U.S. patent application number 14/773553 was filed with the patent office on 2016-01-28 for method and apparatus for determining a leg length difference and a leg offset.
This patent application is currently assigned to Brainlab AG. The applicant listed for this patent is BRAINLAB AG. Invention is credited to Sabine KLING, Luise POITZSCH, Mario SCHUBERT, Melanie WEGNER.
Application Number | 20160022173 14/773553 |
Document ID | / |
Family ID | 48083146 |
Filed Date | 2016-01-28 |
United States Patent
Application |
20160022173 |
Kind Code |
A1 |
SCHUBERT; Mario ; et
al. |
January 28, 2016 |
METHOD AND APPARATUS FOR DETERMINING A LEG LENGTH DIFFERENCE AND A
LEG OFFSET
Abstract
A data processing method, performed by a computer, for
determining a leg length difference and a leg offset difference of
a patient's leg including a femur connected to a pelvis, comprising
the steps of: --determining a first landmark vector between a
femoral landmark and a second landmark at a first point in time;
--determining a second landmark vector between the femoral landmark
and the second landmark at a second point in time which is later
than the first point in time; --calculating an orthogonal
projection of the first landmark vector into a sagittal plane and
using the direction of the orthogonal projection of the first
landmark vector into the sagittal plane as a leg length direction;
--calculating a direction which is perpendicular to the sagittal
plane and using this direction as a leg offset direction; and
--calculating the leg length difference in the leg length direction
and the leg offset difference in the leg offset direction from the
first landmark vector and the second landmark vector.
Inventors: |
SCHUBERT; Mario; (Poing,
DE) ; WEGNER; Melanie; (Kirchseeon, DE) ;
KLING; Sabine; (Unterschlei heim, DE) ; POITZSCH;
Luise; (Hallertau, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BRAINLAB AG |
Feldkirchen |
|
DE |
|
|
Assignee: |
Brainlab AG
Feldkirchen
DE
|
Family ID: |
48083146 |
Appl. No.: |
14/773553 |
Filed: |
September 18, 2013 |
PCT Filed: |
September 18, 2013 |
PCT NO: |
PCT/EP2013/069358 |
371 Date: |
September 8, 2015 |
Current U.S.
Class: |
600/587 |
Current CPC
Class: |
A61B 5/1072 20130101;
A61B 5/4538 20130101; A61B 2034/2048 20160201; G06T 7/0014
20130101; A61B 5/7278 20130101; G06T 7/60 20130101; G06T 2207/30204
20130101; A61B 5/0077 20130101; G01B 11/14 20130101; A61B 5/1127
20130101; G06T 7/30 20170101; A61B 2090/3937 20160201; A61B 34/20
20160201; G06T 7/74 20170101; A61B 5/1075 20130101; G06T 3/60
20130101; A61B 2562/0219 20130101; G06K 9/52 20130101; A61B 5/1079
20130101; A61B 5/1121 20130101; A61B 2034/2055 20160201; A61B
2034/2068 20160201; A61B 2576/00 20130101; G06T 7/73 20170101; G06T
2207/30004 20130101 |
International
Class: |
A61B 5/107 20060101
A61B005/107; A61B 5/11 20060101 A61B005/11; A61B 5/00 20060101
A61B005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2013 |
EP |
PCT/EP2013/057024 |
Claims
1. A data processing method, performed by a computer, for
determining a leg length difference and a leg offset difference of
a patient's leg including a femur connected to a pelvis, comprising
the steps of: determining a first landmark vector between a femoral
landmark and a second landmark at a first point in time;
determining a second landmark vector between the femoral landmark
and the second landmark at a second point in time which is later
than the first point in time; calculating an orthogonal projection
of the first landmark vector into a sagittal plane and using the
direction of the orthogonal projection of the first landmark vector
into the sagittal plane as a leg length direction; calculating a
direction which is perpendicular to the sagittal plane and using
this direction as a leg offset direction; and calculating the leg
length difference in the leg length direction and the leg offset
difference in the leg offset direction from the first landmark
vector and the second landmark vector.
2. The method according to claim 1, wherein a orientation of the
sagittal plane is defined as being horizontal.
3. The method according to claim 1, wherein a orientation of the
sagittal plane is determined as the orientation of a surface of an
operating table on which the patient is located.
4. The method according to claim 1, wherein a orientation of the
sagittal plane is determined from a plurality of inertial sensor
data which are acquired from an inertial sensor attached to a tibia
of the leg while the femur is locked in position and the tibia is
flexed relative to the femur.
5. The method according to claim 3, wherein a first sagittal plane
is determined for the first landmark vector, and a second sagittal
plane is determined for the second landmark vector, and the
orthogonal projections of the landmark vectors into the
corresponding sagittal plane are calculated.
6. The method according to claim 1, wherein calculating the leg
length difference and the leg offset difference involves
calculating a first orthogonal projection of the second landmark
vector into the sagittal plane, calculating a second orthogonal
projection of the first orthogonal projection onto the projection
of the first landmark vector, determining the leg length difference
as the difference in length between the projection of the first
landmark vector and the second projection of the second landmark
vector, and calculating the leg offset difference as the difference
between the components of the first landmark vector and the second
landmark vector in the leg offset direction.
7. The method according to claim 1, wherein calculating the leg
length difference and the leg offset difference involves
calculating an orthogonal projection of the second landmark vector
into the sagittal plane, rotating the orthogonal projection of the
second landmark vector within the sagittal plane such that its
direction matches the direction of the projection of the first
landmark vector, determining the leg length difference as the
difference in length between the projection of the first landmark
vector and the rotated second landmark vector, and calculating the
leg offset difference as the difference between the components of
the first landmark vector and the second landmark vector in the leg
offset direction.
8. The method according to claim 1, wherein calculating the leg
length difference and the leg offset difference involves
calculating a difference vector between the first landmark vector
and the second landmark vector and decomposing the difference
vector into its components in the leg length direction and the leg
offset direction.
9. The method according to claim 1, comprising the steps of
obtaining neutral position data which define a neutral position of
the leg at the first point in time and obtaining second position
data which define a position of the leg at the second point in
time, wherein the second landmark vector is only determined if the
second position data match the neutral position data.
10. The method according to claim 9, wherein the position data are
acquired from an inertial sensor.
11. The method according to claim 1, wherein a landmark vector is
calculated from two landmark reference vectors, wherein each
landmark reference vector represents a vector between the
respective landmark and a common reference point and at least one
of the landmark reference vectors is determined using a light beam
which is emitted from a light beam source and pointed at an offset
point, the light beam source having a known distance from the
landmark and a known orientation relative to the direct line from
the light source to the landmark, by performing the steps of
acquiring the direction of the light beam and the distance between
the light beam source and the offset point, and calculating the
landmark reference vector from the known distance between the light
source and the landmark, the known orientation of the light source
relative to the direct line from the light source to the landmark,
the direction of the light beam, the distance between the light
beam source and the offset point and the reference offset between
the offset point and the reference point.
12. The method according to claim 11, wherein the second landmark
is a virtual landmark, namely the centre of rotation of the
acetabulum; one of the landmark reference vectors is a virtual
landmark reference vector for the virtual landmark and is
determined as the average of two auxiliary reference vectors,
wherein each auxiliary reference vector represents a vector between
an auxiliary landmark and the common reference point and comprising
the steps of obtaining neutral position data which define a neutral
position of the leg at the first point in time and obtaining second
position data which define a position of the leg at the second
point in time, wherein the second landmark vector is only
determined if the second position data match the neutral position
data, wherein the position data are acquired from an inertial
sensor, and wherein the respective auxiliary landmark is used as
the landmark.
13. A non-transitory computer-readable storage medium storing a
program which, when running on a computer, causes the computer to
perform the steps of determining a first landmark vector between a
femoral landmark and a second landmark at a first point in time;
determining a second landmark vector between the femoral landmark
and the second landmark at a second point in time which is later
than the first point in time; calculating an orthogonal projection
of the first landmark vector into a sagittal plane and using the
direction of the orthogonal projection of the first landmark vector
into the sagittal plane as a leg length direction; calculating a
direction which is perpendicular to the sagittal plane and using
this direction as a leg offset direction; and calculating the leg
length difference in the leg length direction and the leg offset
difference in the leg offset direction from the first landmark
vector and the second landmark vector.
14. A device for determining a leg length difference and a leg
offset difference of a patient's leg including a femur connected to
a pelvis, comprising a computer that executes a program that causes
the computer to determine a first landmark vector between a femoral
landmark and a second landmark at a first point in time; determine
a second landmark vector between the femoral landmark and the
second landmark at a second point in time which is later than the
first point in time; calculate an orthogonal projection of the
first landmark vector into a sagittal plane and use the direction
of the orthogonal projection of the first landmark vector into the
sagittal plane as a leg length direction; calculate a direction
which is perpendicular to the sagittal plane and use this direction
as a leg offset direction; and calculate the leg length difference
in the leg length direction and the leg offset difference in the
leg offset direction from the first landmark vector and the second
landmark vector.
15. The method according to claim 4, wherein a first sagittal plane
is determined for the first landmark vector, and a second sagittal
plane is determined for the second landmark vector, and the
orthogonal projections of the landmark vectors into the
corresponding sagittal plane are calculated
Description
[0001] The present invention relates to a data processing method, a
software program and an apparatus for determining a leg length
difference and a leg offset difference of a patient's leg including
a femur, a tibia and a knee, wherein the leg is connected to a
pelvis via a hip joint.
[0002] Some events, such as an accident or surgery, can change the
geometry of the leg and in particular the geometry of the femur. In
such cases, it is desirable to be able to compare the geometry
before and after the event. A leg length difference and/or leg
offset difference are of particular interest. The leg length
difference is the difference in the length of the leg before and
after the event and is typically measured in a direction of the
mechanical axis of the femur, which is identical or approximate to
a sagittal, superior-inferior or cranial-caudal direction. The leg
offset difference is the change or displacement of the leg in a
lateral direction. The leg length difference is given in a leg
length difference direction, and the leg offset difference is given
in a leg offset direction, wherein the leg length difference
direction and the leg offset direction are in particular orthogonal
to each other.
[0003] An overview of some of the terms used in this document shall
now be given in order to render the rest of the document more
comprehensible.
[0004] A landmark is typically a defined element of an anatomical
body part which is preferably always identical or recurs with a
high degree of similarity in the same anatomical body part of
multiple patients. A landmark can be a natural landmark which can
be easily determined, in particular sampled using a pointer or any
other suitable device. Typical natural landmarks are for example
the epicondyles of a femoral bone. A landmark can however also be
an artificial landmark, such as for example a particular structure
which is rigidly attached to the anatomical body part, such as for
example a screw. A landmark can also be a virtual landmark which
cannot be directly sampled but can be determined indirectly. An
example of a virtual landmark is the centre of rotation of the
femur, which is in particular the centre of the femoral head.
[0005] The position of an object or a point, i.e. its spatial
location in up to three spatial dimensions and/or its alignment in
up to three rotational dimensions, is preferably determined in a
global co-ordinate system. Examples of a global co-ordinate system
include an earth-fixed co-ordinate system which can be defined by
parameters such as a gravity vector and the magnetic field of the
earth, and an artificial global co-ordinate system which can be
defined by field generators which generate a magnetic and/or
electric field.
[0006] A reference point is a point with a static position in a
particular co-ordinate system, in particular in the global
co-ordinate system. A landmark reference vector is a vector between
a landmark and the reference point. A landmark vector is a vector
between two landmarks and can be acquired directly or can be
calculated from two landmark reference vectors. The two landmarks
and the reference point either lie on the same line or constitute
the corners of a triangle, hence if the two landmark reference
vectors between the reference point and the respective landmarks
are known, the landmark vector between the two landmarks can be
unambiguously calculated.
[0007] The term "direction" means the alignment or orientation of a
line in which a vector lies. The direction of a vector from a first
point to a second point is unambiguous, while the direction of a
vector between a first point and a second point is ambiguous
because it can be the direction from the first point to the second
point or the direction from the second point to the first point.
However, this ambiguity is irrelevant as long as it is resolved for
any calculation where necessary. A vector has a direction and a
magnitude, which means the vector's length.
[0008] In standard navigated procedures, the leg length difference
and the leg offset difference are determined using markers which
are attached to the femur and the pelvis. The positions of the
markers, and therefore of the femur and the pelvis, are tracked
using a medical navigation system. One popular implementation of a
medical navigation system uses a stereoscopic camera which captures
an image of a marker. The medical navigation system determines the
absolute position of the pelvis and the femur in a particular
co-ordinate system and calculates the positional relationship
between them. The inventors of the present invention have found a
different approach for determining the leg length difference and
the leg offset difference which does not require attaching markers
to the femur and the pelvis.
[0009] In a brief summary, which is not to be understood as
limiting the present invention in any way, it may be said that this
approach analyses the vector between two landmarks at different
points in time. This so-called landmark vector can be acquired
directly or can be calculated from vectors between the respective
landmarks and a common reference point, for example for an easy
integration into the surgical procedure. A landmark reference
vector is in particular determined from the orientation of a laser
beam which is aimed at a particular point, wherein the position of
the laser beam source relative to the landmark is known, for
example by being measured. The point at which the laser beam is
directed is either the common reference point or an offset point
with a known location relative to the common reference point.
[0010] The present invention is defined by the subject-matter of
any appended independent claim. Advantages, advantageous features,
advantageous embodiments and advantageous aspects of the present
invention are disclosed in the following and contained in the
subject-matter of the dependent claims. Different advantageous
features can be combined in accordance with the invention wherever
technically expedient and feasible. In particular, a feature of one
embodiment which has the same or a similar function to another
feature of another embodiment can be exchanged with said other
feature. In particular, a feature of one embodiment which adds an
additional function to another embodiment can be added to said
other embodiment.
[0011] The present invention relates to a method for determining a
leg length difference and a leg offset difference of a patient's
leg including a femur. The method comprises the steps of
determining a first landmark vector between a femoral landmark and
a second landmark at a first point in time; and determining a
second landmark vector between the (identical or same) femoral
landmark and the (identical or same) second landmark at a second
point in time which is later than the first point in time. The
first and second landmark vectors represent the positional
relationship between the two points (the femoral landmark and the
second landmark) at two different points in time. The difference
between the two landmark vectors therefore represents a change in
the geometry of the patient's leg. The leg length difference and
the leg offset difference (between the first and second points in
time) can thus be determined by appropriately analysing the two
landmark vectors, in particular by decomposing the difference
between the two landmark vectors into at least the leg length
direction and the leg offset direction.
[0012] The method further comprises the step of calculating an
orthogonal projection of the first landmark vector into a sagittal
plane and using the direction of the orthogonal projection of the
first landmark vector into the sagittal plane as the leg length
direction. The leg length direction is assumed to lie within a
sagittal plane, such that a lateral component of the first landmark
vector can be ignored by performing the projection.
[0013] In an orthogonal projection, a perpendicular is dropped onto
a plane or line--in this case, the sagittal plane. In other words,
the orthogonal projection of a point onto a plane or line is the
point on the plane or line which has the shortest distance from the
point to be projected.
[0014] The method further comprises the steps of calculating a
direction which is perpendicular to the sagittal plane and using
this direction as a leg offset direction; and calculating the leg
length difference in the leg length direction and the leg offset
difference in the leg offset direction from the first landmark
vector and the second landmark vector.
[0015] The femoral landmark is preferably a point on the greater
trochanter of the femur. The second landmark is preferably either a
point on the pelvis, for example a point on the acetabular rim, or
a landmark in a fix relation to the pelvis, such as a landmark
which coincides with the centre of rotation of the femoral
head.
[0016] The direction of a vector or a line is preferably defined in
a global co-ordinate system. Examples of a global co-ordinate
system include an earth-fixed co-ordinate system which can be
defined by parameters such as the gravity vector and the magnetic
field of the earth, and an artificial global co-ordinate system
which can be defined by field generators which generate a magnetic
and/or electric field.
[0017] It should be noted that a landmark may be accessed through
the skin without any surgical intervention. If a surgical
intervention is required in order to access a landmark, then this
surgical intervention is performed in an independent, preceding
step which is not part of the present invention. The present
invention preferably only relates to processing the data obtained
by sampling and not necessarily to the sampling process itself.
[0018] A major advantage of the present invention is that there is
no need to acquire the absolute position of the landmarks in a
co-ordinate system such as the global co-ordinate system. This
would require an adequate medical navigation system, which is
elaborate and expensive. In addition, a medical navigation system
based on a stereoscopic camera needs an unobstructed field of view
in order to capture stereoscopic images of a marker. The present
invention only requires directions and distances, which can be
acquired using simple sensors. The line-of-sight problem is reduced
considerably as sources and sensors are brought closer
together.
[0019] This invention requires the orientation of the sagittal
plane to be known. In accordance with a first option, the sagittal
plane is defined as a horizontal plane. This is an appropriate
assumption if the patient is in a lateral recumbent position.
[0020] In accordance with a second option, the orientation of the
sagittal plane is determined as the orientation of a surface of an
operating table on which the patient is located. This is also based
on the assumption that the patient is in a lateral recumbent
position on the operating table. However, if the surface of the
operating table is not horizontal, then the sagittal plane of the
patient is also not horizontal. Since it is an appropriate
assumption that the sagittal plane is parallel to the surface of
the operating table, the orientation of the surface of the
operating table can be used as the orientation of the sagittal
plane. The orientation of the surface of the operating table is
preferably determined using an orientation sensor such as an
inertial sensor. In this document, an inertial sensor is capable of
determining its orientation in up to three rotational dimensions,
for example in a known reference system such as the global
co-ordinate system.
[0021] In accordance with a third option, the sagittal plane is
determined from a plurality of inertial sensor data which are
acquired from an inertial sensor attached to the tibia of the leg
while the femur is locked in position, preferably in its neutral
position, and the tibia is flexed relative to the femur. If the
tibia is flexed relative to the corresponding femur about the knee
joint, the tibia moves within a plane which is parallel to the
sagittal plane. If the sensor data provided by the inertial sensor
are sampled for a plurality of positions of the tibia, then the
orientation of the plane in which the tibia moves can be
calculated.
[0022] In one embodiment, a first sagittal plane is determined for
the first landmark vector, and a second sagittal plane is
determined for the second landmark vector. The orthogonal
projections of the landmark vectors into the corresponding sagittal
plane are calculated. This embodiment is particularly useful if the
orientation of the patient has changed, in particular relative to
the global co-ordinate system. In this case, the leg length
direction and the leg offset direction at the first point in time
and second point in time are different. This is compensated for by
projecting the first landmark vector into the first sagittal plane
and the second landmark vector into the second sagittal plane, in
order to determine the respective leg length directions and leg
offset directions. The two sagittal planes are preferably
co-registered in order to harmonise the leg length directions and
the leg offset directions and so incorporate the first and second
landmark vectors into the same reference system.
[0023] In one embodiment, calculating the leg length difference and
the leg offset difference involves calculating a first orthogonal
projection of the second landmark vector into the sagittal plane
and calculating a second orthogonal projection of the first
orthogonal projection of the second landmark vector onto the
projection of the first landmark vector into the sagittal plane.
This compensates for an anterior-posterior shift of the femur
between the first and second points in time. Such an
anterior-posterior shift occurs in the sagittal plane,
perpendicular to the leg length direction.
[0024] The leg length difference is then determined as the
difference in length between the projection of the first landmark
vector into the sagittal plane and the second projection of the
second landmark vector. The leg offset difference is then
calculated as the difference between the components of the first
landmark vector and the second landmark vector in the leg offset
direction, which is the direction perpendicular to the sagittal
plane.
[0025] In another embodiment, calculating the leg length difference
and the leg offset difference involves calculating an orthogonal
projection of the second landmark vector into the sagittal plane
and rotating the orthogonal projection of the second landmark
vector within the sagittal plane such that its direction matches
the direction of the projection of the first landmark vector into
the sagittal plane. This also compensates for an anterior-posterior
shift of the femur between the first and second points in time as
in the previous embodiment, but uses a rotation rather than a
projection in order to align the two projections of the two
landmark vectors. The leg length difference and the leg offset
difference are then determined as in the previous embodiment.
[0026] In the previous embodiments, the two landmark vectors are
decomposed into their components in the leg length direction and
leg offset direction before the leg length difference and leg
offset difference are calculated from these components. In another
embodiment, calculating the leg length difference and the leg
offset difference involves calculating the difference vector
between the first landmark vector and the second landmark vector.
This difference vector is then decomposed into its components in
the leg length direction and the leg offset direction. In other
words, the difference vector between the two landmark vectors is
decomposed into the desired directions, whereas in the previous
embodiment, the landmark vectors are first decomposed into the
desired directions and the desired distances are then calculated
from the difference between the corresponding components.
[0027] In one embodiment, neutral position data are obtained which
define (or correspond to) a neutral position of the leg, and
therefore of the femur, at the first point in time. Preferably, the
neutral position data represent the orientation of the femur when
it is in its neutral position. In particular, the leg is positioned
in its neutral position, and the orientation of the femur is
sampled. Second position data which define a position of the leg,
and therefore of the femur, at the second point in time are also
obtained. In this embodiment, the expression "position of the leg"
means the rotational alignment of the femur about the centre of
rotation of the femoral head. The second landmark vector is only
determined if the second position data match the neutral position
data. In other words, the second landmark vector can only be
determined if the femur is in the same position at both the first
and second points in time. In this embodiment, the word "match" can
mean that the position data are exactly identical but may also
include an allowable deviation between the position of the femur at
the first and second points in time of for example up to 1, 2, 5 or
10 degrees.
[0028] The neutral position of the leg, and thus of the femur,
relative to the pelvis refers to a position of the leg when the
patient is standing at a normal angle and with a normal base of
gait, which is widely used as a reference position for assessing
deviations from the norm. The neutral position is well known in
medical practice.
[0029] In this embodiment, the position data are preferably
acquired from an inertial sensor. The inertial sensor determines
its orientation in a co-ordinate system such as the global
co-ordinate system. The inertial sensor is preferably attached to
the thigh which is to be examined. The inertial sensor can for
example be attached using tape or bandaging.
[0030] In one embodiment of the invention, a landmark vector is
calculated from two landmark reference vectors, wherein each
landmark reference vector represents a vector between the
respective landmark and a common reference point. The two landmarks
and the common reference point generally constitute a triangle. The
two landmark reference vectors constitute two known sides of the
triangle, and the angle between these sides is known, such that the
length and direction of the third side, which together characterise
the landmark vector, can be calculated.
[0031] At least one of the landmark reference vectors is determined
using a light beam which is emitted from a light beam source and
pointed at an offset point. The light beam is in particular a light
beam in the visible spectrum, preferably a laser beam. The light
beam source has a known distance from the landmark and a known
orientation relative to the direct line from the light source to
the landmark. This means that the angle between the light source
and the shortest (direct) line from the light source to the
landmark is known. This can be achieved by attaching the light
source to a pointer which comprises a tip for touching the
landmark. In this case, the distance between the light source and
the tip of the pointer and the orientation of the light source
relative to the pointer are known. Alternatively, a laser range
finder is attached to the light source. The laser beam of the laser
range finder is pointed at the landmark, wherein the relative
position between the light beam source and the laser range finder
is known. This known relative position and the distance determined
using the laser range finder can be used to determine or calculate
the distance between the light source and the landmark and the
orientation of the light source relative to the direct line from
the light source to the landmark. Any distances relating to the
light beam source are preferably determined relative to a light
beam source reference point of the light beam source.
[0032] Determining a landmark reference vector also involves
acquiring the direction of the light beam and the distance between
the light beam source (preferably the light beam source reference
point) and the offset point. The landmark reference vector is then
calculated from the known distance between the light source and the
landmark, the known orientation of the light source relative to the
direct line from the light source to the landmark, the direction of
the light beam, the distance between the light beam source and the
offset point, and the reference offset between the offset point and
the reference point.
[0033] In this approach, a quadrilateral is defined by the
landmark, the light source (preferably defined by the light beam
source reference point), the offset point and the reference point.
Since the orientation and length of three of the sides of the
quadrilateral are known, the direction and the length of the fourth
side, which is the landmark vector, can be calculated.
[0034] In one embodiment, the landmark reference vector between the
second landmark and the common reference point is only determined
once and is used for determining both the first landmark vector and
the second landmark vector. This is in particular possible if the
relative position between the second landmark and the common
reference point is identical at the first and second points in
time.
[0035] The common reference point has a constant position in the
co-ordinate system which remains constant while a landmark vector,
i.e. the first or second landmark vector, is determined. However,
the position of the common reference point can change between the
point in time at which the first landmark vector is determined and
the point in time at which the second landmark vector is
determined. The position of the offset point can also vary for each
determination of a landmark reference vector. This means that the
offset point can be different for each landmark reference
vector.
[0036] The reference point and the offset point are preferably
points on a reference device. The reference device is static in the
global co-ordinate system, at least during the period of time
required to determine all the necessary data such as the landmark
reference vectors. The reference device can for example be a pole
or other structure which has a spatially extended surface. The
reference device preferably comprises at least one label or surface
structure with the aid of which the distance between the offset
point and the reference point can be determined.
[0037] In one embodiment, the reference offset is determined from
an image depicting the reference device. The reference point and
the offset point can be identified in this image. The offset point
is the point at which the light beam is reflected by the reference
device. The reference offset can be determined on the basis of
known properties of the camera used to capture the image. The
distance between the offset point and the reference point can for
example be calculated from the distance between the offset point
and the reference point in the image. This distance can
alternatively or additionally be calculated from the label on the
reference device or the structure of the reference device. The
camera which captures the image of the reference device can be
attached to the light beam source or can be provided independently.
Attaching the camera to the light beam source has the advantage
that the relative orientation between the camera and the light beam
source is known, such that the orientation of the camera, and
therefore the orientation of the image, in the global co-ordinate
system can be calculated from the orientation of the light beam
source.
[0038] The reference offset is preferably determined from the
orientation of the reference device in the global co-ordinate
system. This orientation can be known, for example because it is
vertical, or can be measured using an orientation sensor such as a
gyroscope. The reference offset is then preferably calculated in
the global co-ordinate system from the distance between the offset
point and the reference point, the orientation of the reference
device and the known location of the offset point on the reference
device.
[0039] In one embodiment, the reference offset is zero, i.e. the
light beam points directly at the reference point. The
quadrilateral thus becomes a triangle defined by the landmark, the
light beam source and the reference point.
[0040] In this embodiment, a light beam detector is preferably
arranged at the reference point and detects whether or not the
light beam hits the reference point. If it does, a measurement of
the orientation of the light beam source and the distance between
the light beam source and the reference point is automatically
triggered.
[0041] In the embodiments described thus far, the second landmark
is preferably a pelvic landmark on the pelvis of the patient. The
pelvic landmark is in particular a point of the pelvis. The second
landmark is thus preferably a natural landmark, i.e. a point on the
pelvic bone, or an artificial landmark such as for example a
reference which is pre-inserted into the pelvic bone.
[0042] In another embodiment, the second landmark is a virtual
landmark, namely the centre of rotation of the pelvis. The centre
of rotation of the pelvis is the centre of the acetabulum and
therefore identical to the centre of the femoral head. Such a
virtual landmark cannot be sampled directly, for example using a
pointer or a laser range finder as described above. The landmark
vector between the femoral landmark and the virtual landmark
therefore has to be determined differently, in particular by an
indirect approach.
[0043] In this embodiment, this landmark vector is determined as
the average of two auxiliary landmark vectors, wherein each
auxiliary landmark vector represents a vector between an auxiliary
landmark and the femoral landmark. An auxiliary landmark can be an
actual landmark or an artificial landmark as described above.
Preferably, all the auxiliary landmarks are points on the
acetabular rim of the pelvis. The centre of rotation of the femur
is assumed to be located centrally between the two auxiliary
landmarks.
[0044] If a landmark vector is calculated from two landmark
reference vectors as described above and the second landmark is a
virtual landmark, then the corresponding landmark reference vector
is a virtual landmark reference vector for the virtual landmark,
i.e. a vector between the virtual landmark and the reference point.
The virtual landmark reference vector is determined as the average
of two auxiliary reference vectors, wherein each auxiliary
reference vector represents a vector between one of the auxiliary
landmarks and the common reference point.
[0045] Each of the two auxiliary reference vectors is determined in
the same way as a landmark reference vector as described above, but
with the respective auxiliary landmark replacing the second
landmark. This means that the light beam source has a known
distance from the auxiliary landmark and a known orientation
relative to a direct line from the light source to the auxiliary
landmark. All the other steps are likewise performed as described
above.
[0046] In an alternative, which might be considered as a
self-contained invention, a first mechanical axis vector between
the virtual landmark (i.e. the centre of rotation of the
acetabulum) and a patellar landmark is acquired for the position of
the femur at the first point in time. This is the position of the
femur when the data for determining the first landmark vector are
acquired. The patellar landmark is a landmark on the patella and
can be a natural or an artificial landmark, as described above. The
leg length direction at the first point in time is then the
direction of the first mechanical axis vector, and the leg offset
is the distance of the femoral landmark from the mechanical axis,
which is represented by the first mechanical axis vector. The leg
offset direction is thus the direction of a line which is
perpendicular to the first mechanical axis vector and runs through
the femoral landmark. If the femur is in the same position relative
to the pelvis at the first and second point in time, then the leg
length difference and the leg offset difference can be calculated
based on the leg length direction and the leg offset direction.
[0047] The first mechanical axis vector can be determined directly
or using a reference point as described above. In particular, a
first patellar reference vector between the patellar landmark and
the common reference point is determined. A virtual landmark
reference vector between the virtual landmark and the reference
point is determined as described above. The first mechanical axis
vector is then calculated from the virtual landmark reference
vector and the first patellar reference vector.
[0048] In one embodiment, a second mechanical axis vector between
the virtual landmark (i.e. the centre of rotation of the
acetabulum) and the patellar landmark is acquired for the position
of the femur at the second point in time. This is the position of
the femur when the data for determining the second landmark vector
are acquired. The leg length direction at the second point in time
is then the direction of the second mechanical axis vector, and the
leg offset direction is the direction of a line which is
perpendicular to the leg length direction and runs through the
femoral landmark.
[0049] The second mechanical axis vector can be determined directly
or using a reference point as described above. In particular, a
second patellar reference vector between the patellar landmark and
the common reference point is determined. A virtual landmark
reference vector between the virtual landmark and the reference
point is determined as described above. The second mechanical axis
vector is then calculated from the virtual landmark reference
vector and the second patellar reference vector.
[0050] The difference between the directions of the two mechanical
axis vectors represents a rotation of the femur about the centre of
rotation of the acetabulum, between the first and second points in
time. If the position of the femur relative to the pelvis has
changed between the first and second points in time, then this
change in the relative position has to be considered.
[0051] In one embodiment, the leg length and the leg offset at the
first point in time are calculated based on the leg length
direction and the leg offset direction at the first point in time.
The leg length and the leg offset at the second point in time are
calculated based on the leg length direction and the leg offset
direction at the second point in time. The leg length difference is
then calculated as the difference of the leg lengths at the first
and second points in time and the leg length offset is then
calculated as the difference of the leg offsets at the first and
second points in time.
[0052] In another embodiment, a rotational transformation which
transforms the direction of the second mechanical axis vector into
the direction of the first mechanical axis vector is calculated,
and this rotational transformation is applied to the second
landmark vector before the leg length difference and the leg offset
difference are calculated. Applying the rotational transformation
to the second landmark vector compensates for a rotation of the
femur between the first and second points in time. This has the
advantage that the first and second landmark vectors do not require
the femur to be in the same rotational position relative to the
pelvis at both points in time.
[0053] It shall be noted that this transformation can also be used
in combination with the first embodiments of the invention in which
the leg length direction is calculated by projecting the first
landmark vector into a sagittal plane. In this case, the second
landmark vector is transformed using the rotational transformation
in order to make it comparable to the first landmark vector.
[0054] Determining a vector, such as a landmark vector or a
landmark reference vector, in particular means determining vector
data which represent the length and/or direction of the vector.
[0055] Acquiring a direction in particular means acquiring
direction data which represent the direction. Acquiring a distance
in particular means acquiring distance data which represent the
distance.
[0056] The invention also relates to a program which, when running
on a computer or when loaded onto a computer, causes the computer
to perform one or more or all of the method steps described herein
and/or to a program storage medium on which the program is stored
(in particular in a non-transitory form) and/or to a computer on
which the program is running or into the memory of which the
program is loaded and/or to a signal wave, in particular a digital
signal wave, carrying information which represents the program, in
particular the aforementioned program, which in particular
comprises code means which are adapted to perform any or all of the
method steps described herein.
[0057] The present invention also relates to a computer on which
the aforementioned program is stored or executed.
[0058] The present invention also relates to a device for
determining a leg length difference and a leg offset of a patient's
leg including a femur. The device comprises a computer as described
above.
[0059] Within the framework of the invention, computer program
elements can be embodied by hardware and/or software (this includes
firmware, resident software, micro-code, etc.). Within the
framework of the invention, computer program elements can take the
form of a computer program product which can be embodied by a
computer-usable, in particular computer-readable data storage
medium comprising computer-usable, in particular computer-readable
program instructions, "code" or a "computer program" embodied in
said data storage medium for use on or in connection with the
instruction-executing system. Such a system can be a computer; a
computer can be a data processing device comprising means for
executing the computer program elements and/or the program in
accordance with the invention, in particular a data processing
device comprising a digital processor (central processing unit or
CPU) which executes the computer program elements, and optionally a
volatile memory (in particular a random access memory or RAM) for
storing data used for and/or produced by executing the computer
program elements. Within the framework of the present invention, a
computer-usable, in particular computer-readable data storage
medium can be any data storage medium which can include, store,
communicate, propagate or transport the program for use on or in
connection with the instruction-executing system, apparatus or
device. The computer-usable, in particular computer-readable data
storage medium can for example be, but is not limited to, an
electronic, magnetic, optical, electromagnetic, infrared or
semiconductor system, apparatus or device or a medium of
propagation such as for example the Internet. The computer-usable
or computer-readable data storage medium could even for example be
paper or another suitable medium onto which the program is printed,
since the program could be electronically captured, for example by
optically scanning the paper or other suitable medium, and then
compiled, interpreted or otherwise processed in a suitable manner.
The data storage medium is preferably a non-volatile data storage
medium. The computer program product and any software and/or
hardware described here form the various means for performing the
functions of the invention in the example embodiments. The computer
and/or data processing device can in particular include a guidance
information device which includes means for outputting guidance
information. The guidance information can be outputted, for example
to a user, visually by a visual indicating means (for example, a
monitor and/or a lamp) and/or acoustically by an acoustic
indicating means (for example, a loudspeaker and/or a digital
speech output device) and/or tactilely by a tactile indicating
means (for example, a vibrating element or a vibration element
incorporated into an instrument). A computer as referred to herein
is a technical computer which in particular comprises technical, in
particular tangible components, in particular mechanical and/or
electronic components. Any device mentioned herein is a technical
and in particular tangible device.
[0060] The method in accordance with the invention is in particular
a data processing method. The data processing method is preferably
performed using technical means, in particular a computer. The data
processing method is preferably designed to be executed by or on a
computer and is in particular executed by or on the computer. All
the steps or merely some of the steps (i.e. less than the total
number of steps) of the method in accordance with the invention can
in particular be executed by a computer. The computer in particular
comprises a processor and a memory in order to process the data, in
particular electronically and/or optically. The calculating steps
described are in particular performed by a computer. Determining
steps or calculating steps 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 technical data processing device, in
particular electronic data processing device. A computer can be a
device which is generally thought of as such, for example desktop
PCs, notebooks, netbooks, etc., but can also be any programmable
apparatus, such as for example a mobile phone or an embedded
processor. A computer can in particular comprise a system (network)
of "sub-computers", wherein each sub-computer represents a computer
in its own right. The term "computer" includes a cloud computer, in
particular a cloud server. The term "cloud computer" includes a
cloud computer system which in particular comprises a system of at
least one cloud computer and in particular a plurality of
operatively interconnected cloud computers such as a server farm.
Such a cloud computer is preferably connected to a wide area
network such as the world wide web (WWW) and located in a so-called
cloud of computers which are all connected to the world wide web.
Such an infrastructure is used for "cloud computing", which
describes computation, software, data access and storage services
which do not require the end user to know the physical location
and/or configuration of the computer delivering a specific service.
In particular, the term "cloud" is used in this respect as a
metaphor for the Internet (world wide web). In particular, the
cloud provides computing infrastructure as a service (IaaS). The
cloud computer can function as a virtual host for an operating
system and/or data processing application which is used to execute
the method of the invention. The cloud computer is for example an
elastic compute cloud (EC2) as provided by Amazon Web Services.TM..
A computer in particular comprises interfaces in order to receive
or output data and/or perform an analogue-to-digital conversion.
The data are in particular data which represent physical properties
and/or are generated from technical signals. The technical signals
are in particular generated by means of (technical) detection
devices (such as for example devices for detecting marker devices)
and/or (technical) analytical devices (such as for example devices
for performing imaging methods), wherein the technical signals are
in particular electrical or optical signals. The technical signals
in particular represent the data received or outputted by the
computer. The computer is preferably operatively coupled to a
display device which allows information outputted by the computer
to be displayed, for example to a user. An example of a display
device is an augmented reality device (also called augmented
reality glasses) which may be used as "goggles" for navigating. A
specific example of such augmented reality glasses is Google Glass
(trademark of Google Inc.). An augmented reality device may be used
both to input information into the computer by user interaction and
to display information outputted by said computer.
[0061] The expression "acquiring data" or "obtaining data" in
particular encompasses (within the framework of a data processing
method) the scenario in which the data are determined by the data
processing method or program. Determining data in particular
encompasses measuring physical quantities and transforming the
measured values into data, in particular digital data, and/or
computing the data by means of a computer and in particular within
the framework of the method in accordance with the invention. The
meaning of "acquiring data" also in particular encompasses the
scenario in which the data are received or retrieved by the data
processing method or program, for example from another program, a
previous method step or a data storage medium, in particular for
further processing by the data processing method or program. The
expression "acquiring data" can therefore also for example mean
waiting to receive data and/or receiving the data. The received
data can for example be inputted via an interface. The expression
"acquiring data" can also mean that the data processing method or
program performs steps in order to (actively) receive or retrieve
the data from a data source, for instance a data storage medium
(such as for example a ROM, RAM, database, hard drive, etc.), or
via the interface (for instance, from another computer or a
network). The data can be made "ready for use" by performing an
additional step before the acquiring step. In accordance with this
additional step, the data are generated in order to be acquired.
The data are in particular detected or captured (for example by an
analytical device). Alternatively or additionally, the data are
inputted in accordance with the additional step, for instance via
interfaces. The data generated can in particular be inputted (for
instance into the computer). In accordance with the additional step
(which precedes the acquiring step), the data can also be provided
by performing the additional step of storing the data in a data
storage medium (such as for example a ROM, RAM, CD and/or hard
drive), such that they are ready for use within the framework of
the method or program in accordance with the invention. The step of
"acquiring data" can therefore also involve commanding a device to
obtain and/or provide the data to be acquired. In particular, the
acquiring step does not involve an invasive step which would
represent a substantial physical interference with the body,
requiring professional medical expertise to be carried out and
entailing a substantial health risk even when carried out with the
required professional care and expertise. In particular, the step
of acquiring data, in particular determining data, does not involve
a surgical step and in particular does not involve a step of
treating a human or animal body using surgery or therapy. In order
to distinguish the different data used by the present method, the
data are denoted (i.e. referred to) as "XY data" and the like and
are defined in terms of the information which they describe, which
is then preferably referred to as "XY information" and the
like.
[0062] The invention in particular does not involve, comprise or
encompass an invasive step which would represent a substantial
physical interference with the body requiring professional medical
expertise to be carried out and entailing a substantial health risk
even when carried out with the required professional care and
expertise. In particular, the invention does not comprise a step of
positioning a medical implant in order to fasten it to an
anatomical structure or a step of fastening the medical implant to
the anatomical structure or a step of preparing the anatomical
structure for fastening the medical implant to it. More
specifically, the invention does not involve, comprise or encompass
any surgical or therapeutic activity. For this reason alone, no
surgical or therapeutic activity and in particular no surgical or
therapeutic step is necessitated or implied by carrying out the
invention.
[0063] The invention and potential alternatives shall now be
explained in more detail with reference to the accompanying
drawings, which show:
[0064] FIG. 1 a femur at two different points in time;
[0065] FIG. 2 a geometry of two landmarks and a reference
point;
[0066] FIG. 3 a system for determining a landmark vector, including
a registration tool;
[0067] FIG. 4 a geometry for determining a landmark vector;
[0068] FIG. 5 a first example of determining the leg length
difference and the leg offset difference;
[0069] FIG. 6 a geometry for determining the leg length
difference;
[0070] FIG. 7 an example of determining the orientation of a
sagittal plane;
[0071] FIG. 8 a second example of determining the leg length
difference and the leg offset difference; and
[0072] FIG. 9 an example of determining a virtual landmark.
[0073] FIG. 1 shows a femur 1 at two different points in time. The
femur 1 comprises a femoral shaft and a femoral head 1a which are
connected by a femoral neck. The femoral head 1a rests in a socket,
the acetabulum, of the pelvis 2. The femoral head 1a is located at
the proximal end of the femur 1. At its distal end, the femur 1
comprises a medial condyle, a lateral condyle and an intercondylar
notch between them. The distal end of the femur 1 is connected to a
tibia (not shown) via the knee joint.
[0074] FIG. 1 also shows a sagittal plane 3 which is perpendicular
to the plane of the drawing. The sagittal plane extends in an
up-down (superior-inferior or cranial-caudal) direction and in a
front-back (anterior-posterior or ventral-dorsal) direction
relative to the patient's body. The mechanical axis of the femur 1,
which runs through the centre of the femoral head 1a and the centre
of the intercondylar notch, lies approximately within the sagittal
plane 3.
[0075] FIG. 1 also shows a global co-ordinate system GCS which is
an earth-fixed co-ordinate system. Orientations (also referred to
as directions) of lines or vectors are determined or measured in
this global co-ordinate system.
[0076] The structure of the femur 1 has changed between the first
and second points in time--in particular, the femoral neck has
become longer. The structural change in the femur 1 is illustrated
in an exaggerated manner in order to emphasise it in the
drawings.
[0077] FIG. 1 also shows two landmarks LM1 and LM2. The landmark
LM1 is a point on the greater trochanter of the femur 1, and the
landmark LM2 is a point on the acetabular rim of the socket of the
pelvis 2. The landmark LM1 is also referred to as the femoral
landmark, and the landmark LM2 is an example of a second
landmark.
[0078] A vector between the two landmarks LM1 and LM2 is referred
to as a landmark vector. The position of the landmark LM1 relative
to the landmark LM2 changes between the first point in time and the
second point in time, because the structure of the femur 1 and/or
pelvis 2 has changed, hence the landmark vector also changes
between the first point in time and the second point in time. The
landmark vector at the first point in time is referred to as LMV1,
and the landmark vector at the second point in time is referred to
as LMV2.
[0079] In the present invention, a landmark vector is not
determined by acquiring the exact locations of the landmarks in the
global co-ordinate system and determining the landmark vector from
the difference between the locations. Only the directions and the
lengths of the landmark vectors LMV1 and LMV2 are used in order to
calculate the leg length difference and the leg offset difference.
Methods for determining the landmark vectors include a direct
measurement or a determination from medical image data, such as CT
or MR image data.
[0080] As an option, a landmark vector can be determined from two
landmark reference vectors between the respective landmark and a
common reference point. The principle of this approach is shown in
FIG. 2.
[0081] As shown in FIG. 2, the two landmarks LM1 and LM2 and the
common reference point P.sub.R constitute the corners of a
triangle. The landmark vector LMV is represented by the side of the
triangle between the landmarks LM1 and LM2. The landmark reference
vector between the landmark LM1 and the common reference point
P.sub.R is referred to as RLV1, and the landmark reference vector
between the landmark LM2 and the common reference point P.sub.R is
referred to as RLV2. Once the two landmark reference vectors RLV1
and RLV2 are known, for example by being measured, then the
landmark vector LMV can be calculated.
[0082] In an exceptional case, the two landmarks LM1 and LM2 and
the common reference point P.sub.R may all lie on one line. In this
case, the three points do not constitute a triangle. However, the
landmark vector LMV can still be calculated from the two landmark
reference vectors RLV1 and RLV2.
[0083] FIG. 3 schematically shows a system for determining a
landmark vector. The system comprises a registration tool 4 and a
medical navigation system 10.
[0084] The registration tool 4 comprises a body 5 featuring a
landmark point 6 which is to be held against a landmark, such as
for example the landmark LM1 as shown in FIG. 3. A light beam
source 7 which is capable of emitting a light beam 9 is rigidly
attached to the body 5. The location and orientation of the light
beam source 7 on the body 5 is known, such that the distance
between the light beam source 7 and the landmark point 6 and the
direction of the light beam 9 relative to the body 5 is known. An
orientation sensor 8 for determining the orientation of the
registration tool 4 in the global co-ordinate system is rigidly
attached to the registration tool 4. If the global co-ordinate
system is an earth-fixed co-ordinate system, then the orientation
sensor 8 can for example be an inertial sensor such as a three-axis
gyroscope. The orientation sensor 8 is capable of transmitting the
determined orientation to the medical navigation system 10.
[0085] The medical navigation system 10 comprises a receiving unit
11 for receiving the orientation of the registration tool 4 from
the orientation sensor 8, and a central processing unit 12 which is
adapted to run a program which implements the method described
herein and is connected to the receiving unit 11 in order to
receive the orientation of the registration tool 4 from the
receiving unit 11. The central processing unit 12 is also connected
to a memory device 13 in which the program and/or data for
performing the method is/are stored.
[0086] The navigation system 10 also comprises an input unit 14 for
receiving information and an output unit 15 for displaying
information.
[0087] The principle of determining a landmark direction shall now
be explained with reference to FIG. 4. As shown in this figure, the
landmark point 6 of the registration tool 4 is held at the landmark
LM1, and a reference device 16 is provided in such a way that it is
static in the global co-ordinate system GCS. The orientation of the
reference device 16 is either known and provided to or stored in
the medical navigation system 10 or is determined using an
orientation sensor (not shown) and sent to the medical navigation
system 10. The reference point P.sub.R is defined on the reference
device 16.
[0088] In the present example, the light beam source 7 is a laser
range finder which is capable of determining the distance between
the laser range finder 7 and a point at which the laser beam 9 is
reflected. It is also capable of transmitting the determined
distance to the medical navigation system 10 via the receiving unit
11.
[0089] The registration tool 4 is held such that the light beam 9
hits the reference device 16. The point at which the light beam 9
hits the reference device 16 and is reflected back to the
registration tool 4 is referred to as the offset point P.sub.O.
Since the orientation of the light beam source 7 can be measured
using the orientation sensor 8, and the distance between the offset
point P.sub.O and the light beam source 7 can be measured, the
medical navigation system 10 can calculate the vector d.sub.1 from
the light beam source 7 to the offset point P.sub.O. The light beam
source 7, the landmark LM1, the offset point P.sub.O and the
reference point P.sub.R constitute a quadrilateral, as can be seen
from FIG. 4. The vector RLV1 from the landmark LM1 to the reference
point P.sub.R is the landmark reference vector which is associated
with the landmark LM1. This vector can be calculated if enough
information about the quadrilateral is available.
[0090] The vector d.sub.LS from the landmark LM1 to the light
source 7 can be calculated from the known relative position between
the light source 7 and the landmark point 6 in combination with the
orientation of the registration tool 4 as determined using the
orientation sensor 8. The offset vector d.sub.O can be calculated
from the orientation of the reference device 16 and the location of
the offset point P.sub.O on the reference device 16. This location
can be determined automatically, for example by a camera which
captures an image of the reference device 16 and calculates the
location by identifying the offset point P.sub.O in the image, or
manually by an operator who identifies the location of the offset
point P.sub.O on the reference device 16 and inputs this
information into the medical navigation system 10 using the input
unit 14.
[0091] Once the vectors d.sub.LS, d.sub.1 and d.sub.O are known,
the vector RLV1 representing the landmark reference vector of the
first landmark LM1 can be calculated.
[0092] As compared to the illustration in FIG. 1, the two landmark
vectors LMV1 and LMV2 in FIG. 5 have the landmark LM1 in common (as
a start or end point) instead of the landmark LM2. These two
illustrations are equivalent because both correctly represent the
shift between the landmarks LM1 and LM2 from the first point in
time to the second point in time.
[0093] As in FIG. 1, so also FIG. 5 shows the femur 1 with its
femoral head 1a in the socket of the pelvis 2. FIG. 5 also shows
the mechanical axis 18 of the femur 1. This mechanical axis 18 runs
through the centre of rotation of the femoral head 1a and the
intercondylar notch. The leg length difference is defined in the
direction of the mechanical axis 18, while the direction of the leg
offset, i.e. a lateral shift of the femoral shaft between the first
and second points in time, is defined in a lateral direction which
is perpendicular to the mechanical axis 18 and runs through the
femoral landmark LM1.
[0094] In order to determine the leg length difference and the leg
offset difference, the relative shift between the landmark LM1 and
the landmark LM2 from the first point in time to the second point
in time has to be separated or decomposed into a component in the
leg length direction and a component in the leg offset direction.
In FIG. 5, this shift is represented by a difference vector DV. In
the example shown in FIG. 5, the difference vector DV is decomposed
into the leg length difference d1 and the leg offset difference
d2.
[0095] As outlined above, some embodiments assume that the leg
offset direction is perpendicular to the sagittal plane 3. It is
then necessary to determine the orientation of the sagittal plane
3. Two different approaches for determining the orientation of the
sagittal plane are given below. In addition, it is apparent from
FIG. 5 that the relative shift between the landmark LM1 on the
femur and the landmark LM2 on the pelvis depends not only on the
structural change in the femur 1 but also on a changed rotational
alignment between the femur 1 and the pelvis 2. In order to
correctly determine the leg length difference and the leg offset
difference from the two landmark vectors LMV1 and LMV2, it is
necessary to ensure that there is no such change in the rotational
alignment or that any change in the rotational alignment is
compensated for. Both approaches are described below.
[0096] A detailed description of how the leg length difference d1
is determined according to the present invention, that is using a
sagittal plane 3 as shown in FIG. 1, shall now be given with
reference to FIG. 6. In this figure, the sagittal plane 3 is
equivalent to the plane of the drawing. The line P1 is the
orthogonal projection of the first landmark vector LMV1 into the
sagittal plane 3, while the line P21 is the first orthogonal
projection of the second landmark vector LMV2 into the sagittal
plane 3. If the change in the structure of the femur 1 also shifts
the femoral shaft in the anterior-posterior direction, then the
directions of the lines P1 and P21 are not identical. The
anterior-posterior difference between the first landmark vector
LMV1 and the second landmark vector LMV2 is irrelevant for
calculating the leg length difference. A second line P22 is then
calculated as an orthogonal projection of the first projection
(line P21) of the second landmark vector LMV2 into the sagittal
plane 3 and onto the line P1. In an alternative, the line P21 is
rotated onto the line P1 instead. The directions of the lines P1
and P22 are then identical, and the leg length difference d1 is
calculated as the difference in length between the lines P1 and
P22.
[0097] A first approach for determining the orientation of the
sagittal plane 3 assumes that the patient is in a lateral recumbent
position at both the first and second points in time. It is then
assumed that the sagittal plane 3 is a horizontal plane. In a
second approach, an orientation sensor is used to determine the
orientation of the surface on which the patient lies, such as for
example the surface of an operating table. This orientation is
determined in the global co-ordinate system GCS and provided to the
medical navigation system 10.
[0098] Another approach for determining the orientation of the
sagittal plane 3 is illustrated in FIG. 7. FIG. 7 shows the femur 1
connected to the tibia 19 via a knee joint. The femur 1 is locked
in position, preferably in its neutral position, relative to the
pelvis 2. The tibia 19 is then flexed and/or extended relative to
the femur 1, as indicated by the arrow in FIG. 7. An orientation
sensor 20 attached to the tibia 19 then samples a plurality of
orientation datasets for different alignments between the femur 1
and the tibia 19. The plane in which the orientation sensor 20, and
therefore the tibia 19, moves can be calculated from these
orientation datasets. The orientation of this plane is then used as
the orientation of the sagittal plane 3.
[0099] One approach for ensuring that the relative orientation,
i.e. the rotational alignment, between the femur 1 and the pelvis 2
is identical at the first and second points in time uses an
orientation sensor 17 attached to the patient's leg, as indicated
in FIG. 5. In the example illustrated in FIG. 5, the orientation
sensor 17 is attached to the soft tissue of the patient, for
example using adhesive tape or bandaging. The orientation sensor 17
determines its orientation within the global co-ordinate system GCS
at the first point in time and transmits this to the medical
navigation system 10.
[0100] Before the second point in time, the orientation sensor 17
intermittently or continuously determines its orientation in the
global co-ordinate system GCS and transmits this to the medical
navigation system 10, where it is displayed on the display unit 15.
The orientation of the femur 1 relative to the pelvis 2 can then be
adjusted until the orientation of the orientation sensor 17 in the
global co-ordinate system GCS matches its orientation at the first
point in time, wherein a match can be deemed to have been achieved
if the difference with respect to the orientation at the first
point in time is below a predetermined threshold. The medical
navigation system 10 can optionally indicate on the display unit 15
whether or not the orientations of the orientation sensor 17
match.
[0101] Additionally or alternatively, the medical navigation system
10 can sample the data from the registration tool 4 at the second
point in time only, if the orientations of the orientation sensor
17 match.
[0102] The orientation sensor 17 can be any orientation sensor,
such as for example a gyro sensor, which is capable of transmitting
the raw sensor output data or the orientation in the global
co-ordinate system to the medical navigation system 10. One example
of a device which contains such an orientation sensor is an iPod
such as is produced by Apple, Inc.
[0103] One approach for compensating for a change in the rotational
alignment between the femur 1 and the pelvis 2 between the first
and second points in time is illustrated in FIG. 8. The position of
the femur 1 at the first point in time is shown by the complete
representation of the femur 1 exhibiting the mechanical axis 18a.
The position of the femur 1 relative to the pelvis 2 at the second
point in time is indicated by the partial representation of the
femur 1 exhibiting the mechanical axis 18b. The landmark vector
LMV1 is determined at the first point in time, and the landmark
vector LMV2a is determined at the second point in time.
[0104] The mechanical axes 18a and 18b are determined from the
landmark LM3 and the landmark LM4. The landmark LM3 is the centre
of rotation of the femoral head 1a, which coincides with the centre
of rotation of the acetabulum, and is thus a virtual landmark which
cannot be accessed directly. The landmark LM4 is a point on the
intercondylar notch or on the patella and can therefore be referred
to as the patellar landmark. The landmark vectors representing the
mechanical axes 18a and 18b can be determined using landmark
reference vectors between the landmark LM3 or LM4, respectively,
and a common reference point such as for example the common
reference point P.sub.R, in the same way as has already been
described above. A rotational transformation T is then calculated
which transforms the direction of the mechanical axis 18b into the
direction of the mechanical axis 18a, in particular about the
landmark LM3, i.e. the centre of rotation of the acetabulum. This
rotational transformation T is then applied to the second landmark
vector LMV2a, resulting in a transformed second landmark vector
LMV2. The leg length difference and the leg offset can then be
calculated from the two landmark vectors LMV1 and LMV2, as
described above.
[0105] As outlined above, the virtual landmark LM3 cannot be
accessed directly using the registration tool 4. An indirect
approach to determining a reference landmark vector for a virtual
landmark shall accordingly now be described with reference to FIG.
9.
[0106] This approach uses two auxiliary landmarks AL1 and AL2 on
the acetabular rim of the pelvis 2. The centre of rotation of the
acetabulum, and therefore the virtual landmark LM3, is considered
to be halfway between the two auxiliary landmarks AL1 and AL2. In
order to determine the landmark reference vector RLV3 between the
virtual landmark LM3 and the common reference point P.sub.R, a
first auxiliary landmark reference vector ARV1 between the
auxiliary landmark AL1 and the common reference point P.sub.R and a
second auxiliary landmark reference vector ARV2 between the second
auxiliary landmark AL2 and the common reference point P.sub.R are
determined. The reference landmark vector RLV3 is then calculated
as the average of the first auxiliary landmark reference vector
ARV1 and the second auxiliary landmark reference vector ARV2.
[0107] As an alternative, the mechanical axes 18a and 18b can be
determined by acquiring the vectors, or at least the directions of
the vectors, between the landmark LM4 and the two auxiliary
landmarks AL1 and AL2, respectively, and then calculating the
direction of a mechanical axis as the average of the two directions
of said vectors.
[0108] As an alternative to the approach of using projections of
the landmark vectors into a sagittal plane for determining the leg
length direction, the directions of the mechanical axes 18a and 18b
can be used as the leg length directions at the first and second
point in time, respectively. In one embodiment, the transformation
is applied to the second landmark vector LMV2 as described above,
such that the leg length directions for the first landmark vector
LMV1 and the transformed second landmark vector LMV2 are identical.
The two landmark vectors thus become directly comparable.
[0109] In another embodiment, the first landmark vector LMV1 is
decomposed into components in the leg length direction and the leg
offset direction based on the direction of the mechanical axis 18a
and the second landmark LMV2 is decomposed based on the direction
of the mechanical axis 18b. The corresponding components of the two
landmark vectors are then compared to calculate the leg length
difference and the leg offset difference.
* * * * *