Implantable prosthesis having textured bearing surfaces

An implantable orthopedic prosthesis includes a first component having a first bearing surface, and a second component having a second bearing surface. The second bearing surface is disposed in opposition to the first bearing surface in a sliding bearing relationship. At lease one of the first and second bearing surfaces includes a plurality of substantially evenly distributed plateaus interspersed with valleys. The plateaus comprise at least about 30% of the gross area of the bearing surface. The plateaus are distributed at a density of about 18 to about 25 plateaus per square inch of gross area of the bearing surface. The first and second bearing surfaces are substantially congruent to each other, and the plateaus have a smooth surface finish of less than about 8 .mu.m root mean square. The valleys have a depth of about 0.0002 inch to about 0.002 inch below the plateaus to facilitate lubrication of the articulating surfaces by natural body fluids.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates generally to implantable prostheses for 
replacing human skeletal joints, and relates more particularly to the 
surface characteristics of prosthetic articulation surfaces of such 
implantable prostheses. 
BACKGROUND INFORMATION 
Implantable orthopedic prostheses, in one form, comprise manufactured 
replacements for the ends and articulating surfaces of the bones of the 
skeleton. Such prostheses are implanted to repair or reconstruct all or 
part of an articulating skeletal joint that is functioning abnormally due 
to disease, trauma, or congenital defect. Among the various articulating 
skeletal joints of the human body that are eligible to be fitted with 
implantable orthopedic prostheses, the hip joint and the knee joint are 
most often treated with such prostheses. The hip and knee joints are major 
weight bearing joints and degenerate more quickly than other joints in 
case of abnormality. Also, the hip and knee joints play a critical role in 
ambulation and quality of life, resulting in great demand for surgical 
correction of abnormalities. 
The human hip joint involves two bones: the femur and the pelvis, each 
having a smooth articulation surface arranged for articulation against an 
adjacent articulation surface of the other bone. The femur includes at its 
proximal extremity a head having a convex, generally spherically contoured 
articulation surface. The pelvis, in pertinent part, includes an 
acetabulum having a concave, generally spherically contoured articulation 
surface. The mutually engaging articulation surfaces of the femur and the 
pelvis together form, functionally, a ball-and-socket joint. 
One or both of the articulation surfaces of the hip joint may fail to act 
properly, requiring the defective natural articulation surface to be 
replaced with a prosthetic articulation surface provided by an implantable 
prosthesis. To fit defects of varying scope, while allowing healthy 
portions of the hip joint to be conserved, a range of types of orthopedic 
implants is available. The range extends from total hip prosthesis systems 
for replacing the articulation surfaces of both the femur and the pelvis, 
to less comprehensive systems for replacing only the femoral articulation 
surface. Commonly employed orthopedic hip prostheses include components 
that fall within one of three principle categories: femoral stems, femoral 
heads and acetabular cups. A so-called "total" hip prosthesis includes 
components from each of these categories. The femoral stem replaces the 
proximal end of the femur and includes a distal stem received within the 
medullary canal at the proximal end of the femur. The femoral head 
replaces the natural head and articulating surface of the femur. The 
acetabular cup replaces the natural socket and articulating surface of the 
acetabulum of the pelvis. In some designs, the stem and head are an 
integral, unitary component, but more often the stem and head are separate 
modular components designed to be assembled to suit the anatomical needs 
of the patient. A so-called "bipolar" hip prosthesis includes only femoral 
stem and head components. The femoral part of the hip joint is replaced 
with a femoral stem supporting an artificial femoral head. The latter 
includes an inner head, fixed to the femoral stem, that articulates within 
an outer head. The outer head articulates directly against the natural 
acetabulum. Similarly, a so-called "unipolar" hip prosthesis also includes 
only femoral stem and head components. The femoral part of the hip joint 
is replaced with a femoral stem supporting an artificial femoral head. The 
femoral head articulates directly against the natural acetabulum while 
remaining fixed relative to the femoral stem. 
The human knee joint involves three bones: the femur, the tibia and the 
patella, each having smooth articulation surfaces arranged for 
articulation on an adjacent articulation surface of at least one other 
bone. The femur includes at its distal extremity an articulation surface 
having medial and lateral convex condyles separated posteriorly by an 
intercondylar groove running generally in the anterior-posterior 
direction. The condyles join at the distal-anterior face of the femur to 
form a patellar surface having a shallow vertical groove as an extension 
of the intercondylar groove. The patella includes on its posterior face an 
articulation surface having a vertical ridge separating medial and lateral 
convex facets, which facets articulate against the patellar surface of the 
femur and against the medial and lateral condyles during flexion of the 
knee joint, while the vertical ridge rides within the intercondylar groove 
to prevent lateral displacement of the patella during flexion. The tibia 
includes at its proximal end an articulation surface having medial and 
lateral meniscal condyles that articulate against the medial and lateral 
condyles, respectively, of the femur. The mutually engaging articulation 
surfaces of the femur and the patella together form, functionally, the 
patellofemoral joint, and the mutually engaging articulation surfaces of 
the femur and tibia together form, functionally, the tibiofemoral joint, 
which two functional joints together form the anatomical knee joint. 
One or more of the articulation surfaces of the knee joint may fail to act 
properly, requiring the defective natural articulation surface to be 
replaced with a prosthetic articulation surface provided by an implantable 
prosthesis. To fit defects of varying scope, while allowing healthy 
portions of the knee joint to be conserved, a range of types of orthopedic 
knee implants is available. The range extends from total knee prosthesis 
systems for replacing the entire articulation surface of each of the 
femur, tibia and patella, to simpler systems for replacing only the 
tibiofemoral joint, or only one side (medial or lateral) of the 
tibiofemoral joint, or only the patellofemoral joint. Commonly employed 
orthopedic knee prostheses include components that fall within one of 
three principle categories: femoral components, tibial components, and 
patellar components. A so-called "total" knee prosthesis includes 
components from each of these categories. The femoral component replaces 
the distal end and condylar articulating surfaces of the femur and may 
include a proximal stem received within the medullary canal at the distal 
end of the femur. The tibial component replaces the proximal end and 
meniscal articulating surfaces of the tibia and may include a distal stem 
received within the medullary canal at the proximal end of the tibia. The 
patellar component replaces the posterior side and natural articulating 
surface of the patella. Sometimes, the patellar component is not used, and 
the natural articulating surface of the patella is allowed to articulate 
against the femoral component. A so-called "unicondylar" knee prosthesis 
replaces only the medial or the lateral femoral condylar articulating 
surface and the corresponding tibial meniscal articulating surface. 
The acetabular cup component of a total hip prosthesis is configured to be 
received and fixed within the acetabulum of a pelvis. The pelvis is 
prepared to receive the acetabular cup by reaming a concavity in the 
acetabular bone. The acetabular cup component typically has an outer 
surface conforming to the concavity reamed in the acetabular bone of the 
pelvis, and an inner bearing cavity for receiving the head of the femoral 
component. The head articulates in the bearing cavity as a ball-and-socket 
joint to restore motion to a defective hip joint. 
One known type of acetabular cup involves an acetabular shell made of a 
biocompatible metal such as titanium or a titanium alloy, and a bearing 
insert made of a bio-compatible polymer such as ultra-high molecular 
weight polyethylene. The acetabular shell is shaped generally as a 
hemispherical cup having a dome, or apex, at a proximal end and an annular 
rim at a distal end. As used herein, the words proximal and distal are 
terms of reference that indicate a particular portion of a prosthesis 
component according to the relative disposition of the portion when the 
component is implanted. "Proximal" indicates that portion of a component 
nearest the torso, whereas "distal" indicates that portion of the 
component farthest from the torso. Between the dome and rim, the 
acetabular shell comprises a shell wall defined by a generally convex 
proximal surface and a generally concave distal surface spaced from the 
proximal surface. The concave distal surface defines a shell cavity having 
an opening at the rim of the cup for receiving the bearing insert. The 
bearing insert has a generally convex proximal surface configured to be 
received and fixed within the acetabular shell in generally congruent 
engagement with the concave distal surface of the shell wall. The bearing 
insert also has a bearing cavity that opens distally for receiving the 
head of the femoral component. The bearing cavity is defined by a 
generally spherical concave bearing surface having a radius similar to 
that of the femoral head component. The concave bearing surface 
articulates against the surface of the spherical femoral head component. 
The acetabular shell can be affixed to the acetabular bone by bone screws 
or bone cement. If bone screws are elected, the screws are driven into the 
bone through the screw holes before the bearing insert is placed into the 
shell. The shell also can be affixed by a combination of bone screws and 
bone cement. 
Other known types of acetabular cup vary from the type described above, 
among other ways, by including a bearing insert, or more particularly, an 
articulating surface of a bearing insert, made of a material other than 
polyethylene. Such other materials include metals and metal alloys, such 
as cobalt chrome, and ceramics. 
The tibial component of a total knee prosthesis is configured to be 
received upon and fixed to the proximal end of the tibia. The tibia is 
prepared to receive the tibial component by resecting part of the proximal 
end of the tibia to leave a substantially horizontal planar bony plateau. 
Sometimes the exposed medullary canal at the proximal end of the tibia is 
also reamed to receive a stem portion of the tibial component. The tibial 
component typically includes a plate portion having an inferior planar 
surface conforming to the resected bony plateau at the proximal end of the 
femur. The plate portion may or may not include a depending stem or keel 
for receipt within a prepared tibial medullary canal. Commonly, a meniscal 
bearing insert is received atop the plate portion of the tibial component 
to provide an artificial meniscal articulating surface for receiving the 
condylar surfaces of the femoral component of the total hip prosthesis. 
The femoral condylar articulating surfaces articulate against the tibial 
meniscal articulating surface to restore motion to a defective knee joint. 
One known type of tibial component involves a tibial plate made of a 
biocompatible metal such as titanium or a titanium alloy, and a meniscal 
bearing insert made of a bio-compatible polymer such as ultra-high 
molecular weight polyethylene. The tibial plate is shaped generally as a 
flat plate having a perimeter that generally conforms to the transverse 
sectional perimeter of the resected proximal tibia. The tibial plate 
includes a planar distal, or inferior, surface for engaging the resected 
proximal tibia, and a proximal, or superior, surface for engaging and 
receiving the meniscal bearing insert. One or more screw holes may extend 
through the plate portion from the superior to the inferior surface. The 
bearing insert has an inferior surface that engages the superior surface 
of the plate portion, and may include locking tabs or other means for 
fixing the bearing insert to the plate portion against relative movement. 
The tibial plate can be affixed to the resected tibial bone by bone screws 
or bone cement. If bone screws are elected, the screws are driven into the 
bone through the screw holes before the bearing insert is placed atop the 
plate portion. The plate also can be affixed by a combination of bone 
screws and bone cement. The tibial bearing insert usually is designed to 
be received atop the tibial plate in nonarticulating relative 
relationship. In some total knee prostheses, however, the bearing insert 
is intended to articulate on the tibial plate in sliding or rotating 
relationship. Such knee prostheses are known as "mobile bearing" knees. 
According to a prevailing hypothesis, sliding motion between adjacent metal 
and polyethylene surfaces in implanted joint prostheses generates fine 
polyethylene particulate debris due to frictional wear. The generation of 
such debris is hypothesized to occur even at metal-to-polyethylene 
interfaces that are not designed to articulate. This is believed to occur 
from unintended relative micro-motion between the metal and polyethylene 
surfaces caused by the varying load borne by the implanted prosthesis in 
use. In reaction to this hypothesis, some manufacturers of implantable 
joint prostheses have begun to polish the metal surfaces in their products 
that are in non-articulating engagement with polyethylene components, just 
as they previously polished the metal surfaces that were known and 
intended to articulate against polyethylene. The reason for concern over 
such polyethylene wear debris is that in vitro experiments have shown that 
fine polyethylene particles are osteolytic. Whether this osteolytic action 
occurs in vivo is not known. Nevertheless, given the concern over the 
issue of wear debris being generated at non-articulating interfaces, it 
would be desirable to avoid sliding interfaces between metal surfaces and 
polyethylene surfaces in implantable joint prostheses, at least in those 
applications where the frictional characteristics of a 
metal-to-polyethylene interface is not required for proper operation of 
the artificial joint. 
In the case of the mobile bearing type knee prothesis, it has been proposed 
to bond the polyethylene meniscal bearing insert to a metal substrate 
which slides against the metal tibial plate. This results in a polished 
metal-to-polyethylene interface between the femoral component and the 
meniscal bearing insert, in combination with a metal-to-metal sliding 
interface between the meniscal bearing insert and the tibial plate. One 
problem associated with such a metal-to-metal sliding interface is the 
need to prevent metal wear debris from being generated. Conventionally, 
the solution to this problem is to make each of the metal sliding surfaces 
highly polished, as has been done in some types of prosthetic hip joints. 
Highly polished metal femoral heads articulating against highly polished 
concave spherical acetabular bearing surfaces have been used successfully 
for many years, particularly in Europe. 
The polished metal-to-metal articulating interface, as used in some 
prosthetic hip joints, is known to provide a prosthesis having excellent 
wear resistance. Nevertheless, the sliding friction between such polished 
metal surfaces is also known to be greater than the sliding friction 
between polyethylene and polished metal. The greater friction of the 
polished metal-to-metal interface results in a tendency toward greater 
resistance to rotation of the joint. This phenomenon has been restrained 
within reasonable limits in the case of ball and socket hip joints by 
limiting the diameter of the spherical metal head. That solution is not 
readily applicable to the planar metal-to-metal interface of a mobile 
bearing knee prosthesis, as the area of the interface is dictated by the 
range of sliding motion to be achieved and cannot be limited arbitrarily. 
This problem can be alleviated somewhat by substituting other hard 
materials having lower coefficients of friction. For example, one surface 
of the interface could be made of a polished ceramic material, while the 
other surface is made of a polished metal. Alternatively, both surfaces 
could be made of polished ceramic. This approach is not believed to fully 
address the problem, as the resulting sliding friction would still be 
significantly greater than that of a metal-to-polyethylene interface. 
It would be desirable to provide a sliding interface in an implantable 
joint prosthesis that preserves the desirable wear resistance 
characteristics of a metal-to-metal, metal-to-ceramic, or 
ceramic-to-ceramic interface, while significantly reducing the sliding 
friction of such interfaces. This and other desirable advantages are 
achieved by the present invention. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention, an implantable orthopedic 
prosthesis includes a first component having a first bearing surface, and 
a second component having a second bearing surface. The second bearing 
surface is disposed in opposition to the first bearing surface in a 
sliding bearing relationship. At least one of the first and second bearing 
surfaces includes a plurality of substantially evenly distributed plateaus 
interspersed with valleys. The plateaus comprise at least about 30% of the 
gross area of the bearing surface. 
According to other aspects of the present invention, the plateaus are 
distributed at a density of about 18 to about 25 plateaus per square inch 
of gross area of the bearing surface. The plateaus have a smooth surface 
finish of less than about 8 .mu.m root mean square, and the valleys have a 
depth of about 0.0002 inch to about 0.002 inch below the plateaus. 
It is an object of the present invention to provide an implantable 
orthopedic prosthesis having a sliding interface between components having 
good wear characteristics and low frictional resistance to sliding. 
Other objects and advantages of the present invention will be apparent from 
the following descriptions of the preferred embodiments illustrated in the 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is concerned with the surface texture characteristics 
of opposed bearing surfaces of components of an implantable orthopedic 
prosthesis arranged in a sliding bearing relationship. In general, the 
surfaces of the preferred embodiments provide polished bearing regions 
dispersed substantially evenly over the gross surface area. The polished 
regions comprise plateaus disposed within a narrow-tolerance surface 
profile that is flat, cylindrical, conical or spherical. Between the 
plateau regions are valley regions of sufficient depth to hold and provide 
pathways for the natural lubricating body fluids in the skeletal joint. By 
way of introduction, the features and characteristics of such surfaces 
according to the present invention are described below with respect to 
FIGS. 1a-5. Following the introductory explanation, the preferred 
embodiments of such surfaces are described in the context of exemplary hip 
and knee prosthetic components, with reference to FIGS. 6a-13. 
Referring to FIGS. 1a and 1b, an arbitrary section of a surface 10 is 
shown. Surface 10 can be described with respect to the following 
characteristics, as annotated in FIGS. 1a and 1b with the following 
respective reference symbols: Roughness height, R.sub.h ; plateau height, 
P.sub.h ; and plateau profile, P.sub.p. From a macroscopic perspective, 
surface 10 is considered as having high areas, or plateaus, 12, and low 
areas, or valleys, 14, interspersed between the plateaus 12. The height 
differential between the plateaus and the valleys is the plateau height, 
P.sub.h. From an even larger macroscopic perspective, surface 10 is 
considered as having a plateau profile, P.sub.p, which represents the high 
and low limits between which a region of surface 10 must lie to be 
regarded as a plateau 10. All regions lying below the lower limit of the 
plateau profile P.sub.p are regarded as valley areas 14. From a 
microscopic perspective, the plateau regions 12 of surface 10 are 
considered as having a roughness height, R.sub.h, which represents the 
high and low limits between which adjacent microscopic peaks and troughs 
of surface 10 must lie for the respective plateau region to be regarded as 
having a specified surface finish. Qualitatively, the plateau height 
specification P.sub.h has the broadest limits, the plateau profile 
specification P.sub.p has narrower limits, and the surface roughness 
specification of the plateaus 12, R.sub.h, has the narrowest limits. 
Referring to FIGS. 2-5, plan views of alternative configurations of surface 
10 are illustrated, showing regions of about one square inch. FIGS. 2-5 
illustrate differing percentages of the gross surface area of surface 10 
occupied by plateau regions 12. All gross surface area not regarded as a 
plateau area 12 is regarded as valley area 14. In FIGS. 2 and 4, the 
plateaus 12 occupy 10% of the gross surface area of surface 10. In FIGS. 3 
and 5, the plateaus 12 occupy 40% of the gross surface area of surface 10. 
FIGS. 2-5 also illustrate differing numbers of plateaus 12 per square inch 
of gross surface area of surface 10. In FIGS. 2 and 3, there are twenty 
plateaus 12 per square inch of gross surface area. In FIGS. 4 and 5, there 
are thirty-two plateaus 12 per square inch of gross surface area. By 
comparing all four figures, FIGS. 2-5, the effect of altering the relative 
area occupied by the plateaus 12 and of altering the absolute number of 
plateaus 12 per unit area can be appreciated. 
It is a purpose of the present invention to provide mutually opposed, 
mating, sliding bearing surfaces of an implantable orthopedic prosthesis 
with surface features that reduce the sliding friction between the two 
surfaces, that enhance lubrication of the sliding bearing surfaces by 
natural body fluids, and that maintain a high resistance to wear. This 
purpose is accomplished by fabricating the sliding bearing surfaces 
according to these principles: (1) each surface is held within close 
tolerances with respect to plateau profile, P.sub.p, the plateau profile 
being flat, cylindrical, conical, spherical, or other contour as dictated 
by the required performance of the prosthesis; (2) the roughness height, 
R.sub.h, of the surface within each plateau region is held within close 
tolerances of about 8 .mu.m root mean square; (3) the percentage of gross 
surface area occupied by the plateau regions is at least about 30%; (4) 
the distribution density of the plateau regions is held between about 18 
and about 25 plateau regions per square inch of gross surface area; and 
(5) the distribution pattern of the plateau regions can be regular or 
random, and can be provided on one or both of the opposed bearing 
surfaces, provided that the distribution is such that no mutually 
interlocking arrangement is possible between the patterns of the two 
surfaces. The plateau height, Ph, is preferred to be between about 0.0002 
inch and about 0.002 inch. The material of the bearing surfaces, as 
preferred, is a bio-compatible metal such as titanium, titanium alloy, or 
cobalt chrome alloy, or a ceramic material, or a suitably hard, 
biocompatible coating, e.g., DLC, over a biocompatible substrate. 
According to the present invention, the surface 10 is fabricated by the 
following method steps. First, a metal component, on which surface 10 is 
to be generated, is shaped by conventional casting, forging or milling 
techniques to the desired configuration of the component. The surface 
which is to become surface 10 is then milled or ground to conform to the 
desired plateau profile, Pp. In other words, if the surface profile is 
desired to be flat, for example, the metal surface is made flat within the 
profile tolerance band, Pp. Alternatively, the profile could be 
cylindrical, conical, spherical or another shape, as required by the 
design goals for the component. Second, the valley regions 14 are formed 
by hand scraping, machine tooling, or other appropriate metal forming 
technique to conform to the desired depth specified by the plateau height, 
Ph. Incidental to this step is the formation of the plateau regions 12 
which constitute that portion of the surface generated in the first step 
that is not disturbed by the operation of the second step. The total area, 
distribution, and density of the plateau regions 12 is maintained within 
the desirable ranges discussed above by controlling the valley forming 
operation. Third, the plateau regions resulting from the second step are 
ground or lapped, to remove any burrs or other protrusions that may have 
resulted from the second step, to restore the plateau profile, Pp, within 
specifications. Fourth, the plateau regions are polished to conform to the 
roughness height, Rh, specifications. 
Referring to FIGS. 6a-8, a first preferred embodiment of an orthopedic 
joint prostheses is shown and described in general terms, and with regard 
to specific bearing surfaces incorporating the surface characteristics 
discussed above. Surface features and parameters that correspond to the 
general terms discussed above with regard to FIGS. 1a-5 are indicated by 
like primed reference symbols. 
With particular reference to FIGS. 6a-6c, a tibial component 20 of a total 
knee prosthesis is shown. Tibial component 20 includes a tibial plate 22 
and a distal keel 24 extending distally from plate 22 for receipt within 
the medullary canal at a resected proximal end of a tibia. Plate 22 
includes a distal surface 26 for engagement with and affixation to the 
resected bony plateau at the proximal end of the tibia. Plate 22 also 
includes a proximal sliding bearing surface 10' bounded by an upstanding 
peripheral wall 28. 
Referring now to FIGS. 7a-7c, a mobile, meniscal bearing insert 30 of a 
total knee prosthesis, for use with the tibial component 20 of FIGS. 
6a-6c, is shown. Bearing insert 30 includes a meniscal bearing 32 having a 
proximal articulation surface 34 for articulating against distal femoral 
condyle articulation surfaces 42 of a femoral component 40, shown in FIG. 
9, of a total knee prosthesis. Affixed to the distal side of meniscal 
bearing 32 is a mobile bearing plate 36 having a distal sliding bearing 
surface 10". 
In FIG. 8, the tibial component 20 of FIGS. 6a-6c is shown assembled to the 
mobile, meniscal bearing insert 30 of FIGS. 7a-7c. Proximal sliding 
bearing surface 10' is disposed in opposition to and in sliding engagement 
with distal sliding bearing surface 10". Because the medial-lateral and 
anterior-posterior dimensions of mobile bearing plate 36 are less than the 
corresponding dimensions of the interior surface of upstanding wall 28, a 
perimetrical gap 38 exists between wall 28 and plate 36, permitting 
sliding translation of meniscal bearing insert 30 relative to tibial 
component 20. This characteristic is referred to by the term "mobile" in 
the phrase mobile bearing knee prosthesis. 
As preferred, surfaces 10' and 10" of tibial plate 22 and mobile bearing 
plate 36, respectively, are made of a biocompatible metal such as 
titanium, titanium alloy, or cobalt chrome alloy. Alternatively, one or 
both of surfaces 10' and 10" could be made of a ceramic material, or of a 
suitably hard, biocompatible coating, e.g., DLC, over a biocompatible 
substrate. Surface 34 of mensical bearing 32, as preferred, is made of 
ultra-high molecular weight polyethylene. Other materials, such as metal 
or ceramic, can be substituted for surface 34 according to the performance 
characteristics desired for the articulating interface between the 
meniscal bearing insert 32 and the femoral component 40, shown in FIG. 9. 
As another alternative, only one of surfaces 10' and 10" need be generated 
according to the present invention. The other opposed sliding bearing 
surface could have a plateau profile, Pp, that is substantially zero. In 
other words, one of the opposed bearing surfaces could be textured 
according to the present invention while the other is uniformly polished 
over its entire gross surface area. 
Referring now to FIGS. 10a and 10b, a second embodiment of the present 
invention is illustrated, comprising a tibial component 50 and a mobile, 
meniscal bearing insert 60, also known as a rotating platform. As in the 
description of the first embodiment of FIGS. 6-8, above, surface features 
and parameters that correspond to the general terms discussed above with 
regard to FIGS. 1a-5 are indicated by like primed reference symbols. 
Likewise, the preferred materials of which the bearing surfaces can be 
constructed are the same as those described with respect to that first 
embodiment. Tibial component 50 includes a tibial plate 52 and a distal 
keel 54 extending distally from plate 52 for receipt within the medullary 
canal at a resected proximal end of a tibia. Plate 52 includes a distal 
surface 56 for engagement with and affixation to the resected bony plateau 
at the proximal end of the tibia. Plate 52 also includes a proximal 
sliding bearing surface 10'". Keel 54 is hollow and opens proximally 
through plate 52. An inner wall 58 of hollow keel 54 is shaped as a 
truncated cone. Conical inner wall 58 has a conical sliding bearing 
surface 10.sup.iv. Bearing insert 60 includes a meniscal bearing 62 having 
a proximal articulation surface 64 for articulating against the distal 
femoral condyle articulation surfaces 42 of the femoral component 40, 
shown in FIG. 9. Affixed to the distal side of meniscal bearing 62 is a 
mobile bearing plate 66 having a distal sliding bearing surface 10.sup.v. 
Depending from mobile bearing plate 66 is a frusto-conical stem 68 having 
an outer conical sliding bearing surface 10.sup.vi. As implanted, stem 68 
of bearing insert 60 is received within the hollow of keel 54 of tibial 
component 50, such that bearing surface 10.sup.vi is received in rotary 
sliding engagement with bearing surface 10.sup.iv, while bearing surface 
10.sup.v is disposed in sliding engagement with bearing surface 10'". 
Referring now to FIGS. 11a-12b, a third embodiment of the present invention 
is illustrated, comprising a patellar base component 70 and a mobile 
patellar bearing insert 80. As in the description of the embodiments of 
FIGS. 6-8 and 10a and 10b, above, surface features and parameters that 
correspond to the general terms discussed above with regard to FIGS. 1a-5 
are indicated by like primed reference symbols. Likewise, the preferred 
materials of which the bearing surfaces can be constructed are the same as 
those described with respect to the first embodiment of FIGS. 6-8. 
Patellar component 70 includes a base plate 72 and anterior pegs 74 
extending anteriorly from plate 72 for receipt within corresponding holes 
drilled in a resected posterior surface of the patella. Plate 72 also 
includes a posterior sliding bearing surface 10.sup.vii and a peg 76 
extending posteriorly therefrom. Patellar bearing insert 80 includes a 
posterior articulation surface 84 for articulating against the anterior 
patellar articulation surface 43 of the femoral component 40, shown in 
FIG. 9. Affixed to the anterior side of patellar bearing insert 80 is a 
mobile bearing plate 86 having a distal sliding bearing surface 
10.sup.viii. Recessed through mobile bearing plate 86 is an undercut 
elongate slot 88 in which peg 76 is received in a snap-fit relationship. 
As implanted, bearing surface 10.sup.viii is disposed in sliding 
engagement upon bearing surface 10.sup.vii. 
Referring now to FIG. 13, a further embodiment of the present invention is 
illustrated, comprising a femoral hip stem component 90, a femoral head 
component 92, and an acetabular cup comprising a shell 94 and a bearing 
insert 96. As in the description of the other embodiments described above, 
surface features and parameters that correspond to the general terms 
discussed above with regard to FIGS. 1a-5 are indicated by like primed 
reference symbols. Likewise, the preferred materials of which the bearing 
surfaces can be constructed are the same as those described with respect 
to the first embodiment of FIGS. 6-8. Stem component 90 includes a male 
conical taper neck 98 for receipt within a female conical bore 100 within 
head 92. Shell component 94, shaped generally as a hemispherical cup, 
includes a cavity 102 in which bearing insert 96 is received in fixed 
engagement. Insert 96, preferably made of a ceramic material, includes a 
cavity 104 in which head 92 is received in articulating relationship. The 
spherical surface of head 92 includes a bearing surface 10.sup.ix. 
Likewise, the spherical inner surface of cavity 104 of bearing insert 96 
includes a bearing surface 10.sup.x. As implanted, bearing surface 
10.sup.ix engages and articulates against bearing surface 10.sup.x 
The present invention has been illustrated and described with particularity 
in terms of preferred embodiments. Nevertheless, it should be understood 
that no limitation of the scope of the invention is intended. The scope of 
the invention is defined by the claims appended hereto. It should also be 
understood that variations of the particular embodiments described herein 
incorporating the principles of the present invention will occur to those 
of ordinary skill in the art and yet be within the scope of the appended 
claims.