order to use the pelvis data provided, landmark data are
preferably provided which link at least some of the main points
and/or auxiliary points to the landmarks of the pelvis. The term
"at least some" here means in particular at least two or at least
three of the points, which can be main points and/or auxiliary
points. In other words, the landmark data inform the navigation
system as to which (actual or virtual) (main or auxiliary) point
represents which landmark. This information is given for at least
some of the points.
[0047] It is possible, when attempting to determine the position of
the main plane on the basis of the provided absolute main point
data, the provided auxiliary point data and the provided relative
point data, for this attempt to result in more than one possible
solution. If more than one solution does result, the provided
pelvis data and the provided landmark data are in particular used
to discount one or more of the solutions which are not in line with
the pelvis data, i.e. which do not correspond to the anatomy of the
pelvis and can therefore be discounted. This allows one of the
possible solutions for the position of the main plane to be
selected on the basis of the provided pelvis data and the provided
landmark data. In other words, the solution which is in line with
the constraints given by the pelvis data is selected.
[0048] As mentioned above, it is not obligatory to provide absolute
main point data. However, in that case, preferably at least one
virtual main point is calculated based on the absolute auxiliary
point data. This calculated at least one virtual main point may be
used in the further calculation in the same way as described below
for the one or two actual main points. In other words, the
provision of actual main points can be replaced by the calculation
of virtual main points.
[0049] According to a particular embodiment of the present
invention, the absolute main point data describe a position of only
one main point of the pelvis relative to the marker device or only
one virtual main point has been calculated or can be calculated,
then two (actual or virtual) main points are in particular missing
in order to determine the main plane since only one (actual or
virtual) main point is available for the calculation. In this case,
the absolute auxiliary point data in particular describe the
position of at least two auxiliary points of the pelvis relative to
the marker device. As mentioned above, the auxiliary points are
outside the main plane. In particular, the actual auxiliary points
are on the same side of the main plane. In this way, the data
processing method can be based on data which can be easily obtained
by a surgeon. In particular, the absolute auxiliary point data
describe the position of only two auxiliary points.
[0050] In accordance with another embodiment, the absolute main
point data describe the position of only two main points of the
pelvis relative to the marker device or two virtual main points
have been calculated (or can be calculated) or there is one actual
main point and one virtual main point, then one additional (virtual
or actual) main point is in particular missing in order to
determine the main plane since only two (actual or virtual) main
points are available for the calculation. In this case, the
absolute auxiliary point data preferably describe the position of
at least one auxiliary point of the pelvis relative to the marker
device, in particular the position of only one auxiliary point of
the pelvis relative to the marker device. The auxiliary points are
in particular outside the main plane and in particular on the same
side of the main plane. Thus, the data processing method can in
this case again be performed on the basis of data which are easily
generated.
[0051] In accordance with another embodiment, the relative position
data describe at least one constraint, in particular only one
constraint for the possible positions of the main plane relative to
the at least one auxiliary point, if the absolute main point data
describe the position of two main points, in particular of only two
main points. In this case, there is in particular only one
auxiliary point.
[0052] In accordance with another embodiment, the relative position
data describe at least two constraints for the possible positions
of the main plane relative to the at least one auxiliary point, if
the absolute main point data describe the position of only one main
point. In this case, the absolute auxiliary point data describe the
position of at least two auxiliary points, in particular of only
two auxiliary points.
[0053] As mentioned above, the main plane is the mid-sagittal plane
in accordance with a preferred embodiment.
[0054] In the field of navigated surgery, in particular
computer-assisted surgery or image-guided surgery, a reference
system (for example a coordinate system) in which the pelvis is
located is preferably determined. In order to determine such a
coordinate system, two planes defined by the shape of the pelvis
are preferably determined. One of these planes is in particular the
mid-sagittal plane; the other plane can be the anterior pelvic
plane or--as will be explained in more detail below--a spinae joint
center plane as a new reference plane referred to as "SJCP". The
spinae joint center plane is defined by auxiliary points. In
summary, there are preferably at least two planes which have to be
determined in order to have an adequate basis for providing
navigation information to the surgeon. These two planes and the
resulting reference system are in particular used to define the
position and in particular the orientation of the acetabulum, in
particular for hip surgery.
[0055] The present invention also relates to a data processing
method which includes the above-mentioned data processing method
and not only allows the position of the main plane of the pelvis
but also an auxiliary plane of the pelvis to be determined.
[0056] The data processing method for determining the main plane
and the auxiliary plane preferably uses the above-mentioned data
processing method in order to determine the main plane. The
above-mentioned data processing method will therefore be referred
to as the first data processing method. The data processing method
for determining both the main plane and the auxiliary plane will be
referred to as the second data processing method. The second data
processing method uses the following approach for determining the
auxiliary plane. Absolute auxiliary point data are provided which
describe the position of at least two of the auxiliary points of
the pelvis relative to the marker device. Thus, contrary to the
first data processing method, there is preferably a minimum of two
auxiliary points, the position of which is known from the absolute
auxiliary point data. The auxiliary points are preferably outside
the main plane. The auxiliary points are in particular on the same
side of the main plane. Preferably, at least one of the at least
two auxiliary points is used for calculating the position of the
main plane. In accordance with another embodiment, the at least two
auxiliary points are used for calculating the position of the main
plane. Data are also provided which are referred to as relative
auxiliary plane data. These auxiliary plane data describe the
positional relationship between the auxiliary plane and the main
plane. Furthermore, it is assumed that the auxiliary points lie
within the auxiliary plane.
[0057] In accordance with the second data processing method, the
provided relative auxiliary plane data are preferably used to
calculate the position of the auxiliary plane on the basis of the
assumption that the at least two auxiliary points lie within the
auxiliary plane. In other words, the auxiliary plane is determined
in such a way that it includes the at least two auxiliary points
and fulfils the predetermined positional relationship with respect
to the main plane, said relationship being known from the relative
auxiliary plane data.
[0058] One example of the predetermined positional relationship is
a particular angle between the main plane and the auxiliary plane.
The angle can in particular be within the range of 30.degree. to
150.degree.. The predetermined positional relationship can in
particular be such that the auxiliary plane is perpendicular to the
main plane.
[0059] The invention is also directed to a third data processing
method which comprises the second data processing method explained
above. The third data processing method is a method for determining
the position of the main plane and of a standard plane, wherein the
position of the auxiliary plane has been determined by the second
data processing method. The auxiliary points lying in the auxiliary
plane are in particular landmarks of the pelvis. Examples of
auxiliary points will be given below.
[0060] In accordance with the third data processing method,
relative standard plane data are also provided. The relative
standard plane data describe the expected relative positional
relationship between the auxiliary plane and the standard plane.
The expected relative positional relationship is a positional
relationship which can correspond to an average positional
relationship derived from a statistical analysis of the positional
relationship between the auxiliary plane and the standard plane for
a plurality of different pelvises. In particular, the average
positional relationship which results from this statistical
analysis represents the expected positional relationship. Any kind
of (statistical) method for determining an average can be applied,
for instance the arithmetic mean or the median or the mode. A
generic model of the pelvis can also be used to determine the
relative positional relationship between the auxiliary plane of the
generic model and the standard plane of the generic model. This
positional relationship also represents an example of an expected
positional relationship.
[0061] In accordance with the third data processing method, the
position of the standard plane is determined on the basis of the
position of the auxiliary plane (determined using the second data
processing method) and on the basis of the relative standard plane
data. For instance, the relative standard plane data describe an
angle between the position of the auxiliary plane and the position
of the standard plane. Thus, the determined position of the
auxiliary plane allows the position of the standard plane to be
calculated on the basis of the standard plane data. A standard
plane is in particular a plane with respect to which the position
of the femoral shaft and/or the position (in particular
orientation) of the acetabulum is (usually) described.
[0062] As mentioned above, the auxiliary points can represent
landmarks in any of the data processing methods described above
(i.e. the first, second and/or third data processing method).
Examples of landmarks represented by the auxiliary points include
the sinistral ASIS point or dextral ASIS point. The auxiliary
points can alternatively or additionally represent a landmark which
is defined by the acetabulum and/or by a partial (surface) point of
the acetabulum. The auxiliary point can then in particular be the
rotational center or the deepest part of the acetabulum or the most
proximal part of the acetabulum. The main plane in any of the data
processing methods described above (i.e. the first, second or third
data processing methods) can for example be the mid-sagittal plane.
The standard plane as described with respect to the third data
processing method can for example be the anterior pelvic plane. The
relative standard plane data described in connection with the third
data processing method describe in particular the angle between the
anterior pelvic plane and the auxiliary plane. The auxiliary plane
(for example the SJCP) is in particular a plane described by the
sinistral and dextral ASIS point and an auxiliary point defined by
the acetabulum or a point or part of the acetabulum. The auxiliary
plane is in particular such that the auxiliary plane and the
anterior pelvic plane intersect each other along a line connecting
the sinistral and the dextral anterior superior iliac spine
landmarks of the pelvis. The present application is also directed
to an independent invention which is directed to a data processing
method which relies on the relative standard plane data which
describe the relative position between the anterior pelvic plane
(APP) and the spinae joint center plane (SJCP). Thus, preferably
any data processing method or system or computer etc. is a
subject-matter of the present invention which relies on this kind
of standard plane data. The inventors of the present invention have
first found that the APP and SJCP have a statistically stable
relative positional relationship which may be used in any kind of
data processing methods which processes data related to the pelvis.
Therefore, the present application is also directed to such an
independent invention which may be a subject-matter of a later
divisional application. These data processing methods are in
particular directed to a calculation of positions, in particular
orientations of planes or parts or points of the pelvis for
computer aided navigation in surgery (image guided surgery). Such a
data processing method is in particular as follows: A data
processing method for determining the position of a plane or a part
or a point of pelvis which comprise the steps of: providing
relative standard plane data which describe the expected positional
relationship between the SJCP and the APP; and determining the
position of the APP on the basis of the position of the SJCP and
the relative standard plane data.
[0063] The following embodiment can be combined with the
aforementioned methods or can also represent an independent
embodiment of the invention. In accordance with this embodiment,
additional auxiliary points are determined which are in particular
symmetrical to another auxiliary point if the main plane is taken
as the plane of symmetry. Said other auxiliary point (which
corresponds to the additional auxiliary point) is referred to as
the symmetrical auxiliary point. The (at least one) symmetrical
auxiliary point is included in the absolute auxiliary point data,
whereas the (at least one) additional auxiliary point is not
included. As an independent embodiment of the invention, the (at
least one) symmetrical auxiliary point is provided in a step of the
independent data processing method. In accordance with this
embodiment, relative auxiliary point data are provided. These
relative auxiliary point data describe at least one constraint for
possible positions of the additional auxiliary point relative to
its corresponding symmetrical auxiliary point. When the symmetrical
auxiliary point is on one side of the main plane, the corresponding
additional auxiliary point is in particular on the other side. In
accordance with this embodiment, candidate point data are provided.
The candidate point data describe the position of a candidate point
with respect to the marker device. These candidate points are
candidates for the additional auxiliary point. New candidate point
data which describe the position of new candidate points relative
to the marker device are preferably provided in steps. All the
candidate points provided are preferably on the same side of the
main plane. The new candidate point data are preferably received in
steps, such that candidate point data are provided in steps. A
candidate point received in the current step is referred to as the
current candidate point. In accordance with this embodiment, the
next step consists of checking whether the position of the current
candidate point (i.e. the new candidate point) complies with the at
least one constraint. The at least one constraint is described by
the relative auxiliary point data as mentioned above. An example of
the constraint is in particular a distance between the symmetrical
auxiliary point and its corresponding additional auxiliary
point.
[0064] After the checking step, the current candidate point (new
candidate point) is preferably accepted as an additional auxiliary
point if the position of the current candidate point complies with
the at least one constraint defined by the relative auxiliary point
data, i.e. if the position of the current candidate point is a
possible position. If the position of the current candidate point
does not comply with the at least one constraint, then the
aforementioned steps (of providing and checking) are repeated.
Since the additional auxiliary point is symmetrical to the
symmetrical auxiliary point with respect to the main plane, the
position of the main plane can be determined on the basis of the
additional auxiliary point and the symmetrical auxiliary point. The
new candidate point data are preferably generated by detecting a
point which contacts an additional anatomical structure which is
symmetrical to another anatomical structure (referred to as the
symmetrical structure). This structure is in particular a prominent
structure (for example a rim or crest). A point detected on the
symmetrical structure corresponds to the symmetrical auxiliary
point. A plurality of points detected on the additional anatomical
structure are candidate points for the additional auxiliary point.
During the movement of the pointer along the additional anatomical
structure, new candidate points are generated in steps. In order to
guide the user during the movement of the pointer, indication data
are preferably provided. Both the additional auxiliary point and
the symmetrical auxiliary point are preferably determined on the
basis of the above-mentioned detection points (representing
detected landmarks) and in particular not on virtual auxiliary
points.
[0065] In accordance with another embodiment, the aforementioned
method comprises additional steps which relate to generating
indication data. These indication data are in particular based on
the result of the checking step and indicate whether the position
of the candidate point complies with the at least one constraint.
These indication data are preferably used by an indication method
which comprises the aforementioned data processing method and which
generates indication signals on the basis of the indication data in
order to inform a user (surgeon) as to whether the current
candidate point complies with the constraint. Preferably, a
deviation between a scalar value describing the constraint (for
instance a distance) and a scalar value describing the position of
the current candidate point relative to a main point or the
particular auxiliary point is calculated. The result of the
calculation is then preferably part of the indication data and is
thus also indicated to the user as part of the indication signal. A
surgeon can use the above-mentioned data processing method as an
assistance in finding an auxiliary point when scanning the pelvis
with a pointer.
[0066] The present invention is also directed to a program which,
when it is running on a computer or is loaded onto a computer,
causes the computer to perform the data processing method as
described in any one of the preceding embodiments. The present
invention is also directed to a program storage medium (for
instance a CD, ROM, RAM, harddrive, etc.) on which the program is
stored. The present invention also relates to a computer on which
the program is running, by which the program is executed or on
which the program is loaded. This computer in particular comprises
a memory which stores the program. The present invention is also
directed to a signal wave, in particular a digital signal wave,
which carries the information represented by the program. A signal
wave carries the program for example during a download process when
downloading the program via the internet.
[0067] The present invention is also directed to a navigation
system for computer-assisted surgery. This navigation system
preferably comprises the aforementioned computer for processing the
data provided in accordance with the data processing method as
described in any one of the preceding embodiments. Preferably, the
navigation system comprises a detection device for detecting the
position of the detection points which represent the main points
and auxiliary points, in order to generate detection signals and to
supply the detection signals generated to the computer such that
the computer can determine the absolute main point data and
absolute auxiliary point data on the basis of the detection signals
received. In this way, the absolute point data can be provided to
the computer. The navigation system also preferably comprises a
user interface for receiving the calculation results from the
computer (for example the position of the main plane, the position
of the auxiliary plane and/or the position of the standard plane).
The user interface provides the received data to the user as
information. Examples of a user interface are a monitor or a
loudspeaker. The user interface can use any kind of indication
signal (for example a visual signal, an audio signal and/or a
vibration signal).
[0068] Where data are described here as being "provided", this
means that they are ready for use by the method in accordance with
the invention. The data can achieve this state of being "provided"
by for example being detected or captured (for example by a
detection device) or by being inputted (for example via interfaces)
or by being determined on the basis of input signals or detection
signals or input data. The data can also have this state by being
stored in a memory (for example a ROM, CD and/or hard drive) and
thus ready for use within the framework of the method in accordance
with the invention.
[0069] The expression "providing data" encompasses (within the
framework of a data processing method) in particular that the data
are determined by the data processing method or program. The
meaning of "providing data" in particular encompasses also that the
data are received by the data processing method or program (e.g.
from another program or a data storage), in particular to further
process the data by the data processing method or program. Thus
"providing data" can mean for instance to wait for a reception of
data and to receive the data. The received data can be for instance
inputted by the interface. "Providing data" can also mean that the
data processing method or program performs steps to (actively)
acquire the data from a data source, for instance a data storage
(for instance ROM, RAM, data base, hard disk etc.) or via the
interface (for instance from another computer or a network). The
data can achieve the state of being "ready for use" by performing a
further step before the providing step. According to the further
step, the data are generated for providing the data. In particular
the data are detected or captured (for example by an analytical
device). Alternatively or additionally, the data are input
according to the further step, for instance via interfaces. In
particular, the generated data can be input (for instance in the
computer). According to the further step (before the providing
step), the data can also be provided by performing the further step
of storing the data in a data storage (for example a ROM, RAM, CD
and/or hard drive) and thus ready for use within the framework of
the method or program in accordance with the invention. The step of
providing in particular does not encompass or comprise an invasive
step representing a substantial physical intervention on the body
which requires professional medical expertise to be carried out
and/or which entails substantial health risk even when carried out
with the required professional care and expertise. The step of
providing does in particular not comprise or encompass a surgical
step. The step of providing does in particular not comprise or
encompass a step for treatment of a human or animal body by surgery
or therapy. The same applies for any steps directed to the
determination of data.
[0070] In order to detect points on a plane, for instance actual
main points and/or actual auxiliary points, a tool called plane
point detection tool can be used. The present invention is also
directed to the use of such plane point detection tools in
combination with the above described method and navigation
system.
[0071] In particular, an independent aspect of the present
invention is directed to such a tool called plane point detection
tool. The plane point detection tool (also just called "tool" in
the following) is for detecting the position of at least one actual
main point and/or at least one actual auxiliary point. The plane
point detection tool is preferably constituted to be arranged in a
plane of the anatomical body part (e.g. main plane or auxiliary
plane).
[0072] The present application is in particular directed to the
independent invention of the plane point detection tool which can
be claimed in particular as follows:
[0073] Plane point detection tool comprising
[0074] a proximal side constituted to be fitted to a patient by
contacting the patient along a first direction of length-wise
extension of the tool, the proximal side is constituted to be able
to be aligned with a plane of an anatomical body part along the
first direction of length-wise extension, the direction in which
the proximal side faces and along which the tool is flexibly
bendable for bringing the tool into contact with the patient being
a third direction of extension of the tool;
[0075] a distal side which comprises at least one pointer insertion
member comprising a tapered recess for inserting the tip of a
pointer, the at least one pointer insertion member being
prominently positioned in a first section of the distal side which
is, in the direction of length-wise extension, closer to a first
end of the plane point detection tool than to a second end;
[0076] wherein the distal side further comprises a second section
having a flat surface and being closer to the second end than to
the first end;
[0077] both the proximal side and distal side extending in the
first direction and in a second direction of extension, the second
direction being traverse to the first direction,
[0078] the tool being flexibly bendable in the third direction
while being rigid in the second direction, the second direction
being a direction of width-wise extension of the tool, the
width-wise extension of the tool being smaller than the length-wise
extension.
[0079] The aforementioned tool can in particular be part of the
navigation system of the present invention and can in particular be
used in combination with the method, in particular the data
processing method of the present invention. In particular, the tool
is used for generating detection signals by means of a pointer
which contacts the bottom of the tapered recess of the plane point
detection tool. The tapered recess is in particular constituted
such that the pointer can be pivoted during that contact.
[0080] The plane point detection tool preferably comprises a front
side and a back side. The front side is close to the user (e.g.
member of the medical staff, in particular surgeon) who inserts the
pointer into the at least one tapered recess of the tool. The
backside of the tool is to be attached to the patient and is
therefore called proximal side. Correspondingly, the front side is
called distal side. According to a preferred embodiment for using
the tool, the tool is attached to the backside of the pelvis in the
region of the sacrum along the midsagittal plane. That is, the
direction of longitudinal extension of the tool is aligned with the
midsagittal plane. Preferably, the at least one pointer insertion
member of the distal side is arranged at one end or close to one
end of the longitudinal extension of the tool. In particular, in
longitudinal extension of the tool, the pointer insertion members
are positioned in one section of the distal side (called "first
section") while no pointer members are present in the reminder of
the distal side (called "right section"). In this way, the
detection of points which are lying in a plane, in particular the
midsagittal plane, and which are not accessible can be replaced by
detection of the position of the pointer insertion members which
are accessible since they are placed closed to one end (called
first end) of the tool (in longitudinal direction), in particular
within one half (called first half) of the tool (in the first
section). In particular the second section is within the other half
of the tool or comprises the other half of the tool (called
2.sup.nd half). Thus, the at least one pointer insertion member is
(in longitudinal direction) closer to the first end of the tool
than to the other end (called second end) of the tool. Preferably,
that one of the at least one pointer insertion members which is
closest to the second end has at least a distance of 5 cm, 10 cm,
15 cm or 20 cm from the second end, preferably at least 15 cm. In
this way, the pointer insertion members do not interfere with
fixing devices which are used to fix the anatomical body part in
particular during surgery and which contact the right section but
do not contact the left section.
[0081] Preferably, the plane point detection tool (shortly just
called "tool") comprises at least two pointer insertion members in
order to detect two points in a plane of the anatomical body part.
Preferably, the distance between that one of the two pointer
insertion members which is closest to the first end and that one of
the pointer insertion member which is closest to the second end is
at least 1 cm, 3 cm, 5 cm or 10 cm, preferably at least 3 cm. The
greater the distance, the higher is the accuracy of detection of a
position, in particular orientation of a plane. Preferably, that
one of the at least two pointer insertion members which is closest
to the second end is (still) closer to the first end than to the
second end. According to a further embodiment, two pointer
insertion members have a distance of preferably at least 5 cm or 10
cm or 15 cm (preferably at least 10 cm) and in particular allow a
patient fixing device to be positioned in between. In particular in
that case, that one of the at least two pointer insertion members
which is closest to the second end is closer to the second end than
to the first end.
[0082] Preferably, the distal side comprises a flat surface
section, in particular plane surface section which is closer to the
second end than to the first end. The length of these flat surface
portion (in longitudinal direction of the tool) is preferably
longer than 5 cm, 10 cm, 15 cm or 20 cm. Due to the flat surface
section, pressing of a fixing device to an anatomical body part is
not obstructed. In particular, the flat surface section adopts at
least a part of the second half of the distal side which is closest
to the second end.
[0083] Preferably, the left section comprises a flat section facing
in distal direction. Preferably, the pointer insertion members are
projecting from the flat surface section which is closer to the
first end than to the second end. In this way, the pointer
insertion members are palpable (can be sensed by a member of the
medical stuff) due to difference in height of the flat section and
the distal end of the prominent pointer insertion member, even
through a drape which covers the distal side of the tool. Thus, it
is possible to insert a pointer tip into the pointer insertion
member although there is a drape between the pointer tip and the
tapered recess of the pointer insertion member. In this way, it is
possible that the drape can rest on the patient during detection of
the points lying in the plane of the anatomical body part (in
particular midsagittal plane).
[0084] Preferably, the proximal side is constituted to be fitted to
a patient. The proximal side of the tool is in particular fitted to
a patient by contacting the proximal side with the patient. The
contact is in particular along the first direction of longitudinal
(length-wise) extension of the tool. Preferably, there are a
plurality of contacts, in particular a multiplicity (manifold) of
contacts between the patient and the tool in order to achieve the
fitting. To this end, the proximal side for instance comprises an
adhesive which is constituted to adhere the tool to the patient's
body. According to another embodiment, the proximal side comprises
a plurality of sucking portions (e.g. sucking cups) which allow to
fit (in particular fix) the tool to the body.
[0085] Preferably, the plane point detection tool is flexible if
flexed in proximal direction. This proximal direction is called
third direction. The third direction is in particular normal to the
surface of the proximal side. Since the tool is flexible in the
third direction, the tool can be attached to a curved surface of
the body part. Preferably, the tool is rigid in a second direction.
In other words, the tool is preferably not bendable in the second
direction. The second direction is preferably perpendicular to the
third direction and to the first direction. The second direction is
preferably a width-wise direction of the tool while the first
direction is preferably a length-wise (longitudinal) direction of
the tool. Preferably, the length of the longitudinal extension of
the tool is longer than the length of the width-wise extension of
the tool. Preferably, the length-wise extension of the tool is
longer than three times, five times or ten times of the width-wise
extension of the tool. Due to the longer extension of the tool in
the lengthwise direction than in the width-wise direction an
adaptation of the shape of the tool to the shape of the surface of
the body of the patient is possible although the tool is rigid in
the widthwise direction. Preferably, the width of the tool in
width-wise direction (at least with respect to the second section)
is smaller than 5 cm, 3 cm, 2 cm or 1 cm and/or larger than 1 mm, 2
mm or 5 mm. The width of the tool at the first section can be wider
than at the second section.
[0086] Preferably, the tool is constituted to be fitted to the body
at least within a part of the section which is opposite to the
second section at the proximal side.
[0087] In this way, the tool is attachable to the body at least in
a section of the tool which is assumed to be obstructed by the
aforementioned fixing device while the fixing device can be brought
into contact with the anatomical body part and the second section
without interfering of the plane point detection tool. In
particular, the fixing device can exert pressure on the anatomical
body part via the tool. Preferably, to this end, the thickness of
the plane point detection tool is smaller than the length and in
particular also smaller than the width of the plane point detection
tool. Preferably, the thickness (i.e. the extension of the tool in
the third direction) is preferably smaller than a half, in
particular smaller than a fifth, in particular smaller than a tenth
of the width of the tool. The thickness of the tool (at least with
respect to the second section) is preferably smaller than 1 cm, 5
mm, 2 mm or 1 mm. The aforementioned definitions of the thickness
of the tool apply in particular for the second section of the tool
which is close to the second end and which comprises or consists of
a flat surface. The first section which includes at least one
pointer insertion member can have a maximum thickness which is
larger than the thickness as defined before. In particular, the
thickness at the position of the pointer insertion members can be
more than 1 mm or 2 mm or 5 mm or 1 cm thicker than the thickness
of the second section (in particular due to the height of the
pointer insertion member) and/or can be a half or equal to or twice
the width of the plane point detection tool.
[0088] In the following detailed description, other features of the
present invention are disclosed. The features of different
embodiments can be combined.
[0089] FIG. 1 shows a navigation system in accordance with the
invention;
[0090] FIG. 2 shows the use of main and auxiliary points for
determining the mid-sagittal plane;
[0091] FIG. 3 shows a plane point detection tool;
[0092] FIGS. 4 to 6 show situations of use of the plane point
detection tool.
[0093] FIG. 1 schematically shows a pelvis 10. Attached to the
pelvis 10 is a reference star 20. There is a fixed spatial
relationship between the reference star 20 and the pelvis 10. A
surgeon can contact parts, in particular landmarks, of the pelvis
10 by means of the tip of a pointer 30. Attached to the pointer 30
are markers, in particular another reference star 40. The location
of the reference stars 20 and 40 (and the corresponding markers)
can be detected by the detection device 50. The process of
contacting the pelvis with the pointer and detecting the markers of
the pointer is also referred to here as "scanning". The detection
device 50 supplies the detection signals to the computer 60. The
computer 60 includes a database which stores the relative spatial
relationship between the markers of the pointer and the tip of the
pointer. In the computer 60, the absolute point data are then
determined on the basis of the detection signals received from the
detection device 50. The relative point and/or pelvis data can be
input or can be stored in the database. Due to this determination,
the absolute point data are available and thus provided.
[0094] A keyboard 70 and mouse 80 and a monitor 90 are for example
connected to the computer 60. The monitor 90 serves to display
information to the user such as the results of the processing
performed in accordance with a data processing method of the
invention and/or indication signals as already mentioned above.
[0095] FIG. 2 shows the pelvis 10 in more detail. The attached
reference star 20 is not shown. FIG. 2 also shows a landmark TSP
which is the ASIS point. This landmark TSP has been scanned by the
pointer 30, and the corresponding position information has been
supplied to the computer 60 of the navigation system. The
navigation system comprises the detection device 50, the computer
60 and the monitor 90. Two landmarks MP1 and MP2 have also been
scanned by the pointer 30. These landmarks MP1 and MP2 lie in the
mid-sagittal plane 100. For instance, in the following the points
MP 1 and MP2 are assumed to be points on the sacrum and lumbar
spine. The point TSP is an example of an auxiliary point, and the
points MP1 and MP2 are examples of main points. Another main point
would be the pubic landmark 110. However, in the following, it is
assumed that the absolute main point data describe only the
position of MP1 and MP2 and that the absolute auxiliary point data
describe only the position of TSP. In the following, it will be
described how the position of the main plane (the mid-sagittal
plane 100) is determined on the basis of only two main points and
only one auxiliary point.
[0096] In accordance with one possible procedure, a line 120 is
determined by MP1 and MP2. A line 130 is drawn from the point TSP
and intersects the line 120 at a right angle. Thus, the line 130 is
perpendicular to the line 120. The intersection between the line
120 and the line 130 defines the center of a circle 140. The circle
140 includes the point TSP and another point NTSP. The point NTSP
is a virtual auxiliary point (i.e. NTSP has not been scanned). The
point NTSP is symmetrical to the point TSP with respect to the
mid-sagittal plane 100. In other words, if the point TSP is
mirrored in the mid-sagittal plane 100, this would result in the
point NTSP. The point NTSP is the solution which is to be found.
Relative point data are used to find the point NTSP. The relative
point data describe the distance between NTSP and TSP. This
distance can for example be derived from an x-ray of the pelvis 10.
The point NTSP has to be on the circle 140 and has to fulfill the
constraint, i.e. has to have the predetermined distance from the
point TSP. There are two possible solutions for these constraints,
namely NTSP and the point 150. The point 150 is an incorrect
solution. The pelvis data are used to discount the incorrect
solution 150 from the two possible solutions. The pelvis data
describe in particular anatomical characteristics of the pelvis.
These data describe in particular that the point NTSP is more
anterior than the point MP1. The point 150, however, is more
posterior than MP1. Therefore, this possible solution can be
discounted. The solution NTSP is therefore selected, since this
point complies with the anatomical characteristic that the solution
has to be more anterior than MP 1. The constraint that NTSP has to
be more anterior than MP1 represents an example of the possible
relative positions between MP1 and NTSP. These possible relative
positions are constraints described by the pelvis data. The circle
140 is incidentally also perpendicular to the line 120.
[0097] In accordance with another possible data processing method,
a sphere is constructed which has a radius corresponding to the
distance between MP1 and TSP. The center of this sphere is located
at the point MP1. A second sphere is also constructed which has a
radius corresponding to the distance between MP2 and TSP. The
center of the second sphere is located at the point MP2. The
intersection between these two spheres is the circle 140 which
corresponds to a multitude of possible solutions for the location
of the point NTSP. In the same way as mentioned above, this
multitude of possible solutions is firstly restricted to the point
NTSP and the point 150 by using the constraint which defines the
distance between NTSP and TSP. In a subsequent step, the pelvis
data are then used to discount the point 150 from the possible
solutions, such that the point NTSP remains as the only possible
solution.
[0098] As mentioned above, NTSP and TSP are symmetrical to each
other with respect to the mid-sagittal plane. Therefore, the
mid-sagittal plane is defined as being perpendicular to the line
which connects NTSP and TSP, hence the mid-sagittal plane is
determined.
[0099] In accordance with another preferred step, another
plane--the auxiliary plane--is determined, with the aim of defining
a coordinate system in which the pelvis rests. For this purpose,
another auxiliary point 160 is provided. To this end, the landmark
corresponding to the auxiliary point 160 is preferably scanned by
the pointer 30. The auxiliary plane is then defined as being
perpendicular to the mid-sagittal plane 100 and including the two
auxiliary points TSP and 160. The auxiliary point 160 is preferably
based on the acetabulum. It can for example be the rotational
center of the femoral head in the acetabulum or the bottom of the
acetabulum (fossa) or the center of the rim of the acetabulum, to
name but a few examples. The inventors of the present application
have found that there is a reliable and fixed spatial relationship
between a standard plane (APP) and the aforementioned auxiliary
plane which includes a landmark defined by the acetabulum and the
ASIS point as another auxiliary point. The point TSP preferably
corresponds to the ASIS point, in which case the auxiliary plane
has a defined positional relationship to the anterior pelvic plane
(APP) which is often used as a reference for defining the
orientation of the acetabulum. This standard plane is also
perpendicular to the mid-sagittal plane and has an angle of about
40.degree. with respect to the aforementioned plane which includes
the ASIS point and the acetabulum landmark point. This auxiliary
plane is also referred to here as the spinae joint center plane
(SJCP).
[0100] In the manner described above, at least two planes are
defined which allow a coordinate system to detected defined. The
planes can for example be the mid-sagittal plane and the auxiliary
plane (in particular the SJCP) and/or can be the mid-sagittal plane
and the APP.
[0101] In the following, the data processing method is described
for the scenario in which there is only one main point on the
mid-sagittal plane. In this case, the relative point data
preferably comprise two constraints or more specifically two scalar
values. One scalar value describes the distance between the point
NTSP and the point TSP. The other scalar value describes the
distance between the acetabular or auxiliary point 160 and the
symmetrical acetabular or auxiliary point 160'. This distance is in
particular the aforementioned inter-fossa distance. In other words,
the symmetrical acetabulum 160' is symmetrical with respect to the
mid-sagittal plane, i.e. the point 160' is a mirrored point of the
acetabular point 160.
[0102] The two scalar values, i.e. the two distances, are in
particular known from x-ray images. The distance between the point
TSP and the point 160 is also known due to the absolute auxiliary
point data. In a subsequent step, a line can be constructed on the
basis of the TSP point and the point 160. This line crosses the
mid-sagittal plane 100. The distance between the point 160 and the
mid-sagittal plane 100 along this line can be calculated on the
basis of geometric laws by using the aforementioned two distances
between the point TSP and the point NTSP and between the point 160
and the point 160'. Thus, the position at which the line which
includes the point TSP and the point 160 crosses the mid-sagittal
plane can be calculated. Thus, a second point on the mid-sagittal
plane is known. The procedure already known from the above
description can then be applied in order to determine the position
of the point NTSP or the point 160'. If these positions are known,
then the mid-sagittal plane is defined as being perpendicular to
the line connecting the point NTSP and the point TSP and/or
perpendicular to the line connecting the point 160' and the point
160. The SJCP or the APP can also then be calculated in the manner
described above.
[0103] The above-mentioned inter-teardrop distance can be measured
in an anterior/posterior x-ray image of the pelvis. Within an
extensive analysis of CT data sets it had be found by the inventors
that the inter-teardrop distance as well as the inter-fossa
distance are almost constant across various populations. Between
males and females there are significant differences. Thus, a
gender-specific value for the distances is preferably used as
relative point data. For fine-tuning purposes, the values could
also be adjusted to certain populations (e.g. Asian, European, US).
In particular, a fixed value for the distance can be assumed for a
specific population in order to provide the relative point
data.
[0104] The inter-teardrop and inter-fossa distances are in
particular defined as described in the following. The teardrop
figure is a well-known structure which can be identified in
anterior/posterior x-rays of the pelvis. Within the mentioned CT
data analysis, artificially generated x-ray images (i.e. digitally
reconstructed radiographs-DRRs) were used to reproduce the teardrop
figure for evaluation purposes. The teardrop figure depicts a
structure close to the medial wall of the acetabulum and can be
determined on each side of the pelvis. In particular the
medial-lateral position of the structure can be determined very
well. The inter-teardrop distance is a distance between the
teardrop figures on a medial-lateral line through the pelvis.
[0105] The fossa is a region within the acetabulum which is deeper
than the rest of the approximately spherical structure of the
acetabulum. It basically lies on the medial side of the acetabulum.
The inter-fossa distance is the distance between the fossa regions
on both sides of the pelvis in a medial/lateral direction. The
fossa region can be easily identified intra-operatively, e.g.
[0106] palpated with a pointer device and acquired with a
navigation system. For evaluation purposes, the fossa region can be
identified directly in the CT datasets.
[0107] The fossa region is an anatomical structure which
significantly influences the position of the teardrop figure (see
also Bowerman J W, Sena J M, Chang R: The teardrop shadow of the
pelvis; anatomy and clinical significance. Radiology 1982 Jun;
143(3):659-62). Thus, both distance values (inter-fossa distance
and inter-teardrop distance) are strongly correlated. For practical
purposes, the inter-teardrop and/or the inter-fossa distance can be
used.
[0108] Typical values for the distances are 112 mm (inter-teardrop)
and 116 mm (inter-fossa) for males. For females, the standard
values are 121 mm (inter-teardrop) and 125 mm (inter-fossa). For
Asian people, the values are a bit lower (110 mm inter-teardrop and
113 mm inter-fossa for males, 117 mm inter-teardrop and 119 mm
inter-fossa for males). Additionally, other procedures to determine
orientations of the pelvis or other planning parameters can be
based on the proposed distances as well. In particular the
orientation according to rotations around a cranial/caudal axis can
be determined. In particular information about the anteversion of
the acetabulum/cup implant e.g. for THR surgeries can be
determined.
[0109] Using the SJCP to define a coordinate system can be
advantageous, since the APP might be less reliably determined by
scanning, i.e. using a pointer to detect the landmarks on the APP.
The pubic landmark 110 in particular can introduce uncertainties
into the method due to differing amounts of fat and other soft
tissue above the pubic area. These tissues can prevent the position
of the point 110 from being accurately detected by means of a
pointer.
[0110] The coordinate systems defined in accordance with the method
of the present invention can be used for registration and also for
other tasks within planning and navigation steps. The method of the
present invention (i.e. the data processing method) is in
particular used for planning surgery and for computer-assisted
navigation. Because of the reliable and consistent relationship
between a coordinate system defined by the mid-sagittal plane and
SJCP and standard coordinate systems, all information can be
transferred back and forth between the coordinate systems.
[0111] In accordance with another embodiment, one main point is
scanned by the pointer on the mid-sagittal plane. In other words,
one main point is provided to the data processing method. One
additional (first) point on a structure, in particular a prominent
structure, of the pelvis is determined as an auxiliary point
(symmetrical auxiliary point). An example of the prominent
structure is the posterior aspect of the iliac crest. The data
processing method of the present invention, performed in particular
on the navigation system of the invention, then assists the user in
finding another (second) auxiliary point by issuing the indication
signal mentioned above. This second point is symmetrical to the
first point with respect to the mid-sagittal plane. For this
purpose, the navigation system shows additional information
(indication signals) such as distance values or angles. The
rationale behind this approach is that the user is assisted in
finding the second point which has the same relationship (for
example distance/angle) to the mid-sagittal plane as the first
point. The user can thus proceed along the prominent structure
until the system informs the surgeon that the desired relationship
has been found. In general, this approach can be used to find new
reference points (new auxiliary points) on an anatomical object
from given points and the specific relationships (geometric
features/constraints) between the given point and the points to be
determined. The navigation system according to the invention
assists in finding the new points (new auxiliary points) by
providing information concerning the geometric
features/constraints. The user can thus find these new auxiliary
points by locating them on a prominent structure. This prominent
structure can be close to the mid-sagittal plane.
[0112] The above-mentioned embodiment can in particular be used in
the case of lateral pelvic registration. The embodiment can be used
to find a second point which lies symmetrical to the first point
with respect to the mid-sagittal plane. The line between these two
points represents the medial-lateral direction. This can be used as
an additional constraint. It provides not only a one-dimensional
but a two-dimensional constraint. Thus, only one point on the
mid-sagittal plane is required in this case. In the same way as in
the previous embodiments, additional information (relative point
data and/or pelvis data) can be used to make the registration more
robust.
[0113] FIGS. 3a, 3b and 3c show an embodiment of a plane point
detection tool. FIG. 3a is a perspective view of the embodiment.
FIG. 3b is a side view of the embodiment. FIG. 3c is a top view of
the embodiment.
[0114] FIG. 3c shows the embodiment of the plane point detection
tool from the bird's eye view (top view). The arrow A shows the
direction of longitudinal (length-wise) extension of the tool. The
arrow B shows the direction of width-wise extension of the tool.
The width-wise extension of the tool is for example 1 cm and
preferably has an extension larger than 3 mm and/or smaller than 4
cm. This upper end and/or lower limit applies in particular for the
section "f" (second section) which is the section on the right side
in FIG. 3c and which extends from the second end of the tool over a
major part of the tool towards but not up to the first end. The
remaining part of the tool is called section "p" (first section).
This section p extends from the first end of the tool at least
until it includes that one of the pointer insertion elements which
is closest to the second end. According to an embodiment, the
section p (first section) and section f (second section) are
adjoining to each other. Section f includes in particular a major
part of the tool (in longitudinal direction of the tool). In
particular the section f is preferably constituted to be fitted to
a patient. To this end, preferably the distal side and/or the
proximal side of the tool is flat, in particular plane. FIG. 3c
shows the distal side of the tool 200. In particular, two pointer
insertion tools 210 and 220 are prominent, in particular prominent
from a flat portion 230 which preferably extends from the first end
to the second end. The flat portion 230 comprises in particular
opposite flat surfaces S1 and S2 (see FIG. 3b). The surface S2
faces towards the distal side and the surface S1 faces towards the
proximal side. The flat portion 230 has preferably a tape like
shape. In particular the surface Si and S2 have a rectangular
shape. Preferably the extension of the flat portion in the
direction A is longer than the extension in the width-wise
direction B for more than a factor 2, in particular 3, preferably
5. The flat portion 230 is preferably rigid in the direction B
while it can be bent in a direction C. That is, the flat portion
230 is preferably flexible in the direction C. That is, in the side
view from a direction as shown in FIG. 3b, the tool 200 can adopt a
curved or undulated shape as is indicated by the perspective view
shown in FIG. 3a. Preferably, the tool is rigid in the direction B,
i.e. cannot be bent. Rigid means in particular that the tool cannot
be bent (e.g. by at least 1 cm in the direction B at the second
end) by a force applyable by human being which is in particular a
force of more than 10 N and less than 1000 N. The force is applied
on one end of the tool (e.g. second end) while the other end (e.g.
first end) is fixed. Preferably, such a force which results in a
bending (e.g. by at least 1 cm in the direction (e.g. at the second
end) is smaller than 1000 N, in particular smaller than 100 N or 10
N if the force is applied in the direction C (at one end, e.g.
2.sup.nd end of the tool while fixing the other end, e.g. first end
of the tool).
[0115] Preferably, the pointer insertion elements 210 and 220 are
attached to the flat portion, for instance by gluing or by means of
a fixing element (for instance a screw or a rivet). The pointer
insertion tools have preferably prominent outside walls 211 and 222
(see FIG. 3b) which are preferably projecting from the flat surface
S2 into the distal direction. Inside the walls there is preferably
a recess, in particular pointed recess which allows to insert the
tip 310 of a pointer 300 (see FIG. 6). The recess has in particular
conical shape and is preferably tapered from the distal end of the
pointer insertion elements towards the second surface (i.e. in
proximal direction).
[0116] Besides the rectangular shape shown in FIG. 3, the tool can
for instance also have oblong or elliptical shape.
[0117] FIG. 4 shows the tool 200 in use. The tool 200 is attached
to the back of a patient is aligned with the midsagittal plane of
the pelvis. The alignment is possible by sensing the spinus process
of the sacrum. In particular, the tool 200 is attached by means of
an adhering layer which comprises the surface Si of the tool 200.
In this way, the tool 200 can be attached to the skin as shown in
FIG. 4. In other words the tool 200 has preferably the properties
of a sticky tape, in particular is constituted as a sticky tape.
Since the tool is flexible in the direction C, the tool can be
smoothly (snugly) fitted to the curved surface of the patient's
skin. Since the tool is small in width-wise direction, the rigid
property of the tool in the width-wise direction does not hinder
the attachment of the tool to the curved skin. Additionally, the
rigid property assures that the pointer insertion elements 210 and
220 are aligned in the same direction as the longitudinal extension
of the section f of the tool. Thus, if the longitudinal extension
of the section f is aligned with the midsagittal plane also a
virtual line which connects the at least two pointers 210 and 220
is aligned with the midsagittal plane. Thus the at least one
pointer insertion element (210 and 220) are positioned in the
midsagittal plane.
[0118] FIG. 5 shows the attachment of a patient fixing device 400
comprising a cushion 410 and a support structure 420 for the
cushion which is fixed to the couch on which the patient is lying.
The cushion 410 is in contact with the back skin of the patient and
the area of the pelvis and thus obstructs at least a part of the
section f of the tool 200. The fixing device is also called
posterior support.
[0119] Thereafter, as shown in FIG. 6 the patient can be draped as
usual. In particular a marker device (reference star 500) can be
fixed to the pelvis of the patient. After fixing the reference star
500 it is of advantage to teach the navigation system the position
of the plane (in particular midsagittal plane) relative to the
reference star 500 in order to help the surgeon during surgery by
means of computer assisted surgery (navigation). Without the tool
200, the problem would be here that the projecting parts of the
sacrum cannot be sensed (palpated) or are difficult to be sensed by
the surgeon or any other member of the medical staff. However, due
to the tool 200, the pointer insertion elements 210 and 220 can be
sensed (palpated) even through the drape since the pointer
insertion tools are prominent and their recesses are sensible. In
particular, the dimension of the opening of the recess in the
pointer insertion member is preferably larger than 1 mm, 3 mm or 5
mm and in particular smaller than 2 cm or 1 cm. Therefore, the
operator (surgeon or any other member of the medical staff) is able
to insert the tip 310 of the pointer 300 into the recess of the
pointer insertion member 210 and 220. In this way, the operator can
teach to the navigation system the positions of the tips of the
recesses 210 and 220 which are positioned in the midsagittal plane.
In particular the detection of the position of the pointer (by
detecting the markers of the pointer) allows to determine the
position of the tip of the pointer (due to data describing the
relative position between the markers of the pointer and the tip of
the pointer) and to thus generate absolute main point data.
* * * * *