Note: Descriptions are shown in the official language in which they were submitted.
WO 96103939 PCTIUS95/09705
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ASYMMETRIC FEMORAL PROSTHESIS
The present invention relates to a medical prosthetic device and more
particularly to an orthopedic medical prosthetic device. Even more
particularly,
the present invention relates to an improved asymmetric knee prosthesis and
method of surgically implanting same wherein the prosthetic medial femoral
condyles include a thinner medial posterior condyle and a thicker lateral
posterior
condyle, resulting in an elevation of the posterior medial femur joint line
with the
knee in flexion. The present invention also has a concave proximal anterior
region.
Arthroplasty is the making of an artificial joint. In total knee arthroplasty
there is a difficulty in reproducing the exact anatomy. The tibia is usually
cut at
ninety degrees (90°) to its axis in the frontal plane but the tibial
plateau is at about
eighty seven degree (87°). The disparity is due to the difficulty in
accurately
reproducing an eighty seven degrees (87°) cut. Also, it has been
documented
that with an eighty seven degrees (87°) or varus cut the tibial
component has a
tendency to loosen. (Proceedings of the Knee Society 1985-1986, Laskin, Varus
Knee deformity). (Surgery of the Knee, 6nsall et al, 1993).
With a ninety degree (90°) cut, more bone is removed on the lateral
side
than on the medial side of the tibia. Neutral is defined as, parallel to a
fine
tangent to the intact (not wom) posterior femoral condyles. When neutral
femoral
cuts are performed, and implants with equal medial and lateral thicknesses are
used, there is a laxity of the lateral collateral ligament at ninety degrees
(90°) of
flexion. The lateral collateral ligament is lax in flexion because the
resection of
the tibia and the posterior resection of the femur are not
parallel, but the prosthesis implanted has equal medial and lateral
thicknesses on
the tibial component and posterior condyles of the femoral component. This
results in less space on the medial side and more space on the lateral side,
which
causes unbalanced ligaments.
The current solution to this problem is to rotate the cutting block so that
more bone is removed from the posterior medial femoral condyle, this is
referred
to as external rotation. By externally rotating (clockwise rotation for a left
knee
when viewed from the distal end), the posterior femoral resection is parallel
to the
WO 96103939 PCT/US95109705~
2
ninety degree (90°) tibia) cut. This results in the collateral
ligaments being
balanced in extension and flexion when the prosthesis is implanted. The
current
industry standard is three degrees (3°) of external rotation which
corresponds to .
the three degree (3°) difference between the eighty seven degree
(87°) ,
anatomical angle of the tibia) plateau and the ninety degree (90°)
angle of the
tibia) resection.
The benefits of this surgical procedure have been mentioned but there are
some drawbacks. The femoral component is no longer aligned with the tibia)
component in full extension. The femoral component is rotated about three
degrees (3°) with respect to the tibia when it is aligned parallel to
the lateral
plane. This malalignment potentially could cause increased wear of the tibia)
insert. A possible solution to this malalignment could be to externally rotate
the
tibia) component, but this would result in reduced tibia) coverage, which is
not
desired. Another possible solution may be to design the insert at an angle,
but
both turning the tibia) tray externally or designing it into the insert have
the
problem of malalignment in flexion. With externally rotating the femoral
component, there will be malalignment with the tibia) insert either in flexion
or
extension whether the tibia) tray or tibia) insert is aligned straight or
externally
rotated.
A second problem with traditional external rotation is the chance of notching
the lateral anterior femoral cortex. "Notchlng° occurs when more none
Is
removed anterolaterally than with the neutral resections resulting in a notch
being
created in the anterior cortex of the femur. Notching greatly increases the
chances of the femur fracturing. A related problem to this is poor
anteromedial
implant coverage or even a gap between the implant and bone. In order to
reduce the chances of notching anteriorly, the lateral side is placed flush
with the
cortex and a gap develops between the implant and bone anteromedially.
Another problem with traditional external rotation is the increased
complexity and difficulty in instrumentation. The alignment of the cutting
blocks
must be variable and there is a different setup for left and right knees.
Also, it
can be difficult to accurately judge three degrees (3°) of external
rotation when
performing the surgery.
WO 96103939 2 ~ 9 6 6 9 6 PCT/U595/09705
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Currently there are both symmetric and asymmetric femoral components
available. The symmetric components all have a patella femoral groove that is
located along the centerline of the component. The asymmetric components
typically have a patella femoral groove which is angled but still straight.
The
GENESIS knee, available from Smith & Nephew Richards Inc. is an example of
this type of asymmetric femoral design. The problem with femoral components
which have either design is that the patella tends to sublux laterally or to
the pull
toward the lateral side. This is because the patella groove is located on the
centerline of the component and this is a medial to where the anatomical
patella
groove is located. Even with an angled patella grrove toward the lateral side
the
patella does not get to track a far lateral as it does normally. Most
traditional
femoral components have a thicker lateral anterior flange. This causes tension
in
the lateral retinaa~larwhich pulls.the patella lateral. The current surgical
solution
is a soft tissue release to allow the patella to track properly.
With external rotation, the patella tracking is altered. By rotating the
component as described, the lateral anterior flange is lowered and the patella
groove is sliifted laterally. This helps in reducing the tension in the
lateral
retinacular and to help locate the patella groove in a more anatomical lateral
position. From zero degrees (0°) to ninety degrees (90°) of
flexion benefits have
been found because of lateralizing the patella in this flexion range. However,
after ninety degrees (90°) of flexion, the patella will be medialized
which can
inaease the lateral force and shear force on the patella. This can lead to
higher
stresses at the bone implant intertace and lead to more wear of the patella
implant on the patella.
Current femoral prosthesis have convex proximal anterior regions from a
lateral view. This results in.the patella andlor ligaments being displaced
anteriorly. By having a concave anterior region, the patella and the ligaments
are
displaced anteriorly less because there is less metal in the concave region.
This
more closely resembles the anatomical femur and helps to improve patella
tracking.
The objectives of the present invention are to balance the flexion and
extension space, maintain proper alignment with the tibia, and to not notch
the
CA 02196696 2005-07-20
c
4
anterior femoral cortex when a ninety degree (90°) tibial resection and
symmetric
thickness tibial component is used. A further object of the present invention
is to
provide an improved femoral prosthesis which allows proper patella tracking
during a normal range of knee movement.
Tfie present invention provides an improved asymmetric femoral prosthesis
for use in total knee arthroplasty and an improved method of implanting a
femoral
prosthesis on a patient's distal femur.
According to the invention, a knee prosthesis comprises a femoral
component having an anterior articulating surface and distal and posterior
mtiarlating portions comprising lateral and medial condylar surfaces and
an internal non-articulating surface; and a tibial component having concave
articulating surfaces that receive the articulating surfaces of the femoral
component during use; wherein the distance between the internal non-
articulating
surface and the articulating of the lateral condylar portion is different from
the distance between the internal non-articulating surface and the
articulating
surface of the medial condylar portion of the femoral component over at least
a
part of the articulating surface of the femoral component, whereby the angle
of
femoral rotation about the madianical axis changes when going through a normal
rage of motion of the knee.
According to one aspect of the present invention, there is provided a knee
prosthesis comprising: (a) a femoral component having distal and posterior
articulating surface portions comprising lateral and medial condylar surfaces
and internal non-articulating surfaces; and (6) a tibial component having
concave articulating surfaces that receive the articulating surfaces of the
femoral component during use, wherein the distance between the internal non-
articulating surface and the posterior articulating surface of the lateral
condylar
surface is greater than the distance between the internal non-articulating
surface and the posterior articulating surface of the media! condylar surface
of
the femoral component over the -posterior but not the distal surface of the
articulating surface of the femoral component, whereby an angle of femoral
rotation about a femoral mechanical axis changes when going through a
normal range of motion of the knee.
CA 02196696 2005-07-20
According to a further aspect of the present invention, there is provided a
knee prosthesis according to the above described one aspect, wherein the
femoral
component further comprises an anterior articulating surface which, in use,
articulates with a patella component or natural patella of the knee; wherein a
centerline of the femoral component is defined centrally between the distal
condyles of the component; said femoral component has a patella groove in its
anterior articulating surface within which said patella component may track
during
normal articulating motion of the knee; the longitudinal axis of said groove
being
located laterally with respect to said centerline in a superior region of the
anterior
articulating surface and curving in a medial direction in the distal region of
the
anterior articulating surtace of the femoral component.
According to another aspect of the present invention, there is provided a
knee prosthesis according to the above described one aspect, wherein the
femoral component further comprises an anterior articulating surface portion,
where an anterior proximal portion has an arc center that is anterior to the
anterior
surtace portion; wherein the femoral component is concave in an anterior
proximal
region in a lateral view.
The preferred embodiment includes a femoral prosthesis having anterior,
distal, and posterior articulating surface portions. The posterior
articulating surtace
portions include a pair of condylar surfaces that are positioned at differing
distances from the anterior non-articulating or internal surtace portion. The
posterolateral articular surface is a greater distance from the anterior non-
articulating surtace than the posteromedial side. The medial and lateral
articular
surfaces in the distal region are the same distance from a tine transverse to
the
component.
The femoral prosthesis has an internal non-articulating surface that includes
a plurality of surtaces for receiving a resected distal femur. Preferably, the
non-
articulating surtace includes distal, anterior and posterior surtaces as well
as a
pair of chamfer surtaces. Additional cut surtaxes may be provided. The
posterior
WO 96103939 2 ~ R ~ (~ ~ ~ PCTIUS95/09705
non-articulating surface preferably defines a single plane that registers
against a
similarly configured resected surface of the distal femur.
The tibia) component inGudes concave articulating surtaces that receive the
femoral component articulating surfaces during use.
5 The improved femoral prosthesis of the present invention solves the above
discussed problems that have attended traditional femoral components aligned
in
neutral or external rotation.
The femoral prosthesis of the present invention aligns properly with the tibia
in extension because it is not rotated like the externally rotated component.
This
results in improved femorotibial articulation and reduces the chance of severe
wear. Since the tibia) component is not externally rotated there is also not
rotational incongruity between the femoral and tibia) components in flexion.
The angle of femoral rotation generally about the mechanical axis gradually
changes when going through a normal range of motion while maintaining
alignment of the femoral and tibia) articular surfaces. A gradual change is
defined
as a change that does not produce a step or immediate shift in the articular
geometry. A~normal range of motion that a patient would use could range from
negative ten degrees (-10°) to one hundred thirty degrees
(130°). During walking
on a level surface a person would typically range from 0 degrees to 70
degrees.
The entire movement associated with walking is referred to as gait. The range
of
motion required far going up stairs is about 0 to 60 degrees and to go down
stairs
is about 0 to 90 degrees. Getting up out of a chair can range from 0 to 90
degrees and deep knee bending could range from 0 to 130 degrees.
Hyperextension, or a negative flexion could occur while standing of about 10
degrees.
The anterolateral cortex should not be notched because no additional bone
is removed. Neutral cuts are used which do not notch the anterior femur. There
is improved anterior medial bone coverage because the prosthesis is not
rotated
(there is no gap on the anteromedial side).
The instruments for putting in the new femoral component are simpler to
use because the same procedure is used for a right or left knee and the work
of
positioning the three degree (3°) resection is eliminated.
WO 96103939 21 g b 6 R b P~T~S95109705
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An alternative prosthesis and method according to the invention uses a
ninety degree (90°) tibia) resection and a tibia) component that is
thicker on the
lateral side than the medial side. The femoral component then preferably
provides a thinner lateral distal condyle than the medial distal condyle. The
posterior condyles are preferably of equal thickness at about ninety degrees
(90°)
of flexion in this embodiment.
With the present invention the thickness could be altered both distally and
posteriorly, simply by making an angled distal resection in the frontal plane
or
making an angled posterior resection with respect to neutral rotation. If the
anterior resection was made neutral and the posterior resection was rotated
then
the cross section from anterior to posterior would be trapezoidal in shape.
Differences between the_ positions of the medial and lateral articular
surfaces are independent of the box geometry and any variation of distal or
posterior condyle thickness. The distal or posterior cuts could be made at an
angle or at different levels in order to produce the same design objective but
maintain constant condyle thicknesses or to maintain different condyle
thicknesses. ~ This would produce the same results of balancing the flexion
and
extension spaces, with proper alignment with the tibia, and have different
thicknesses than in the description of this new femoral prosthesis.
The femoral component could also be designed with more or less external
rotation incorporated. With traditional neutral resections of the distal and
posterior femur, in order to design for more external rotation there would be
a
larger difference in the thickness of the posterior condyles with the
preferred
embodiment. The medial posterior condyle is thinner than the lateral posterior
condyle of the femoral component so that at ninety degrees (90°) of
flexion the
femoral joint line is angled at three (3°) of rotation along its
mechanical axis.
Other angles could be designed into the femoral ranging from about one degree
(1 °) to ten degrees (10°) at about ninety degrees (90°)
of flexion. The articular
geometry of the condyles changes such that the femoral component articulates
with the tibia) component during filexion and extension without requiring the
femoral component to turn relative to the alignment of the tibia) insert in
the
transverse plane.
WO 96/03939 219 6 6 9 6 PCT~S95109705
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In order to solve the problem of improper patella tracking, the invention
provides a knee prosthesis comprising a femoral component having internal non-
articulating surfaces and external articulating surfaces, including an
anterior
articulating surface and distal and posterior lateral and medial condylar
articulating surfaces, a centerline of the femoral component being defined
centrally between the distal condyles of the component; a patella component or
natural patella having an articulating surface which articulates with the
anterior
articulating surface of the femoral component in use, said femoral component
having a patella groove in its anterior articulating surface within which said
patella
component may track during normal articulating motion of the knee; the
longitudinal axis of said groove being located laterally with respect to said
centerline in the superior region of the anterior articulating surface and
curving in
a medial direction in the distal region of the anterior articulating surface
of the
femoral component.
The patella groove has bean shifted laterally to allow the patella to track in
its more anatomical location. The groove is located lateral of the centerline
in the
anterior region and then curves back, preferably to the centerline of the
component. The lateral shift of the patella groove allows the patella to track
in its
anatomical location and it reduces the tension on the ligaments that pull the
patella laterally. The lateral shift is only limited by the width of the
component.
The new asymmetric femoral component may also have a concave proximal
anterior region in a lateral view. This results in the femoral component being
thinner in this region. The benefits are that the thickness of the implant
more
closely matches the thickness of the bone that is removed. The patella and the
ligaments can function more anatomically because extra metal is not being
added
to the anterior cortex of the femur. The concave region has an arc center that
is
located anterior to the component where a convex region would have an arc
center that is posterior to the anterior surtace of the component.
The present invention also provides an improved method of implanting a
femoral prosthesis on a patient's distal femur. The distal femur is first
resected
with a plurality of five cuts including anterior, posterior, distal and a pair
of
chamfer cuts. The femoral prosthesis is affixed to the patient's resected
distal
WO 96103939 - PCT/US95109705
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femur. The femoral prosthesis is an improved asymmetric prosthesis having
anterior, posterior, and distal non-articulating portions that closely fit the
anterior,
posterior and distal cuts on the distal femur.
The patient's tibia is reseded to receive a tibia) prosthesis. The femoral and
tibia) cuts are generally parallel in full extension and relative to the
tibia)
prosthesis and to the femoral . prosthesis so that the angle of femur rotation
generally about the patient's mechanical axis relative to the tibia gradually
changes when going through a normal range of motion of the patient's knee from
about minus twenty degrees (i.e. -20°) of flexion to about one hundred
thirty
degrees (130°) of flexion while maintaining alignment of the femoral
and tibia)
articular surfaces.
The femoral posterior resections are made parallel to a line tangent to the
intact posterior femoral condyles with approximately the same amount of bone
being removed off of each posterior condyle, but the prosthesis has a thinner
medial posterior condyle than its lateral posterior condyle. The distal
thickness is
the same beiween the medial and lateral condyles in the preferred embodiment.
This results in balancing both the flexion and extension spaces similar to
that of
externally rotating a traditional femoral prosthesis.
For a further understanding of the nature and objects of the present
invention, reference should be had to the following detailed description,
which is
given only as an example of the present invention, taken in conjunction with
the
accompanying drawings, in which like parts are given like reference numerals,
and wherein:
FIGURE 1 is a lateral view of the preferred embodiment of the present
invention showing the medial and lateral posterior condyle thickness and the
concave anterior region;
FIGURE 2 is a frontal view of the preferred embodiment of the apparatus of
the present invention;
FIGURE 3 is a frontal view illustrating human femur, knee joint, and tibia
and the leg mechanical axis;
FIGURE 4 is a frontal view of a human knee joint showing a ninety degree
(90°) tibia resection and a parallel femoral distal resection;
R'O 96103939 ~ I ~ ~ ~ ~ ~ PCTlUS95/09705
9
FIGURE 5 is a frontal view of a human knee joint in ninety degree
(90°)
flexion illustrating the anatomical angle of the tibia at eighty seven degree
(87°)
and the tibia) resection at ninety degrees (90°);
FIGURE 6 is a frontal view of a human knee joint in ninety degrees
(90°) of
flexion illustrating neutral femoral posterior and anterior resections with a
ninety
degree (90°) tibia) resection;
FIGURE 7 is a frontal view of a human knee joint in ninety degrees
(90°) of
flexion showing a symmetric thickness tibia) and femoral prosthesis in neutral
alignment;
FIGURE 8 is a frontal vigw of a human knee joint illustrating externally
rotated resections on the anterior and posterior femur and showing more bone
cut from the anterolateral femur and from the posteromedial femur,
FIGURE 9 is a frontal view of a knee joint in ninety degrees (90°) of
flexion
with a symmetric thickness tibia) and femoral prosthesis externally rotated
and
the balanced collateral ligaments;
FIGURE 10 is a lateral view illustrating a femur that has been notched in the
anterior corteX as part of a prior art knee joint replacement surgical
procedure;
FIGURE 11 illustrates knee joint resections using the method of the present
invention and showing the extension space parallel;
FIGURE 12 illustrates the neutral femoral resections using the method of
the present invention, showing the flexion space that is trapezoidally shaped;
FIGURE 13 is a frontal view of a knee joint in ninety degrees (90°) of
flexion
showing neutral femoral resections of the anterior cortex and posterior
condyles;
FIGURE 14 is a frontal view of the preferred embodiment of the present
invention of a knee joint ninety degrees (90°) in flexion showing a
thinner
posteromedial condyle than the posterolateral and balanced collateral
ligaments;
FIGURE 15 is a lateral view of the preferred embodiment of the asymmetric
component of the present invention showing the different medial and lateral
curves posteriorly;
FIGURE 16 is a frontal view of a knee in ninety degree (90°) of
flexion of an
alternate embodiment of the method and apparatus of the present invention
showing neutral femoral resections;
WO 96/03939 _- ~ ~~ ~ PCTlUS95109705
FIGURE 17 is a frontal view of a knee joint in ninety degrees (90°) of
flexion
showing the alternate embodiment of the apparatus of the present invention
with
a tibial component that has a thicker lateral section that it's medial
section;
FIGURE 18 is a lateral view of the alternate embodiment of the asymmetric
5 femoral component of the present invention showing the different medial and
lateral curves distally; and
FIGURE 19 is a lateral view of a femoral component showing both the
traditional convex anterior and the concave anterior region.
FIGURE 20 is a frontal view of a prior art femoral component showing a
10 centrally located patella groove.
FIGURE 21 is a frontal view of a prior art femoral component showing an
angled patella groove.
FIGURE 22 is a frontal view of an improved femoral component of the
invention.
FIGURE 23 is a similar view showing the position of the patella
FIGURE 24 is a distal view of the improved femoral component showing the
location of the patella groove.
FIGURE 25 is a medial view of an improved femoral component showing
the patella groove.
Figures 1 and 2 show generally the preferred embodiment of the apparatus
of the present invention designated generally by the numeral 10. The
asymmetric
femoral component 10 includes a prosthesis articulating surface 11 that
includes
an anterior articulating surface portion 12, distal articulating surface 15,
and a pair
of condylar surfaces including lateral posterior condyle 13 having posterior
condylar surface 13A and medial posterior surface condyle 14 having posterior
condylar surface 14A.
The proximal side of asymmetric femoral prosthesis 10 provides a recess 16
that receives a patient's distal femur after the distal femur has been
resected to fit
the plurality of surfaces 17-21. Anterior proximal surtace 17 is a generally
flat
surface that intersects proximal anterior chamfer 18. Anterior chamfer 18
extends between anterior proximal surface 17 and proximal surface 19. Proximal
posterior chamfer 20 extends between surface 19 and posterior proximal surface
WO 96103939 ~ ~ ~ ~ ~ ~ ~ pCTIfTS95/09705
11
21. In Figure 2, asymmetric femoral prosthesis 10 has a central recess portion
22 between lateral distal surface 23 and medial distal surtace 24.
In Figure 1, there can be seen a difference in the position between the
surfaces 13A-14A. A line drawn tangent to the most posterior portion of
lateral
posterior condyle 13, generally parallel to the frontal plane, is defined by
the line
13B. A line drawn tangent to the most posterior point on posterior condylar
surface 14A (generally parallel to frontal plane) is the line 14B. A distance
25
between the lines 13B and 14B shows that the lateral posterior condyle 13 is
thicker than the medial posterior condyle 14 whereas the inner surface 21 of
each
of the condyles 13 and 14 is the same flat surface 21, defining a plane that
accepts corresponding resected surfaces on the patient's distal femur. The
frontal plane or coronal plane is shown in Figure 1 as a plane going through
line
12C and perpendicular to the page. It is a plane that splits the body into a
front
and a rear half. The transverse plane is typically a plane that goes through
the
waist area and cuts the body into a top and a bottom half. For ease of use it
is
shown in Figure 1 as a plane going through line 12d and perpendicular to the
page. This plane is parallel to the typical transverse plane and for this
description
is functionally the same.
This femoral component 10 essentially has three degrees (3°) of
external
rotation built into it. This is in contrast to the current procedure of the
surgeon
cutting the femur at three degrees (3°) of external rotation. On the
femoral
component 10 of the present invention, the angle of femoral rotation generally
about the mechanical axis 32 gradually changes when going through a normal
range of motion of the patient's knee, typically from about zero degrees
(0°) of
flexion to about ninety (90°) to one hundred thirty degrees
(130°) of flexion. A
normal range of motion that a patient would use could range from negative ten
degrees (-10°) to one hundred thirty (130°) degrees. During
walking on a level
surtace a person would typically range from 0 degrees to 70 degrees. The
entire
movement associated with walking is referred to as gait. The range of motion
required for going up stairs is about 0 to 60 degrees and to go down stairs is
about 0 to 90 degrees. Getting up out of a chair can range from 0 to 90
degrees
and deep knee bending could range from 0 to 130 degrees. Hyperextension, or
W O 96103939 PC1'IUS95109705
12
a negative flexion could occur while standing of about 10 degrees. The
rotation
balances the flexion gap between the femoral and tibia) components as well as
aligning the femoral component articular surface parallel to the lateral plane
in
flexion and extension. The three degrees (3°) of external rotation is
generally
along the mechanical axis 32 of the femur.
In total knee arthroplasty there is a difficulty in reproducing the exact
anatomy. As illustrated in Figures 4 and 5, the patient's tibia 31 is usually
cut at
ninety degrees (90°) to its axis in the frontal plane but the tibia)
plateau is at
eighty seven degrees (87°). The disparity is due to the difficulty in
accurately
reproducing an eighty seven degree (87°) cut. Also, it has been
documented that
with an eighty seven degree '(87°) or varus cut the tibia) component
has a
tendency to loosen. (Proceedings of the Knee Society 1985-1986, Laskin, Varus
Knee deformities).
With a ninety degree (90°) cut, more bone is removed on the lateral
side 33
than on the medial side 34 of the tibia 31 as shown in Figures 3, 4, and 5. In
Figure 4, the tibia) cut is designated as 35. Because of this, at ninety
degrees
(90°) of flexion, when neutral femoral cuts are performed the resection
35 of the
tibia and the posterior resections 38 and 39 of the femur are not parallel.
This
results in less space on the medial side 34 and more space on the lateral side
33,
which causes unbalanced ligaments 40 and 41 when traditional tibia) and
femoral
components are used that have symmetric thicknesses on the medial and lateral
sides (see Figure 7).
The current solution to this problem is to rotate the cutting block so that
more bone is removed from the posteromedial femoral condyle. This is referred
to in the art as external rotation. By externally rotating (clockwise rotation
for a
left knee when viewed from the distal end), the posterior femoral resection is
parallel to the ninety degree (90°) tibia) cut (see Figure 8). This
results in the
collateral ligaments 40 and 41 being balanced in extension and flexion when
the
prosthesis is implanted as shown in Figure 9. The current industry standard is
three degrees (3°) of external rotation.
One problem with traditional external rotation is the chance of notching the
anterolateral femoral cortex as illustrated in Figure 10 with prior art
femoral
WO 96103939 219 0 ~ ~ ~ PCT~S95/09705
13
prosthesis F and tibial prosthesis T. Figure 8 shows that more bone is removed
anterolateral and posteromedial than with the femoral neutral resections shown
in
Figure 6. Figure 10 illustrates the anterior cortex 42 with a notch 43, which
greatly increases the chance of the femur fracturing.
In the preferred embodiment, the femoral posterior resections are made
neutral with approximately the same amount of bone being removed off of each
posterior condyle, but the prosthesis has a thinner medial posterior condyle
14
than its lateral posterior condyle 13 as seen in Figure 14. The distal
thickness is
the same between the medial and lateral condyles in the preferred embodiment
as shown in Figure 15. This results in balancing both the flexion and
extension
spaces similar to that of externally rotating a traditional femoral
prosthesis.
Figures 11 and 12 show neutral femoral resections as used in the method of the
present invention. The distal femoral resection is indicated as 44, the tibial
resection as 45, defining a parallel extension space 46. The anterior
resection is
indicated as 48 in Figures 11 and 12. In Figure 12, posterior femoral
resections
38 and 39 appear when the knee is in flexion, defining a trapezoidal flexion
space
47. '
An alternative method and apparatus is shown in Figures 16 - 18. The
alternate embodiment of the method and apparatus of the present invention uses
a ninety degree (90°) tibial resection 51 and a tibial component 50
that is thicker
on the lateral side than the medial side. The femoral component 60 would then
have a thinner lateral distal condyle 61 than the medial distal condyle 62.
The
posterior condyles 63 and 64 would then have the same thickness at about
ninety
degrees (90°) of flexion in the alternate embodiment.
With the present invention the thickness could be altered both distally and
posteriorly simply by making an angled distal resection in the frontal plane
or
making an angled posterior resection with respect to neutral rotation. If the
anterior resection was made neutral and the posterior resection was angled
then
the cross section from anterior to posterior would be trapezoidal in shape in
the
transverse plane.
The asymmetric femoral prosthesis components 10 and 60 shown in
Figures 13 -18 show that the difference between the medial and lateral sides
is
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14
independent of the box geometry and any variation of distal or posterior
condyle
thickness. The distal or posterior cuts could be made at an angle or at
different
levels in order to produce the same design objective but maintain constant
condyle thicknesses or to maintain different condyle thicknesses. This would
produce the same results of balancing the flexion and extension spaces, with
proper alignment with the tibia, and have different thicknesses in the
condyles
than in the description of this new femoral prosthesis.
The femoral component could also be designed with more or less external
rotation built in. With traditional neutral resections of the distal and
posterior
femur, there would be a larger difference in the thickness of the posterior
prosthesis condyles to produce more external rotation.
The new femoral component design is intended to be placed to compensate
for a ninety degree (90°) tibial resection. Resetting the tibia at
ninety degrees
(90°) in the frontal plane and placing a prosthetic tibial component,
which has the
same thickness medially and laterally, results in an elevated medial
compartment
of the tibia. To compensate for this, the femoral component of the present
invention, iri conjunction with the resections of the distal and posterior
femur,
elevate the media( joint line relative to the femur to compensate for the
elevation
that occurs in the tibia.
On the posterior femur, this is accomplished by resetting the posterior
femur roughly parallel to the posterior femoral condyle joint line. The
prosthetic
medial femoral condyle is thinner than the resection that occurs medially.
This
results in an elevation of the posterior medial femoral joint line with the
knee in
flexion.
The distal resection of the femur is accomplished by removing asymmetrical
amounts of bone from the medial and lateral condyles of the femur. The distal
medial and lateral surfaces are then replaced with equal thicknesses of
prosthetic
medial and lateral surfaces. Because additional bone has been removed from
the medial distal femoral condyle, this results in an elevation of the medial
distal
femoral joint line with the knee in extension. This too compensates for
elevation
of the medial compartment of the tibial that occurs due to the ninety degree
(90°)
tibial resection in the frontal plane.
W096/03939 2 ~ g d 6 ~ ~ PCTIUS95109705
Figure 19 shows a lateral view of a femoral component. The anterior region
65 shows the concave region of the new femoral where the arc center is located
anterior to the component. The anterior region 66 shows the convex region of a
traditional femoral component where the aro center is located posterior to the
5 anterior surface. The concave anterior results in less metal anteriorly as
shown
in Figure 19 which more closely replaces the resected bone with the same
amount of metal.
Figures 20 and 21 show prior art prostheses in which the patella groove 67
is either centrally positioned with respect to the centerline 68 of the
femoral
10 component (fig 20) or is angled (fig 21 ).
By contrast, the improved femoral component of figure 22 has a patella
groove which is positioned lateral of and about parallel to centerline 68 in
the
anterior region 65 of the component and then curves back to the centreline 68
at
the intracondylar notch 70. The depth of the groove 67 is shown in fig 25 by
line
15 72. By lateralizing the patella femoral groove, the ligaments are more
balanced
and the patella potentially tracks smoother and properly in the patella
femoral
groove. The t~ew design is a patella femoral groove which is aligned virtually
straight in the anterior region but shifted lateral to the centerline and
which
gradually transitions into the intracondylar notch region. This type of
patellofemoral groove design causes the patella to track much more lateral at
higher degrees of flexion than compared to a centerline or angled patella
femoral
groove design. This has a clinical advantage because it allows the patella to
track properly which should avoid the need for soft tissue lateral releases.
The
patella is allowed to track laterally up to about 40 to 50 degrees of flexion
before
it gradually transitions to the centerline of the component in the
intracondylar
notch as shown in figure 24. It transitions into the intracondylar notch so
that
there is good contact between the femoral component and the patella at high
degrees of flexion.
The following table fists the parts numbers and parts descriptions as used
herein and in the drawings attached hereto.
WO 96103939 ~ ~ (~ ~ ~ ~ 6 PCTIUS95109705
16
PARTS LIST
Part Number Description
10 asymmetrical femoral prosthesis
11 prosthesis articulating surface
12 anterior articulating surface '
12p anterior tangent line
12B anterior concave surtace
12C frontal plane
12D transverse plane
13 . lateral posterior condyle
13A posterior condylar surface
13B posterolateral tangent line
14 medial posterior condyle
14A posterior condylar surface
14B posteromedial tangent line
15 distal surtace
16' proximal recess
17 anterior proximal surtace
18 proximal anterior chamfer
19 proximal surtace
20 proximal posterior chamfer
21 posterior proximal surtace
22 recess
23 lateral distal surface
24 medial dital surface
25 offset distance
26 medial distal surtace
27 lateral distal surface
2B angle
29 anatomical angle tibia
30 femur
31 tibia
WO 96103939 PCTIU595109705
2196b96
17
y 32 mechanical axis
33 lateral side
34 medial side
35 ninety degree tibia) resection
36 distal femoral resection
37 extension space
38 femoral resection - posteromedial
39 femoral resection - posterolateral
40 lateral collateral ligament
41 medial collateral ligament
42 anterior femoral cortex
43 notch
44 distal femoral resection
45 ninety degree tibia) resection
46 extension space - parallel
47 flexion space - trapezoidal
48 ' anterior resection
49 angle
50 tibia) component
51 tibia) resection
F femoral prosthesis
T tibia) prosthesis
60 femoral component
61 lateral distal condyle
61A lateral distal surface
62 medial distal condyle
62A medial distal surface
63 posterolateral condyle
64 posteromedial condyle
65 concave anterior region of new asymmetric
femoral
66 convex anterior region of traditional femoral
67 patella groove
WO 96/03939 PCTlUS95/09705
18
68 centerline of the femoral component
69 longitudinal axis of patella
groove
70 intracondylar notch
71 patella component
72 line showing depth of patella
groove
Because many varying and different embodiments may be made within the
scope of the inventive concept herein taught, and because many modifications
may be made in the embodiments herein detailed in accordance with the
descriptive requirement of the law, it is.to be understood that the details
herein
are to be interpreted as illustrative and not in a limiting sense.
