Rotatable joint prosthesis with axial securement

A joint prosthesis system includes first and second components that are joinable to each other in a manner to inhibit the second component from withdrawing from the first component. The prosthesis system includes a securing mechanism that has at least one first surface feature (e.g. a negative surface feature), located on the first component, and at least one second surface feature (e.g. a positive surface feature) located on the second component. The first and second surface features are engageable so that, upon engagement, the second component is axially secured to the first component and the second component is able to rotate with respect to the first component. Preferably, the first surface feature and the second surface feature are adapted and dimensioned to engage and form a snap fit arrangement. In one embodiment, the joint prosthesis system is a knee joint prosthesis and the two components are a tibial tray and a tibial bearing insert.

BACKGROUND OF THE INVENTION 
The invention relates to joint prostheses. More particularly, the invention 
is directed to tibial components of knee joint prostheses that have a 
tibial bearing insert that is axially secured to a tibial tray, and 
rotatable with respect to the tibial tray upon which it is mounted. 
Joint replacement surgery is quite common and enables many individuals to 
function normally when otherwise it would not be possible to do so. 
Artificial joints are normally composed of metallic, ceramic and/or 
plastic components that are fixed to existing bone. 
Knee arthoplasty is a well known surgical procedure by which a diseased 
and/or damaged natural knee joint is replaced with a prosthetic knee 
joint. Typical knee protheses include a femoral component, a patella 
component, a tibial tray or plateau, and a tibial bearing insert. The 
femoral component generally includes a pair of laterally spaced apart 
condylar portions, the distal surfaces of which articulate with 
complementary condylar elements formed in a tibial bearing insert. 
The tibial tray is mounted within the tibia of a patient. Typically, the 
tibial bearing insert, which is usually made of ultra high molecular 
weight polyethylene (UHMWPE) is mounted upon the superior surface of the 
tibial tray. Load and stress are placed upon the knee prosthesis, and 
particularly on the tibial bearing insert, during normal daily use. These 
forces may lead to the displacement or dislocation of the insert from the 
tibial tray. To accommodate these forces, and to reduce the chances for 
dislocation, some tibial components of knee prostheses have been designed 
to allow rotation of the tibial bearing insert relative to the proximal or 
superior surface of the tibial tray, about the longitudinal axis of the 
prosthesis. Such rotation, when controlled, can increase the contact area 
between the femoral condyles and the tibial bearing insert throughout the 
range of knee motion, thus reducing stress on the tibial bearing insert. 
Some knee prosthesis tibial components accommodate insert rotation without 
providing axial securement of the tibial bearing insert within the tibial 
tray. That is, some tibial bearing inserts that are able to rotate with 
respect to a tibial tray are not fully secured within the tibial tray. 
Certain forces to which the knee is subjected, particularly forces with 
axially directed components, may cause the tibial bearing insert to 
separate from the tibial tray. 
Various designs for rotatable tibial components of knee joint prostheses 
are known in the art. For example, U.S. Pat. No. 4,219,893 (Noiles) and 
U.S. Pat. No. 4,301,553 (Noiles) disclose knee joint prostheses in which 
the tibial component comprises a tibial tray having a bearing surface with 
a recessed region within which the tibial bearing insert may rest. A 
sufficient clearance is provided in the bearing surface of the tibial tray 
to allow some medial-lateral rotation of the tibial bearing insert with 
respect to the tray. Other patents that disclose tibial components of knee 
joint prostheses in which a tibial bearing insert is rotatable with 
respect to the tibial tray are disclosed in U.S. Pat. Nos. 5,059,216 
(Winters); 5,071,438 (Jones et al); 5,171,283 (Pappas et al); and 
5,489,311 (Cipolletti). 
Despite the existing designs for knee joint prostheses having a rotatable 
tibial component, there remains a need for prostheses that allow rotation 
of the tibial bearing insert to accommodate the stresses placed upon the 
knee. At the same time, such tibial bearing inserts should possess 
sufficient axial securement so as to decrease or eliminate the possibility 
of subluxation of the tibial bearing insert. 
SUMMARY OF THE INVENTION 
The invention is directed to a joint prosthesis system having a first 
component and a second component, wherein the second component is 
rotatable with respect to the first component while maintaining axial 
securement of the second component to the first component. The term "axial 
securement" refers to the ability of the second component to resist 
withdrawal or separation from the first component when subjected to a 
separation force. In a preferred embodiment, the joint prosthesis system 
is a knee joint prosthesis in which the first component is a tibial tray 
and the second component is a tibial bearing insert for use in knee joint 
prostheses. 
The prosthesis system of the invention includes a first component (e.g., a 
tibial tray) having a superior mounting surface and an inferior bone 
contacting surface. The bone contacting surface includes an anchor stem 
having outer, implantable side and distal walls. Preferably, a cavity is 
formed in the mounting surface and extends into the anchor stem. This 
cavity is defined by an inner side and distal walls. 
A second component (e.g., a tibial bearing insert) of the prosthesis system 
has a superior articulation surface and an inferior surface that is 
mountable within the cavity of the first component. The inferior surface 
includes a mating stem that is mountable within the cavity of the first 
component and which has a size and shape complementary to the cavity. 
The prosthesis system also includes a securing mechanism that has at least 
one first surface feature (e.g., a negative surface feature) located on 
the interior side wall of the first component and at least one second 
surface feature (e.g., a positive surface feature) located on the second 
component. The first and second surface features are engageable so that, 
upon engagement, the second component is rotatable with respect to the 
first component while remaining axially secured to the first component. In 
one embodiment, the first surface feature and the second surface feature 
are adapted and dimensioned to engage and form a snap fit arrangement. 
The prosthesis system may also include an axial bore formed in the superior 
bearing surface of the second component that extends into the mating stem 
of the second component. The bore has a size and dimensions sufficient to 
receive an elongate reinforcement pin that can be mounted within the bore. 
In a further embodiment, insertion of the reinforcement pin can effect 
engagement of the positive and negative surface features. 
In another embodiment the positive surface feature is an eccentrically 
shaped rib adapted to mate with a negative surface feature (e.g., a 
groove). The rib enables one prosthesis component to mate with another 
prosthesis component to provide axial securement and controlled rotation 
of the one component with respect to the other component.

DETAILED DESCRIPTION OF THE INVENTION 
The invention provides a prosthesis system 10 that has first and second 
components that can be axially secured to one another while maintaining 
the ability of one component to rotate with respect to the other. For 
purposes of illustration the system 10 is shown as the tibial component of 
a knee joint prosthesis. It is understood, however, that the invention is 
applicable to other prostheses. 
Referring to FIGS. 1 and 2, the system 10 includes a first component in the 
form of a tibial tray 14, upon which is mounted a second component, i.e., 
tibial bearing insert 12. The mounting of the tibial bearing insert 12 to 
the tibial tray 14 is such that the tibial bearing insert is able to 
rotate with respect to the proximal or superior surface 32 of the tibial 
tray while remaining axially secured to the tibial tray. 
The tibial bearing insert 12 has an anterior side 13, a posterior side 15, 
a superior articulation surface 16 and an inferior mating surface 18. The 
superior surface 16 may have one or more condylar elements 20 that are 
adapted to articulate with complementary condyle(s) of a femoral component 
(not shown) of a knee joint prosthesis. The inferior surface 18 preferably 
includes a mating stem 22 that protrudes from the inferior mating surface 
18 and that is adapted to mate selectively with tibial tray 14. 
The tibial tray 14 includes an anterior side 17, a posterior side 19, a 
superior mating surface 32 and an inferior bone contacting surface 34. The 
bone contacting surface 34 has a first portion 36 that represents an area 
of the inferior surface that mounts upon the proximal surface of a 
resected tibia (not shown). A second portion 38 of the bone contacting 
surface 34 extends from the first portion 36 and is adapted to extend into 
a cavity (not shown) formed within a patient's tibia. Preferably, the 
second portion 38 is an elongate tibial stem 39 that extends from the 
first portion 36. The tibial stem 39 has outer side and distal walls 40, 
41. The outer side walls 40 of the tibial stem 39 may have irregular 
surface features (such as steps 42) to enhance bone fixation. 
The superior surface 32 of the tibial tray 14 includes an aperture 72 
(which may be of any suitable shape, e.g., substantially circular) that 
communicates with a mating cavity 44. In an illustrated embodiment, the 
mating cavity 44 is a blind cavity, defined by interior side walls 45 that 
extend into the tibial stem. The mating cavity 44 terminates in an 
interior distal wall 46 that may be substantially cone-shaped, or formed 
in another shape that is suitable for receiving a mating stem 22. 
The mating stem 22 of the tibial bearing insert 12 is adapted to fit within 
the mating cavity 44 of the tibial tray. A securing mechanism ensures that 
the mating stem 22 is secured within the mating cavity 44 in such a way 
that the tibial bearing insert is axially secured to the tibial tray. 
Moreover, the tibial bearing insert must be able to rotate relative to the 
tibial tray while the two components are secured to one another. 
One of ordinary skill in the art will readily appreciate that the 
dimensions of cavity 44 and mating stem 22 may vary. In one embodiment the 
cavity 44 has a diameter that tapers from proximal 51 to distal 53 ends 
thereof at an angle in the range of about 0.25.degree. to 5.degree.. The 
diameter at the proximal end 51 is in the range of about 5 to 40 mm and 
the diameter at the distal end 53 is in the range of about 3 to 39 mm. The 
cavity 44 preferably has a depth in the range of about 5 to 75 mm. 
The mating stem 22 should have a size and shape complementary to the cavity 
44. Accordingly, the diameter of stem 22 should taper from about 6 to 38 
mm at a proximal end to about 3 to 30 mm a distal end. The length of stem 
22 preferably is in the range of about 4 to 75 mm. 
The superior surface 16 of the tibial bearing insert 12 may optionally 
include a blind bore 52 of the type illustrated in the embodiment of FIGS. 
15-18. The blind bore 52 is substantially centrally located and is of a 
size and shape sufficient to receive a reinforcement pin 54 of the type 
shown in FIG. 19. Such reinforcement pins are well known in the art and 
may be substantially cylindrically shaped and made of a metal or metal 
alloy. Such pins may also have knurled or grooved surface features (not 
shown) as is known in the art. In one embodiment bore 52 is cylindrical, 
having a diameter of about 1 to 12 mm and a depth of about 5 to 75 mm. 
As shown in FIGS. 1-13, the securing mechanism of the tibial tray 14 and 
the tibial bearing insert 14 includes at least one negative surface 
feature 24 and at least one positive surface feature 26 which, upon 
engagement, form a snap fit arrangement that allows rotation of the second 
component with respect to the first component while axially securing the 
two components to each other. Preferably, the axial securement is 
sufficient to inhibit withdrawal of the tibial bearing insert 12 from the 
tibial tray 14 when a separating force, e.g., an upwardly-directed force, 
acts on the tibial bearing insert 12. Although the tibial bearing insert 
is shown to include a negative surface feature while the tibial tray 
includes a positive surface feature, it is understood that this 
arrangement can be reversed so that the positive surface feature is 
present on the tibial tray and the negative surface feature is present on 
the tibial bearing insert. 
The negative surface feature 24 may be in the form of one or more grooves 
28 formed in mating cavity 44. Grooves 28 may be formed in the cavity 44 
at the proximal end 51, distal end 53, or at intermediate locations. In 
one embodiment, separate grooves 28 are formed on each of the anterior and 
posterior sides of mating cavity 44. Each groove 28 is in the form of an 
arc that extends along a portion of the perimeter of the mating cavity 44. 
Preferably, each groove forms an arc of about 5.degree. to 60.degree.. 
Each groove 28 is defined by a first vertical surface 56, a horizontal 
shoulder 58, recessed vertical surface 60 and a bottom surface 62. The 
horizontal shoulder 58 may extend from the recessed vertical surface 60 at 
an angle of between about 45.degree. and 90.degree., and most preferably 
at an angle of 90.degree.. The length (L.sub.1) of horizontal shoulder 58, 
which is the distance by which the recessed vertical surface 60 is offset 
from the first vertical surface 56, should be sufficient to ensure and 
maintain engagement of the positive surface features 26 with the grooves 
28. Preferably, this length (L.sub.1) is about 0.25 to 3.00 mm. 
One of ordinary skill in the art will appreciate the height (H) of recessed 
vertical surface 60 may vary depending upon the requirements of a given 
application. In any event, this height must be sufficient to accommodate 
the positive surface features. As described below, the height need not be 
constant. Instead, the height can taper from a maximum height (H.sub.max) 
at a central portion 30 of the groove to a minimum height (H.sub.min) at 
end portions 31 of the groove. The use of a groove having a tapered height 
can gradually restrict rotational movement of the tibial bearing insert 
12. 
As noted above, at least one groove 28 is disposed on the tibial tray. The 
groove(s) may be present on any part of the tibial tray, as long as they 
are able to engage with a corresponding positive surface feature on the 
tibial bearing insert. The groove(s) preferably are located within the 
mating cavity 44, on either the inner side walls 45 or the distal wall 
thereof. In an embodiment illustrated in FIGS. 1 to 13, the grooves are 
located at a proximal end of mating cavity 44. 
Although only one, continuous groove may be used, it is preferable to 
utilize two opposed grooves. For purposes of illustration, grooves 28 are 
shown to be on opposed anterior 17 and posterior 19 sides of the tibial 
tray 14. It is understood, however, that opposed grooves may instead be 
disposed on other locations of the tibial tray, including on medial and 
lateral sides of the tibial tray. 
One of ordinary skill in the art will readily appreciate that suitable 
negative surface features may include structures other than the grooves 
described herein. 
Positive surface features 26 include virtually any structures protruding 
from the tibial bearing insert 12 that are able to mate with the negative 
surface features 24 (e.g., grooves 28) of the tibial tray 14 to enable the 
tibial bearing insert 12 to rotate with respect to the tibial tray 14 and 
to axially secure these components to each other. 
Referring to FIGS. 1 to 13 the system 10 includes at least one positive 
surface feature, each of which includes protruding members that are formed 
on a portion of the tibial bearing insert 12 such that they are able to 
mate with grooves. In one embodiment each positive surface feature is a 
snap member 64, which may be slightly deformable or deflectable, that 
forms an arc over a portion of the surface of the tibial bearing insert 
12. Each snap member 64 forms an arc that is smaller than the 
corresponding arc of groove 28 with which it is to engage, in order to 
facilitate rotation of the tibial bearing insert 12 with respect to the 
tibial tray 14 when the grooves 28 and snap members 64 are mated. The arc 
formed by the snap members 64 should be about 5.degree. to 90.degree. less 
than that of grooves 28. Preferably, the snap members 64 form an arc of 
4.degree. to 85.degree.. 
For purposes of illustration, the snap member 64 is shown to be formed on a 
portion of the inferior surface 18 of the tibial bearing insert 12 that is 
in proximity to the mating cavity 44. It is understood, however, that the 
snap member may be disposed at alternative locations on tibial bearing 
insert 12. 
As shown in FIGS. 4, 7 and 11, each snap member may include a cantilevered 
wall 66 that extends from the tibial bearing insert 12. A distal end 68 of 
cantilevered wall extends radially outwardly from wall 66 and has a radius 
wall 70 and an engaging shoulder 74. In an illustrated embodiment, the 
cantilevered wall 66 extends vertically downwardly from inferior surface 
18 and the engaging shoulder 74 has a horizontal surface 76 that is 
orthogonal to the cantilevered wall 66. The cantilevered wall 66 
preferably is separated from the adjacent surface 65 of mating stem 22 by 
a distance of about 0.25 to 3.00 mm. 
One of ordinary skill in the art will appreciate that the dimensions of 
snap member may vary. Generally, however, the cantilevered wall 66 has a 
length of about 0.25 to 3.00 mm. The horizontal surface 76 of the engaging 
shoulder 74 preferably extends over a distance of about 0.25 to 3.00 mm. 
The radius wall 70 should have sufficient curvature to promote the mating 
of the snap members 84 within groove 28. In one embodiment, the radius 
wall 70 has a radius in the range of about 0.10 mm to 1.00 mm. 
The system 10 is assembled by forcing the tibial bearing insert 12 within 
the tibial tray 14. As the radius wall 70 contacts the tibial tray 14 and 
as force is applied to the tibial bearing insert 14 radius wall 70 slides 
past the first vertical surface 56 of groove 28. In one embodiment, the 
cantilevered wall 66 of the snap member 64 deflects by a minimal amount 
sufficient to accommodate the mating of the snap member and the groove. 
Once the snap member is fully inserted within the groove, the cantilevered 
wall 66 returns to its original position and the horizontal surface 76 of 
the engaging shoulder 74 of the snap member abuts the horizontal shoulder 
58 of the groove, thereby providing axial securement of the tibial bearing 
insert 12 to the tibial tray 14. 
At the same time, some degree of rotation of the tibial bearing insert with 
respect to the tibial tray is possible because the arc defined by the 
grooves is greater than the arc defined by the snap members. Preferably, 
the tibial bearing insert is able to rotate in the range of about 
1.degree. to 30.degree.. 
In one embodiment, shown in FIGS. 12 and 13, the grooves 28 may have a 
height that tapers from a maximum height (H.sub.max) at a central portion 
30 of the groove 31 to a minimum height (H.sub.min) at end portions of the 
groove. By way of example, the maximum height may be about 0.25 mm and the 
minimum height may be about 30 mm. The use of a tapered groove can be 
advantageous as it allows increasing resistance to rotation, thereby 
gradually restricting rotation. 
One of ordinary skill in the art will appreciate that each snap member 
should have dimensions suitable to enable it to fit snugly within grooves 
28 with substantially no axial play. The fit of the snap members within 
the grooves should not be so snug, however as to present excess friction 
upon rotation of the tibial bearing insert. If desired a seal member (not 
shown) may be disposed within grooves 28 to reduce axial play and or 
radial movement. 
It is understood that the negative and positive surface features may be 
disposed at any location on the tibial bearing insert 12 and tibial tray 
14 that enables these surface features to be suitably engaged with each 
other to properly orient the tibial bearing insert 12 with respect to the 
tibial tray 14. In one embodiment, at least two positive surface features 
are present on the tibial bearing insert, at locations opposite one 
another. 
One of ordinary skill in the art will appreciate that the securing 
mechanism of the tibial tray and the tibial bearing insert (or any other 
joint system of the invention) may be modified by any structure that 
facilitates the axial securement of the tibial bearing insert to the 
tibial tray while preserving rotatability of the tibial bearing insert. 
FIGS. 14-18 illustrate an embodiment in which the distal end 78 of the 
mating stem 22 includes positive surface features in the form of 
expandable wedge-like elements 80, each of which is separated by a slot 
81. Each wedge-like element 80 has a protruding shoulder 82 on an exterior 
surface thereof. The expandable wedge-like elements 80, in cooperation 
with slots 81, enable the diameter of the distal end 78 of the mating stem 
22 to expand from a first diameter (D.sub.1) to a second diameter 
(D.sub.2). The first diameter (D.sub.1), as shown in FIG. 17, is 
insufficient to enable the protruding shoulders 82 of the wedge-like 
members 80 to engage a corresponding negative surface feature (e.g., 
groove 84) in mating cavity 44. However expansion of the mating stem 22 to 
the second diameter (D.sub.2), as shown in FIG. 18, causes interaction 
between the protruding shoulders 82 and the groove 84 that prevents 
removal of the tibial bearing insert 12 from the tibial tray 14. Despite 
this axial securement, the tibial bearing insert 12 is still able to 
rotate relative to the tibial tray 14. 
The expansion of the mating stem from the first diameter to the second 
diameter may be accomplished by the insertion of a reinforcement pin 54 
within a blind bore 52 formed in the superior articulation surface 16 of 
the tibial bearing insert 12. The mating of pin 54 within bore 52 causes 
expansion of the distal end of the mating stem from the first to the 
second diameter. 
The reinforcement pin 54 may be of the type described above and shown in 
FIG. 19. The blind bore 52 is preferably located substantially centrally 
on the superior articulation surface 16 of the tibial bearing insert 12 
and is of a size and shape sufficient to receive a reinforcement pin 54. 
In one embodiment, the bore 52 is cylindrical and has a diameter of about 
1 to 12 mm and a depth of about 12 to 76 mm. The reinforcement pin may 
have a diameter of about 1 to 10 mm and a length of about 5 to 60 mm. 
FIGS. 14-18 illustrate four wedge-like elements 80 formed at the distal end 
78 of mating stem 22. It is understood, however, that an alternative 
number of wedge elements 80 may be present. For example, mating stem 22 
may have only two or three wedge elements, or more than four. 
Alternatively, the tibial bearing insert 12 may include wedge-like elements 
80 that are biased to a first diameter that enables axial securement with 
tibial tray 14. Upon mating of the tibial bearing insert 12 with tibial 
tray 14, the wedge-like elements are compressed to a second, smaller 
diameter. Upon proper seating of the tibial bearing insert the wedge-like 
elements return to the first diameter. 
The embodiment of FIGS. 14-18 may be further modified by omitting 
wedge-like elements 80 and groove 84. Instead axial securement can be 
provided by the frictional engagement of mating stem 22 with the inner 
wall of mating cavity 44 when reinforcement pin 54 is inserted into bore 
52. Surface features (not shown) analogous to wedge-like elements 80 may 
be provided at a proximal portion of mating stem 22 to interact with 
complementary surface features on a top portion of cavity 44 when the 
reinforcement pin 54 is inserted. The surface features on the mating stem 
22 can be of dimensions smaller than the surface features on the cavity 44 
so as to limit the degree of rotation of the tibial bearing insert 12. 
FIGS. 27 and 28 illustrate an alternative embodiment in which wedge-like 
elements 80 are disposed at a more proximal portion of mating stem 22. 
FIGS. 20-26 illustrate an alternative securing mechanism for the tibial 
tray 14 and the tibial bearing insert 12 that includes at least one 
negative surface feature 85 and at least one positive surface feature 87, 
that are matable to axially secure the tibial bearing insert 12 to the 
tibial tray 14 while permitting rotation of the tibial bearing insert 12 
with respect to the tibial tray 14. Although the tibial tray is shown to 
include a negative surface feature 85 while the tibial bearing insert 
includes a positive surface feature 87, it is understood that this 
arrangement can be reversed. 
The negative surface feature 85 may be in the form of at least one 
circumferential groove 90 that extends partially or entirely around the 
perimeter of the mating cavity 44. The circumferential groove 90 may 
extend in a linear fashion about the interior side walls 45 of fashion 
about the perimeter of mating cavity 44, or it may follow another path, 
such as an eccentric, sinusoidal, parabolic, or wave. Further, the 
circumferential groove(s) may be present at any location on the tibial 
tray, as long as it is able to engage with a corresponding positive 
surface feature on the tibial bearing insert 12. The circumferential 
groove(s) preferably are disposed on the inner side walls 45 of the mating 
cavity 44. Circumferential groove 90 may be located at any position 
between the proximal and distal ends 51, 53 of mating cavity 44. 
The dimensions of circumferential groove(s) 90 may vary within limits 
readily appreciated by one of ordinary skill in the art. The 
circumferential groove 90 may have a substantially constant opening width 
(W) in the range of about 1 to 20 mm. The depth of circumferential groove 
90 generally is in the range of 0.25 to 2.00 mm. 
One of ordinary skill in the art will readily appreciate that suitable 
negative surface features may include structures other than the 
circumferential groove described herein. 
Positive surface feature 87 also includes virtually any structure that 
protrudes from the tibial bearing insert 12 that is able to mate with the 
negative surface feature 85 (e.g., the circumferential groove 90) of the 
tibial tray 14 to enable the tibial bearing insert 12 to rotate with 
respect to the tibial tray 14 and to axially secure these components to 
each other. 
As shown in FIGS. 20-26, the positive surface feature 87 is in the form of 
an eccentric rib 86 that extends over a portion of the surface of the 
tibial bearing insert 12. The term "eccentric rib" refers to the property 
of the rib 86 that enables it to form a substantially sinusoidal, 
parabolic, or wave-like path about all or part of the outer surface of 
mating stem 22. Preferably, the difference between high and low points in 
the oscillation of rib 86 is about 1 to 10 mm. The rib 86 may extend about 
the circumference of the mating stem 22 over a range of about 1 to 4 
oscillation cycles. 
The dimensions of rib 86 may also vary and depend to a large extent on the 
dimensions of the circumferential groove 90. The rib 86 must be of 
dimensions that enable it to fit within circumferential groove 90 to 
axially secure tibial bearing insert 12 to tibial tray 14. There must also 
exist sufficient clearance to enable some side-to-side movement of the rib 
86 within circumferential groove 90 to permit some rotation of the tibial 
bearing insert 12. The rib 86 must also have a thickness (T) that is less 
than the width (W) of circumferential groove 90. Generally, the thickness 
(T) of rib 86 ranges between about 1 and 15 mm. The eccentric rib 86 
should protrude from mating stem 22 by a distance that is less than the 
depth circumferential groove 90. 
The eccentric rib 86 may be disposed at various locations on tibial bearing 
insert 12. Preferably, however, the eccentric rib 86 is formed on a 
surface of mating stem 22. Such that it extends partially or fully about 
the circumference of mating stem 22. 
Upon mating of rib 86 within circumferential groove 90, the tibial bearing 
insert 12 and tibial tray are axially secured to each other. The tibial 
bearing insert 12 is also able to rotate with respect to the tibial tray 
14. However, the degree of rotation is generally limited to about 
5.degree. to 90.degree.. As rotation progresses the high point 102 and/or 
low point 104 of rib 86 impinges upon the walls of the groove, prohibiting 
further rotation, or greatly increasing the force necessary to effect 
rotation. In one embodiment, the rib 86 and groove 90 are generally of the 
same shape. 
One of ordinary skill in the art will appreciate that the rotation of the 
tibial bearing insert may further be controlled by varying other 
dimensions of the circumferential groove 90 and/or the eccentric rib 86, 
such as the length of the eccentric rib, the distance by which the 
eccentric rib extends into the circumferential groove, and the width of 
the circumferential groove. 
FIGS. 22 through 26 illustrate the ability of groove 90 and rib 86 to limit 
rotation of tibial bearing insert 12. In FIG. 22, the tibial bearing 
insert is rotated by approximately 5.degree.. Upon doing so, the high 
points 102 of rib 86, as shown in FIGS. 23 and 24, impinge upon upper 
walls 106 of groove 90, thereby preventing further rotation, or greatly 
increasing the force necessary to effect further rotation. At the same 
time, as shown in FIGS. 25 and 26, the low points 104 of rib 86 impinge 
upon lower walls 108 of groove 90. 
One of ordinary skill in the art will appreciate that the components of the 
system 10 of the invention can be made from a variety of known materials. 
The tibial bearing insert typically is made of a polymeric material such 
as ultra high molecular weight polyethylene. The tibial bearing insert can 
be made of a variety of known metals and metal alloys that are suitable 
for implantable prostheses. 
One of ordinary skill in the art will further appreciate that minor 
modifications may be made to the invention described herein without 
departing from its intended scope. All references noted herein are 
expressly incorporated by reference in their entirety.