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Description  |
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FIELD OF THE INVENTION
The present invention relates to a method of designing and manufacturing
individualized prostheses for human joints. More particularly it relates
to a method of generating prostheses by computer aided design and
customizing prostheses to match specific bone shapes of individuals to
produce a minimum of stress between the prosthesis and the supporting
bone, and a maximum of stability therebetween.
BACKGROUND OF THE INVENTION
Recent efforts to improve the long-term fixation of femoral stems have
focused primarily on press-fit designs often with some form of porous
coating for biological ingrowth. Whether this approach will be successful
and widely applicable is currently difficult to assess due to a number of
issues. Firstly, the early results of contemporary cemented femoral
components have improved since the introduction of modern cementing
techniques (Cornell and Ranawat, 1986; Harris and McGann, 1986; Roberts,
et al, 1986). Secondly, contemporary cementless prostheses may not be
optimal in terms of design and in the use of the ingrowth materials, one
consequence of which is inconsistency of bone ingrowth (Thomas, et al,
1987; Jasty, et al, 1987.) The early experience with a variety of
cementless designs has led to some uncertainty about the relative
importance of the many variables that must be tested in an effort to
improve the clinical results.
A general conclusion that can be drawn from the previous work is that the
clinical results improve with the degree of fit achieved at surgery,
indicating that immediate stable fixation is of paramount importance to
clinical success. (Ring, 1978; Engh, 1983; Ring, 1983; Itami, 1983;
Morsher, 1983; Bombelli, 1984.) It has become clear, however, that certain
design strategies for immediate fixation are inferior to others. For
example, fixation achieved with long stems that are fully coated with an
ingrowth material may produce deleterious proximal bone resorption (Brown
and Ring, 1985). It is generally believed that if micromotion occurs
between the stem and the bone during weightbearing, fibrous tissue will be
formed (Cameron, et al, 1973) which can lead to eventual clinical
loosening of the device. Primarily for this reason, the main thrust of the
research in the United States has been to achieve primary biologic
fixation with bone by the application of porous coating. To date, however,
retrieval studies have reported that only a fraction of the acetabular and
femoral surfaces achieve bone ingrowth.
Our initial approach to cementless stem design is based on the proposition
that a stem shape that closely resembles the anatomy of the femoral canal,
particularly in the proximal region, can achieve intimate contact and
stability and approximate the load transmission patterns of the normal
femur. The fit achieved with such an anatomic design, with the emphasis on
maximum fit in certain priority, areas of contact, should result in
maximal load transfer to cortical bone and resist not only axial and
bending loads, but the important torsional loads as well.
A non-anatomic stem design can result in an apparently stable interface
with a benign layer of fibrous tissue. (Kozinn, et al, 1986.) But, if the
stem is much more inherently stable due to anatomic fit, and is
constructed from an appropriate biocompatible material such as titanium
alloy, it may be possible to achieve an interface of bone upgrowth with no
interposition of fibrous tissue. (Lintner, et al, 1986; Linder, et al,
1983.)
The purpose of the present invention is to develop a method for the design
and evaluation of a prosthesis including the combination of all of the
significant elements on a given side of a human joint, for maximum
geometric compatibility with the supporting bone structure of the
recipient of the prosthesis. The object is to provide in such a prosthesis
articulating surfaces and supporting elements therefore in combination to
allow press-fit implantation and achieve sufficient stability to induce a
stable biologic interface. A further purpose is to establish appropriate
design parameters for initial stabilization, and thereby make it so that
any ingrowth material at the interface of a joint which is made up of
components so designed on both sides of the joint will thereby maximize
formation of bone ingrowth and permit better evaluation in the future. An
additional object of providing an anatomic design (including a relatively
smooth articulating surface) is to increase versatility, whereby
downsizing of the supporting elements can be performed to render them
suitable for fixation with cement, and thereby to provide a uniform mantle
with respect to cortical bone. A further object of the invention is to
provide a substantially customized anatomical fit for prostheses for each
patient, but to do it in a practically achievable and economical way.
BRIEF DESCRIPTION OF THE INVENTION
In the accomplishment of these and other objects of the invention in a
preferred embodiment thereof a three dimensional model of an average joint
bone including both the exterior surfaces and the cortical canal is
developed using a statistically significant number of samples and by
generating a piecewise mathematical analog thereof. The preferred method
of calculating the surfaces and supporting elements comprises sectioning
of embedded cadaver joints into a multiplicity of closely spaced segments
(for example, 25 segments). (Alternatively, CT scan can be used to define
the sections). The sections are then (copied and) digitized into a
computer, using, for example, 30 to 40 points per section with a greater
point density around regions of particular importance. Corresponding
sections of equivalent levels of all of the bones are then averaged using
a contour averaging routine. The digitalized representation of this
average joint bone is then stored in the computer's memory. When it has
been determined that an implant for a femur or other bone is required in a
patient, the size and shape of the corresponding bone in the patient is
determined by tomography or radiographs taken with the bone in fixed
anterior-posterior, lateral-medial, and longitudinal axes. The
relationships between the dimensions of the patient's bone on these axes
and the stored average bone (scaled to correspond to the scale of the
patient's bone) are then measured and input to the computer which then
generates a three-dimensional shape which represents the average bone
distorted to conform to the principal contours and dimensions of the
patient's bone. The bone shape so generated, although derived from key
linear dimensions of the patient's bone is essentially a synthesis of the
average bone. The prosthesis shape, while replicating the predicted bone
shape in certain areas, is relieved in other areas to enable it to be
surgically insertable. The resulting computer generated shape is then
fabricated using CAD/CAM and CNC techniques to provide prosthesis which
closely approximates the patient's bone in the critical areas and thereby
provides a maximum of stress free stability in the joint. It is a feature
of the invention that the prosthesis, although it conforms accurately to
the patient's bone in major ways and thereby provides a near optimum
anatomical fit, is essentially derived from a synthesis of the stored
average bone. Thus the process provides a virtually customized bone
implant, but avoids the much more hazardous and costly task of attempting
to determine the true shape (externally and internally) of the patient's
bone by CAT scan and to duplicate it.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention selected solely for purposes of illustration
is shown in the accompanying drawings in which:
FIG. 1 is three-dimensional line view in perspective showing an average
femur with outer surface to the left, the inner surface in the middle, and
the outer and inner surfaces superimposed, to the right;
FIG. 2. is a three-dimensional line view in perspective showing the
cortical canal, and the regions to which priority is assigned;
FIG. 3. shows cross-sectional cuts arranged sequentially, illustrating the
optimal stem shape;
FIG. 4. shows the results of different stems fitted in bones for proximal,
mid-stem, and distal regions, with the left hand representing
individualized fit, the center optimal fit, and the right, a symmetric
fit;
FIG. 5. is a bar graph comparing the percent of fit of prostheses made by 4
different methods;
FIG. 6A is a view of a patient's bone taken along the anterior-posterior
axis illustratively showing the location of two representative sections
used in making the comparisons of the method of the invention;
FIG. 6B is a view of the average of a plurality of bones similar to the
bone of FIG. 6A taken along the anterior-posterior axis illustratively
showing the location of two representative sections used in making the
comparisons of the method of the invention;
FIG. 7A is a view of a patient's bone taken along the lateral-medial axis
illustratively showing the location of two representative sections used in
making the comparisons of the method of the invention;
FIG. 7B is a view of the average of a plurality of bones similar to the
bone of FIG. 7A taken along the lateral-medial axis illustratively showing
the location of two representative sections used in making the comparisons
of the method of the invention;
FIG. 8A is an X-ray derived view of the frontal section of an optimal-fit
(profile) stem cemented into a bone showing a continuous cement mantle;
FIG. 8B is an X-ray derived view of the sagittal section of the bone and
stem shown in FIG. 8A;
FIG. 8C is an X-ray derived view of a transverse section of the bone and
stem shown in FIG. 8A taken along the line A--A;
FIG. 8D is an X-ray derived transverse section of the bone and stem of FIG.
8A taken along the line B--B;
FIG. 8E is an X-ray derived transverse section of the bone and stem of FIG.
8A taken along the line C--C; and
FIG. 9 is a flow chart setting forth the major steps of the process
including alternative paths for input data concerning the patient's bone
obtained from radiographs or from CT scans.
DETAILED DESCRIPTION OF THE INVENTION
The preferred embodiment of the present invention is described herein in
connection with the design and manufacture of a prosthesis of a human
femur, but it will be understood that the same process can be applied to
any other joint, and either to both halves of the joint or simply to one
half thereof.
The first step is to develop an average bone corresponding to the bone in
question, in this instance, the femur. In the preferred embodiment herein
described it was developed in the following manner:
The femurs of twenty-six human cadavers, aged in the 5th to 8th decades,
were individually clamped in a jig to a reproducible axis system based on
the center of the femoral head, and the straight part of the shaft just
below the lesser trochanter. The center line of the shaft defined the
vertical y-axis, the medial-lateral axis intersected the y-axis and the
center of the femoral head, and the a-p axis was mutually perpendicular. A
neck cut was made at 30 degrees from the horizontal, starting from the
piriformis fossa. The cut surface was photographed to show the shape of
the canal opening with respect to the orientation of the head and neck.
The head was then reattached. The femurs were embedded and sectioned
transversely, perpendicular to the y-axis. The section spacing was
proportional to the length of the femur, measured from the top of the
femoral head to the distal medial condyle. For a femur length of 500 mms,
the proximal 21 sections were spaced at 5 mms, the distal 4 sections at 20
mms.
The sections were photographed and the cortical-cancellous interface of the
femoral canal was identified for each section. Proximally, the inner and
outer contours were virtually coincident due to the small cortical
thickness. The inner and outer contours were then sequentially digitized
into a computer.
The boundary coordinates were scaled triaxially, according to equations
derived from overall measurements of femurs from a skeleton bank (Terry
Collection at the Smithsonian Museum). For each section, splining was used
to determine the perimeter and then 40 uniformly spaced points were
calculated with defined start- and mid-points. The section numbers were
adjusted so that the level of the trochanteric fossa corresponded for all
femurs. Compensation was next made for the location of the cente of the
femoral head. This was done by using an alogorithm to stretch the
coordinates to bring the center of the femoral heqad of each particular
femur into the average location of the center of the femoral head. For the
26 bones, for each section, the coordinates of corresponding point numbers
were then averaged. This produced an average femur which had a natural
appearance and where each section was aligned smoothly to adjacent
sections (FIG. 1, left hand view).
Next, the internal contour was determined. The goal here was to design an
optimal-fit stem that maximized implant-bone fit while recognizing that
the stem must be surgically insertable, but maintain fit in mechanically
important regions. Three interactive software modules were developed to
design a hip stem to fit the average 3-D bone model (Garg, et al, 1985;
Nelson, 1985).
The first module simulated the surgical bone preparation to prepare the 3-D
bone model for stem insertion. Interactive simulation of femoral head and
neck resection and canal reaming and rasping were performed in this
module. Inner cortical areas that were needed for efficient stem-bone load
transfer and for minimizing stem motion were identified and assigned a
priority score in this module. The particular high priority regions were
the proximal-medial and distal-lateral walls (FIG. 2). Once this
preparation was completed, the edited 3-D model was submitted to the
second software module.
The second module performed the actual stem design. The guiding principle
was that, due to the three-dimensional curvature of the femoral canal, a
stem that completely filled the canal could not be inserted. Thus, the
stem had to be shaved down in certain regions. The design process began by
setting the stem shape equal to the shape of the edited inner-cortical
model. The stem was translated proximally an incremental amount along the
vertical axis. Movements were then made in the other five degrees of
freedom. At each given orientation, a stem-canal surface overlap score was
determined, calculated as the point by point sum of the overlap distance
multiplied by the priority value. Minimization of the overlap score was
obtained by applying a modification of Newton's optimization algorithm.
The stem was then moved to the orientation of minimal score and the
overlapping stem regions removed by redefining them at their intersections
with the bone canal. The stem was then elevated to the next level and the
process repeated until the entire stem shape was withdrawn from the neck
cut. The resultant stem shape described the optimal stem-canal fit which
was still insertable (FIG. 3).
The third and final module was for verification and editing of the stem
design. Computer graphics methods were used for qualitative assessment of
implant-bone fit, while finite element analysis (FEA) was used to examine
the mechanical environment of the implant and bone. Following these
examinations, the computer-designed femoral stem could then be
manufactured using CNC.
A standardized optimal-fit hip stem was thus designed for the average
femoral geometry of the twenty six femurs described earlier. By using the
average femur, it was assumed that the average misfit of the optimal stem
would be minimized over a large number of femurs. As an interim measure,
the stem was then scaled to produce six sizes and mirrored to produce
rights and lefts.
In the preferred process of this invention, the computerized average femur
is stored and used thereafter to generate a prosthesis by the following
steps:
By tomography or by the use of radiographs, the profiles of the patient's
femur are determined on one of two views, such as the anterior-posterior
and/or medial-lateral axes. To accomplish this, the patient's bone is
positioned to align the plane subtended by the longitudinal axis of the
femur and the median prominences of the greater and lesser trochanters
normal to the axis of the camera. The resultant radiograph is placed on a
graphics tablet (which can be a precision digitizer utilizing a
microprocessor to calculate the cursor position from information detected
by circuitry placed in the surface), and, using a mouse, the center axis
(i.e., the Y axis) of the patient's femur is determined by establishing
the coordinates of the outer contours of a plurality of points below the
lesser trochanter. Once the center axis of the bone has been determined, a
first scale-determining plane normal to the Y axis and passing through the
lowermost extremity of the piriformis fossa is established, and then a
second scale-determining plane is established normal to the Y axis of the
femur and passing through the median prominence of the lesser trochanter.
The axial dimension between these two scale-determining planes is used to
adjust the scale of the average femur so that the upper scale-determining
plane corresponds to the twenty-fifth section of the average femur, and
the second scale-determining plane corresponds to the fifteenth section of
the average femur. This scale-adjusted shape can be referred to as a
"first synthesized shape". Then, having established the scale, the mouse
is guided around the entire contour of the cortical canal and upper part
of the femur. At each point of its passage, the coordinates are entered
and compared to the corresponding points of the average femur and each
section of the average femur is adjusted to conform around its entire
periphery to the difference on this one axis between the patient's femur
at that section and the average femur. This is done by placing the
negative of the patient's femur on the graphics tablet and rapidly running
the mouse around the patient's femur as represented by the radiograph.
Once the scan has been completed, the computer determines the adjusted
average femur which can be referred to as a "second synthesized shape". At
this point a printout of it can be made, similar to FIG. 1, except that it
will be adjusted to conform substantially to the femur of the patient, but
with additional relief in certain areas of the stem to facilitate
insertion. This can be regarded as the "3-D Optimal-fit Design" (see FIG.
9). The data thereof is then stored and used in the production of the
implant by CAD/CAM and CNC techniques. The entire process from radiography
of the patient's bone to production of the finished product can be
performed in a few hours.
A special additional feature of the process is that the surgeon can
improvise as he deems necessary in order to provide a joint which may be
calculated, for example, to correct for previous deterioration of the
patient's joint in order to restore the joint to correct dimensions which
previously existed. In this case, the patient's bone is X-rayed and then
the surgeon superimposes on the image the adjustments he deems necessary
and the distortion of the average femur (i.e., the second synthesized
shape) is based on the adjusted contour, rather than the actual
radiograph.
In addition, the process permits an implant to be made, examined and
further adjusted before actual implantation. Thus, if the surgeon, upon
examining the radiographs together with an actual sample of the implant
(or even before a first sample has been fabricated), detects that an
insertion problem is presented, due to peculiarities of a given patient's
cortical canal, he can rapidly have the sample adjusted, or a new sample
fabricated. The process is, in fact, sufficiently rapid to have the
equipment standing by to perform an adjustment during a surgical
procedure.
It will be understood that the dimensions of the patient's bone can be
radiographed on additional axes for fine tuning. In one embodiment, the
patient's femur is radiographed additionally on the orthogonally disposed
medial-lateral axis, and the same procedures of adjusting the average
femur are carried out to include this further input. The resultant
prosthesis is even closer to the patient's natural bone. Further "fine
tuning" is not considered necessary.
Having thus described a preferred embodiment of our invention, it will be
apparent to those skilled in the art that various modifications can be
made without departing from its spirit. For example, while it is
well-suited for designing and fabricating the femur, it can be used for
other joints. In addition, additional levels longitudinally of the bone
can be useful to detect differences in the angle of the cortical canal.
Further, the stem portion of the prosthesis can be scaled down to permit
the use of cement rather than a press-fit, while still benefitting from
advantages of the invention. In addition, while we have described
developing the average femur by sectioning the femurs of 26 humans, it
will be understood that the important thing is to take a sufficient number
of samples to establish an adequate bell curve to be statistically
significant, and, thereby, produce a reliable average femur, or other
bone, as the case may be. In addition, while tomography and radiography
have been mentioned as the method of determining the contour of the
patient's bone, other methods may be used. Accordingly, it is not intended
to confine the invention to the precise form herein described, but to
limit it only in terms of the appended claims.
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