Abstract:
A joint prosthesis ( 10 ) including a first component ( 12 )for cooperation with a first long bone ( 14 ) and a second component ( 16 ) for cooperation with a second long bone ( 20 ) is provided. The joint prosthesis ( 10 ) also includes a bearing component ( 222 ) positionable between the first component ( 12 ) and the second component ( 16 ) and cooperable with the first ( 12 ) and second ( 16 ) components. The bearing component ( 222 ) has a reinforcing component ( 236 ) having a first end ( 286 ) and a second end ( 294 ) and a polymeric material ( 207 ). The polymeric material ( 207 ) surrounds at least 99% of the surface area of the reinforcing component ( 236 ) and is molded to the reinforcing component ( 236 ) so that the material may be sterilized by a predominately surface sterilizing technology. The bearing component ( 222 ) defines a first peripheral region ( 271 ) and a second peripheral region ( 282 ). The first peripheral region ( 271 ) is adjacent to the first end (286) of the reinforcing component ( 236 ) and the second peripheral region ( 282 ) is adjacent the second end ( 244 ) of the reinforcing component ( 236 ). The first peripheral region ( 271 ) is cooperable with said first component ( 12 ) and the second peripheral region ( 282 ) is cooperable with the second component ( 16 ).

Description:
CROSS REFERENCE TO U.S. PROVISIONAL PATENT APPLICATION  
       [0001]    This application is a Utility Application based upon U.S. Provisional Patent Application, Serial No. 60/302,115 filed Jun. 30, 2001, entitled SURFACE STERILIZABLE JOINT REPLACEMENT PROSTHESIS COMPONENT WITH INSERT.  
       CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0002]    Cross reference is made to the following applications:  
         [0003]    DEP 677 titled “JOINT PROSTHESIS MOLDING METHOD AND DIE FOR PREFORMING THE SAME” and DEP 676 titled “JOINT REPLACEMENT PROSTHESIS COMPONENT WITH NON LINEAR INSERT” filed concurrently herewith which are incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0004]    The present invention relates generally to the field of orthopaedics, and more particularly, to an implant for use in joint arthroplasty.  
         BACKGROUND OF THE INVENTION  
         [0005]    The invention relates to joint prostheses. More particularly, the invention is directed to tibial components of knee joint prostheses that can be configured to be either rotatable or non-rotatable.  
           [0006]    Joint replacement surgery is quite common and it enables many individuals to function normally when otherwise it would not be possible to do so. Artificial joints usually comprise metallic, ceramic and/or plastic components that are fixed to existing bone.  
           [0007]    Knee arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural knee joint is replaced with a prosthetic knee joint. A typical knee prostheses 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.  
           [0008]    The tibial plateau 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 plateau. The geometry and structure of the tibial bearing insert varies depending upon the needs and joint condition of a patient. Some tibial bearing inserts are designed to be used with joint prostheses that are implanted during procedures that retain one or both of the cruciate ligaments. Others are implanted after removal of one or both of the cruciate ligaments, and are thus structured to compensate for the loss of these ligaments. Yet other tibial bearing inserts are used with prostheses that provide enhanced stabilization to the knee joint.  
           [0009]    Recent total knee prostheses have been designed which allow for increased freedom of rotation between the femur and the tibia. To allow for this rotational motion, tibial bearing inserts have been designed which allow for rotation of the insert on the tibial tray or plateau. Typically the tibia bearing inserts have a central stem which rotationally engages centrally in the tibial stem of the tibial tray implant, thereby providing for the rotational motion. Typically, there are no rotational constraints between the tibial tray implant and the tibial bearing insert. Frequently, during total knee arthroplasty, the posterior cruciate ligaments are sacrificed and a substitute for the posterior cruciate ligaments is required. Orthopaedic implants for total knee arthroplasty have been developed which provide for the substitution of the posterior cruciate ligament. Examples of such implants include the PFC Sigma RP as described in U.S. Pat. No. 4,298,992 incorporated herein by reference, and the LCS Complete total knee prosthesis, both of which are sold by DePuy Orthopaedics, Inc., Warsaw, Ind.  
           [0010]    These total knee prostheses are designed with tibial components and femoral components which have in conjunction with their articulating surface, a spine and cam mechanism, which is used as a posterior cruciate substituting feature when the posterior cruciate of the knee is sacrificed.  
           [0011]    Such total knee replacement prostheses, which include a spine and cam mechanism, typically contain tibial bearing components manufactured from suitable plastic, usually UHMWPE. One such construction use for a class of total knee replacement prosthesis, which are known as constrained prosthesis, often incorporate metal reinforcement rods in the construction of the plastic bearing component. The bearing insert is constructed so that the metal rod lies within the bearing, and thus provides additional support for the central spine element of the bearing. Such components are typically manufactured by machining or molding the bearing component, drilling a central hole, and press fitting the reinforcing metal rod. An example of such a component is described in U.S. Pat. No. 5,007,933 to Sidebotham et al. hereby incorporated in its entirety by reference.  
           [0012]    In order to allow for desired kinematics of the knee during a full range of motion, the spine and cam mechanism on the tibial bearing insert may be placed in a suitable position, preferably anterior to the center line of the insert in the anterior/posterior direction. Designs of tibial inserts are available to help reconstruct knees where the stabilizing soft tissue compromises have been made or occurred due to various reasons. In such cases, the tibial bearing inserts are required to experience greater loads in the anterior/posterior and the medial/lateral directions. The constrained inserts may be reinforced with a metal rod, as mentioned earlier, to help distribute the loads experienced by the spine of the polyethylene tibial bearing.  
           [0013]    Total knee joint prostheses have been designed with the spine and cam mechanism on the tibial bearing insert placed in a position that the central axis of the distal stem portion of the insert that engages the tibial tray, and the axis of the superior spine portion that engages the cam of the femoral component, are not necessarily collinear.  
           [0014]    Unfortunately, this design does not allow for a straight rod, commonly employed for reinforcement of tibial bearing inserts, to be used.  
           [0015]    It should be appreciated that a first rod could be inserted inside the spine, and a second rod could be inserted in the stem of the tibial tray portion of the bearing insert. However, the load on the first rod would be transferred through the polymer portion of the insert to the second rod. The polymer strength would then limit the load carrying capacity of this configuration. Such a configuration may not provide the required strength to sufficiently support and reinforce the spine.  
           [0016]    The present invention is directed to providing a tibial bearing insert with sufficient strength at the spine to withstand the loads of the knee prosthesis in the anterior/posterior and medial/lateral direction, while preserving bearing wear resistance when the central axis of the distal stem of the insert and the axis of the superior spine are not necessarily co-linear.  
         SUMMARY OF THE INVENTION  
         [0017]    The present invention is directed to an improved joint prosthesis for total knee replacement which includes a spine and cam mechanism. The cam mechanism being on the femoral component and the spine being on the bearing component. The mechanism is capable of withstanding the greater loads experienced in the anterior/posterior and medial/lateral direction caused by the substitution of the cam and spine for the posterior cruciate ligament which may be sacrificed during total knee arthroplasty while preserving bearing wear resistance.  
           [0018]    The spine on the tibial bearing insert, according to the present invention, is placed anterior to the centerline of the insert in the anterior/posterior direction. Therefore, the distal stem portion of the insert which engages the tibial tray and the superior spine portion which engages the cam of the femoral component are not in the same plane. The tibial bearing insert of the present invention thus includes a rod placed internal to the tibial bearing insert which includes an offset feature.  
           [0019]    The knee prosthesis of the present invention thus includes a first polymeric component and a reinforcing component including a first portion on a first center line and a second portion on a second center line such that the first portion may engage the tibial tray and the second portion may be cooperating with the cam mechanism in the femoral component of the knee prosthesis.  
           [0020]    According to one embodiment of the present invention, there is provided a joint prosthesis including a first component for cooperation with a first long bone and a second component for cooperation with a second long bone. The joint prosthesis also includes a bearing component positionable between the first component and the second component and cooperable with the first and second components. The bearing component has a reinforcing component having a first end and a second end and a polymeric material. The polymeric material surrounds at least 99% of the surface area of the first component and is molded to the first component so that the material may be sterilized by a predominately surface sterilizing technology. The bearing component defines a first peripheral region and a second peripheral region. The first peripheral region is adjacent to the first end of the reinforcing component and the second peripheral region is adjacent the second end of the reinforcing component. The first peripheral region is cooperable with said first component and the second peripheral region is cooperable with the second component.  
           [0021]    According to another embodiment of the present invention, there is provided a knee prosthesis including a femoral component for attachment to a femur and a tibial tray for attachment to a tibia. The knee prosthesis also includes a bearing component positionable between the femoral component and the tibial tray for cooperation with the femoral component and the tibial tray. The bearing component includes a reinforcing component with a first end and a second end. The bearing component also includes a polymeric material. The polymeric material surrounds at least 99% of the surface area of the reinforcing component and is and molded to the reinforcing component, so that the material may be sterilized by a predominately surface sterilizing technology. The bearing component defines a first peripheral region and a second peripheral region. The first peripheral region is adjacent to the first end of the reinforcing component and the second peripheral region is adjacent to the second end of the reinforcing component. The first peripheral region is cooperable with the femoral component and the second peripheral region is cooperable with the tibial tray.  
           [0022]    According to yet another embodiment of the present invention, there is provided a bearing component. The bearing component is for use in knee joint arthroplasty. The bearing component is positionable between a femoral component and a tibial tray for cooperation with the femoral component and said tibial tray, the bearing component including a reinforcing component having a first end and a second end. The bearing component also includes a polymeric material surrounding at least 99% of the surface area of the reinforcing component, so that the material may be sterilized by a predominately surface sterilizing technology. The bearing component defines a first peripheral region and a second peripheral region. The first peripheral region is adjacent the first end of the reinforcing component and the second peripheral region is adjacent the second end of the reinforcing component. The first peripheral region is cooperable with the femoral component and the second peripheral region is cooperable with the tibial tray.  
           [0023]    According to another embodiment of the present invention, there is provided a method of manufacturing a polymeric bearing component for use in joint arthroplasty and for cooperation with a first joint component and a second joint component. The method includes the steps of providing a reinforcing support having a first end and a second end thereof and providing a molding die adapted for manufacturing the bearing component for use in joint arthroplasty and having a first mold portion and a second mold portion. The first mold portion is adapted to provide a first surface to cooperate with the first joint component and the second mold portion and is adapted to provide a second surface to cooperate with the second joint component. The method also includes the steps of providing a positioning member for cooperation with the reinforcing support and molding die and positioning the support in a desired position within the molding die. One of the first end and at the second end is located in the first mold portion. The method further includes the steps of maintaining the position of the support with the positioning member in intimate contact with the support and adding moldable polymeric material into the molding die. The method also includes the steps of substantially surrounding the support with the moldable material, heating and pressurizing the mold, and permitting the moldable material to cool to form the bearing component and during the cooling and forming of the moldable material, removing the positioning member from the support while maintaining a lower amount of one of heat and pressure on the mold and allowing the polymeric material to replace the space occupied by the positioning member The method also includes the step of removing the component from the molding die.  
           [0024]    If a total knee prosthesis requires removal from the patient and replacement with a new prosthesis, such replacement prosthesis typically engages further into the medullary canals of the femur and tibia. Such prostheses are called revision prosthesis. During the prosthesis replacement, cruciate ligaments are much more often sacrificed than in an initial or primary total knee arthroplasty. Currently, no revision tibial bearing inserts with rotational features include a spine which centerline is not aligned with the center of the distal stem portion of the insert which rotationally engages the tibial tray.  
           [0025]    Attempts have been made to reinforce polyethylene bearings. One such attempt is that as shown in U.S. Pat. No. 5,989,472 Ashby et al, incorporated herein by reference. The polyethylene bearing in Ashby includes a reinforcement feature for bone attachment. The reinforcement feature is to assist in eliminating motion between the polyethylene and the metal backing.  
           [0026]    Another attempt at reinforcing a polyethylene bearing is described in U.S. Pat. No. 4,997,445 to Hodoreck incorporated herein by reference. This patent describes a metal backed prosthesis implant with enhanced bonding of polyethylene to the metal base.  
           [0027]    Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in connection with the accompanying drawings, in which:  
         [0029]    [0029]FIG. 1 is a perspective view of the knee system including the bearing component of the present invention showing the femoral component and the tibial tray component with the tibial bearing showing the knee system in extension;  
         [0030]    [0030]FIG. 2 is an elevation view from the anterior of FIG. 1;  
         [0031]    [0031]FIG. 3 is a side view of the assembly shown in FIGS. 1 and 2;  
         [0032]    [0032]FIG. 4 is an exploded side view showing the plastic bearing component partially removed from the tibial tray or plateau;  
         [0033]    [0033]FIG. 5 is an elevation view from the posterior of FIG. 1;  
         [0034]    [0034]FIG. 6 is an exploded elevation view from the anterior showing the plastic bearing component partially removed from the tibial tray or plateau;  
         [0035]    [0035]FIG. 7 is an exploded perspective view showing the plastic bearing component partially removed from the tibial tray or plateau;  
         [0036]    [0036]FIG. 8 is a fully exploded side view showing the plastic bearing component removed from the tibial;  
         [0037]    [0037]FIG. 9 is a fully exploded elevation view from the anterior showing the plastic bearing component removed from the tibial;  
         [0038]    [0038]FIG. 10 is a plan view of a reinforcing rod for use with the bearing component for an embodiment of the prosthesis of the present invention;  
         [0039]    [0039]FIG. 10A is a view of the reinforcing rod of FIG. 10 along the line  10 A- 10 A in the direction of the arrows;  
         [0040]    [0040]FIG. 11 is a plan view of the reinforcing rod of FIG. 10 located in a molding die for use in manufacturing the bearing component for the prosthesis of the present invention;  
         [0041]    [0041]FIG. 12 is a plan view of the reinforcing rod of FIG. 10 located in a molding die shown partially in cross section for use in manufacturing the bearing component for the prosthesis of the present invention showing the molding die in greater detail;  
         [0042]    [0042]FIG. 13 is a bottom view of the molding die of FIG. 12;  
         [0043]    [0043]FIG. 14 is a plan view of the bearing component made from the reinforcing rod of FIG. 10 utilizing the molding die of FIG. 12;  
         [0044]    [0044]FIG. 15 is a plan view of a reinforcing rod for use with the bearing component for another embodiment of the prosthesis of the present invention;  
         [0045]    [0045]FIG. 15A is a view of the reinforcing rod of FIG. 10 along the line  15 A- 15 A in the direction of the arrows;  
         [0046]    [0046]FIG. 16 is a plan view of the reinforcing rod of FIG. 15 located in a molding die for use in manufacturing the bearing component for the prosthesis of the present invention;  
         [0047]    [0047]FIG. 17 is a plan view of the reinforcing rod of FIG. 15 located in a molding die shown partially in cross section for use in manufacturing the bearing component for the prosthesis of the present invention showing the molding die in greater detail;  
         [0048]    [0048]FIG. 18 is a bottom view of the molding die of FIG. 16;  
         [0049]    [0049]FIG. 19 is a plan view of the bearing component made from the reinforcing rod of FIG. 15 utilizing the molding die of FIG. 16;  
         [0050]    [0050]FIG. 20 is a process flow chart for a method of manufacturing the prosthesis component of FIG. 21;  
         [0051]    [0051]FIG. 21 is a side view of the assembly shown in FIGS. 1 and 2 showing the assembly in flexion;  
         [0052]    [0052]FIG. 22 is a perspective view of the knee system of FIG. 1 including the bearing component of the present invention showing the femoral component and the tibial component with the tibial bearing showing the knee system in flexion;  
         [0053]    [0053]FIG. 23 is an elevation view from the anterior side of the assembly shown in FIGS. 1 and 2 showing the assembly in flexion; and  
         [0054]    [0054]FIG. 24 is an elevation view from the posterior side of the assembly shown in FIGS. 1 and 2 showing the assembly in flexion. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0055]    Embodiments of the present invention and the advantages thereof are best understood by referring to the following descriptions and drawings, wherein like numerals are used for like and corresponding parts of the drawings.  
         [0056]    According to the present invention and referring now to FIG. 8, a joint prosthesis in the form of knee prosthesis  10  as shown. The knee prosthesis  10  includes a femoral component or first joint component  12  for attachment to femur or first long bone  14 . The prosthesis  10  further includes a tibial tray or second joint component  16  for attachment to tibia or second long bone  20 . The femoral component  12  and the tibial component  16  are shown in greater detail in FIGS.  1 - 9  and  21 - 24 . The femoral component  12  and the tibial component  16  are made of any suitable durable material which are biologically compatible with the human anatomy. The femoral component  12  and the tibial component  16  may, for example, be made of a metal alloy, for example, cobalt-chromium-molybdenum, a titanium and its alloys, or be made of stainless steel.  
         [0057]    The knee prosthesis  10  further includes a bearing component  222 . The bearing component  222  is positionable between the femoral component  12  and the tibial tray  16 . The bearing component  222  cooperates with the femoral component  12  and the tibial tray  16  to provide for the kinematics of the knee prosthesis.  
         [0058]    The prosthesis, as shown in FIGS.  1 - 9  and  21 - 24 , are commonly referred to as a mobile bearing prosthesis or a mobile bearing knee. Such mobile bearing knees have been provided by DePuy Orthopaedics, Inc. under the trade name LCS since about 1977. Mobile bearing knees of this type are different than fixed bearing knees in that the tibial component  20  and the bearing component  222  may be physically separated from each other. The bearing component is also allowed to have rotational freedom about the tibial tray component. The use of mobile bearing knees may require that the patient have satisfactory cruciate collateral ligaments and tendons necessary to maintain the proper relationship of the femoral component to the bearing component. In those cases where the cruciate ligaments are either severely damaged or have been sacrificed or removed during a knee surgery, provisions must be made within the prosthesis to constrain the femoral component with respect to the tibial tray.  
         [0059]    Referring now to FIGS. 21 and 22, one solution to restraining the femoral component  12  with respect to the tibial tray  16  is by the use of a mechanism in the form of a spine  24  located on the bearing component  222  which mates with cam  26  located on femoral component  12 . As shown in FIGS. 21 and 22, to provide medial/lateral support for the knee prosthesis  10  preferably the femoral component  12  includes femoral face  30  which cooperate with spine faces  32  on the spine  24 . The spine faces  32  define a spine width SW which is related to the femoral width CW defined by femoral faces  32 . The relation behind SW &amp; CW define the level of constraint in the prosthesis in the medial-lateral direction.  
         [0060]    Referring now to FIG. 8, to provide anterior support the spine  24  includes a cam cooperating face  34  with which the spine cooperating face  35  of the cam  26  cooperates (see FIG. 21). It should be appreciated that for patients in which the posterior cruciate is severely damaged or missing the forces on the spine  24  both anterior/posterior and medial/lateral can be quite severe.  
         [0061]    Preferably, and as shown in FIG. 8, the bearing component  222  is made of a polymeric material, for example, polyethylene. Preferably, the bearing component  222  is made of UHMWPE. The bearing component  222  may be further processed to improve the wear properties of contact surface  40  of the bearing component. The contact surface  40  is the surface that is in contact with the laterally spaced condylar outer periphery  42  of the femoral component  12 . Methods of improving the wear properties of UHMWPE include a process known as Gamma Vacuum Foil (GVF) as disclosed in U.S. Pat. No. 5,577,368 to Hamilton, et al, and a process known as the Marathon® process as disclosed in U.S. Pat. No. 6,017,975 and U.S. Pat. No. 6,242,507 to Saum et al and in U.S. Pat. No. 6,228,900 to McKellop et al. These patents are incorporated herein by reference.  
         [0062]    Referring again to FIG. 8 and according to the present invention, the bearing component  222  of the prosthesis  10  includes a first component or reinforcing component  236 . The reinforcing component  236  serves to strengthen the bearing component  222  so that the spine  24  may withstand the forces that are present in the spine of the knee prosthesis  10  when the posterior cruciate and collateral ligaments cannot support the knee properly.  
         [0063]    Since the bearing component  222  is preferably made of a polymer and since the reinforcing component  236  is to strengthen the bearing component  222 , the reinforcing component  236  is preferably made of a higher strength material than polymer, preferably a material with a higher modulus of elasticity. For example, the reinforcing component  236  may be made of a metal that is a material compatible with the human anatomy, for example, stainless steel, a titanium and its alloys or a cobalt-chromium-molybdenum alloy.  
         [0064]    Applicants have found that desired kinematics of the knee during a full range of motion may require that an optimum design of the components that comprise a knee prosthesis, for example, those of FIG. 8, may include a tibial tray  16  having a central pivot axis  44  which is not coincident with center line  46  of the spine  24  of the bearing component  222 . Since the prosthesis  10  including the bearing component  222  will be implanted into the human body, it is essential that the prosthesis  10  including the bearing component  222 , be sterilized. Several effective methods of sterilization are possible for the prosthesis  10  including the bearing component  222 .  
         [0065]    For example, the bearing component  222  may alternatively be sterilized by subjecting the bearing component  222  to gamma irradiation. The subjection of the bearing component  222  to gamma irradiation may lead to the presence of free radicals within the polymer or polyethylene with which the bearing component  222  is typically manufactured. The presence of free radicals within the bearing component  222  may lead to early degradation of the bearing component  222  through an oxidation process.  
         [0066]    To minimize the negative effect of the free radicals generated from gamma sterilization, the bearing component  222  preferably is barrier packaged in vacuum or inert gas to keep the oxygen out and also to trap hydrogen gas inside the package. Such treatment precludes early oxidation of the bearing material and sufficient sterilization for the bearing component  222 .  
         [0067]    According to the present invention, a preferred method of sterilization is gas plasma sterilization. Gas plasma sterilization is predominantly a surface sterilizing technology. Gas plasma sterilization has limited ability to sterilize internal surfaces which have limited exposure to the outer surfaces of the component.  
         [0068]    Therefore, and according to the present invention, there is the need for a bearing component designed to be amenable to gas plasma sterilization and yet have the reinforced spine necessary for use of a constrained mobile bearing knee prosthesis for use with patients having compromised or sacrificed cruciate ligaments.  
         [0069]    According to the present invention and now referring to FIGS. 15 through 19, an embodiment of the present invention is shown as bearing component  222 .  
         [0070]    Referring now to FIG. 19, the bearing component  222  of the present invention is shown in greater detail. The bearing component  222  is a component that may be molded as a net shaped molding including a reinforcing component or reinforcing rod  236  to provide sufficient strength for the spine  224  and the distal stem. The reinforcing rod includes a first end  286  and an opposed second end  294 . The bearing component  222  is designed to not include bearing component openings in the polyethylene portion of the bearing component to expose the reinforcing rod to atmosphere. The technology that permits this configuration will be described in greater detail herein.  
         [0071]    By providing the bearing component  222  with no external exposure to the reinforcing rod, the bearing component  222  may be gas plasma sterilized. By gas plasma sterilizing the bearing component  222 , the bearing component  222  may be sterilized without providing free radicals which could lead to oxidative degradation of the bearing material.  
         [0072]    Referring now to FIG. 19 and according to the present invention, the bearing component  222  of the prosthesis  10  includes the reinforcing component  236  which is designed to accommodate the fact that centerline  44  of the central pivot stem of the tibial tray  16  (see FIG. 8) and is offset from centerline  46  of the spine  24 .  
         [0073]    Thus, as shown in FIG. 19, the reinforcing component  236  is designed with a first centerline  250  which is not coincident with second centerline  252 . As shown in FIGS. 8 and 10, the first centerline  250  of the reinforcing component  236  is coincident with central pivot stem centerline  44  of tibial tray  16 . Similarly the second centerline  252  of the reinforcing component  236  is coincident with the centerline  46  of the spine  24 .  
         [0074]    Continuing to refer to FIG. 19, the reinforcing component  236  includes a first portion  254  which defines the first centerline  250  thereof. The reinforcing component  236  further includes a second portion  256  thereof which-defines-the second centerline  252  thereof. The first centerline  250  and the second centerline  252  are non-coincidental.  
         [0075]    As shown in FIG. 19, the first centerline  250  may be parallel and spaced from the second centerline  252 . It should be appreciated, however, that the first centerline  250  and the second centerline  252  may, in fact, be skewed or converging or diverging. As shown in FIG. 19, however, the first centerline  250  and the second centerline  252  are separated and offset a distance COO which is similar to the offset SOO between the centerline of  46  of spine  24  and the centerline  44  of the tibial tray  16  (see FIG. 8).  
         [0076]    As shown in FIG. 19, the reinforcing component  236  includes a connecting portion  260  positioned between first portion  254  and second portion  256 . The connecting portion  260  may have any suitable shape but preferably for strength and simplicity the connecting portion  260  is an arcuate portion. In such a configuration, the shape of the connecting portion  260  is defined by a pair of radii, RR 1  and RR 2  which may, for example, be similar.  
         [0077]    While it should be appreciated that the reinforcing component  236  may have any suitable shape capable of providing for support with a pair of offset centerlines, it should be appreciated that for simplicity, and as shown in FIG. 15A, the reinforcing component  236  may have a uniform cross section. For example, the cross section of the reinforcing component may be square, triangular, hexagonal or as shown in FIG. 15A, may be circular. A circular cross section may provide for optimum bending strength in a variety of directions for a given weight or size of the reinforcing component  236 .  
         [0078]    The reinforcing component  236  may be hollow or as shown in FIG. 18, may be made of a generally solid material. Due to space constraints, the reinforcing component  236  may be solid as shown in FIG. 18.  
         [0079]    As can be readably apparent by FIGS. 15 and 19, in particular, the bearing component  222  including the reinforcing component  236  may be made by a number of methods but cannot simply and easily be made by first making the bearing component  222  and then preparing an opening or conduit for installing the reinforcing component  236  therein. Therefore, typical methods of providing a reinforcing rod to a bearing component  222  in the form of drilling a hole in the bearing component  222  and inserting a straight cylindrical rod therein is not possible.  
         [0080]    Referring now to FIG. 19, the reinforcing component or reinforcing rod  236  is shown in greater detail. The bearing component  222  includes the reinforcing rod  236  which is placed into a mold and the polymeric material is molded around the reinforcing rod  236 . Thus, the bearing component  222  requires that the mold provide provisions for the proper placement of the reinforcing rod  236  within the molding die. Therefore, and as shown in FIG. 19, the reinforcing rod  236  includes an orientation and location feature  202  which provides both orientation and location. The location and orientation feature  202 , as shown in FIG.  15 , include a first recess or through hole  204  and a second recess or through hole  206 .  
         [0081]    Preferably, the first recess  204  and the second recess  206  are small. The first recess and second recess  204  and  206  in the reinforcing rod  236  are preferably both located on the same portion of the rod. By placing the recesses on the same portion, for example second portion  256 , the recesses may be both positioned in the base or bottom mold  266  of the die  262  (see FIG. 17) to assist in the proper operation of the invention. The value of having the recesses on the same end of the rod will be described in greater detail herein.  
         [0082]    Referring now to FIG. 17, a molding die  262  is shown for molding the bearing component  222 . Molding die  262  is utilized in the direct compression molding process. The bearing component  222  is molded in the molding die  262  in reverse or upside down order to provide for the positioning of the recesses  204  and  206  in the base or bottom mold  266 .  
         [0083]    The advantage of positioning the location and orientation features  202  in the base or bottom mold  266  will be described in greater detail later.  
         [0084]    As shown in FIG. 17, the molding die  262  includes base or bottom mold  266 . The bottom mold  266  is utilized to form bottom bearing surface  280  and rotating shaft or second peripheral region  282  of the bearing component  222 . Extending upwardly from the bottom mold  266  is the body or side mold  272 . The side mold  272  is utilized to form curved profile  274  of the bearing component  222 . Slidably positioned within the side mold  272  is plunger or top mold  270 . The plunger or top mold  270  is utilized to form articular surface or first peripheral region  271  of the bearing component  222 . The molds  270 ,  272  and  266  serve to provide an inner forming surface  264  which conforms to the outer periphery of the bearing component  222  with provisions for accommodating the shrinkage dimensions that are well known in the art.  
         [0085]    The inner forming surface  264  defines an internal cavity  208 .  
         [0086]    The reinforcing rod  236  needs to be properly positioned within the cavity  208  of the molding die  262 . Preferably, thus, the molding die  262  includes a positioner  284  for proper repositioning of the reinforcing rod  236  within the cavity  208  of the molding die  262 . For example and as shown in FIG. 17, the positioner  284  is in the form of a first pin  290  and a second pin  292 . The pins  290  and  292  cooperate with first recess  204  and second recess  206  of the reinforcing rod  236  (see FIG. 19).  
         [0087]    Preferably, and according to the present invention, the pins  290  and  292  have a very small dimension with respect to the reinforcing rod  236 . For example, if, as shown in FIG. 17, the pins  290  and  292  are cylindrical, the pins  290  and  292  may have a diameter D which is much smaller than diameter DD of the second portion  256  of the reinforcing rod  236 . For example for a reinforcing rod  236  having a diameter DD of, for example, approximately 10 millimeters. The corresponding diameter D of the pins  290  and  292  may be, for example, 0.5 to 2.0 millimeters.  
         [0088]    It is preferred to have the pins  290  and  292  made of materials that have a high melting point in order to resist the heat and pressure experienced in the mold during the molding process. Pins may be made of metals, ceramics or pyrolytic carbons. The molding process for the molding die  262  to mold the bearing component  222  as shown in FIG. 17 includes first separating the top mold  270  from the bottom mold  266  and adding powder  207  similar to powder  112  of the process as described for the molding die  62  of FIG. 12. After the required powder  207  is added, the top mold  270  is placed within the side mold  272  and lowered in the direction of the bottom mold  266  until the molds  266 ,  270  and  272  forming surface  264  correspond to the periphery of the bearing component  222 .  
         [0089]    Towards the end of the compression molding cycle when the UHMWPE material has almost assumed full density and completely fills the mold the pins  290  and  292  are withdrawn from the cavity preferably in a direction normal to the centerlines  250  and  252  of the reinforcing rod  236 . For example, as shown in FIG. 17, the first pin moves from a position as shown in solid to the position shown in phantom. As the first pin  290  and second pin  292  are retracted to the position in phantom, a small pin cavity  238  is left behind where the pin  290  was withdrawn from. Since the compression cycle has not ended, the melted polymer still under pressure quickly fills the pin cavity  238  thereby eliminating the pin cavity  238 .  
         [0090]    Since the powder  207  within the mold cavity  208  has obtained a high viscosity at the point in the compression molding cycle when the UHMWPE material has assumed full density and completely fills the mold, the reinforcing rod  236  remains in its previous position even after the pins  290  and  292  have been fully retracted and no longer support the rod  236 .  
         [0091]    Preferably, and as shown in FIG. 17, the pins  290  and  292  are preferably spaced apart along second centerline  250  a distance P of, for example, twice the distance DD of the diameter of the rod  236 . The larger the dimension P, the greater the stability and accuracy of the positioning of the rod  236  within the molding die  262 .  
         [0092]    Preferably, and as shown in FIG. 17, the pins  290  and  292  are positioned perpendicularly to the second centerline  250  and preferably at an angle with respect to each other, preferably at 90 degrees or perpendicular to each other. Such positioning optimizes the effectiveness of the pins  290  and  292  to properly position the reinforcing rod  236  in more than 3 degrees of freedom. After appropriate cooling, the plunger or top mold  270  is opened and the completed bearing component  222  is removed from the molding die  262 .  
         [0093]    It should be appreciated that other approaches may be taken to position the reinforcing rod  236  within the molding die  262  and yet provide for a complete encapsulation of the reinforcing rod with the polyethylene. For example, the pins  290  and  292  may be made of a polyethylene identical to that of the powder  207 . The pins  290  and  292  may then be left fully extended and not retracted. The pins  290  and  292  then would melt and form with the powder  207 , and yet have sufficient strength early on in the forming process to properly locate the rod  236  within the molding die  262  until the polyethylene becomes sufficiently viscous to support the rod.  
         [0094]    Other approaches for properly supporting the rod yet allowing for complete encapsulation of polyethylene around the rod  236  may fall within the scope of the present invention.  
         [0095]    According to the present invention and now referring to FIGS. 10 through 14, another embodiment of the present invention is shown as bearing component  22 .  
         [0096]    Referring to FIG. 8 it should be appreciated that the bearing component  22  of FIG. 10 may be substituted for the bearing component  222  for the prosthesis  10 . The bearing component  22  is made of similar materials and has similar strength and load carrying capacity of bearing component  222  as well as similar contour dimensions such that bearing component  22  can readily replace bearing component  222  in the prosthesis  10 .  
         [0097]    Referring now to FIG. 10 an alternate embodiment of the bearing component of the present invention is shown as the bearing component  22  which may alternatively be used in prosthesis  10 . Bearing component  22  includes the reinforcing component  36  which is designed to accommodate the fact that centerline  44  of the central pivot stem of the tibial tray  16  is offset from centerline  46  of the spine  24  (see FIG. 8). Thus as shown in FIG. 10, the reinforcing component  36  is designed with a first centerline  50  which is not coincident with second centerline  52 . As shown in FIGS. 8 and 10, the first centerline  50  of the reinforcing component  36  is coincident with central pivot stem centerline  44  of tibial tray  16 . Similarly the second centerline  52  of the reinforcing component  36  is coincident with the centerline  46  of the spine  24 .  
         [0098]    Continuing to refer to FIG. 10, the reinforcing component  36  includes a first portion  54  which defines the first centerline  50  thereof. The reinforcing component  36  further includes a second portion  56  thereof which defines the second centerline  52  thereof. The first centerline  50  and the second centerline  52  are non-coincidental.  
         [0099]    As shown in FIG. 10, the first centerline  50  may be parallel and spaced from the second centerline  52 . It should be appreciated, however, that the first centerline  50  and the second centerline  52  may, in fact, be skewed or converging or diverging. As shown in FIG. 10, however, the first centerline  50  and the second centerline  52  are separated and offset a distance CO which is similar to the offset SO between the centerline of  46  of spine  24  and the centerline  44  of the tibial tray  16  (see FIG. 8).  
         [0100]    As shown in FIG. 10, the reinforcing component  36  includes a connecting portion  60  positioned between first portion  54  and second portion  56 . The connecting portion  60  may have any suitable shape but preferably for strength and simplicity the connecting portion  60  is an arcuate portion. In such a configuration, the shape of the connecting portion  60  is defined by a pair of radii, R 1  and R 2  which may, for example, be similar.  
         [0101]    While it should be appreciated that the reinforcing component  36  may have any suitable shape capable of providing for support with a pair of offset centerlines, it should be appreciated that for simplicity, and as shown in FIG. 10A, the reinforcing component  36  may have a uniform cross section. For example, the cross section of the reinforcing component may be square, triangular, hexagonal or as shown in FIG. 10A may be circular. A circular cross section may provide for optimum bending strength in a variety of directions for a given weight or size of the reinforcing component  36 .  
         [0102]    The reinforcing component  36  may be hollow, or as shown in FIG. 10A may be made of a generally solid material. Due to space constraints the reinforcing component  36  may be solid as shown in FIG. 10A.  
         [0103]    As can be readably apparent by the FIGS. 8 and 10, in particular, the bearing component  22 , including the reinforcing component  36 , may be made by a number of methods but cannot simply and easily be made by first making the bearing component  22  and then preparing an opening or conduit for installing the reinforcing component  36  therein. Therefore, typical methods of providing a reinforcing rod to a bearing component  22  in the form of drilling a hole in the bearing component  22  and inserting a straight cylindrical rod therein is not possible.  
         [0104]    Therefore, referring to FIGS. 11, 12 and  13 , the bearing component  22  is preferably made by a molding process for example a compression molding process or any molding process by which the polymeric material may be processed.  
         [0105]    Referring to FIGS. 11, 12 and  13 , the bearing component  22  is preferably made in molding die  62 . While the bearing component  22  may be manufactured utilizing any suitable molding technique preferably and as shown in FIG. 12, the molding die  62  is for use with direct compression molding. Plastic powder is placed into the molding die  62 , the die is closed and pressure is applied to compress, heat, and cause flow of the plastic to be conformed to the cavity shape.  
         [0106]    The molding die  62  is made in a shape including an inner forming surface  64  which is made in the shape of the final finished bearing component  22 . Preferably, the inner forming surface  64  is sized to allow for appropriate shrinking dimensions as is known in the art.  
         [0107]    The molding die is made in several pieces. Typically, a base or bottom mold  66  is utilized to form articular surface  70  of the bearing component  22 . The molding die  62  also includes a body or side mold  72 . The body  72  is utilized to form the curved lateral surfaces  74  of the bearing component  22 . Also the molding die  62  further includes a plunger assembly  76 . The plunger assembly  76  is utilized to form bottom bearing surface  80  and the rotating shaft  82 . One mold may be used to obtain varying thickness of the bearing component  22 .  
         [0108]    In order to manufacture the bearing component  22  according to the present invention, the molding die  62  is modified to support reinforcing component  36  in the form of, for example, a reinforcing rod.  
         [0109]    Preferably, and as shown in FIG. 12, reinforcing rod or component  36  is position spaced from the inner forming surface  64 . Preferably, and as shown in FIG. 12, the reinforcing rod  36  is kept spaced from the inner forming surface  64  by use of a support feature  84  as initially designed to provide the offset between the spine and distal stem of the bearing component  22 . The support feature  84  is utilized to space, support or position the reinforcing rod  36  within the molding die  62 . The positioner or support feature  84  may support or secure the reinforcing component  36  at any suitable position on the reinforcing component  36 . For simplicity, and as shown in FIG. 12, the positioner  84  may be located on first end  86  of the reinforcing rod  36 .  
         [0110]    The positioner  84  may include a sole positioning member which interacts with first end  86  of the reinforcing rod  36 . If the positioner is located only on one end and the rod is held at that one end, that portion of the die including the positioner either at the base or bottom mold  66  or the plunger or top mold  76  must provide rigid temporary attachment of the reinforcing rod  36  to the positioner  84 .  
         [0111]    While the present invention may be practiced utilizing a sole positioner located on one end of the reinforcing rod  36  such a configuration may have some problems in that the tolerance between the positioner and the reinforcing rod may be such that the accuracy of the position of the reinforcing rod  36  within the molding die  62  may not be sufficiently accurate resulting in the misposition of the reinforcing rod  36  within the finished reinforcing component  36 . Misposition may occur either in the anterior-posterior or medial-lateral direction. Additionally, the reinforcing pin  36  may be rotationally mispositioned with respect to the superior spine and distal stem.  
         [0112]    Preferably, and as shown in FIG. 12, the positioner  84  is in the form of a first positioner  90  located at the first end  86  of the reinforcing rod  36  and a second positioner  92  located at second end  94  of the reinforcing rod  36 . If the reinforcing rod  36  is held at both the first end  86  and the second end  94  of the rod  36 , then one end, for example, end  86  must be a rigid temporary attachment and the other end, for example, second end  94  or second positioner  92  must be a sliding temporary attachment. A sliding temporary attachment is necessary as the two ends of the molding die approach and separate from each other during each molding cycle. Additionally, the sliding temporary attachment may provide for rotational alignment to obtain the optimal position of the reinforcing component  36  in the spine by allowing equal polymeric material around the reinforcing component  36 .  
         [0113]    To improve the accuracy of the positioning of the reinforcing rod  36  within the molding die  62 , optionally, the molding die may include an orientation feature  100  to optimally angularly orient the reinforcing rod  36  with respect to the inner forming surface  64  and eventually the reinforcing component  36 . The orientation feature  100  may, for example, be included with the positioners  90  and  92  and may, as shown in FIG. 12, be in the form of flat  102  located on the second positioner  92 . As shown in FIG. 12, the orientation feature  100  is in the form of six equally spaced flats, three of which are shown. Therefore the positioner  84  and the orientation features are in the form of a hexagonal rod. An additional flat may help better fine tune the position of the reinforcing element with respect to the mold components.  
         [0114]    Referring again to FIG. 10, preferably, and as shown in FIG. 10, the reinforcing rod  36  includes positioning features in the form of, for example, first recess  104  which is located on first end  86  of the rod  36  and second recess  106  which is located on second end  94  of the rod  36 . The first recess  104  matingly receives the first positioner  90  while the second recess  106  receives the second positioner  92  (see FIG. 11). Preferably, and as shown in FIG. 10, the second recess  106  includes a recess flat  110  which mate with flat  102  on second positioner  92 .  
         [0115]    Referring now to FIG. 14, the bearing component  22  is shown having been molded on the molding die  62  (see FIG. 12). In order that the first positioner  90  and the second positioner  92  may be removed from the cavity  114  and from the bearing component  22  when it is removed from the cavity  114  of the molding die  62 , the bearing component  22  includes a first bearing component opening  120  located in line and above the first recess  104  of the reinforcing rod  36 . Likewise, the bearing component  22  further includes a second bearing component opening  122  extending outwardly from the second recess  106  of the reinforcing rod  36 . The first bearing component  120  and the second bearing component opening  122  provide for access to the reinforcing rod  36  from the outside of the bearing component  22 .  
         [0116]    Referring again to FIG. 12, plastic powder  112  is added in the proper amount into cavity  114  of the molding die  62 . The molding die  62  is closed by the positioning of the plunger assembly or top mold  76  over the body or side mold  72  of the molding die  62 .  
         [0117]    The bearing component  22  is fully formed by subjecting the molding die  62  to the well known conditions of pressure and temperature required to consolidate the powder  112 . After appropriate cooling, the molding die  62  is opened by the removal of the plunger assembly or top mold  76  from the body or side mold  72 . The bearing component  22  including the reinforcing rod  36  is then removed from the cavity  114  of the molding die  62 . After proper cleaning an additional reinforcement rod and additional powder  112  is added to the cavity  114  and the process is repeated in order to obtain a second bearing component.  
         [0118]    Referring now to FIG. 14, the bearing component  22  of the present invention includes first bearing component opening  120  and second bearing component opening  122  which expose the bearing component  22  to access the reinforcing rod  36 . The reinforcing rod thus has internal surfaces which have limited exposure or connection to the outside surfaces of the bearing component  22 .  
         [0119]    Therefore, because the reinforcing rod,  36  is exposed to the surface of the component via the holes  120  and  122  through which it was inserted or by the method of holding the post using the mold which holds the post during the molding process, the bearing component  22  is not amenable to sterilization by techniques which are predominantly surface sterilizing technology, for example, gas plasma sterilization.  
         [0120]    In order to utilize the bearing component  22  with gas plasma sterilization, steps can be taken to fill the holes  120  and  122  with polyethylene plugs or the positioners  90  and  92  can be made of polyethylene and not retracted once the bearing  22  is removed from the die  62  (see FIG. 12).  
         [0121]    Referring now to FIG. 20, a process for molding a bearing component with a reinforcing rod is described more fully. First step  120  of the process described in FIG. 20 is the step of providing a component of a durable material. The durable material may, for example, be in the form of cobalt chrome alloy, stainless steel or titanium and its alloys. The component may be in the form of, for example, an elongated member, for example, a rod. The rod as described in the present invention is in the form of a bent rod or a rod having two substantially linear portions with the portions being skewed or non-linear with respect to each other.  
         [0122]    Second step  122  of the process, as described in FIG. 20, is the step of providing a molding die adapted for manufacturing a component for use in total joint arthroplasty.  
         [0123]    Third step  124  in the process is the step of placing the reinforcing component into the molding die in the desired position. Fourth step  126  of the process is placing moldable material powder into the molding die. Fifth step  130  in the process for making a bearing component is the step of substantially surrounding the component with moldable material. Sixth step  131  of the process is the step of heating and pressurizing the mold, thus the moldable material. Seventh step  132  of the process is the step of permitting the moldable material to cool to form the component and the eighth step  134  of the process is the step of removing the component from the molding die.  
         [0124]    By utilizing the non-linear reinforcement component of the present invention, a knee may be provided with improved load carrying capacity in the anterior-posterior and medial-lateral directions for the spine and cam mechanism in situations in which the center line of the insert which engages the tibial tray and the superior spine portion which engage the cam of the femoral component are not in the same plane. In such situations where these planes are different, the kinematics of the knee may be improved.  
         [0125]    By providing a tibial bearing insert with an insert that has most of its entire periphery encapsulated in polyethylene, a tibial bearing insert can be made that has improved strength and can be gas plasma sterilized.  
         [0126]    By providing a non-linear re-inforcing component to the tibial bearing insert, the non-linear support rod may be properly positioned within the tibial bearing insert to optimize the load transfer mechanism through the spine.  
         [0127]    By providing a tibial bearing insert including a nonlinear support including an orientation feature, the support rod may be adjusted with respect to the tibial bearing insert during the manufacturing of the tibial bearing insert.  
         [0128]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.