U.S. patent application number 10/677321 was filed with the patent office on 2005-01-27 for apparatus for digitizing intramedullary canal and method.
Invention is credited to Amiot, Louis-Philippe, Cossette, Sebastien, Jansen, Herbert Andre, Jutras, Sebastien, Pelletier, Benoit.
Application Number | 20050021043 10/677321 |
Document ID | / |
Family ID | 32738083 |
Filed Date | 2005-01-27 |
United States Patent
Application |
20050021043 |
Kind Code |
A1 |
Jansen, Herbert Andre ; et
al. |
January 27, 2005 |
Apparatus for digitizing intramedullary canal and method
Abstract
An apparatus for obtaining an axis of an intramedullary canal of
an exposed bone with a position tracking system in
computer-assisted surgery, comprising a detectable device trackable
in space for position and orientation. A stem portion is secured to
the detectable device so as to be tracked for position and
orientation. The stem portion has a leading end insertable in an
intramedullary canal of the bone through an opening in the bone,
and is adapted to be handled by a following end thereof. A tip
portion is provided at the leading end of the stem portion. The tip
portion is positionable in a determined way with respect to a
surface of the intramedullary canal, such that reference points
with respect to the intramedullary canal are calculable as a
function of the position and orientation of the detectable device.
The reference points are related to define an axis of the
intramedullary canal. A method is provided for digitizing the
intramedullary canal axis.
Inventors: |
Jansen, Herbert Andre;
(Montreal, CA) ; Amiot, Louis-Philippe;
(Hampstead, CA) ; Cossette, Sebastien; (Laval,
CA) ; Jutras, Sebastien; (Montreal, CA) ;
Pelletier, Benoit; (Montreal, CA) |
Correspondence
Address: |
OGILVY RENAULT
1981 MCGILL COLLEGE AVENUE
SUITE 1600
MONTREAL
QC
H3A2Y3
CA
|
Family ID: |
32738083 |
Appl. No.: |
10/677321 |
Filed: |
October 3, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60415809 |
Oct 4, 2002 |
|
|
|
60465805 |
Apr 28, 2003 |
|
|
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Current U.S.
Class: |
606/102 |
Current CPC
Class: |
A61B 34/70 20160201;
A61B 2090/363 20160201; A61F 2002/4681 20130101; A61B 5/064
20130101; A61B 17/1717 20130101; A61B 17/15 20130101; A61B
2034/2072 20160201; A61F 2002/4632 20130101; A61B 2034/2055
20160201; A61F 2/32 20130101; A61F 2002/4658 20130101; A61B 5/6878
20130101; A61B 17/1666 20130101; A61B 2034/108 20160201; A61F
2002/4668 20130101; A61B 34/25 20160201; A61F 2/4609 20130101; A61B
2034/107 20160201; A61B 2034/2068 20160201; A61B 90/36 20160201;
A61F 2/34 20130101; A61F 2002/4696 20130101; A61B 2034/252
20160201; A61B 2034/105 20160201; A61B 34/10 20160201; A61B
2034/102 20160201; A61B 2034/104 20160201; A61F 2/4607 20130101;
A61F 2002/30952 20130101; A61B 5/103 20130101; A61F 2/4657
20130101; A61B 2090/3983 20160201; A61B 2090/062 20160201; A61B
17/175 20130101; G09B 23/30 20130101; A61B 17/1735 20130101; G09B
23/28 20130101; A61B 17/1703 20130101; A61B 5/4528 20130101; A61B
17/1746 20130101; A61B 34/20 20160201; A61F 2002/3611 20130101;
A61B 2034/256 20160201 |
Class at
Publication: |
606/102 |
International
Class: |
A61B 017/88; A61F
002/46 |
Claims
We claim:
1. A method for digitizing an intramedullary canal axis of a bone
in computer-assisted surgery, comprising the steps of: performing
an opening in a bone to expose an intramedullary canal of the bone;
providing a tool trackable in space for position and orientation
and a frame of reference on the bone, said tool having a leading
end thereof being positionable in a determined way with respect to
a surface of the intramedullary canal; and obtaining the axis of
the intramedullary canal with respect to the frame of reference by
calculating and relating reference points in the intramedullary
canal by inserting the leading end of the tool at given depths in
the intramedullary canal and calculating a reference point of the
intramedullary canal for each said given depths as a function of a
position and orientation of said tool having the leading end
positioned in said determined way.
2. The method according to claim 1, wherein the method is performed
on an anatomical bone model or on a cadaver.
3. An apparatus for obtaining an axis of an intramedullary canal of
an exposed bone with a position tracking system in
computer-assisted surgery, comprising: a detectable device
trackable in space for position and orientation; a stem portion
secured to the detectable device so as to be tracked for position
and orientation, the stem portion having a leading end insertable
in an intramedullary canal of the bone through an opening in the
bone, and being adapted to be handled by a following end thereof;
and a tip portion at the leading end of the stem portion, the tip
portion being positionable in a determined way with respect to a
surface of the intramedullary canal, such that reference points
with respect to the intramedullary canal are calculable as a
function of the position and orientation of the detectable device,
said reference points being related to define an axis of the
intramedullary canal.
4. The apparatus according to claim 3, wherein the tip portion has
two fingers actuatable from the following end of the stem portion
to extend radially from the stem portion to center the leading end
of the stem portion in the intramedullary canal.
5. The apparatus according to claim 4, wherein the fingers are
pivotally mounted to one another.
6. The apparatus according to claim 4, wherein the fingers are
biased to be retracted radially, so as to facilitate an insertion
of the stem portion in the intramedullary canal.
7. The apparatus according to claim 3, further comprising a flared
adapter slidingly mounted on the stem portion, the flared adapter
being flared toward the following end of the stem portion to engage
with a surface of the intramedullary canal at the opening of the
intramedullary canal, to center the stem portion in the
intramedullary canal.
8. The apparatus according to claim 3, wherein the stem portion is
graduated on an outer surface thereof to indicate a depth of
insertion of the stem portion in the intramedullary canal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority on U.S. Patent Application
No. 60/415,809, filed on Oct. 4, 2002 by the present Applicants,
the subject matter of which is incorporated herein by reference.
This application also claims priority on U.S. Patent Application
No. 60/465,805, filed on Apr. 28, 2003 by the present Applicants,
the subject matter of which is incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention generally relates to computer-assisted
hip replacement surgery and, more precisely, to hip replacement
surgery with minimal preoperative procedures.
BACKGROUND OF THE INVENTION
[0003] Total hip replacement surgery involves the introduction of
an artificial hip joint in a patient. The artificial hip joint
typically consists of a pelvic implant and a femoral implant. The
pelvic implant is a cup received in the acetabulum. The femoral
implant consists of a spherical portion received at an end of a
longitudinal implant portion. The longitudinal implant portion is
introduced into the intramedullary canal of the resected femur,
with the spherical portion being generally centered with respect to
the previous position of the femoral head. Therefore, the femoral
head (i.e., spherical portion of the femoral implant) and the cup
(i.e., pelvic implant) coact to create the artificial hip
joint.
[0004] Different output values are of concern in hip replacement
surgery. In order to reproduce a natural and/or improved gait and
range of motion to a patient, the position and orientation of the
implants, the offset of the femur and the limb length must be
considered during surgery. The work of the surgeon during hip
replacement surgery will have a direct effect on these output
values.
[0005] Known computer-aided hip replacement surgery techniques
presently involve preoperative computerized tomography (CT) steps
that enable acquisition of data related to the skeletal
configuration of the patient. This data is used, for instance, to
determine implant size and to guide the surgeon intraoperatively
into reproducing the ideal output values mentioned above.
[0006] There are inconveniences to the use of CT. Firstly, it is
desired to reduce preoperative steps to a minimum. Ideally, hip
replacement surgery would be free of preoperative steps, as this
would be logistically preferred. Cost saving issues are involved in
the logistic simplification of hip replacement surgery. Moreover,
CT uses X-rays, which are known to be hazardous to health.
[0007] Accordingly, it is believed that computer-aided hip
replacement surgery techniques having minimal preoperative steps
would be of great appeal to surgeons and to management of health
institutions.
SUMMARY OF THE INVENTION
[0008] It is an aim of the present invention to provide an
apparatus and method for digitizing a center of a bone canal.
[0009] It is a further aim of the present invention to provide a
novel method for guiding an operator in inserting implants in hip
joint replacement with computer assistance.
[0010] It is a still further aim of the present invention to
provide a method of performing hip joint replacement with minimal
preoperative procedures.
[0011] It is a still further aim of the present invention to
provide a computer-assisted surgery system for guiding an operator
in inserting implants in hip joint replacement.
[0012] Therefore, in accordance with the present invention, there
is provided a method of doing surgical treatment with a position
tracking system in computer-assisted surgery for guiding an
operator in inserting a femoral implant of a hip joint implant in a
femur as a function of a limb length and orientation of the femoral
implant, comprising the steps of: obtaining a frame of reference of
the femur, the frame of reference being trackable in space for
position and orientation; providing a digital model of a femoral
implant; calculating a desired implant position for the femoral
implant with respect to the frame of reference of the femur, as a
function of the limb length; and guiding an operator in altering
the femur for a subsequent insertion of the femoral implant in the
femur by providing information about a current implant position and
orientation with respect to the desired implant position, the
current implant position and orientation being calculated as a
function of the digital model of the femoral implant and of a
real-time tracking for position and orientation of at least one
surgical tool altering the femur for receiving the femoral
implant.
[0013] Further in accordance with the present invention, there is
provided a method of doing surgical treatment in computer-assisted
surgery for guiding an operator in inserting a pelvic implant of a
hip joint implant in an acetabulum of a pelvis as a function of a
position and orientation of the pelvic implant with respect to the
pelvis, comprising the steps of: creating a frame of reference of a
pelvis by registering points on the pelvis, the frame of reference
being trackable in space for position and orientation; creating a
digital model of a surface of an exposed acetabulum of the pelvis
by registering points on the surface of the acetabulum as a
function of the frame of reference; providing a digital model of a
pelvic implant; calculating an initial center of rotation of the
acetabulum as a function of the digital model of the surface of the
acetabulum; and guiding an operator in altering the acetabulum for
a subsequent insertion of the pelvic implant in the acetabulum by
providing information about a current implant position and
orientation-with respect to the initial center of rotation of the
acetabulum and the frame of reference, the current implant position
and orientation being calculated as a function of the digital model
of the pelvic implant and of a real-time tracking for position and
orientation of at least one surgical tool altering the acetabulum
for receiving the pelvic implant.
[0014] Still further in accordance with the present invention,
there is provided a method for digitizing an intramedullary canal
axis of a bone in computer-assisted surgery, comprising the steps
of: performing an opening in a bone to expose an intramedullary
canal of the bone; providing a tool trackable in space for position
and orientation and a frame of reference on the bone, said tool
having a leading end thereof being positionable in a determined way
with respect to a surface of the intramedullary canal; and
obtaining the axis of the intramedullary canal with respect to the
frame of reference by calculating and relating reference points in
the intramedullary canal by inserting the leading end of the tool
at given depths in the intramedullary canal and calculating a
reference point of the intramedullary canal for each said given
depths as a function of a position and orientation of said tool
having the leading end positioned in said determined way.
[0015] Still further in accordance with the present invention,
there is provided an apparatus for obtaining an axis of an
intramedullary canal of an exposed bone with a position tracking
system in computer-assisted surgery, comprising: a detectable
device trackable in space for position and orientation; a stem
portion secured to the detectable device so as to be tracked for
position and orientation, the stem portion having a leading end
insertable in an intramedullary canal of the bone through an
opening in the bone, and being adapted to be handled by a following
end thereof; and a tip portion at the leading end of the stem
portion, the tip portion being positionable in a determined way
with respect to a surface of the intramedullary canal, such that
reference points with respect to the intramedullary canal are
calculable as a function of the position and orientation of the
detectable device, said reference points being related to define an
axis of the intramedullary canal.
[0016] Still further in accordance with the present invention,
there is provided a computer-assisted surgery system for guiding an
operator in inserting a femoral implant of a hip joint implant in a
femur as a function of a limb length and orientation of the femoral
implant with respect to the femur, comprising: a reference tool
securable to the femur and trackable in space for position and
orientation; a registration tool trackable in space for position
and orientation and handled by the operator to register surface
information; a bone altering tool trackable in space for position
and orientation; a sensing apparatus, for tracking any one of the
tools for position and orientation; a controller connected to the
sensing apparatus, the controller being provided to: i) register a
frame of reference of the femur by at least one of calculating
surface information provided by the registration tool as a function
of the position and orientation of the registration tool provided
by the sensing apparatus, and retrieving in a database a model of
the femur; ii) calculate a desired implant position with respect to
the frame of reference as a function of the limb length; and iii)
calculate a current implant position and orientation in relation to
the desired implant position with respect to alterations being
performed in the femur with the bone altering tool, as a function
of the position and orientation of the bone altering tool provided
by the sensing apparatus and of a digital model of a femoral
implant provided by the database; and the database connected to the
controller for the controller to store and retrieve information
relating to an operation of the controller.
[0017] Still further in accordance with the present invention,
there is provided a computer-assisted surgery system for guiding an
operator in inserting a pelvic implant of a hip joint implant in an
acetabulum as a function of an orientation of the pelvic implant
with respect to the pelvis, comprising: a reference tool securable
to the femur and trackable in space for position and orientation; a
registration tool trackable in space for position and orientation
and handled by the operator to register surface information; a bone
altering tool trackable in space for position and orientation; a
sensing apparatus, for tracking any one of the tools for position
and orientation; a controller connected to the sensing apparatus,
the controller being provided to: i) register a frame of reference
of the pelvis by calculating surface information provided by the
registration tool as a function of the position and orientation of
the registration tool provided by the sensing apparatus; ii)
register a digital model of a surface of an exposed acetabulum of
the pelvis with respect to the frame of reference by calculating
surface information provided by the registration tool as a function
of the position and orientation of the registration tool provided
by the sensing apparatus; iii) calculate an initial center of
rotation of the acetabulum as a function of the digital model; and
iv) calculate a current implant position and orientation in
relation to the initial center of rotation of the acetabulum and
the frame of reference with respect to alterations being performed
in the acetabulum with the bone altering tool, as a function of the
position and orientation of the bone altering tool provided by the
sensing apparatus and of a digital model of an acetabular implant
provided by the database; and the database connected to the
controller for the controller to store and retrieve information
relating to an operation of the controller.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These and other features, aspects and advantages of the
present invention will become better understood with regard to the
following description and accompanying drawings wherein:
[0019] FIG. 1 is a front elevation view of leg bones involved in a
hip replacement method in accordance with the present
invention;
[0020] FIG. 2 is a block diagram of a computer-assisted surgery
system in accordance with the present invention;
[0021] FIG. 3 is a flow chart of a method of hip replacement
surgery in accordance with the present invention;
[0022] FIG. 4 is a front elevation view of a canal digitizer in
accordance with the present invention;
[0023] FIG. 5A is a longitudinal cross-section view of the canal
digitizer;
[0024] FIG. 5B is a side elevation view of a handle portion of the
canal digitizer;
[0025] FIG. 6 is a front elevation view of fingers of the canal
digitizer;
[0026] FIG. 7 is a front elevation view of one of the fingers
having a stopper; and
[0027] FIG. 8 is a front elevation view of the fingers of the canal
digitizer centering the canal digitizer.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0028] According to the drawings, and more particularly to FIG. 1,
bones of the leg that will be involved in the hip replacement
surgery of the present invention are generally shown at 1. FIG. 1
is provided as reference for the description of the steps of the
hip replacement surgery method described herein. The bones are the
pelvis 10, the femur 20, the tibia 30 and the fibula 40.
Hereinafter, parts of these bones will each be referenced by
numerals from the same numeric decade. For instance, parts of the
pelvis (e.g., the acetabulum 11) will bear reference numerals
between 11 and 19.
[0029] Referring to FIG. 2, a computer-assisted surgery system is
generally shown at 50 (hereinafter CAS system 50) and generally
consists of a CAS controller 52 connected to sensor apparatus 54.
The sensor apparatus 54 tracks for position and orientation tools
56, to be described in detail with the description of the hip
replacement surgery method of the present invention. The controller
52 is typically a PC unit that has user interfaces by which a
surgeon will receive or send information that will guide him during
the hip replacement surgery. For instance, monitors, keyboard,
mouse, and foot pedals are a few of the user interfaces that can be
provided with the controller 52. A database of the controller 52 is
illustrated separately as database 58, and is typically the hard
disk drive of the controller 52. A discussion of the preferred
system configuration will follow the description of the method
100.
[0030] Referring to FIG. 3, a method for hip replacement surgery in
accordance with the present invention is generally shown at 100.
Although the method 100 is referred to in the singular, various
choices of procedure will be given to the surgeon, as will be set
forth in the forthcoming description, according to the preferences
of the surgeon. A plurality of methods can be derived from the
method 100 according to the decisions of the surgeon.
[0031] In Step 102, preparative steps for surgery are effected.
Namely, general patient information can be entered into the CAS
system 50 (FIG. 2) for opening a patient file. For instance, a
general patient profile can be entered, consisting of the name,
birth date, identification number, sex and the like, as well as
more specific data pertaining to the surgery, such as leg length
discrepancy (with the identification of the longer leg), if
applicable. For instance, the leg length discrepancy is measured
using X-rays of the hip joint. More precisely, the leg length
discrepancy is measured from the vertical comparison between the
lesser trochanters. These X-rays are typically taken during the
diagnostic stages leading to surgery, so they are usually available
for hip joint surgery. The calibration of the various surgical
tools to be used is done. For instance, a calibration base and
method, as set forth in International Publication No. WO 01/67979
A1 by Jutras et al., can be used for the calibration. Also,
correspondence between the tracking of the tools 56 and the display
on the CAS controller 52 can be verified in further calibration
steps included in Step 102.
[0032] It is pointed out that the general patient information can
be entered preoperatively. Moreover, the entering of the general
patient information is straightforward such that the surgeon need
not be involved. However, in order to minimize the preoperative
procedures, all steps of method 100 can be performed at the
beginning of the surgical session, during short time span preceding
the surgery.
[0033] Surgery is initiated between Step 102 and subsequent
Decision 104-, by the surgeon exposing the hip joint. No computer
assistance is required thereat.
[0034] In Decision 104, the surgeon is given the choice of
proceeding either with (A) the dislocation of the femur from the
pelvis, or with (B) the resection of the femoral head from the
femur. Both (A) and (B) will lead to the obtainment of frames of
reference for both the femur and the pelvis, as will be explained
hereinafter, but both (A) and (B) have advantages over the other.
For reference purposes, flow chart lines have been identified as
(A) or (B) for the method 100 in FIG. 3, in accordance with the
choice of the surgeon. The dislocation of the femur from the pelvis
(A) is preferred, as centers of rotation of the acetabulum 11 and
the femoral head 21 (FIG. 1) will be digitized independently from
one another. In (B) resecting the femoral head 21 from the femur 20
to then remove the femoral head 21 from the acetabulum 11, the
femoral head 21 will not be exposed out of the acetabulum 11 until
it is resected, whereby the center of rotation thereof cannot be
digitized with respect to the rest of the femur 20. Therefore, a
calculation based on an assumption will enable the calculation of a
theoretical femoral center of rotation.
[0035] On the other hand, dislocation (A) of the femur 20 shows
some level of difficulty, and involves risks such as fracture of
the femur 20 (e.g., at the neck 22), and damage (e.g.,
hyperextension) to ligaments and muscle. Resection (B) of the
femoral head 21 can be preferred by surgeons as a safer
procedure.
[0036] Procedure (A) will initially be described in detail,
followed by a description of procedure (B).
[0037] Referring to FIG. 1 (for bone parts reference) and FIG. 3,
in Step 106, following the choice of (A) dislocating the femur 20
from the pelvis 10, the femoral head 21 is removed from the
acetabulum 11. Therefore, both the acetabulum 11 and the femoral
head 21 are exposed.
[0038] In Step 108, tracking references (included in the tools 56)
are secured to the pelvis 10 and the femur 20. Therefore, both the
pelvis 10 and the femur 20 can be tracked for position and
orientation in space simultaneously as a function of their
respective tracking references, by the CAS system 50 of FIG. 2. The
tracking references will remain anchored to their respective bones
throughout the computer-assisted steps of surgery. It is pointed
out that Step 108 could have been performed prior to Step 106 in
procedure (A), although it is preferred that the tracking
references not interfere with the dislocation of the femur 20. The
CAS system 50 must thus be adapted to track at least two tracking
references simultaneously, and in real-time.
[0039] It is pointed out that the tracking reference may take the
form of a marked point on one of the bone elements. For instance, a
tracking reference, of the type being screwed to the bone element,
can be secured to the pelvis 10 to be tracked for position and
orientation, while the femur 20 is solely marked with a point
(e.g., from a physical or visual marker). In order to relate the
marked point on the femur 20 to the tracking reference on the
pelvis 10, the marked point is registered when the pelvis 10 and
the femur 20 are in a known and reproducible posture. Thereafter,
to update the position and orientation information relating to the
femur 20 during surgery, the known and reproducible posture is
reproduced and the marked point is digitized with a registration
pointer.
[0040] It is also contemplated to provide a portable tracking
reference that is positionable on a bone element in a reproducible
way. For instance, instead of having a single marked point, the
femur 20 may be marked with three nonlinear points to be used to
position the portable tracking reference thereon. With such a
portable tracking reference, position and orientation information
for the femur 20 can be obtained by positioning the portable
tracking reference with respect to the three nonlinear points. The
portable tracking reference is advantageous in that no screw holes
are performed in the bone element.
[0041] Nonetheless, the method 100 will be described with a
tracking reference anchored to the femur 20 in accordance with a
preferred embodiment of the present invention. Yet it is
appreciated that the above described alternatives could be used
instead of the anchored tracking reference of the femur 20.
[0042] Referring to FIGS. 1 to 3, in Step 110, digitization of a
center of the femoral head 21 is performed. For instance, a
registration pointer (from the tools 56) having its tip tracked in
space is used to register points on the surface of the femoral head
21. Therefore, points of contact between the tip and a given
surface can be registered as a function of the tracking reference
(Step 108). As tracking references have been secured to the femur
20 and the pelvis 10 in Step 108, the points on the surface of the
femoral head 21 are known as a function of the tracking of the
respective tracking reference of the femur 20. The CAS controller
52 is equipped with software that will enable it to calculate the
center of a sphere (i.e., the femoral head 21) from the collected
points. Moreover, the center calculation software is adapted to
perform a validation of the center calculation with the standard
and maximum deviations. A rejection criterion for the validation of
the center calculation can be recorded, for instance, during the
setting of parameters in Step 102.
[0043] Although the above described Step 110 is preferred for
obtaining a 3D model from which the femoral center of rotation will
be calculated, other equivalent methods are contemplated for
obtaining the femoral center of rotation. For instance,
photogrammetric scans can be used to rapidly create 3D models, or
preoperative computerized tomography images can be gathered, from
which the femoral head geometry can be established.
[0044] The diameter of the sphere (i.e., of the femoral head 21)
can be displayed, as such information can guide the surgeon in the
choice of a femoral implant. The femoral center of rotation and the
diameter of the femoral head 21 can be used to correct displays on
the CAS controller 52. Although CT images can be shown on the
display unit of the CAS controller 52, the method 100 is preferably
free of CT images. Accordingly, general visual images of the pelvis
10 and of the femur 20 can be displayed for general reference, but
these images can be scaled as a function of the center of rotation
and diameters, calculated in Step 110 for the femur 20 in procedure
(A), and to be calculated in Step 114 for the pelvis 10.
[0045] In Step 112, with the center of the femoral head 21 now
identified in procedure (A) as a function of the tracking reference
(Step 108), resection of the femoral head 21 is performed.
[0046] Referring to FIGS. 1 to 3, in. Step 114, digitization of the
acetabular center of rotation is performed, by taking reference
points on the surface of the acetabulum 11, and using the center
calculation software of the CAS controller 52, set forth in Step
110 to find the acetabular center of rotation. The acetabular
center of rotation is therefore known as a function of the tracking
reference on the pelvis 10. It is noted that in procedure (A) the
digitization of the acetabular center of rotation (Step 114) is
done independently from the digitization of the femoral center of
rotation (Step 110).
[0047] Step 116 consists in the digitization of the acetabular and
femoral coordinate systems, i.e., the acetabular and femoral frames
of reference.
[0048] The acetabular coordinate system is digitized with the
registration pointer. In a preferred embodiment of the invention,
three points are taken on the pelvis 10 to create the acetabular
coordinate system. Referring to FIG. 1, there is one point on the
iliac crest 12 of the operated side, one point on the contra
lateral iliac crest 13, and one point on one of the two pubic
tubercles 14 of the pelvis 10. To be generally aligned, the points
digitized on the iliac crests 12 and 13 are taken at the outermost
anterior point of the iliac crests 12 and 13. The points digitized
on the iliac crests 12 and 13 are preferably taken directly on the
soft tissue covering the bone pelvis on the iliac crests, as the
soft tissue is relatively thin thereon. The point on the pubic
tubercle 14 completes a first plane, the frontal plane. A second
plane, the transverse plane, is perpendicular to the frontal plane
and includes the points on the iliac crests. A third plane, the
sagittal plane, is perpendicular to the frontal and transverse
planes.
[0049] Supplemental information regarding the frontal plane can be
obtained for various postures of a patient. For instance, trackable
references can be used to gather information about sitting,
standing and walking postures. This information can be used to
adjust the orientation of the frontal plane, as these postures can
provide information not available from the typical lying posture in
which a patient is during surgery. This information can influence
the anteversion positioning of the implants.
[0050] It is possible to obtain anteversion and/or inclination
values of the acetabulum of the patient, to be used as a reference
(e.g., comparison basis) later in the surgery. To do so, points are
digitized (using a registration pointer from the tools 56) on the
generally circular edge of the acetabulum 11 and a plane is defined
from these points. A normal to this plane is projected onto the
acetabular transverse plane (as defined in Step 116) to give an
anteversion angle with the intersection of the acetabular frontal
plane and transverse plane. The normal to this plane is projected
onto the acetabular sagittal plane (as defined in Step 116) to give
an inclination angle with a cranial-caudal axis (y) on the
acetabular sagittal plane.
[0051] In a preferred embodiment of the present invention, the
femoral coordinate system is digitized in Step 116 by providing
five points of reference on the leg to the CAS controller 52, which
is equipped with software that will create the femoral coordinate
system. Referring to FIG. 1, a first point is taken on the tip of
the greater trochanter 23 of the femur 20, and will be defined as a
starting point of an anatomical axis of the femur 20. Thereafter,
points are taken on the medial and lateral epicondyles 24 and 25 of
the femur 20, respectively. A midpoint between the medial
epicondyle and lateral epicondyle points, in alignment therewith,
is defined as an endpoint of the anatomical axis of the femur. The
fourth and fifth points are taken on the medial malleolus 31 of the
tibia 30 and on the lateral malleolus 41 of the fibula 40, with the
leg being bent at the knee. By having the leg bent at the knee, the
tibia 30 stands on the posterior condyles 26 of the femur 20.
Therefore, an assumption is made wherein an aligned midpoint of the
medial and lateral malleoli points is said to define a plane (i.e.,
sagittal plane) with the anatomical axis, with an axis of the knee
being normal to the sagittal plane. The frontal plane is
perpendicular to the sagittal plane, with the anatomical axis lying
therein. The transverse plane is perpendicular to the sagittal and
frontal planes, and can be positioned at any height. With the
anatomical axis and the midpoint of the malleolus region digitized,
the femoral coordinate system, i.e., the femoral frame of
reference, is complete. It is noted that it is not required to
measure two points to obtain a midpoint of the malleolus region. As
this latter point will be in the sagittal plane, the only
requirement is that a point is taken at a midpoint of the malleolus
region, and may thus be placed approximately by the operator.
[0052] It is pointed out that the projection values described
herein (e.g., inclination, anteversion, etc.) are based on the
acetabular and the femoral coordinate systems. As it is
contemplated to use alternative methods of digitizing the
acetabular and the femoral coordinate systems, in addition to the
preferred methods of Step 116, the projection values would be
related to the alternative acetabular and femoral coordinate
system.
[0053] Now that the procedure (A) has been described, the procedure
(B), involving the resection of the femur 20 without the
dislocation thereof, will be described. It is pointed out that some
of the steps described for procedure (A) are also performed in
procedure (B), as readily seen in FIG. 3. Therefore, unless stated
otherwise, corresponding steps will involve the same features and
will not be described in detail for procedure (B). As shown in
FIGS. 1 and 3, the first step of the surgical procedure following
the decision to proceed with the resection (B) is Step 108, wherein
tracking references are secured to the pelvis 10 and to the femur
20.
[0054] Referring to FIGS. 1 and 3, Step 200 consists in a
registration of a relative position between the pelvis 10 and the
femur 20, as a function of the tracking references on each. The leg
is simply left in a straight position, and a relative position is
acquired between tracking references secured to their respective
bones.
[0055] In procedure (B), Step 112 of resecting the femoral head 21
from the femur follows Step 200. Accordingly, the acetabulum 11 of
the pelvis 10 is exposed, and Step 114, consisting in the
digitization of the acetabular center of rotation, follows. As
mentioned previously, the digitization of the acetabular center of
rotation is as a function of the tracking reference secured to the
pelvis 10.
[0056] Step 202 includes the calculation of the femoral center of
rotation. In this calculation, an assumption is made that the
acetabular center of rotation, calculated in the previous Step 114
in procedure (B), coincides with the femoral center of rotation.
However, as the pelvis 10 and the femur 20 are separated due to the
resection of the femoral head 21 in Step 112, the position of the
femoral center of rotation is calculated as a function of the
relative position between the pelvis 10 and the femur 20, obtained
in Step 200.
[0057] Step 116 follows, with the digitization of acetabular and
femoral coordinate systems, i.e., acetabular and femoral frames of
reference.
[0058] Therefore, procedures (A) and (B) have now generally
gathered the same data. At this point, Decision 150 follows,
wherein the surgeon is given the choice of proceeding first with
(C) the insertion of the pelvic implant, or with (D) the
preparation of the femur for the insertion of the femoral
implant.
[0059] Once more, procedures (C) and (D) will each have their
respective advantages. Procedure (C) is the preferred embodiment
and will most likely involve at least one fewer step. Some surgeons
may prefer the procedure (D) as it involves work on the femur 20,
which has just been resected to lose its femoral head 21 (Step
112).
[0060] Procedure (C) will initially be described, followed by a
description of procedure (D).
[0061] Step 152 is the first step of procedure (C), and consists in
the preparation of the acetabulum for insertion of the pelvic
implant. Typically, the preparation of the acetabulum is performed
by a reamer (from amongst the tools 56 of the CAS system 50). The
previous acetabular center of rotation is known as a function of
the tracking reference secured to the pelvis 10, as it was acquired
in previous. Step 114. Preferably, the reamer is tracked for
position and orientation, such that an axis of actuation of the cup
tool on the reamer is displayed on the CAS controller 52.
[0062] In Step 152, the diameter of the pelvic implant chosen by
the surgeon will be used to display a position of the new
acetabular center of rotation in comparison to the digitized
acetabular center of rotation (Step 114). For instance, the
distance between the centers of rotation can be displayed
numerically (e.g., in mm) as a function of the acetabular
coordinate system digitized in previous Step 116. Also, the
anteversion and inclination of the actuation axis of the reamer,
both as a function of the acetabular coordinate system, can be
given numerically (e.g., in degrees) to guide the surgeon in the
reaming. More precisely, the anteversion is calculated as the angle
between an intersection of the acetabular frontal plane and
transverse plane and the projection of the axis of the reamer on
the acetabular transverse plane, and the inclination is the angle
between the reamer axis and a cranial-caudal axis (y) on the
sagittal plane of the acetabular coordinate system (Step 116).
[0063] Step 154 consists in the insertion of the pelvic implant in
the acetabulum 11. A tracked impactor (from amongst the tools 56)
is preferably used. As the pelvic implant size has been chosen, the
diameter thereof and the known relation between the impactor and
the pelvic implant is used with the tracking of the impactor to
give the anteversion and the inclination of the pelvic implant.
Also, the distances between the current and the digitized centers
of rotation can be displayed. Therefore, the surgeon is guided
during the use of the impactor so as to position the pelvic implant
to a given position of the center of rotation thereof, and to a
given orientation [with respect to anteversion and inclination
(Step 152)] to provide a maximal range of motion and stability of
the leg.
[0064] Although the pelvic implant is secured at this point to the
pelvis 10, it is possible to adjust the position and orientation of
the pelvic implant. Firstly, the tracked impactor may be
reconnected to the pelvic implant to serve as a lever in
manipulating the pelvic implant with the tracked impactor, allowing
position and orientation information (e.g., anteversion and
inclination) to be calculated from the tracking of the impactor.
Alternatively, points on the circular edge of the pelvic implant
may be digitized to define a plane, with the normal to this plane
being used to calculate the anteversion and the inclination, as
suggested previously to obtain this information for the acetabulum.
This alternative approach is well suited for pelvic implants having
screw holes for additional fixation, through which the implants can
be altered in position and orientation.
[0065] Step 156 consists in the creation of a femoral target
height. The target height is a desired position for the femoral
center of rotation, and is calculated as follows:
(target height)=(.DELTA..sub.PELVIC COR)-(initial
.DELTA..sub.LL),
[0066] where (.DELTA..sub.PELVIC COR)is the deviation of the
implant center of rotation (i.e., Step 154) with respect to the
digitized acetabular center of rotation (i.e., Step 114), in
cranial-caudal (y) direction (with a cranial deviation having a
positive value), and (initial .DELTA..sub.LL)is the initially
acquired limb length discrepancy (Step 102).
[0067] Referring to FIGS. 1 and 3, Step 158 consists in the
preparation of the femur 20 for the insertion of the femoral
implant. More precisely, an axis of the intramedullary canal 27 is
digitized as a function of the tracking reference secured to the
femur 20 by relating points in the canal 27. The intramedullary
canal 27 is exposed by the resection of the femoral head 21
performed in Step 112. In an embodiment of the present invention,
the points in the canal 27 are digitized by the insertion and
tracking of a pointer (from amongst the tools 56) in the canal 27,
to various depths. It is also contemplated to use a tool, such as
an awl, that is trackable in space for position and orientation,
and whose axis is known with respect to the tracking. The awl can
be inserted in the intramedullary canal to a depth wherein the
inner diameter of the intramedullary canal is generally equivalent
to the awl. At such a depth, the awl can be considered to be
generally centered in the intramedullary canal, whereby the axis of
the awl can be registered as being the axis of the intramedullary
canal.
[0068] Referring to FIG. 4, a canal digitizer in accordance with a
preferred embodiment of the present invention is generally shown at
300. The canal digitizer 300 can be used to accurately position a
tip thereof in the center of the intramedullary canal 27, and hence
an axis centered in the canal 27 can be digitized. The canal
digitizer 300 will be described in further detail hereinafter.
[0069] Step 158 also includes the rasping of the intramedullary
canal 27 in view of the insertion of the femoral implant therein.
The rasp is part of the tools 56 of the CAS system 50, and is
therefore tracked for position and orientation. The tooling portion
of the rasp, which will alter the intramedullary canal 27, and the
femoral implant both have a predetermined geometry. It is preferred
to have rasps each having a tooling end with a generally similar
geometry to the bone-engaging portion of the femoral implants.
Moreover, the rasps can be provided in different sizes, with an
equivalent femoral implant for each size of rasp. Therefore, the
tracking of the rasp for position and orientation relative to the
femoral coordinate system during alteration of the canal is used to
calculate the current position and orientation of the femoral
implant. A plurality of guiding parameters are displayed to the
surgeon to quantify the current position and orientation of the
femoral implant, and are listed below.
[0070] The anteversion of the femoral implant as calculated from
the tracked rasp can be displayed numerically (e.g., in degrees)
based on the femoral coordinate system calculated in Step 116. It
is represented by the angle between the intersection of the frontal
plane and the transverse plane and a projection of the neck axis
(anticipated for the femoral implant) onto the transverse plane
(Step 116).
[0071] Another guiding parameter to be provided to the surgeon is
the varus/valgus angle of the femoral implant, which is equivalent
to the varus/valgus angle of the tracked rasp. The angle is
measured between the projection of the intramedullary canal axis
and the projection of the longitudinal rasp axis onto the femoral
frontal plane (Step 116), and is displayed to the surgeon in
degrees.
[0072] Another guiding parameter to be provided to the surgeon is
the distance between the previous femoral center of rotation [i.e.,
digitized in Step 106 for the dislocation procedure (A), or
calculated in Step 202 for the resection procedure (B)] and the
current femoral center of rotation. The current femoral center of
rotation is calculated as a function of the femoral implant
geometry (e.g., the ball head size) and the tracking of the rasp.
The distance can be given in X, Y and Z values (e.g., in mm)
according to the femoral coordinate system (Step 116).
[0073] Another guiding parameter to be provided to the surgeon is
the current leg length discrepancy. The current leg length
discrepancy, (current .DELTA..sub.LL), is calculated as
follows:
(current .DELTA..sub.LL)=(initial
.DELTA..sub.LL)-(.DELTA..sub.PELVIC COR)+(.DELTA..sub.FEMUR
COR),
[0074] where (.DELTA..sub.FEMUR COR) is the Y value calculated
above in Step 158, and where (.DELTA..sub.PELVIC COR) and (initial
.DELTA..sub.LL) have been calculated in Step 156. The current leg
length discrepancy can be displayed by the CAS system 50 as an
overall leg length, or as a relative value between leg lengths,
with the value 0 representing legs of equal length.
[0075] Another guiding parameter is the offset of the femoral
implant. The offset is the distance between the acetabular center
of rotation and the axis of the implant (i.e., the anatomical axis
of the femur as defined previously) on the transverse plane, and is
thus directly related to the size of the femoral implant. It is
pointed out that a type of femoral implant contemplated for the
above described method 100 can be provided in various sizes, and a
size of femoral implant can have various femoral centers of
rotation along the neck of the implant. A proper identification of
the size of the femoral implant must therefore be provided as it
will have an effect on both the offset and the limb length.
[0076] It may also be desired to display the sum of the measured
femoral and acetabular distances in the X-axis.
[0077] Once the surgeon is satisfied with the rasped intramedullary
canal 27 in procedure (C), the femoral implant is inserted. Step
160 consists in the insertion of the femoral implant in the
intramedullary canal 27. It is anticipated that the femoral implant
will be positioned and orientated substantially as calculated from
the tracking of the tool 56 that has previously altered the
intramedullary canal. However, it is also contemplated to track the
femoral implant for position and orientation with a position of the
center of rotation of the implant being calculable by the CAS
controller 52 as a function of the tracking of the femoral implant.
Such a step would ensure a precise position of the implant. The CAS
controller 52 can change its display of the femur 20 upon the
insertion of the femoral implant, or of the pelvis 10 upon the
insertion of the pelvic implant (Step 154).
[0078] Step 162 consists in the analysis of the range of motion of
the hip joint. The range of motion can be assessed by the following
parameters: the angles of flexion/extension, of
adduction/abduction, and the internal/external rotation of the leg.
These angles are measured based on both coordinate systems
digitized in Step 116, and the minimal and maximal angle values can
be recorded as part of the patient profile.
[0079] Step 164 signals the end of the computer assistance to the
hip replacement surgery.
[0080] Now that procedure (C) has been described in detail,
procedure (D), involving the femur preparation prior to the pelvic
implant insertion, will now be described. In procedure (D), Step
250 follows Decision 150, and consists in the same operations
effected in Step 158. Namely, the femur 20 is prepared for the
insertion of the femoral implant. However, an assumption is made
that the actual pelvic implant center of rotation will be
coincident with the digitized acetabulum center of rotation (Step
114). The digitized acetabulum center of rotation will thus be used
in providing the various guiding parameters to the surgeon.
[0081] Following Step 250, procedure (D) comprises Steps 152, 154
and 156, as performed in procedure (C). It is pointed out that in
Step 154, the actual pelvic implant center of rotation is
obtained.
[0082] In Decision 252, a comparison is made between the actual
pelvic implant center of rotation and the digitized acetabulum
center of rotation, that has been used for the preparation of the
femur in Step 250. If there is a difference between these values,
femoral correction may be required, as shown in Step 158.
Otherwise, the femoral implant can be inserted in the femur.
[0083] Thereafter, Steps 162 and 164 follow to end procedure
(D).
[0084] Now that the method 100 has been described in detail, the
CAS system 50 will be described in accordance with the preferred
embodiment of the present invention.
[0085] Referring to FIG. 2, an operator (e.g., surgeon) is
illustrated at S and is guided in performing surgery by the CAS
system 50. More specifically, the operator S interacts with the
controller 52 of the CAS system 50 using the user interfaces of the
controller 52 (e.g., mouse, display unit, keyboard, sound emitter).
As shown in FIG. 2, the controller 52 will provide guiding
information on the method 100 to the operator S throughout CAS. The
guiding information is for instance retrieved by the controller 52
from the database 58, and will guide the operator S in handling the
tools 56.
[0086] The tools 56 are each trackable in space for position and
orientation by the sensing apparatus 54, such that a position
and/or orientation of given components thereof are calculable. As a
general basic requirement, the tools 56 include the reference
tools, such as the trackable references securable to the bones
(Step 108), for the creation of frames of reference of the bones.
Another one of the required tools 56 is a registration tool that
will enable to gather surface information about the bones (e.g.,
Steps 110, 114, 116, etc.). As mentioned previously, the
registration tool can be a registration pointer, a tracked
photogrammetric sensor, or the like. Finally, a bone altering tool
is included in the required tools 56, such as a reamer and a rasp,
for which uses have been described previously in Steps 152 and 158,
respectively. Also, the tools 56 include the pelvic (impactor) and
the femoral implant, that can be tracked for position and
orientation, to guide the operator during the insertion. It is
pointed out that information relating to the tools (e.g., geometry,
position of tip) is either known by the controller 52 (or
retrievable from the database 58) or determinable using various
steps of calibration.
[0087] The sensing apparatus 54 is connected to the controller 52,
and transfers position and orientation tracking to the controller
52. The position and orientation trackings are used by the
controller 52 to calculate parameters pertaining to the CAS. More
precisely, the position and orientation trackings of the reference
tool and registration tool are used to create frames of reference
of the pelvis and the femur, as described in Steps 110, 114 and
116. As shown in FIG. 2, the frame of reference information is
provided to the operator S, for instance using the display unit of
the controller 52.
[0088] For the pelvic implant, an initial center of rotation is
calculated with respect to the frame of reference, as described in
Step 114. The acetabular center of rotation will be used with the
pelvic frame of reference as references for the alteration of the
acetabulum in view of the insertion of the pelvic implant therein.
A desired femoral implant position is calculated by the controller
52 in accordance with the Step 156, and will be used as a reference
for the alteration of the intramedullary canal in view of the
insertion of the femoral implant therein. The database 58 stores
information that is retrieved by the controller 52 to make the
calculation.
[0089] The current pelvic and femoral implant positions and
orientations are calculated as a function of the position and
orientation tracking of the bone altering tools, and of the
geometry of the respective implants. This is performed in
accordance with Steps 154 and 158/252 of the method 100. Once more,
the controller 52 uses the output of the sensor apparatus 54 and
information stored in the database 58 for the calculations, that
will be displayed for guiding the operator S.
[0090] The CAS system 50 can operate with active or passive
tracking. In a preferred embodiment of the present invention, the
sensor apparatus 54 is a NDI Polaris.RTM. optical tracking
apparatus, with appropriate operating software in the controller
52. With the Polaris.RTM. optical tracking apparatus, passive
detectable devices, such as retro-reflective spheres, are used in
patterns to be tracked in space for position and orientation. Each
one of the tools 56 that requires to be tracked has an own
detectable pattern.
[0091] The CAS system 50 must guide the surgeon throughout the
method 100, and relevant information is displayed to ensure the
surgeon follows the proper Steps of operation. For instance, when
leg length discrepancy values are given, the cranial-caudal
convention can be displayed to explain the reading obtained.
Animations can be initiated automatically to guide the surgeon, for
example, in taking reference points on the various bones, such that
the reference points are taken in a given order, or at the right
locations.
[0092] Referring to FIGS. 4, 5A and 5B, the canal digitizer 300 is
shown having an elongated shaft 302, a handle portion 304, a piston
306, a detectable device base 308, and a centering mechanism
309.
[0093] The shaft 302 is hollow, and has an outer surface 310 and a
free open end 312. The shaft 302 is graduated such that when the
digitizer 300 is inserted in a canal, the tool handler has an
indication of the depth of insertion of the digitizer 300. As shown
in FIG. 5A, a splitter 314 is provided at the free open end 312,
and is held between a pair of extensions 316.
[0094] The handle portion 304 has a pair of radial flanges 320. The
detectable device base 308 projects outwardly from a distal one of
the radial flanges 320. The proximal flange 320 is adjacent to the
piston 306, and is used as leverage by the tool handler to push the
piston 306 inwardly. A flared tip 322 is adjacent to the distal
flange 320, and will enable the digitizer 300 to be centered when
abutting against walls of a canal, if the elongated shaft 302 is
completely inserted in the canal. The handle portion 304 also
defines an inner cavity 324, having a guiding channel 325 adjacent
to the proximal flange 320, for a purpose to be described
hereinafter.
[0095] As shown in FIG. 4, a flared adapter 323 can be optionally
provided in sliding engagement on the shaft 302. The flared adapter
323 serves the same purpose as the flared tip 322 of the handle
portion 304, but is displaceable on the shaft 302 so as be used at
various depths of insertion of the shaft 302 in the canal.
[0096] The centering mechanism 309 has a slender rod 326,
concentrically disposed in the shaft 302. The rod 326 is connected
to the piston 306 at a proximal end thereof, and has a pair of
fingers, 328 and 330, pivotally mounted thereto at a distal end
thereof, as best seen in FIG. 6. Returning to FIGS. 5A and 5B, a
spring 332 surrounds a proximal portion of the rod 326, and
interacts with a surface of the inner cavity 324 of the handle
portion 304 such that the rod 326 is biased in the proximal
direction, i.e., the piston 306 is held away from the handle
portion 304. Accordingly, when the piston 304 is pushed in by the
tool handler, the rod 326 moves in the distal direction, but
retracts back proximally upon release of the piston 306. The spring
332 is fixed at a proximal end thereof to the piston 306, for
instance, by welding therebetween.
[0097] The piston 306 has a pin 327 at a distal end thereof. The
pin 327 cooperatively engages into the guiding channel 325 of the
cavity 324 of the handle portion 304. Accordingly, the assembly of
the piston 306/centering mechanism 309 can be readily separated
from the shaft 302/handle portion 304 assembly by guiding the
former from the latter until the pin 327 is out of the guiding
channel 325. The guiding channel 325 has an elbow portion 325'
(FIG. 5B) and a straight portion 325", such that once the pin 327
is past the elbow portion 325' (FIG. 5B) in the cavity 324, the
piston 306/centering mechanism 309 assembly is captive and free to
translate in the straight portion 325", being held captive with
respect to the handle portion 304. This assembly is preferred to
facilitate the sterilization of the inner cavity 324 and of the
interior of the shaft 302.
[0098] Referring to FIGS. 6 to 8, the fingers 328 and 330, at the
distal end of the rod 326, are kept with their respective tips 329
and 331 separated from one another by a stopper 334 protruding from
the finger 328. Therefore, the fingers 328 and 330 define a slot
336. The fingers 328 and 330 are held by pivot 338. The tips 329
and 331 are both at a same distance from the pivot 338.
[0099] When the piston 306 is pushed inwardly, the rod 326 moves
towards the free open end 312 of the shaft 302, such that the
splitter 314 engages with the slot 336. In doing so, the fingers
328 and 330 are guided away from one another, as best seen in FIG.
8. If the digitizer 300 is in a canal, as shown at 340 in FIG. 8,
the fingers 328 and 330 will center the digitizer 300 in the canal.
Other configurations are possible. For instance, a pivotless
solution can also be imagined where fingers 328 and 330 would be
two blades protruding from a same solid and the flexion of the
material would split the blades apart when engaged on the splitter
314. It has also been thought to equip the elongated shaft 302 with
an inflatable end that will be self-centered in the intramedullary
canal upon being inflated.
[0100] The rod 326 has a given length, such that the fingers 328
and 330 at an end thereof do not interfere with the splitter 314
when the assembly of the piston 306/centering mechanism 309
(including the rod 326) is screwingly inserted in the shaft
302/handle portion 304. Such interference would prevent the pin 327
from going past the elbow portion 325' of the guiding channel 325
in the inner cavity 324. Another possible configuration is to
provide an axial rotational degree of freedom between the piston
306 and the rod 326. Therefore, an engagement of the fingers 328
and 330 with the splitter 314 would not prevent the engagement of
the pin 327 of the piston 306 in the guiding channel 325 of the
inner cavity 324. This can be achieved by providing an annular
groove 339 on an end of the rod 326, and corresponding engagement
pins 341 between the grooves 339 and the piston 306.
[0101] The digitizer 300 is used with a tracking system, such as
CAS system 50 (FIG. 2). As CAS system 50 is preferably equipped
with an optical tracking system, the digitizer 50 is shown having
the detectable device base 308 consisting of a support for
detectable devices. More precisely, the detectable device base 308
has an arm 342 with a support plate 344 at a free end thereof.
Snap-fit fingers 346 are provided for receiving detectable devices
in snap-fit engagement therewith. For instance, the detectable
devices can be retro-reflective detectable spheres, one of which is
shown at 348 in FIG. 4 on the verge of being snap-fitted to one of
the fingers 346. It is obvious that the digitizer 300 may also be
equipped with an active detectable device, provided that the
tracking system is configured therefor.
[0102] The tracking system used with the digitizer 300 must know
the relation between the detectable device on the base 308 and the
tip of the shaft 302. The relation can be determined in calibration
using a calibration base (as explained previously for the tools 56
of FIG. 2). Accordingly, when the digitizer 300 is stabilized in
the canal, the position and orientation of the detectable device
can be registered, and a center point of the canal can be
calculated thereafter, where the tip of the shaft 302 is
located.
* * * * *