U.S. patent application number 10/415962 was filed with the patent office on 2007-08-09 for system for determining the position of a knee prosthesis.
This patent application is currently assigned to Perception Raisonnement Action en Medecine. Invention is credited to Stephane Lavallee.
Application Number | 20070185498 10/415962 |
Document ID | / |
Family ID | 8856078 |
Filed Date | 2007-08-09 |
United States Patent
Application |
20070185498 |
Kind Code |
A2 |
Lavallee; Stephane |
August 9, 2007 |
SYSTEM FOR DETERMINING THE POSITION OF A KNEE PROSTHESIS
Abstract
The invention concerns a system for determining the ideal
theoretical position of a knee prosthesis comprising, for
determining the position of a tibial prosthesis, means for:
determining the shape of the tibia proximal part and its position
relative to the centre of the ankle joint; determining a high point
of the tibial plateau; calculating the position of the ankle, the
orientation and the size relative to the tibia of the tibial
prosthesis and the section plane corresponding to the tibial
prosthesis taking into account the following parameters: the
perpendicular to the section plane passing through the centre of
the prosthesis also passes through the centre of the ankle joint;
the section plane is at a distance from said high point equal to
the height of the prosthesis to be fixed; the large end of the
prosthesis is centred on the large end of the tibia cross-section
in the section plane; the front edge of the small end of the
prosthesis is at a predetermined distance from the small end of the
tibia cross-section in the section plane; and determining the
orientation in the section plane of the prosthesis such that the
large end of the prosthesis should be parallel to the horizontal
axis of the knee.
Inventors: |
Lavallee; Stephane; (Saint
Martin D'uriage, FR) |
Correspondence
Address: |
DARBY & DARBY P.C.
P.O. BOX 770
Church Street Station
New York
NY
10008-0770
US
|
Assignee: |
Perception Raisonnement Action en
Medecine
4, Avenue de l'Obiou
La Tronche
FR
38700
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20050101966 A1 |
May 12, 2005 |
|
|
Family ID: |
8856078 |
Appl. No.: |
10/415962 |
Filed: |
October 22, 2003 |
Current U.S.
Class: |
606/102; 600/587;
623/20.14 |
Current CPC
Class: |
A61B 2034/104 20160201;
A61F 2/4657 20130101; A61B 6/032 20130101; A61B 2034/2068 20160201;
A61B 2034/2055 20160201; A61B 34/20 20160201; A61B 2034/102
20160201; A61B 2090/3916 20160201; A61B 2034/105 20160201; A61B
2090/3958 20160201; A61B 2090/363 20160201; A61B 90/36 20160201;
A61B 34/10 20160201; A61B 17/157 20130101; A61B 17/155 20130101;
A61B 2034/108 20160201; A61B 17/154 20130101; A61B 2034/107
20160201; A61B 2090/3983 20160201; A61B 2034/2072 20160201; A61B
2090/3962 20160201; A61B 2090/3979 20160201; A61B 5/062 20130101;
A61B 17/158 20130101; A61B 90/39 20160201 |
Class at
Publication: |
606/102;
623/020.14; 600/587 |
International
Class: |
A61B 5/103 20060101
A61B005/103; A61F 2/38 20060101 A61F002/38; A61B 5/117 20060101
A61B005/117 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 6, 2000 |
FR |
00/14173 |
Claims
14. A computer assisted orthopedic surgery system in support of an
arthroplasty surgery of a patient's knee joint, comprising: a first
locatable element attachable to a first bone on one side of the
patient's knee joint; a sensor comprising a second locatable
element, the sensor being movable in proximity of the patient's
knee joint; a tracking device locating a plurality of positions of
the sensor relative to the first locatable element; a model of a
knee joint; a computer configured to deform the model in response
to the plurality of positions of the tracking device and to
determine a position for a knee prosthesis on the deformed model;
and a display connected to the computer so as to output the
determined position upon the deformed model.
15. The system of claim 14, further comprising a third locatable
element attachable to a second bone on another side of the
patient's knee joint.
16. The system of claim 14, wherein the tracking device comprises
at least one of a camera, a magnetic tracker, a mechanical tracker,
and an acoustic tracker.
17. The system of claim 14, wherein the tracking device locates the
plurality of positions in response to an operator's orders.
18. The system of claim 14, wherein the locatable element comprises
at least three markers, and wherein the markers comprise at least
one of a reflective disk, a reflective sphere, and an infrared
diode.
19. The system of claim 14, wherein the sensor comprises at least
one of a palpation device, an echographic device, and a
radiographic device or combination thereof.
20. The system of claim 19, wherein the palpation device has a
ball-shaped end.
21. The system of claim 20, wherein the computer is configured to
respond to sliding movement of the ball-shaped end of the sensor on
a bone surface to locate the plurality of positions.
22. The system of claim 14, wherein the model of the knee joint
comprises at least one of: a model of a femoral bone; a model of a
tibial bone; a model of a patella; and a model of positions of
attachments of a medial and lateral ligament on the femoral and
tibial bone.
23. The system of claim 14, wherein the model comprises a set of
triangular facets connected to one another.
24. The system of claim 14, wherein the computer is further
configured to determine section planes on the deformed model.
25. The system of claim 24, wherein the section planes correspond
to section planes for implanting a femoral prosthesis.
26. The system of claim 24, wherein the section planes correspond
to section planes for implanting a tibial prosthesis.
27. The system of claim 14, wherein the computer is further
configured to determine an accuracy of the model based upon a
density of the plurality of positions located by the tracking
device.
28. The system of claim 27, wherein the accuracy of the model is
indicated on the display using at least one of various colors and
line thicknesses.
29. The system of claim 14, wherein the locatable elements are
geometrically differentiated on the display.
30. The system of claim 29, wherein each locatable element
comprises: a shaped portion; and at least three markers, wherein
the shaped portion supports the at least three markers so as to
represent the shaped portion on the display.
31. The system of claim 14, further comprising: a drilling guide
having a third locatable element; and a section guide assembled
into threads formed by the drilling guide.
32. The system of claim 31, wherein the computer is further
configured to determine a location of the drilling guide on the
deformed model.
33. The system of claim 15, wherein the first bone is a femur and
the second bone is a tibia, the tibia and femur are connected by a
medial and a lateral ligament, and wherein the computer is further
configured to determine an elongation of the medial and lateral
ligaments in response to displacement of the medial and lateral
ligaments as the tibia is displaced.
34. The system of claim 33, wherein the computer is further
configured to determine when the elongation limit of the medial and
lateral ligaments has been exceeded.
35. The system of claim 14, further comprising a fourth locatable
element attachable to a patella, wherein the first bone is a femur
and the second bone is a tibia, and wherein the patella, tibia, and
femur are connected, and wherein the computer is further configured
to determine a trajectory of the patella as the knee is moved from
extension to flexion.
36. The system of claim 35, wherein the computer is further
configured to determine a point on an internal patella surface,
wherein the point on the internal patella surface better coincides
with a center of a groove of a femoral prosthesis.
37. The system of claim 14, wherein the first bone is a tibia and
the computer is further configured to determine the position,
orientation, and size of a tibial prosthesis for implantation into
the tibial bone.
38. The system of claim 14, wherein the first bone is a tibia and
the computer is further configured for performing the steps of:
determining a reference point on a superior surface of the tibia;
calculating a position of a section plane for implanting a tibial
prosthesis, wherein the position is a distance from the reference
point equal to a thickness of the superior surface of a tibial
prosthesis and wherein the section plane is perpendicular to a
plane crossing a center of the tibial prosthesis and a center of an
ankle joint; and calculating an orientation of the tibial
prosthesis such that an anterior edge of the tibial prosthesis is
at a predetermined distance from an anterior edge of the section
plane; whereby the calculated position and orientation coincides
with the position determined for the knee prosthesis.
39. The system of claim 14, wherein the first bone is a femur and
the computer is further configured to determine the position,
orientation, and size of a femoral prosthesis.
40. The system of claim 14, wherein the first bone is a femur and
the computer is further configured for performing the steps of:
determining a most distal point on the femur, a most posterior
point on the femur, and a trajectory of a center of a patella
during knee flexion; calculating a position of a distal section
plane wherein the position is a distance from the most distal point
equal to a thickness of a distal portion of a femoral prosthesis;
calculating a position of a posterior section plane wherein the
position is a distance from the most posterior point equal to a
thickness of a posterior condyle of the femoral prosthesis;
calculating a maximum size of the femoral prosthesis to be
implanted; and calculating a position of implantation of the
femoral prosthesis such that a trochlea of the femoral prosthesis
is in alignment with the trajectory of the center of the patella;
whereby the calculated position and size coincides with the
position determined for the knee prosthesis.
41. A computer assisted orthopedic surgery system in support of an
arthroplasty surgery of a patient's knee joint, comprising: a first
locatable element attachable to a first bone on one side of the
patient's knee joint; a sensor comprising a second locatable
element, the sensor being movable in proximity to the patient's
knee joint; a means for locating a plurality of positions of the
sensor relative to the first locatable element; a model of the knee
joint; a means for deforming the model in response to the plurality
of positions provided by the locating means; a means for
determining a position, orientation, and size of a knee prosthesis
on the deformed model; and a means for outputting the deformed
model and indicating the determined position for the knee
prosthesis on the deformed model.
42. The system of claim 41, further comprising a means for
determining section planes on the deformed model.
43. The system of claim 41, further comprising a means for
determining an accuracy of the deformed model.
44. The system of claim 43, further comprising a means for
indicating the accuracy of the deformed model.
45. The system of claim 41, further comprising: a drilling guide
having a third locatable element; and a section guide assembled
into threads formed by the drilling guide
46. The system of claim 45, further comprising a means for
determining a location of the drilling guide on the deformed
model.
47. The system of claim 41, further comprising a means for
geometrically differentiating the locatable elements.
48. The system of claim 41, further comprising a third locatable
element attachable to a second bone on another side of the
patient's knee joint.
49. The system of claim 47, wherein the first bone is a femur and
the second bone is a tibia, and wherein the tibia and femur are
connected by a medial and a lateral ligament, further comprising a
means for determining an elongation of the medial and lateral
ligaments in response to displacement of the medial and lateral
ligaments as the tibia is displaced.
50. The system of claim 48, wherein the computer is further
configured to determine when the elongation limit of the medial and
lateral ligaments has been exceeded.
51. The system of claim 48 further comprising: a means for
determining motion limits of a patient's knee joint; and a means
for outputting a simulated image of the motion limits.
52. The system of claim 47, further comprising: a fourth locatable
element attachable to a patella; a means for determining a
trajectory of the patella as the knee is moved from extension to
flexion; and a means for outputting the trajectory of the
patella.
53. The system of claim 51 further comprising a means for
determining a point on an internal patella surface, wherein the
point on the internal patella surface better coincides with a
center of a groove of a femoral prosthesis.
Description
[0001] The present invention relates to a system for determining
the position of a prosthesis enabling assisting a surgeon in the
fitting of knee prostheses, by means of the computer system present
at the operating suite.
[0002] The general aim of computer-assisted knee surgery is to
determine an optimal position of the prostheses on the tibia, on
the femur, and possibly on the patella according to geometric and
dynamic criteria, and to provide devices enabling effective placing
of the prostheses at the optimal place. Ideally, an alignment of
the centers of the hip, of the knee and of the ankle is attempted
to be restored for a knee placed in extension, while balancing the
position of the patella and the tensions of the ligaments in a
flexional motion, and while having good adjustments of the
prostheses with the bones. In some cases, only a portion of the
knee is replaced, for example, one of the two condyles of the
femur, but the criteria remain identical.
[0003] Conventional methods use mechanical ancillaries adjustable
according to the radiological data of each patient. Such methods
are inaccurate and do not enable performing an ideal prosthesis fit
in all cases, which sometimes leads the surgeon to progressively
adjusting the positions of the prostheses, which is long and
difficult, or to be content with an average quality result.
[0004] To overcome these disadvantages, computer-assisted surgery
systems using position sensors, computers and possibly robots have
been developed.
[0005] Generally, in conventional computer-assisted surgery
systems, a remarkable element, also known as the measurement mark,
to a bone or an anatomic structure and its motions are followed by
means of a position sensor, also known as a locating system or a
tridimensional positioning system. Such a position sensor may be a
set of cameras which locate the position and orientation of
measurement marks each formed of at least three infrared diodes or
reflective markers. Many acoustic, mechanical, or magnetic
technologies enabling locating of the relative position and
orientation of two position marks attached on anatomic structures,
surgical instruments, digitizing palpation devices or various
sensors such as described in chapter 1 of book "Computer Integrated
Surgery", MIT Press, 1996, R. Taylor ed., entitled "Multimodal
Information for Computer-Integrated Surgery", pages 5-21, by R.
Moesges and S. Lavallee may be used. One of the position marks may
also be used as a position sensor, as is the case in small-size
magnetic systems. By means of these position sensors, a palpation
device may be used to digitize points on the surface of the
structures equipped with a position mark, and motions between two
bones equipped with position marks may also be recorded.
[0006] Most of the existing computer-assisted surgical systems for
assisting the fitting of a total knee prosthesis require use of
medical images acquired before an intervention by powerful means
such as the tomodensimetric (TDM) scanner or magnetic resonance
imaging (MRI), as described in the patents of ORTHOSOFT Inc (WO
99/60939), Eric Brosseau et al., or of Scott Delp et al. (U.S. Pat.
No. 5,682,886). Based on these images, a doctor can plan and
partially simulate a surgical intervention, then complex techniques
of adjustment between the patient's images and the patient's
position on the operation table enable following and reproducing
the planned strategies. However, the acquisition of such images in
clinical routine is complicated to manage, it has a non-negligible
cost, error risks are associated with the adjustment methods, and
the use of a scanner generates an X-ray dose which is
non-negligible for the patient. Further, such systems do not use
the cinematic and dynamic information which can be easily acquired
at the beginning of an operation, they thus do not enable directly
and easily obtaining the ideal position of the prosthesis.
[0007] An alternative consists of only using information acquired
just before the beginning of the operation by means of position
sensors. Such methods are however limited in their use since they
only enable collecting very simple information to remain within
reasonable time limits.
[0008] In this category, the most widely used methods are based on
the search for an alignment of the three rotation centers of the
knee, of the hip, and of the tibia, as described for example in
documents WO-A-95/00075 of ANDRONIC and WO-A-98/40037 of AESCULAP.
However, in such methods, the determination of the knee center is
made difficult and inaccurate by the strong anatomic variations of
each knee which must undergo an intervention, the very definition
of a pathological knee being the subject of debates and
controversies.
[0009] The present invention aims at using a simple equipment
including a computer, a screen, a tridimensional positioning
system, a position mark attached on each bone of the joint, and a
position mark placed on a drilling guide.
[0010] An object of the present invention is to provide an
automatic system for determining the best size, position and
orientation for each implant laid on one or several osseous section
planes, such as a conventional tibia and femur prosthesis.
[0011] Another object of the present invention is to provide such a
system in which the complete surface of each bone is obtained by
deformation of a generic model at the operating suite, without MRI
or TDM images.
[0012] Another object of the present invention is to provide such a
system in which the degree of accuracy at any point of the image of
the bone surface is indicated by a color.
[0013] Another object of the present invention is to provide such a
system in which ligamentary information are taken into account to
balance the tensions between each bone of the knee joint.
[0014] Another object of the present invention is to provide such a
system in which a drilling guide particularly simple to place to
then ensure the accurate positioning of a section guide is
provided.
[0015] An advantage of the present invention is that it enables
avoiding determination of a knee center on the patient. Instead of
aligning any center of the pathological knee with the center of the
hip and of the ankle, the present invention provides aligning the
center of the prosthesis of the pathological knee with the center
of the hip and of the ankle.
[0016] Another advantage of the present invention is that it
enables automatically determining an ideal theoretical position of
all the implants to be attached on each bone, taking into account
all the essential criteria and then letting the surgeon modify the
parameters with respect to the so-called ideal reference, just
before performing the surgical acts enabling placing the
implants.
[0017] Another advantage of the present invention is that it no
longer needs all the TDM or MRI methods of medical image
acquisition and adjustment.
[0018] To achieve these objects, the present invention provides a
system for determining the ideal theoretical position of a knee
prosthesis including, for the determination of the position of a
tibial prosthesis, means for:
[0019] determining the shape of the proximal portion of the tibia
and its position with respect to the center of the ankle joint;
[0020] determining a high point of the superior surface of the
tibia;
[0021] calculating the position, the orientation, and the size with
respect to the tibia of the tibial prosthesis and of the section
plane corresponding to the tibial prosthesis, taking the following
parameters into account:
[0022] the perpendicular to the section plane crossing the center
of the prosthesis also crosses the center of the ankle joint,
[0023] the section plane is at a distance from said high point
equal to the height of the prosthesis to be attached,
[0024] the large side of the prosthesis is centered on the large
side of the section of the tibia in the section plane,
[0025] the anterior edge of the small side of the prosthesis is at
a predetermined distance from the anterior edge of the small side
of the tibia section in the section plane; and
[0026] determining the orientation in the section plane of the
prosthesis so that the large side of the prosthesis is parallel to
the horizontal axis of the knee.
[0027] According to an embodiment of the present invention, the
system further includes, to determine the position of a femoral
prosthesis, means for:
[0028] determining the relative position of the distal portion of
the femur with respect to the center of the hip joint;
[0029] calculating the position, the orientation and the size with
respect to the femur of the femoral prosthesis and of the section
plane corresponding to the femoral prosthesis, taking the following
parameters into account:
[0030] the perpendicular to the section plane crossing the center
of the prosthesis also crosses the center of the hip joint,
[0031] the distal section plane is at a distance from the most
distal point on one of the condyles equal to the thickness of the
prosthesis to be attached,
[0032] the posterior section plane is at a distance from the most
posterior point on one of the condyles equal to the thickness of
the prosthesis to be attached,
[0033] the large side of the prosthesis is centered on the large
side of the section of the femur in the section plane,
[0034] the maximum size of the prosthesis is such that the edge of
the prosthesis is as close as possible but inside of the femur
surface; and
[0035] determining the orientation in the section plane of the
prosthesis so that the large side of the prosthesis is parallel to
the horizontal axis of the knee.
[0036] According to an embodiment of the present invention, the
orientation and the lateral position of the femoral prosthesis are
adjusted with an alignment of the plane of the trochlea of the
femoral prosthesis with the trajectory of the center of the
digitized patella during a knee flexion motion by means of a
position mark placed on the external surface of the patella.
[0037] According to an embodiment of the present invention, this
system further includes, to determine the shapes and positions of
the tibia, of the femur, of the center of the ankle joint and of
the center of the hip joint:
[0038] elements locatable in position in a tridimensional
positioning system, attachable at least to the tibia and to the
femur,
[0039] a palpation device, an echographic device, and/or a
radiographic device to locate in said tridimensional positioning
system the position of various points of the tibia and of the femur
with respect to said locatable elements, and means for adjusting a
preestablished model of the leg bones by using the collected
position information.
[0040] According to an embodiment of the present invention, said
model also includes a modeling of the positions of the attachments
of the ligaments of the knee joint, this modeling being adapted to
the patient at the same time as the model of the bone surface, and
said model also includes the values of the maximum elongations of
each ligament, experimentally determined by having the still
non-operated patient's leg move to locate the displacement limits
linked to existing ligaments.
[0041] According to an embodiment of the present invention, the
system further includes means for:
[0042] simulating on the image of the femur-tibia assembly the
positioning of the prostheses linked together for different flexion
positions, and the position of the knee ligaments;
[0043] deducing therefrom the motion limits that the leg provided
with the prosthesis will have with the existing ligaments; and
[0044] taking this information into account to modify the
theoretical ideal position of the knee prosthesis, and/or to
suggest an intervention on the ligaments.
[0045] According to an embodiment of the present invention, said
palpation device has a ball-shaped end and the locating by the
palpation device is performed dynamically as said ball is displaced
against the portion to be analyzed, the tridimensional positioning
system being designed to determine the instantaneous positions of
the ball center and a system of surface reconstitution from a
deformable model being designed to perform a correction
corresponding to the ball radius.
[0046] According to an embodiment of the present invention, an
image of the adjusted model is formed and this image is displayed
by assigning its various areas colors or thicknesses characterizing
the density of points found in this area by the palpation device,
which indicates the degree of accuracy of the model of the
corresponding area.
[0047] According to an embodiment of the present invention, the
system includes means for:
[0048] determining by calculation locations at which the
tibia/femur must be drilled into to accommodate a section
guide;
[0049] pointing these locations on the displayed image of the
tibia/femur,
[0050] permanently displaying on the restored image of the
tibia/femur the image of a drilling guide provided with locating
means in said tridimensional positioning system, this guide
including tubes separated by the same interval as tubes of the
section guide intended to be assembled on threads fixed in the
bone,
[0051] and the drilling guide includes a central point that can
bear against the bone and the image of which must coincide with a
target point pre-positioned on the displayed image of the
tibia/femur.
[0052] According to an embodiment of the present invention, the
drilling guide further includes adjustment means, operable once the
guide has been brought to its position.
[0053] According to an embodiment of the present invention, the
locatable elements respectively associated with the tibia, with the
femur and with the sensor are geometrically differentiated.
[0054] According to an embodiment of the present invention, the
locatable element associated with the tibia has the shape of letter
T, the locatable element associated with the femur has the shape of
letter F, the locatable element associated with the palpation
device has the shape of letter P.
[0055] According to an embodiment of the present invention, the
position of the patella component of the prosthesis is obtained by
means for:
[0056] determining in a tridimensional positioning system the
trajectory of an element having a locatable position attached to
the external surface of the patella in a knee flexion motion,
[0057] determining the point of the internal patella surface which
better coincides with the center of the groove of the prosthesis of
the femur for a selected angular flexion area;
[0058] guiding the placing of a drilling thread towards said
point.
[0059] The foregoing objects, features and advantages of the
present invention will be discussed in detail in the following
non-limiting description of specific embodiments in connection with
the accompanying drawings.
[0060] FIGS. 1A and 1B show a front view and a side view of a
femur-tibia assembly;
[0061] FIGS. 2A and 2B illustrate a femur and an associated
sensor;
[0062] FIGS. 3A and 3B show a distal end portion of a femur and
colored areas thereon;
[0063] FIG. 4 shows a model of a femur-tibia assembly and of
associated ligaments;
[0064] FIGS. 5A to 5C illustrate the assembly of a prosthesis
according to the present invention;
[0065] FIGS. 6A to 6B illustrate steps implemented by the system
according to the present invention;
[0066] FIG. 7 shows a drilling tool according to the present
invention;
[0067] FIG. 8 shows targets for determining the position of a
drilling object according to the present invention;
[0068] FIG. 9 shows an alternative of the drilling tool according
to the present invention;
[0069] FIG. 10 shows another alternative of the drilling tool
according to the present invention; and
[0070] FIG. 11 shows another alternative of the drilling tool
according to the present invention.
[0071] To perform an intervention according to the present
invention, the bones are placed in the field of a tridimensional
positioning system. Then, as illustrated in FIG. 1A in side view
and in FIG. 1B in front view, elements locatable by the
tridimensional positioning system respectively attached on femur 1
and on tibia 2 of a patient, respectively 3 and 4, are analyzed.
These locatable elements are generally points screwed in the bones,
one end of which is provided with markers which may be reflective
disks, reflective spheres, or infrared diodes. As illustrated in
FIG. 2A, to avoid any error risk, the present invention provides
for locatable element 3 attached on the femur to be F-shaped, with
at least three markers attached on the ends of the branches and on
the corners of the F. This device enables having the largest
possible letter, which avoids any possible confusion, with markers
which are as distant as possible, which increases the system
accuracy, while forming a compact shape. Similarly, the locatable
element attached to the tibia is T-shaped with three markers
attached to the ends of the branches of the T. All locatable
elements may thus be associated with a letter, P for the palpation
device, R for the patella, H for the humerus, etc. containing
markers on the ends of its branches and on its corners.
[0072] Reference number 5 designates the center of rotation of the
hip between the femur and the ilium, reference number 6 designates
the center of the ankle between the tibia and the foot, and
reference number 7 designates the joint of the knee between the
femur and the tibia. In FIG. 1B, the patella has been designated
with reference number 8 and a locatable element attached to the
patella on its external surface has been designated with reference
number 9.
[0073] The present invention provides using means for determining
the deformation of a generic model to follow digitized points on
the real bone. The model is generally formed of a set of several
hundreds of triangular facets connected to one another on their
edges and vertexes. Methods may be used, which are described in
chapter 16 of book "Brain Warping", Toga ed., Academic Press, 1999,
entitled "Elastic registration and Interference using Oct-Tree
Splines", pages 282-296, S. Lavallee et al., consisting of
deforming as little as possible a volume meshing which encompasses
and drives along the digitized surface points to have them coincide
with the model surface, then inverting the transformation function
thus obtained to deform the generic model towards the digitized
points. In such a method, it is first searched for a general
deformation which brings some specific anatomic points palpated by
the user to their homologous points of the model, after which the
deformation is refined by progressively decreasing the sum of the
squares of the distances between all the palpated points and the
model surface. Methods of distance calculation between points and a
surface represented by triangular facets are widely known in
computerized tridimensional geometry literature. A deformation
function F transforming the coordinates (X,Y,Z) of a point
expressed in the position reference system into coordinates
(X',Y',Z') of the point expressed in a reference system associated
with the model is thus obtained. To obtain the deformed model, the
antecedent point (X,Y,Z) of each point of the model (XM,YM,ZM) over
function F is searched by iteratively minimizing the interval
between (XM,YM,ZM) and F(X,Y,Z) and the geometric links applied
between the model points. Methods described in article "Building a
complete surface model from sparse data using statistical shape
models: application to computer assisted knee surgery" by M. Fleute
and S. Lavallee, published in Medical Image Computing And
Computer-Assisted Intervention--MICCAI '98, Spinger-Verlag LNCS
Series, pages 880-887, October 1998 may also be used. Such methods
are robust since they use statistical models. An intelligent
interpolation of the points enabling constructing complex surfaces
by means of a few points, and thus easily and rapidly, is then
achieved. The two mentioned approaches may also be combined by
starting with searching for the deformation of a statistical model
according to the method described in Fleute's previously-mentioned
article, then by carrying on with a deformation of a volume meshing
according to the method described in S. Lavallee's article
published in previously-mentioned book "Brain Warping".
[0074] To determine and digitize points on the bone surface, a
palpation device locatable in the tridimensional positioning system
is used. Conventionally, a palpation device with a pointed end may
be used to obtain points on the surface with a good accuracy, but
such a palpation device clings onto the bone and it is difficult to
digitize many points on the fly.
[0075] The present invention provides, as illustrated in FIGS. 2A
and 2B, using a sensor 10 having its end in contact with the area
to be detected including a specific portion with a radius that can
be greater than 0.5 mm (preferably, from 0.5 mm to 3 mm). From a
time when the surgeon or another operator sends an order by a
pressure on a foot or hand control, or by voice control, many
points can then be digitized on the fly by sliding the spherical
portion on the bone surface, but points shifted by a value equal to
the radius of the sphere outside of the real surface are recorded.
This shifting will have to be compensated for. To compensate for
the sensor sphere radius, the present invention provides means for
progressively minimizing the sum of the distances between each
point and the model surface to which the value of the sensor radius
is subtracted provided that the distances to the surface are
positive outside of the surface and negative inside of the
surface.
[0076] Despite the use of the sphere, it would be tedious to ask
the operator to digitize all the useful parts of the bone surface.
This may require a significant manual work and the reconstructed
surfaces may be quite inaccurate, or even erroneous.
[0077] In cases where the surface points cannot be directly
accessed to, an echographic probe equipped with a position mark may
be used to locate points on the bone through the skin, as described
in chapter 32 of book "Computer Integrated Surgery", MIT Press,
1996, R. Taylor ed., entitled "Computer-assisted spinal surgery
using anatomy-based registration", pages 434-437, by S. Lavallee et
al. The surface may also be constructed by using a few radiographic
images obtained at the operating theatre suite or in the radiology
room. If radiographs acquired outside of the operating suite are
used, the models thus reconstructed must then be adjusted on the
operating data. Such techniques are described in article "Nonrigid
3D/2D registration of images using statistical models" by M. Fleute
and S. Lavallee, published in Medical Image Computing And
Computer-Assisted Intervention--MICCAI '99, Spinger-Verlag LNCS
Series 1679, pages 138-147, October 1999. This article also
describes how to combine radiological information and position
information to construct 3D surfaces.
[0078] The surface reconstructed by the deformation method cannot
be perfect and inaccuracies remain, especially in regions where
points have not been palpated. It is important for the surgeon to
be informed of the inaccuracies of the surface reconstruction. As
illustrated in FIG. 3A, to give an indication of this inaccuracy to
the surgeon, the present invention provides displaying on a screen
of the computer the surfaces reconstructed in 3 dimensions with
surface colorings according to the estimated inaccuracy. For
example, in regions 21, which are critical for the prosthesis,
where many points have been palpated, the surface is of a first
color. In regions 22 and 23, which are less critical for the
prosthesis, where less points have been palpated, the surface is of
a second color. For the rest of the femur, in regions 24 where
little or no sensing has been performed, the image is substantially
that of the initial model at another scale factor and this is
indicated by a third color. The portion of the surface contained in
spheres of X mm around the digitized points may also be colored.
More or less brightly colored areas are thus obtained according to
the amount of sensing. A color gradation may be assigned according
to the increasing values of X to obtain a continuous effect. The
values of the inaccuracy of the reconstruction on the surface may
be obtained by other means, what matters being to given a notion
thereof to the surgeon.
[0079] When a planar section of the reconstructed surface is
displayed, inaccuracy colors may further be displayed on the
portions of the intersection contours between the plane and the
surface. Preferably, contours are displayed around the calculated
intersection contour with a thickness depending all along the
contour on the inaccuracy of the estimated surface in the
considered region. As illustrated in FIG. 3B, if a point of the
intersection contour between the plane and the surface has an
inaccuracy of X mm, a thickness of X mm is given to the contour of
this point.
[0080] As illustrated in FIG. 4, it is known to define and
construct models of the ligamentary structures (also including
tendons, muscles, cartilages, and others) attached to the surfaces
of the modeled bone structures. Such models are relatively faithful
approximations of the reality which enable predicting the general
behavior of the knee structures, according to the different
positions of the prostheses, as described in article "A
strain-energy model of passive knee kinematics of surgical
implantation strategies" by E. Chen et al., published in Medical
Image Computing And Computer-Assisted Intervention--MICCAI '2000,
Spinger-Verlag LNCS Series 1935, pages 1086-1095, October 2000. The
main difficulty of known methods is the construction of the models
adapted to each patient. The attachment points of the ligaments may
be located on MRI images but this is inaccurate and tedious, it
does not provide the elastic properties of the ligaments, and does
not translate the general behavior of the knee including the sum of
all unlocated minor structures. For simplicity, a model formed of a
femur 1 and of a tibia 2 connected together by ligaments 31, 32, 33
is considered. For example, the lateral ligaments, the crossed
ligaments and the ligaments of the rear capsule may be modeled by
simple straight lines or elastic curves or else by pencils of
straight lines or curves, with a maximum elongation for each
straight line, curve or fiber. Having used the previously-described
method to deform the model on the surface points digitized on the
real bone, the result of the deformation may be applied to the
attachment points of the ligamentary structures on each bone of the
model so that they are now known in the reference system associated
with each bone. Given the strong inaccuracies of the models and the
strong variations of each individual, it would be inaccurate to use
the elasticity parameters of the ligaments of the model and to
apply them as such to the patient's data. To avoid such errors,
prior to the intervention, once the position marks are in place in
each bone, the surgeon or another operator exerts motions on the
tibia in all possible directions and for several knee flexions to
reach multiple extreme positions of the tibia with respect to the
femur. The operator exerts strong but reasonable forces on the
bones, to place the ligaments close to their maximum elongation.
The computer memorizes all the relative positions of the tibia with
respect to the femur during these motions. For each ligament
connecting a point A of the femur to a point A' of the tibia, all
the distances between points A and A' for each of the memorized
relative positions are then calculated. Only the largest distance
is retained. This distance is that corresponding to the maximum
ligament length. At the end of this step, each ligament model is
known by its two attachment points on the femur and on the tibia,
as well as by its maximum length when extended. Based on the
maximum length of the ligament in extended position, and assuming
that the manual strain imposed to the ligament by the surgeon is
approximately known, models known in biomechanics may be applied on
the extension of the ligaments to assign them a non-linear curve
characterizing the elongation according to the tension strain, but
such a very complete model will have to only remain qualitative and
the maximum elongation model will generally be that used. In
practice, the system according to the present invention includes
means for memorizing the maximum extensions of the ligaments for
different angular knee flexion areas to compensate for the errors
linked to the position of the ligament attachments in each of these
areas, these angular areas may be the area of a flexion from 0 to
10.degree., the area of a flexion from 10 to 30.degree., and the
area of a flexion from 70 to 120.degree.. For each area, it is also
possible to only reconstruct the model of the ligaments which are
known to be involved in this area.
[0081] To complete the acquisition of the data necessary for what
follows, the position of the ankle center must be determined in the
referential system of the tibia and the hip center must be
determined in the referential system of the femur. For this last
point, known methods are used, for example, the method described in
French patent application FR-A-2785517 by P. Cinquin et al.
entitled "Procede et dispositif de determination du centre d'une
articulation". To determine the ankle center, the present invention
provides digitizing characteristic points on the ankle and defining
the ankle center by a geometric rule using these characteristic
points, as a complement of the data acquired on preoperative
radiographs. For example, the bumps of the internal and external
malleolae may be palpated, after which the ankle center can be
considered as the right-hand point with a relative distance equal
to the relative distance measured on a front radiograph of the
ankle.
[0082] The present invention provides using the various
above-mentioned measurement and position determination means, to
determine an ideal theoretical position of femoral and tibial
implants. It should be clear that the sequence of the different
steps may be modified in many ways according to each surgical
technique.
[0083] As illustrated in FIGS. 5A to 5C, the position of a portion
of the prosthesis on a bone, for example, tibial prosthesis 41, is
first set. Said prosthesis is generally formed of a fixed or mobile
superior surface of a tibia 42, its position is defined by the 3
parameters defining a section plane and by the 3 parameters
defining the position of the implant in the section plane (2
translations, 1 rotation). The implant size must also be chosen
from among a wider or narrower range.
[0084] To begin with, the tibial prosthesis has a center T
mechanically known by construction of the prosthesis. The tibial
prosthesis also has a main plane which corresponds to the section
plane on which it will bear. It is imposed as a constraint that the
straight line crossing prosthesis center T and perpendicular to the
section plane crosses the center of ankle C. The plane of the
tibial prosthesis is then forced to be tangent to spheres centered
on ankle center C. This constraint sets 2 of the 6 parameters.
[0085] Then, a reference point PT is palpated on the tibia and the
section plane is placed at a distance E from point PT, the value of
E being equal to the thickness of the superior surface of the
tibia. This constraint sets 1 of the 6 parameters.
[0086] There remains to determine the ideal position of the implant
in the section plane. A horizontal axis of the knee is first
determined either by searching the rotation axis between the two
extreme flexion and extension positions of the knee, or by
connecting two specific anatomic points such as the epicondyles,
defined by direct palpation or defined on the model adjusted to the
patient. The angle between this knee axis and the axis of the
tibial prosthesis is established by default at 0.degree.. The two
remaining translation parameters are set to respect a geometric
bulk in each considered section plane.
[0087] As illustrated in FIG. 6A, for the smallest given prosthesis
size, an arbitrary initial position of the prosthesis is given, and
the intersection contours between the surface and the corresponding
section plane are calculated and compared to the prosthesis contour
in this plane. For each of the two contours, the rectangle of
minimum size encompassing all the contour points is calculated,
imposing an orientation of an edge of the rectangle parallel to
horizontal axis AH. The intervals between the edges of each of the
two rectangles are minimized, which gives a new position of the
prosthesis in the section plane, and this process is repeated until
it converges. The values of X and X' are thus measured between the
lateral edges of the prosthesis and the lateral edges of the
contour of the section on the surface, and the lateral position is
adjusted to equalize X and X'. Similarly, the distance from back to
front Y is adjusted so that the prosthesis comes at Y mm from the
anterior edge of the intersection contour, Y being a value set by
default by the surgeon. The entire process is repeated, searching
for the largest size for which all edges of the prosthesis are
inside of the edges of the intersection contour, including the
posterior edges. The position, the size and the orientation of the
tibial prosthesis are thus totally automatically determined. If
need be, the operator may then move each of the parameters,
preferably by means of a tactile screen (or any other mouse
equivalent), to align on values of his choice according to each
surgical technique recommendation.
[0088] The position of the femoral prosthesis is determined in an
approximately equivalent manner, taking into account center H of
the hip instead of the ankle center. As illustrated in FIG. 6B, the
anteroposterior position of the prosthesis is automatically
calculated to equalize thickness Ep of the posterior condyle of the
prosthesis with distance Dp calculated between the posterior
section plane of prosthesis PCP and the most posterior point PP of
the surface of one of the two posterior condyles of the knee
selected by the surgeon. Posterior point PP is automatically
determined by the computer as being the point of the condyle
considered on the model and adapted to the patient having the
smallest coordinate Y in the direction of a vector Y from the
posterior to the anterior. This principle is applicable to the
search for all the endmost points in a given direction. The size of
the largest prosthesis is determined by the computer so that the
distance between the endmost point E which is most proximal to the
prosthesis is close to the femur surface while remaining inside of
this surface. This criterion is fulfilled by iteratively minimizing
the distance between the endmost point of the memorized prosthesis
and all the triangular facets forming the surface. The distal
position of the prosthesis is determined by the computer to
equalize the thickness of the distal condyle of prosthesis Ep with
distance Dp calculated between the distal section plane of the
prosthesis and the most distal point of the surface of one of the
two distal condyles of the knee selected by the surgeon.
[0089] To adjust the lateral position of the femur prosthesis, the
lateral edges may be used as described for the tibia.
Preferentially, the present invention provides memorizing the
trajectory of the midpoint R of the patella surface in a knee
flexion motion, starting with the complete extension. A small and
light position mark attached on the external portion of the patella
by means of small thin and shallow pins is used for this purpose.
For a position of patella 8 closed on femur 1 (obtained for example
at the beginning of the operation), the motions of the patella mark
with respect to the femur are recorded. Point R thus describes a
trajectory which is known in the femur reference system. The femur
prosthesis contains a groove to accommodate the patella during the
knee flexion. The middle of the groove is in a plane.
[0090] As illustrated in FIG. 6C, the position of the femur
prosthesis is determined to have the plane of the groove coincide
at best with the trajectory of point R. A rotation parameter may be
used to optimize this alignment. To optimize all criteria at the
same time, the process starts from an empirically-determined
arbitrary initial position close to the searched solution, then the
criteria are minimized one by one by modifying the value of the
position, orientation, or size parameter of the prosthesis which
influences it most, iteratively, until obtaining a stable
convergence of the ideal position, orientation and size of the
implant. When several parameters influence a criterion,
multidimensional optimization methods well known in mathematical
literature are used.
[0091] Up to now, it has been assumed that the positions of the
tibia and of the femur were independent. In reality, the prostheses
of the femur and of the tibia are designed by defining an ideal
trajectory of the tibial prosthesis on the femoral prosthesis from
flexion to extension, at least for 3 or 4 flexion positions, for
example, 0.degree., 30.degree., 90.degree., and 120.degree.. For
each of these angles, the ideal relative position between the
femoral component and the tibial component of the prosthesis is
totally determined. Some prostheses are said to be non-congruent
and allow for more complex motions than a simple trajectory of
flexion of the tibial component with respect to the femoral
component, but the average neutral kinematics significantly
describe an ideal searched component, it being also possible for
the method implemented by the present invention to take into
account the intervals with respect to the neutral kinematics. At
this stage, all the relative positions of the implants and of the
bones for different motions chosen according to the considered
prosthesis can thus be simulated. The model then enables predicting
that the ligaments do not exceed their limiting value for all these
relative positions. If some ligaments exceed their limits, the
surgeon may use this information to provide surgically relaxing the
ligament. At any time, the surgeon can intervene on the ligaments
and randomly acquire again the extreme positions to recalibrate the
parameters of the ligaments and restart the prediction.
[0092] For any global position, the present invention provides
simulating relative motions of the tibia and of the femur by
rotations around contact points existing on the condyles to
simulate positions in which the ligaments are in maximum elongation
when forces approximately equivalent in intensity and direction to
the forces applied upon measurement of the maximum elongations of
the ligaments are applied. The ligamentary balance can thus be
predicted for different flexion angles characterized by the
amplitude of the motions and by the dissymmetry of the possible
motions around the neutral position.
[0093] The surgeon can then freely determine the essential
parameters which most influence the setting of the ligamentary
balance, without having made any bone section yet. He can for
example choose to keep a perfect alignment of the hip and ankle
centers with the prosthesis center in extension, but inclining the
line perpendicular to the section planes with respect to the
mechanical axis, thus creating oblique interlines between the
prostheses. Any type of surgical technique can thus be simulated by
using the system according to the present invention.
[0094] Once each section plane of a bone (tibia or femur) has been
determined, it is provided to attach to this bone a section guide
in which a saw blade is engaged to perform the section with
accuracy according to the angle determined by the computer
(possibly altered by the surgeon). A section guide generally
includes at least two cylindrical bushes enabling placing threads
fixed in the bone. A section guide is a relatively bulky and heavy
instrument, and supports adjustable with screws, wheels or wedges
to hold and place these section guides generally have to be used,
said supports being themselves attached to the patient by various
rods which are heavy, bulky and often invasive, that is,
significantly deteriorating the bone in its healthy parts. Ideally,
it would be desired to be able to avoid using such supports and
place the section guides directly, but this is very difficult to
perform manually. The direct alignment of a plane on an ideal plane
or of a solid on an ideal solid is a delicate operation, even using
visual alignments on a screen which shows the real and desired
positions of each structure according to various graphical
representation modes. The present invention, instead of trying to
directly position a section guide, provides previously using a
drilling guide including two cylindrical bushes having exactly the
same spacing as those of the cross-section guide and a punctual
point used as a mechanical constraint.
[0095] FIG. 7 shows a drilling guide according to an embodiment of
the present invention. This drilling guide includes a handle 51.
The two cylindrical bushes are designated by references 52 and 53.
To this drilling guide is rigidly associated a position mark 55
visible by the tridimensional positioning system monitoring the
operation scene. Mark 55 is for example provided with 3 reflective
markers and is Y-shaped. According to a significant feature of the
present invention, the drilling guide includes a pointed point 54.
The general shape of this drilling guide, that is, the relative
position of point 54 and of bushes 52 and 53 in the Y-shaped
position mark is memorized in the computer used to determine the
position and the orientation of the section plane with respect to
the considered bone. As shown in FIG. 8, this computer displays a
target including a central target C1 and lateral targets C2 and C3.
The display of target C1 is a parallel perspective view in the
direction of the point axis. The display of targets C2 and C3 are
parallel perspective views in the direction of the bush axis.
First, the image of point 54 must be brought exactly on the center
of target C1. The point must for this purpose be slid on the bone
surface according to two translations, which is easy. Once the good
position of the point has been found, the point is very slightly
driven into the bone so that it remains stable thereafter. All the
guide motions are then forced to turn around this fixed point. The
images of bushes 52 and 53 are then brought on lateral targets C2
and C3, which is easily done by setting three rotations around a
fixed and mechanically stable point. These operations are performed
by the surgeon or another operator by displacing the drilling guide
on the bone and by monitoring its image on the computer which
substantially appears as shown in FIG. 8A. Thus, at the step of
FIG. 8B, point 54 is put in contact with central target 61. At the
step of FIG. 8C, bushes 52 and 53 are brought in front of targets
C2 and C3 and, at the step of FIG. 8D, the bushes are placed
according to the proper vertical orientation. Once the drilling
guide is in position, threads inserted in bushes 52 and 53 are
fixed in the bone.
[0096] FIG. 9 shows an alternative of the drilling guide of FIG. 7
provided with means for blocking in position to avoid any sliding
upon placing of the threads in the bone. For this purpose, on
handle 51 is assembled a horizontal frame 61 which carries at its
two ends, on either side of handle 51, two auxiliary vertical
bushes 62 and 63. Each of bushes 62, 63 receives a pointed rod,
respectively 64, 65 which crosses this bush. Inside of frame 61 is
provided a blocking means operated by a button 67. Such a blocking
can be performed by a spring which maintains a pressure on the
rods. When the button is pressed, rods 64, 65 freely slide in
bushes 62, 63. Thus, they come down until stopping against the bone
above which they are placed. As soon as button 67 is released, the
rods are blocked in position. The surgeon, as soon as he has
reached the position illustrated in FIG. 8D, can then, without
having to keep on monitoring the target appearing on the computer
screen, assemble the threads while being ensured that the drilling
guide does remain in its position, since it bears upon 3 fixed
points.
[0097] Once the drilling threads are positioned in the bone, the
drilling guide is removed and the first section guide is assembled
on the threads which have just been set. For a prosthesis such as
the femur which includes several section planes, a series of
section guides calibrated to perform all sections corresponding to
the bearing planes of the prosthesis, propping up against the two
reference threads placed in the bone, may then be added. For each
of these section guides, the relative position of the two bushes
intended to contain the holding threads and the section planes is
precisely known. It is important to note, on this regard, that the
point imposes a degree of liberty of the section guides since it is
in contact with the surface. The choice of the point position is
thus very important since it conditions the final position of the
prosthesis.
[0098] FIG. 10 shows another drilling guide 70 specifically
applicable to the femur prosthesis in which the holding point 71 is
off-centered with respect to the drilling bushes 72, 73, to bear
against the anterior surface of femur 1, to mechanically force the
drilling guide to respect the anterior position of the prosthesis
to be with accuracy.
[0099] FIG. 11 shows another drilling guide 80 specifically
applicable to the femur prosthesis in which holding point 81 is
off-centered with respect to drilling bushes 82, 83 to bear against
the posterior surface of one of the two condyles of femur 1, to
mechanically force the drilling guide to respect the posterior
position of the prosthesis to be with accuracy.
[0100] Of course, this structure is likely to have various
alterations and modifications which will occur to those skilled in
the art. For example, several points and a single blocking rod
could be provided. All the drilling guides described in the present
invention will be made in shapes and materials to be as light as
possible, to be easily handled.
[0101] Once a number of section planes have been formed, wedges can
be introduced between the femur and the tibia bearing against these
sections at the level of the two condyles of the femur to check
that the ligamentary balance will be correct once the prostheses
have been placed. Conventionally, the surgeon mechanically
displaces the tibia from left to right to evaluate its angular
play, but this measurement remains inaccurate. The system according
to the present invention enables accurately measuring the angular
variation and displaying it on the computer screen. The two signed
angles Bd and Bg obtained by an extreme displacement to the left
and to the right with respect to the neutral position of the knee
at rest are displayed. The difference between the absolute values
of the two angles Bd and Bg is a measurement of the ligamentary
imbalance from left to right. The sum of the absolute values of
angles Bd and Bg is a measurement of the general ligamentary
balance. Indicators Bd+Bg and Bd-Bg are measured and displayed for
various flexion angles. Preferentially, angles Bd and Bg are
graphically shown on a screen in a vertical segment of a straight
line corresponding to the neutral position and by two segments
intersecting the neutral segment at its vertex and forming angular
values of Bd and Bg with the neutral segment. By means of these
measurements, the surgeon can extend or relax the ligaments, or add
or subtract thicknesses between the prostheses of the femur and of
the tibia to obtain a good balance for all degrees of flexion.
[0102] According to another preferential embodiment, the present
invention provides displaying the distances between the closest
points facing one another on the femur and on the tibia equipped
with their simulated or real prosthesis, for each of the two
condyles, and for several flexion angles, when the knee is
submitted to external stress of swinging from left to right.
[0103] Once the femoral prosthesis is in place, many surgical
techniques invite to place a small prosthesis of button type on the
internal surface of the patella. The system according to the
present invention enables determining the ideal position of this
patellar button. The position of the groove of the femoral
prosthesis may be determined in the position reference system
associated with the femur, either by using the previously predicted
position, or by digitizing in fine the groove by palpation on the
fitted prosthesis. The relative positions of the position mark
attached to the patella are known and memorized for several flexion
angles. For a chosen flexion position, the digitized groove
position can then be displayed on screen by projecting it on a view
corresponding to the internal patella surface. Or conversely, the
patella trajectory obtained for all flexions can be obtained
according to an axial or front view of the femur trochlea. After
having conventionally cut the patella across its thickness, the
surgeon can then use a drilling guide and aim at a point on the
internal surface of the patella which coincides with the groove on
the femur, either globally, or for a given angular flexion area, so
that the patella does penetrate at the middle of the groove towards
20 degrees of flexion, and to balance the lateral forces which will
be exerted between the patella and the femoral prosthesis. This
drilling into the bone will then be used as a centering to place
the final patellar button.
[0104] The method and the system described in the present invention
may be applied to any type of knee prosthesis and they are
compatible with most surgical knee prosthesis setting techniques,
and they may be extended to other joints such as the elbow or the
shoulder.
* * * * *