U.S. patent application number 10/242122 was filed with the patent office on 2004-03-18 for posterior stabilized knee with varus-valgus constraint.
Invention is credited to Dietz, Terry, Running, Don, Vosler, Marc C., Wyss, Joe.
Application Number | 20040054416 10/242122 |
Document ID | / |
Family ID | 31946380 |
Filed Date | 2004-03-18 |
United States Patent
Application |
20040054416 |
Kind Code |
A1 |
Wyss, Joe ; et al. |
March 18, 2004 |
Posterior stabilized knee with varus-valgus constraint
Abstract
A femoral component of a knee prosthesis has spaced condyle
surfaces defining a notch therebetween. The notch defines an
elongated cam housing having an anterior cam and a posterior cam at
opposite ends of the housing. The tibial component of the knee
prosthesis includes a platform and a bearing supported on the
platform, the bearing defining bearing surfaces configured to
articulate with the condyle surfaces. The tibial component includes
a spine projecting superiorly from the bearing that defines an
anterior face and a posterior face. The posterior face and the
posterior cam define complementary curved surfaces configured for
cooperative engagement when the femoral component and the tibial
component are at a predetermined flexion angle. The cam housing is
configured to form a gap between the posterior cam and the spine
when the knee is normally extended. In another feature, the spine
includes a stiffening pin extending therethrough.
Inventors: |
Wyss, Joe; (Fort Wayne,
IN) ; Dietz, Terry; (Columbia City, IN) ;
Vosler, Marc C.; (Columbia City, IN) ; Running,
Don; (Warsaw, IN) |
Correspondence
Address: |
Paul J. Maginot
Maginot, Moore & Bowman, LLP
Bank One Center/Tower
111 Monument Circle, Suite 3000
Indianapolis
IN
46204-5115
US
|
Family ID: |
31946380 |
Appl. No.: |
10/242122 |
Filed: |
September 12, 2002 |
Current U.S.
Class: |
623/20.27 ;
623/20.29 |
Current CPC
Class: |
A61F 2/3836 20130101;
A61F 2220/0033 20130101; A61F 2220/0025 20130101; A61F 2002/30398
20130101; A61F 2002/30332 20130101; A61F 2002/30364 20130101; A61F
2/3886 20130101; A61F 2002/30604 20130101; A61F 2/3868
20130101 |
Class at
Publication: |
623/020.27 ;
623/020.29 |
International
Class: |
A61F 002/38 |
Claims
What is claimed is:
1. A knee prosthesis comprising: a femoral component configured to
be attached to the distal end of a femur and having a medial and a
lateral condyle surface spaced apart to define a notch
therebetween, said notch defining an elongated cam housing having a
posterior cam at one end of said cam housing; and a tibial
component including a platform configured for attachment to the
proximal end of a tibia and a bearing supported on said platform,
said bearing defining medial and lateral bearing surfaces
configured to articulate with said medial and lateral condyle
surfaces, and a spine projecting superiorly from said bearing
within said cam housing when said condyle surfaces are in
articulating contact with said bearing surfaces, wherein said spine
defines an anterior face and an opposite posterior face facing said
posterior cam, said posterior face and said posterior cam defining
complementary curved surfaces configured for cooperative engagement
when said femoral component and said tibial component are rotated
relative to each other to at least a predetermined flexion
angle.
2. The knee prosthesis according to claim 1, wherein said
complementary curved surface of said posterior face of said spine
is concave at a radius, and said complementary curved surface of
said cam is curved at substantially said radius.
3. The knee prosthesis according to claim 1, wherein: said cam
housing defines an anterior cam at an opposite end thereof; and
said anterior face of said spine facing said anterior cam is
substantially flat.
4. The knee prosthesis according to claim 3, wherein said anterior
cam defines a substantially flat surface complementary to said
anterior face of said spine.
5. The knee prosthesis according to claim 1, wherein said cam
housing defines a width sufficient to provide a clearance ranging
between about 0.0 mm to about 0.13 mm on either side of said spine
when said spine projects into said cam housing, whereby said spine
and said cam housing interact to restrict varus-valgus pivoting
between said femoral component and said tibial component.
6. The knee prosthesis according to claim 1, wherein said cam
housing each define a width sized relative each other so that said
spine and said cam housing interact to restrict varus-valgus
pivoting between said femoral component and said tibial component
to between 0.5.degree. and 2.5.degree..
7. The knee prosthesis according to claim 1, wherein said spine has
a height of about 16.0-24. mm and said cam housing includes a roof
and is sized relative to said condyle surfaces so that said spine
cannot contact said roof when said condyle surfaces are supported
on said bearing surfaces.
8. The knee prosthesis according to claim 7, wherein said
complementary curved surface of said posterior face of said spine
is concave at a radius and has a length sized so that said curved
surface extends along substantially the entire height of said
spine.
9. The knee prosthesis according to claim 8, wherein said spine
terminates in a rounded posterior peak.
10. The knee prosthesis according to claim 8, wherein said
posterior cam has a length from an anterior end to a posterior end
that is substantially equal to the length of said curved surface of
said posterior face of said spine.
11. The knee prosthesis according to claim 1, wherein said cam
housing defines an anterior-posterior distance between said curved
surface of said posterior cam at said one end and an opposite end
of said cam housing that is substantially greater than an
anterior-posterior dimension of said spine.
12. The knee prosthesis according to claim 11, wherein the
anterior-posterior distance defined by said cam housing is about
1.5 times greater than the anterior-posterior dimension of said
spine.
13. The knee prosthesis according to claim 1, wherein said bearing
is mounted on said platform to permit relative movement
therebetween.
14. The knee prosthesis according to claim 13, wherein said tibial
component includes: a socket defined in said platform; and a stem
extending from said bearing configured for rotating engagement
within said socket.
15. A knee prosthesis comprising: a femoral component configured to
be attached to the distal end of a femur and having a medial and a
lateral condyle surface spaced apart to define a notch
therebetween, said notch defining an elongated cam housing having a
posterior cam at one end of said cam housing; a tibial component
including a platform configured for attachment to the proximal end
of a tibia and a bearing supported on said platform, said bearing
defining medial and lateral bearing surfaces configured to
articulate with said medial and lateral condyle surfaces, and a
spine projecting superiorly within said cam housing when said
condyle surfaces are in articulating contact with said bearing
surfaces, wherein said spine defines a posterior face facing said
posterior cam and configured for cooperative engagement when said
posterior cam, said spine further defining a bore therethrough; and
a pin configured to be disposed within said bore, said pin formed
of a material different from the material of said spine.
16. The knee prosthesis according to claim 15, wherein said pin is
configured to be press-fit into said bore.
17. The knee prosthesis according to claim 15, wherein tibial
component includes: a socket defined in said platform; and a stem
extending from said bearing configured for rotating engagement
within said socket.
18. The knee prosthesis according to claim 17, wherein said bore
extends into at least a portion of said stem and said pin is
configured to extend into said portion of said stem.
19. The knee prosthesis according to claim 15, wherein said pin is
formed of a mechanically stiffer material than said spine.
20. The knee prosthesis according to claim 19, wherein said spine
is formed of a plastic and said pin is formed of a metal.
21. A knee prosthesis comprising: a femoral component configured to
be attached to the distal end of a femur and having a medial and a
lateral condyle surface spaced apart to define a notch
therebetween, said notch defining an elongated cam housing having
an anterior cam and a posterior cam at opposite ends of said cam
housing; and a tibial component including a platform configured for
attachment to the proximal end of a tibia and a bearing supported
on said platform, said bearing defining medial and lateral bearing
surfaces configured for rotating contact with said medial and
lateral condyle surfaces, and a spine projecting superiorly from
said bearing within said cam housing when said condyle surfaces are
in articulating contact with said bearing surfaces, and said spine
defining an anterior face facing said anterior cam and a posterior
face adapted for articulating contact with said posterior cam,
wherein said cam housing is configured to define an
anterior-posterior distance between said anterior cam and said
posterior face of said spine when said femoral component and said
tibial component are in a normally extended position relative to
each other, whereby said posterior face of said spine is in
articulating contact with said posterior cam only at a first
predetermined flexion rotation angle between said femoral component
and said tibial component.
22. The knee prosthesis according to claim 21, wherein said first
predetermined flexion angle is about 50.degree..
23. The knee prosthesis according to claim 21, wherein said
posterior cam and said posterior face define complementary curved
surfaces, whereby said complementary surfaces articulate relative
to each other at flexion angles between said femoral component and
said tibial component greater than said first predetermined flexion
angle.
24. The knee prosthesis according to claim 23, wherein said
posterior cam includes a rounded anterior end that is arranged to
contact said posterior face first at said first predetermined
flexion angle.
25. The knee prosthesis according to claim 23, wherein the
complementary curved surface of said posterior cam is arranged on
said posterior cam so that complementary curved surface of said
posterior cam is substantially nested within the complementary
curved surface of said posterior face of said spine only after said
femoral component and said tibial component rotate relative to each
other to a second predetermined flexion angle greater than said
first predetermined flexion angle.
26. A knee prosthesis, comprising: a femoral component configured
to be attached to the distal end of a femur and having a medial and
a lateral condyle surface spaced apart to define a notch
therebetween, said notch defining an elongated cam housing having a
posterior cam at one end thereof; and a tibial component including
a platform configured for attachment to the proximal end of a tibia
and a bearing supported on said platform, said bearing defining
medial and lateral bearing surfaces configured for rotating contact
with said medial and lateral condyle surfaces, and a spine
projecting superiorly from said bearing within said cam housing
when said condyle surfaces are in articulating contact with said
bearing surfaces, and said spine defining a posterior face adapted
for articulating contact with said posterior cam, wherein the
rotating contact between said bearing and said condyle surfaces is
configured to carry all of the anterior-posterior shear load of the
knee prosthesis from a full extension angle to a predetermined
flexion angle between said femoral component and said tibial
component, and said spine is arranged relative to the rotating
contact between said bearing and said condyle surfaces so that said
spine contacts said posterior cam at said predetermined flexion
angle to share a portion of the anterior posterior shear load of
the knee at flexion angles greater than said predetermined flexion
angle.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a knee prosthesis and more
particularly to a mobile bearing knee providing posterior
stabilization of the anterior-posterior translation of the femoral
component relative to the tibial component.
[0002] Flexion and extension of the normal human knee involves
complex movements of the femur, the tibia and the patella. During
flexion (i.e., when the knee is bent), the distal end of the femur
and the proximal end of the tibia roll and glide relative to each
other, with the center of rotation of the joint moving posteriorly
over the condyles of the femur. This complex movement is typically
referred to as rollback. During extension (i.e., when the leg is
straightened), the tibia and femur follow a reverse path.
Simultaneous with the movements of the tibia and femur, the patella
moves over the surface of the femoral condyles, while remaining a
constant distance from the tubercle of the tibia.
[0003] Damage or disease can deteriorate the bones, articular
cartilage and ligaments of the knee, which can ultimately affect
the ability of the natural knee to function properly. To address
these conditions, prosthetic knees have been developed that are
mounted to prepared ends of the femur and tibia. Among the many
knee prostheses, a mobile bearing knee simulates the condylar and
bearing surfaces of the knee to emulate the natural movement of the
knee during flexion and extension. The tibial component is
configured to permit rotation about the axis of the tibia to
accurately replicate the effects of differential rollback in the
transverse plane.
[0004] In one type of mobile bearing knee, the tibial component
includes an upward projecting spine that translates within an
intercondylar notch formed in the femoral component. The spine can
contact cam surfaces at the anterior and posterior ends of the
notch to limit the relative anterior-posterior movement between the
two bones. The spine also operates to provide varus-valgus
stability of the joint and to resist dislocation or subluxation at
high angles of flexion. An exemplary mobile bearing knee is
disclosed in U.S. Pat. No. 6,443,991, the disclosure of which is
incorporated herein by reference. Other exemplary mobile bearing
knees are embodied in the LCS.TM. System and the PFC Sigma RP.TM.
knee system marketed by Depuy Orthopaedics, Inc., of Warsaw,
Ind.
[0005] While mobile bearing knees are thought to most accurately
mimic the natural movement of the intact knee, the design of knee
systems requires the introduction of features to maintain the
stability of the artificial joint. Thus, modern knee systems
provide additional stability to posterior stabilized devices to
prevent hyperextension. The articulating and rotating components of
the knee system must do so smoothly and accurately. At the same
time, the natural knee permits a certain amount of movement and
pivoting in the transverse and coronal planes that should be
approximated in the prosthetic knee system. The development of knee
systems has attempted to harmonize the need for preserving a full
range of motion with the need for maintaining the strength of the
joint.
SUMMARY OF THE INVENTION
[0006] The present invention contemplates an improved knee
prosthesis comprising a femoral component configured to be attached
to the distal end of a femur and having a medial and a lateral
condyle surface spaced apart to define a notch therebetween. The
notch defines an elongated cam housing having an anterior cam and a
posterior cam at opposite ends of the cam housing.
[0007] The prosthesis further includes a tibial component including
a platform configured for attachment to the proximal end of a tibia
and a bearing supported on the platform. The bearing defines medial
and lateral bearing surfaces configured to articulate with the
medial and lateral condyle surfaces, and a spine projecting
superiorly from the bearing within the cam housing when the condyle
surfaces are in articulating contact with the bearing surfaces.
[0008] The spine defines an anterior face facing the anterior cam
and a posterior face facing the posterior cam. In one feature of
the invention, the posterior face and the posterior cam defining
complementary curved surfaces configured for cooperative engagement
when the femoral component and the tibial component are rotated
relative to each other to at least a predetermined flexion angle.
In certain embodiments, that predetermined angle corresponds to
about 50.degree. of flexion of the knee joint.
[0009] The complementary curved surfaces of the posterior cam and
posterior face of the spine are preferably curved at a common
radius, while the anterior cam and the anterior face of the spine
are substantially flat.
[0010] In one aspect of the knee prosthesis the cam housing defines
a width sufficient to provide a predetermined clearance on either
side of the spine, when the spine projects into the cam housing, to
limit varus-valgus movement or pivoting of the joint. In a
preferred embodiment, the widths of the spine and cam housing are
sized to limit varus-valgus pivoting to
0.5.degree.-1.5.degree..
[0011] In addition, the cam housing can be configured so that a gap
exists between the posterior cam and the spine when the knee is in
its normally extended position. The spine does not contact the
posterior cam until the knee is flexed to the predetermined angle.
In another aspect, the complementary surfaces of the spine and
posterior cam do not nest or coincide until the knee is flexed
further to another predetermined angle. The posterior cam can
include a blunt or rounded anterior end that contacts the spine
first when the knee is flexed. The spine and posterior cam produce
roll-back for the knee prosthesis.
[0012] In yet another aspect of the invention, the spine has a
greater height than prior spine designs. The spine height is
calibrated to prevent subluxation of the joint at high flexion
angles. In a preferred embodiment, the spine height is about 24.6
mm. The cam housing includes a roof that is sized relative to the
condyle surfaces so that the spine cannot contact the roof when the
condyle surfaces are supported on the bearing surfaces.
[0013] The invention also contemplates a knee prosthesis comprising
a femoral component configured to be attached to the distal end of
a femur and having a medial and a lateral condyle surface spaced
apart to define a notch therebetween, the notch defining an
elongated cam housing having an anterior cam and a posterior cam at
opposite ends of the cam housing. The prosthesis also comprises a
tibial component including a platform configured for attachment to
the proximal end of a tibia and a bearing supported on the
platform, the bearing defining medial and lateral bearing surfaces
configured to articulate with the medial and lateral condyle
surfaces, and a spine projecting superiorly within the cam housing
when the condyle surfaces are in articulating contact with the
bearing surfaces, wherein the spine defines an anterior face facing
the anterior cam and a posterior face facing the posterior cam and
configured for cooperative engagement when the posterior cam.
[0014] In this embodiment, the spine further defines a bore
therethrough that receives a pin configured to be disposed within
the bore. The pin is formed of a material different from the
material of the spine to add stiffness or bending strength to the
spine. The pin can be configured to be press-fit into the bore. In
certain embodiments, the spine is formed of a plastic and the pin
is formed of a metal.
[0015] In still another aspect of the invention, a knee prosthesis
comprises a femoral component configured to be attached to the
distal end of a femur and having a medial and a lateral condyle
surface spaced apart to define a notch therebetween, the notch
defining an elongated cam housing having an anterior cam and a
posterior cam at opposite ends of the cam housing. A tibial
component includes a platform configured for attachment to the
proximal end of a tibia and a bearing supported on the platform,
the bearing defining medial and lateral bearing surfaces configured
for rotating contact with the medial and lateral condyle surfaces.
A spine projects superiorly from the bearing within the cam housing
when the condyle surfaces are in articulating contact with the
bearing surfaces, the spine defining an anterior face facing the
anterior cam and a posterior face adapted for articulating contact
with the posterior cam.
[0016] With this embodiment, the cam housing is configured to
define an anterior-posterior distance between the anterior cam and
the posterior face of the spine when the femoral component and the
tibial component are in a normally extended position relative to
each other. With this configuration, the posterior face of the
spine is in articulating contact with the posterior cam only at a
first predetermined flexion rotation angle between the femoral
component and the tibial component. In a specific embodiment, the
first predetermined flexion angle is about 50.degree..
[0017] This embodiment further contemplates that the posterior cam
and the posterior face define complementary curved surfaces,
whereby the complementary surfaces articulate relative to each
other at flexion angles between the femoral component and the
tibial component greater than the first predetermined flexion
angle. The posterior cam can include a rounded anterior end that is
arranged to contact the posterior face first at the first
predetermined flexion angle. The complementary curved surface of
the posterior cam can further be arranged on the posterior cam so
that complementary curved surface of the posterior cam is
substantially nested within the complementary curved surface of the
posterior face of the spine only after the femoral component and
the tibial component rotate relative to each other to a second
predetermined flexion angle greater than the first predetermined
flexion angle.
[0018] It is one object of the present invention to provide a
prosthetic knee that accurately and efficiently emulates the
kinematics and function of a normal, health knee. A more specific
object is to accomplish these functions with a posterior stabilized
knee that can create proper joint roll-back.
[0019] Another object is accomplished by features of the invention
that restrict varus-valgus movement or pivoting, as well as provide
resistance to subluxation. Other objects and certain benefits of
the invention can be appreciated from the following written
description together with the accompanying figures.
DESCRIPTION OF THE FIGURES
[0020] FIG. 1 is an exploded side view of a mobile bearing knee
system according to one embodiment of the present invention.
[0021] FIG. 2 is an anterior view of the knee system shown in FIG.
1.
[0022] FIG. 3 is a lateral view of the knee system shown in FIGS. 1
and 2.
[0023] FIGS. 4a-4c are cross-sectional view of the knee system
shown in FIG. 2, taken along line 4-4, with the knee system shown
in its hyper-extended, normally extended, and flexed
configurations. FIG. 4c includes a partial cut-away of the spine on
the bearing.
[0024] FIG. 5 is an enlarged diagram illustrating roll-back of the
contact point between the femoral and tibial components of the
mobile bearing knee system shown in FIG. 4c.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] For the purposes of promoting an understanding of the
principles of the invention, reference will now be made to the
embodiments illustrated in the drawings and described in the
following written specification. It is understood that no
limitation to the scope of the invention is thereby intended. It is
further understood that the present invention includes any
alterations and modifications to the illustrated embodiments and
includes further applications of the principles of the invention as
would normally occur to one skilled in the art to which this
invention pertains.
[0026] Referring to FIG. 1, a knee system 10 is depicted that
includes a femoral component 12 and a tibial component 14. The
tibial component includes a tibial platform 16 from which extends a
tibial stem 18 that is configured for engagement within the
prepared end of the tibia. A bearing 20 is rotatably mounted on the
platform 16 by way of a bearing stem 22 that fits within a
complementary socket 24 within the platform.
[0027] The bearing 20 defines an upper bearing surface that
supports the femoral component. More specifically, the bearing 20
includes a lateral bearing surface 26 and a medial bearing surface
28. These bearing surfaces 26, 28 are configured for articulating
support of corresponding condyle bearings 30, 32 of the femoral
component 12. This articulating or sliding support is best seen in
FIGS. 2 and 3.
[0028] The femoral component 12 is configured to emulate the
configuration of the femoral condyles. Thus, the component 12
includes an anterior portion 34 and a posterior portion 36 that are
curved in the manner of the natural condyles. The anterior portion
34 defines a patellar groove 49 that is configured to orient a
patellar implant (not shown) in a manner known in the art.
[0029] The femoral component 12 utilizes a number of surfaces to
fix the component to the prepared end of the femur. The inner
surface 37 of the anterior and posterior portions 34, 36, are
configured to directly interface over the prepared end of the
femur. In addition, a stem 38 can be provided that is fixed within
the femur. In addition, the femoral component 12 can include an
intercondylar notch 40 formed by a pair of opposite side walls 44
and a roof 46.
[0030] As thus far described, the prosthetic knee 10 can assume a
variety of known configurations. For instance, the femoral
component 12 and tibial component 14 as described above can have
the configuration of like components of the mobile bearing knee
described in U.S. Pat. No. 6,443,991, the description of which is
incorporated by reference.
[0031] As with the prior mobile bearing knee of the '991 Patent,
the knee 10 of the present invention includes a spine 60 that
projects from the upper surface 25 of the bearing 20. The spine 60
resides within a cam housing 42 (FIG. 3) that is essentially formed
by the walls of the intercondylar notch 40. In one aspect of the
present invention, the spine 60 is sized relative to the cam
housing 42 to provide a measured degree of varus-valgus constraint.
The spine 60 has a width W.sub.1 that is slightly less than the
width W.sub.2 of the cam housing 42 at the intercondylar notch 40
(FIG. 1). These two widths are sized relative to each other to
limit varus-valgus movement or pivoting to a range of about
0.5.degree.-2.5.degree.. In a specific preferred embodiment of the
invention, the width W.sub.1 is sized relative to the width W.sub.2
to provide 0.13 mm clearance on each side of the spine. Preferably,
this clearance should be limited to from about 0.12 mm to about
0.50 mm per side to avoid excessive varus-valgus movement of the
knee components 12, 14 relative to each other. In the specific
embodiment, this clearance permits about 1.25.degree. of
varus-valgus pivoting.
[0032] As with other known prosthetic knees, each of the components
must be sized to the skeletal dimensions of the patient. Thus, it
is contemplated that the femoral component 12 and tibial component
14 can be provided in several sizes, and preferably in six sizes
ranging from small to extra-large. For a medium sized knee, the
tibial spine 60 can have a width W.sub.1 of about 17.5 mm. In
accordance with the specific embodiment discussed above (i.e., with
a 0.13 mm clearance on each side), the cam housing 42 would have a
width W.sub.2 of 17.76 mm to provide the proper side-to-side
clearance for the spine 60. The dimensions of the spine and the cam
housing can be appropriately proportioned for other sizes of knee
components.
[0033] In addition to providing a measured degree of varus-valgus
constraint, the spine 60 interacts with the cam housing 42 to
prevent subluxation of the knee 10. In particular, the spine 60
defines a subluxation height from the bearing surface 25 that
corresponds to the distance that the femoral component must be
raised relative to the tibial component until the femoral component
is clear of the top of the spine. Subluxation is generally not a
problem when the knee is straightened (as shown in FIGS. 3 and 4b),
but can be problematic when the knee is flexed (as shown in FIG.
4c). Thus, the subluxation height is measured at a certain degree
of flexion, most typically at 120.degree. of flexion. (For
comparison, the knee in FIG. 4c is shown at approximately
80.degree. of flexion).
[0034] In accordance with the preferred embodiment of the present
invention, the spine has an effective height of between 16-24 mm,
and most preferably 19.3 mm, when the prosthesis is at 90.degree.
flexion. Thus, the femoral component must rise off the tibial
bearing 20 by at least 19.3 mm to cause a dislocation of the knee.
The natural ligaments and surrounding soft tissues of the knee
provide sufficient resistance to femoral lift-off greater than this
subluxation height, especially at high flexion angles.
[0035] Referring now to FIGS. 3 and 4a-c, a further feature of the
present invention is depicted. In particular, the cam housing 42
defines an anterior cam 50 having a cam face 51. This anterior cam
50 is adjacent the anterior portion 34 of the tibial component 12.
As seen in FIGS. 3 and 4a, the cam face 51 is substantially flat.
Similarly, the spine 60 has an anterior face 62 that is also
substantially flat. The cam face 51 and anterior face 62 are
arranged to restrict extension of the knee in the anterior
direction (as designated by the arrow A in FIG. 4a). Thus, as the
tibia, and hence the tibial component 14, moves anteriorly relative
to the femur and femoral component 12, the spine 60 can contact the
anterior cam 50 to prevent further movement in the anterior
direction. In the illustrated embodiment, this contact can occur at
about 5.degree. hyperextension. However, tension in the ligaments
supporting and surrounding the knee will prevent hyper-extension of
the knee, and ideally will prevent contact between the spine and
the anterior cam 50.
[0036] The cam housing further defines a posterior cam 55 at the
opposite end of the intercondylar notch 40 from the anterior cam
50, as shown in FIG. 3. The posterior cam 55 defines a curved
surface 56 that cooperates with a curved posterior face 64 of the
spine, as best shown in FIG. 4c. These cooperating surfaces are
configured for optimum roll-back characteristics of the prosthetic
knee 10. As the knee is flexed from the neutral position depicted
in FIG. 4b to the position shown in FIG. 4c, it is desirable for
the contact point between the tibial and femoral components, as
well as the axis of rotation of the tibia relative to the femur, to
shift posteriorly (as designated by the direction arrow P in FIG.
4a). This posterior shift optimizes the moment arm and reduces the
strain on the quadricep muscle responsible for flexing the
knee.
[0037] As shown in FIG. 4b, the cam housing 42 is elongated between
the anterior and posterior cams so that the bearing 20, and
particularly the spine 60, can articulate as the knee is initially
flexed and the condyle bearings 30, 32 rotate on the bearing
surfaces 26, 28. As the knee continues to rotate to about
50.degree. of flexion, the posterior face 64 of the spine 60
contacts the posterior cam 55. This contact between spine and cam
provides posterior stability to the knee 10 as the knee continues
to flex. In order to accommodate continued femoral roll-back, the
mating surfaces are complementary curved, as best illustrated in
FIGS. 4a-4b. Specifically, the posterior face 64 of the spine 60 is
concave from the bearing surface 25 of the bearing 20 to the
posterior peak 66 at the top of the spine. The posteriorly directed
peak 66 provides additional posterior stability and resistance to
subluxation at high flexion angles of 120.degree. and beyond.
[0038] The curvature or concavity of the posterior face 64 is
selected to permit a predetermined amount of roll-back, while
maintaining the posterior stability afforded by the spine-to-cam
contact. This roll-back is depicted in FIG. 5. The contact point
designated C.sub.1 corresponds to the initial contact between the
spine and the posterior cam. When the knee is flexed further, the
contact point shifts posteriorly to the contact point C.sub.2. In a
preferred embodiment, the posterior face 64 is configured to permit
roll-back of between 0.0 mm up to about 5.0 mm. Most preferably
this roll-back is about 4.2 mm. Thus, as the knee continues to flex
from the 50.degree. point of contact between spine and posterior
cam, the curved surface 56 of the cam nestles into the curved
posterior face 64. At the same time, the contact point between the
femoral component 12 and the tibial component 14 shifts
posteriorly. Continued flexing causes the curved cam surface 56 to
articulate within the concave posterior face 64 of the spine, which
further shifts the contact point in the posterior direction.
[0039] In a preferred embodiment, the curved posterior face 64 of
the spine 60 is concave at a radius of between 28-32 mm. Most
preferably, the radius of the posterior face is about 30 mm. The
curved posterior face 64 transitions into the posterior peak 66,
which is preferably rounded. In a preferred embodiment, this peak
is formed at a radius of about 5 mm. Since the curved surface 55 of
the posterior cam is complementary to the posterior face 64, it too
has a most preferred radius of 30 mm.
[0040] At least the anterior end 57 of the posterior cam 55 is
blunted or rounded to provide a smooth transition when the cam
contacts the spine 60. The opposite posterior end of the cam 55 can
also be rounded, as shown in the figures. This rounded anterior end
57 is the first portion of the posterior cam to contact the spine
as the knee is flexed to the predetermined flexion angle.
Nominally, the anterior end 57 will initially contact the spine 60
below the rounded posterior peak 66.
[0041] The curved posterior face 64 of the spine is curved along
substantially the entire height of the spine 60. Moreover, the
posterior cam 55, or more particularly the curved surface 56 of the
cam, has a length that is substantially equal to the length or
height of the curved face of the spine. At about 120.degree. of
flexion, the curved surface of the posterior cam is completely
nested within the concave posterior face of the spine. This
complementary interface can then operate as a fulcrum or pivot
point for further relative rotation between the tibial and femoral
components. While the condyle bearings and bearing surfaces
continue to articulate relative to each other, the greater share of
the shear load can now be borne by the complementary interface
between the posterior cam 55 and the posterior face 64 of the spine
60. This interface can thus preserve the mechanical advantage of
the quadricep muscle through high flexion angles. In addition, the
kinematics of this spine/cam interface allows a patellar implant to
easily follow the patellar track 49 without placing undue stress on
the patellar tendons.
[0042] Referring to FIG. 4b, it can be seen that the spine 60 has
an anterior-posterior dimension that is significantly less than the
distance between the anterior and posterior cams 50, 55 in the cam
housing 42. From the limit of extension, shown in FIG. 4a, to the
normal straight leg position of FIG. 4b the spine does not contact
the cam housing and therefore does not either bear any knee loads
or dictate any knee motion. In one feature of the invention, the
cam housing 42 is elongated with a distance between anterior and
posterior cams that is significantly greater than the
anterior-posterior (a-p) dimension of the spine. In a preferred
embodiment, the distance between cams is about 1.5 times the a-p
dimension of the spine. In one specific embodiment, the a-p
dimension of the spine is about 10.0 mm at the posterior peak 66,
and the distance between the anterior cam face 51 and the anterior
end of the posterior cam 55 is about 15.0 mm.
[0043] With this configuration, the knee load is carried solely by
the articulating interface between the condyle bearings 30, 32 and
the bearing surfaces 26, 28. As the knee starts to flex from the
straightened position shown in FIG. 4b the quadricep muscles enjoy
their greatest moment arm and mechanical advantage. As the knee
continues to flex, the femoral component 12 moves posteriorly
relative to the tibial component 14 so the quadricep mechanical
advantage gradually decreases.
[0044] In order to preserve the quadricep mechanical advantage, the
present invention contemplates that the posterior face 64 of the
spine will contact the posterior cam 55 after a pre-determined
amount of flexion. In a preferred embodiment, this pre-determined
amount of flexion of about 50.degree.. At this point, the spine and
posterior cam cooperate to provide an additional articulating
bearing interface to not only share in the shear loads, but also to
provide a fulcrum or reaction surface working against the quadricep
muscle to preserve the flexion moment arm and mechanical advantage.
As the flexion continues, the posterior cam 55 becomes fully seated
within the concave posterior face 64 of the spine to maximize the
bearing contact between these two components.
[0045] In another aspect of the invention, the stem 22 of the
bearing 20 defines a central bore 70 at least partially
therethrough, as shown in FIG. 4b. A stiffening pin 72 can be
pressed into the bore 70, as shown in FIG. 4c. The pin 72 can be
formed of a stiff metal, such as a cobalt chrome alloy.
[0046] In accordance with accepted practice, the prosthetic
components designed to engage the natural bone, such as the femoral
component 12 and the tibial platform 16, are formed of a
biocompatible metal, such as cobalt chrome alloy. The bone engaging
surfaces of these components can be textured to facilitate
cementing the component to the bone, or can be porous coated to
promote bone ingrowth for permanent fixation.
[0047] However, the bearing 20 is most preferably formed of a
material that allows for smooth articulation and rotation between
the bearing and the other components. The material is selected to
meet several criteria, such as producing as little friction as
possible between the articulating/rotating surfaces, providing as
much wear resistance as possible, and remaining as strong as
possible. One preferred material is ultra-high molecular weight
polyethylene (UHMWPe) because it optimizes these three and other
criteria.
[0048] One concern posed by the material used for the spine 60 is
that the spine must bear significant loads in the transverse and
coronal planes--i.e., lateral to the spine axis. In one approach,
the spine 60 can be provided as a separate component that mates
with the remainder of the bearing 20. With this approach, the spine
can be formed of a high strength metal, such as the cobalt alloy
mentioned above. This approach adds to the complexity of the knee
construct and adds the problem of interfacing the spine to the
remainder of the bearing.
[0049] It is preferred that the spine 60 be integrally formed with
the bearing 20, which means that the spine will be formed of the
same material. Where this material is UHMWPe, transverse or shear
strength, and even bending stiffness, becomes a design
consideration, particularly for active patients. To address this
concern, the stiffening pin 72 can extend through the axis of the
spine 60 to add bending stiffness and shear resistance to the
spine. The pin 72 can be provided in different lengths depending
upon the desired effect. For instance, the pin can be sized for
insertion from the top of the spine 60 and to only extend for the
height of the spine. Alternatively, as shown in FIG. 4c, the pin
can be introduced from the bottom of the bearing stem 22 and can
include a stepped diameter to be press-fit into a comparable
stepped diameter bore 70 of the stem 22.
[0050] While the invention has been illustrated and described in
detail in the drawings and foregoing description, the same should
be considered as illustrative and not restrictive in character. It
is understood that only the preferred embodiments have been
presented and that all changes, modifications and further
applications that come within the spirit of the invention are
desired to be protected.
[0051] For instance, the preferred embodiment contemplates one form
of mobile bearing knee in which the tibial bearing rotates relative
to the tibial platform. Other mobile bearing knees are
contemplated, including knee prostheses in which the bearing slides
on the platform. Of course, the inventive concepts can also be
implemented in knee prosthesis in which the bearing does not move
or is incorporated into the tibial platform.
[0052] In addition, the illustrated embodiments contemplate that
the spine projects from the bearing. The inventive concepts can be
implemented where the spine is separate from the bearing, whether
as a separate insert or integrated with the tibial platform.
* * * * *