Patent Publication Number: US-RE44476-E

Title: Constrained prosthetic knee with rotating bearing

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 09/771,061, filed Jan. 29, 2001, now U.S. Pat. No. 6,485,519. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to prosthetic joints, and, more particularly to a constrained prosthetic knee having a modular hinge post and a rotating bearing. 
     2. Description of the Related Art 
     Generally, the knee is formed by the pair of condyles at the distal portion of the femur, the lower surfaces of which bear upon the correspondingly shaped proximal surface plateau of the tibia. The femur and tibia are connected by means of ligaments such as, the posterior cruciate ligament, the lateral collateral ligament, the medial collateral ligament, and the anterior cruciate ligament. These ligaments provide stability to the joint formed by the femur and tibia (i.e., the knee). 
     In a broad sense, prosthetic knee joints can be considered either constrained or unconstrained. For the purposes of this discussion, constrained prosthetic knees include femoral and tibial prosthetic components which are mechanically linked or constrained to each other by a hinge structure. An unconstrained prosthetic knee includes femoral and tibial components which are not mechanically linked. An unconstrained knee utilizes the patient&#39;s existing ligaments to provide joint stability. With this in mind, constrained prosthetic knees have particular applicability to cases in which a patient has experienced ligament loss and/or the existing ligaments do not provide adequate support and stability to the knee. 
     Tibial components of a prosthetic knee can be formed as a one-piece configuration in which the tibial tray forms the meniscal component of the prosthetic knee. Various other prosthetic knees utilize a modular meniscal component separate from the tibial component. Devices utilizing modular meniscal components include those in which the meniscal component (i.e., tibial bearing surface) is fixed to the tibial tray portion of the tibial component and is incapable of movement relative thereto. Alternative devices utilize a modular meniscal component capable of movement relative to the tibial tray. Devices in which relative rotational movement occurs between the meniscal component and the tibial component are typically referred to as rotating bearing knees. Rotating bearing knees thus allow movement between the bearing (i.e., meniscal component) and the tibial tray, as well as movement between the femoral component and the tibial bearing. 
     Constrained knees of the prior art include constructions in which a hinge post extension is first positioned within a tibial component (with an end protruding therefrom) and is thereafter connected to the femoral component by positioning the hinge post (rotatably attached to the femoral component) over the top of the protruding end of the hinge post extension and thereafter connecting the hinge post extension to the hinge post, e.g., by threading the hinge post extension into the hinge post. After making this connection, the meniscal component is thereafter slid into position between the femoral component and the tibial component. Meniscal components utilized with these prior art prosthetic knees were fixed to the tibial component. 
     The present invention is directed to a constrained knee prosthesis with a rotating bearing. The knee prosthesis of the present invention is structured to facilitate implantation thereof. The present invention is further directed to a prosthetic knee implant set having a plurality of matched modular hinge post and meniscal component pairs. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved constrained knee prosthesis having a cannulated hinge post facilitating implantation of the knee prosthesis in a relatively minimally invasive procedure. The prosthetic knee implant set of the current invention includes a separately packaged femoral component, a separately packaged tibial component, and a third package containing a hinge post extension and the meniscal component. Packaging the individual components of a knee prosthesis in this fashion insures that the appropriate hinge post extension is readily available. A bearing box is interposed between the hinge post and the femoral component. The bearing box includes a hyperextension stop which cooperates with the hinge post to prevent hyperextension of the knee prosthesis. Various structures are utilized to prevent the disengagement of the constrained knee prosthesis of the present invention. 
     A prosthetic knee constructed in accordance with the present invention includes a femoral component having a pair of condyler surfaces and a hinge post rotatably connected to the femoral component between the condyler surfaces. The hinge post is cannulated and accommodates insertion of a hinge post extension shaft therein. The hinge post and hinge post extension include cooperating locking tapers for locking the hinge post extension to the hinge post. Additionally, the hinge post includes internal threads so that a set screw may be threaded therein to further hold the hinge post extension in place. The tibial component includes a hinge post extension aperture into which the hinge post is seated. The meniscal component similarly includes an aperture to accommodate the hinge post and hinge post extension. The meniscal component of the current invention is free to rotate about the hinge post during flexion and extension of the knee joint. 
     Having a cannulated hinge post through which a hinge post extension may be anteriorly positioned and secured advantageously allows for a relatively minimally invasive knee replacement procedure. 
     The present invention advantageously provides a constrained prosthetic knee having a rotating bearing flush with the condyler surfaces of the femoral component. 
     Another advantage of the present invention is the packaging of the prosthesis components and specifically the packaging of the appropriate hinge post extension together with a meniscal component. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining of them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of an assembled knee prosthesis in accordance with the present invention; 
         FIG. 2  is an exploded view thereof; 
         FIG. 3  is a cutaway, exploded view illustrating assembly of the knee prosthesis of the current invention including the anterior positioning of the hinge post extension into the hinge post; 
         FIG. 4  is a cutaway view illustrating securement of the hinge plug (i.e., set screw) in the hinge post to facilitate locking of the hinge post extension therein; 
         FIG. 5  is a cutaway, exploded view illustrating removal of the hinge post extension; 
         FIG. 6  is a bottom elevational view of the meniscal component of the present invention; 
         FIG. 7  is a front elevational view thereof; 
         FIG. 8  is a top elevational view of a tibial component in accordance with the present invention; 
         FIG. 9  is a sectional view of a hinge plug in accordance with the present invention; 
         FIG. 10  is a side elevational view of a bearing box in accordance with the present invention; 
         FIG. 11  is a front elevational view thereof; 
         FIG. 12  is a top elevational view thereof; 
         FIG. 13  is a cutaway, exploded view of an alternative embodiment of the knee prosthesis of the present invention; 
         FIG. 14  is a cutaway view of an assembled knee prosthesis in accordance with the embodiment illustrated in  FIG. 13 ; 
         FIG. 15  is a fragmentary, cutaway view of an alternative embodiment of the hinge post extension and tibial bushing of the present invention; 
         FIG. 16  is a fragmentary, cutaway view of the embodiment of  FIG. 15  illustrating insertion of the hinge post extension into the tibial bushing; 
         FIG. 17  is a fragmentary, cutaway view of the embodiment of  FIG. 15  illustrating the hinge post extension fully inserted into the tibial bushing; 
         FIG. 18  is an exploded view of an alternative embodiment of the knee prosthesis of the current invention; 
         FIG. 19  is a sectional view of a meniscal component in accordance with an alternative embodiment of the present invention; and 
         FIG. 20  is an elevational view of a hinge post in accordance with an alternative embodiment of the present invention. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the invention, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explain the invention. The exemplifications set out herein illustrate embodiments of the invention, in alternative forms, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION 
     Referring now to the drawings and particularly to  FIG. 2 , knee prosthesis  20  in accordance with the present invention is illustrated. Knee prosthesis  20  generally includes femoral component  22 , tibial component  24 , and meniscal component  26 . Hinge post  40  is rotatably connected to femoral component  22  and includes elongate hinge post extension aperture  112  ( FIGS. 3-6 ,  13 , and  14 ). Elongate aperture  112  accommodates placement of hinge post extension  42  therein. Hinge post extension  42  thereafter traverses hinge post aperture  114  in meniscal component  26  and hinge post extension aperture  110  ( FIGS. 3-6 ,  13  and  14 ) in tibial component  24 . Elongate hinge post extension aperture  112  of hinge post  40  advantageously allows for anterior placement of hinge post extension  42  during surgical implantation of knee prosthesis  20  of the present invention. 
     As illustrated in  FIG. 2 , hinge post extension  42  includes locking taper  46  and cylindrical extension  48 . Hinge post extension aperture  112  includes a mating locking taper to cooperate with locking taper  46  and lock hinge post extension  42  to hinge post  40 . After positioning of hinge post extension  42  through apertures  112 ,  114 , and  110 , hinge plug  38  may be threaded into hinge plug threads  54  in elongate aperture  112  of hinge post  40  ( FIG. 4 ). Hinge plug  38  abuts the end of hinge post extension  42  and thereby facilitates locking of morse taper  46  in elongate aperture  112 . In one exemplary embodiment, locking taper  46  comprises a two degree locking taper. When prosthetic knee  20  is assembled as illustrated in  FIG. 1 , condyler bearing surfaces  28 ,  30  abut bearing surfaces  86 ,  88  (see, e.g.,  FIG. 2 ) in meniscal component  26 . 
     Hinge post extension  42  is typically formed as a one-piece construction of an inert metal such as, e.g., a cobalt-chromium alloy. Hinge post extension  42  may, however, be constructed of other bio-compatible metals or alloys, such as titanium. Throughout this document reference will be made to various components formed of a cobalt-chromium alloy. Any such component may also be constructed of other bio-compatible metals or alloys such as titanium, as is well-known. As illustrated in  FIG. 4 , hinge plug wrench  102  is utilized to thread hinge plug  38  into hinge plug threads  54  of hinge post  40 . As illustrated in  FIG. 9 , hinge plug  38  includes locking material  100  to provide a locking connection between hinge plug  38  and hinge plug threads  54  in hinge post  40 . Hinge plug  38  is, in one exemplary embodiment formed of a cobalt-chromium alloy. Locking material  100  comprises any suitable biocompatible polymer such as, e.g., ultra-high molecular weight polyethylene (UHMWPE). 
     As illustrated, e.g., in  FIG. 2 , femoral component  22  includes condyler bearing surfaces  28 ,  30  with bearing box wall  76  positioned therebetween. Femoral component  22  further includes external side walls  82 , only one of which can be seen in  FIG. 2 . Condyler bearing surfaces  28 ,  30  are smooth and highly polished, generally spheroidally shaped and extend outwardly from external side walls  82 , as is well known in the industry. Femoral component  22  further includes modular femoral stem  32  for insertion into femur  116  ( FIGS. 3-5 ,  13 , and  14 ), as is known in the art. Femoral component  22  further includes internal side walls  80 , only one of which is illustrated in  FIG. 2 . Internal side walls  80  are substantially perpendicular to bearing box wall  76  and extend outwardly therefrom. Femoral component  22  is typically formed as a one-piece construction of an inert metal such as, e.g., a cobalt-chromium alloy. 
     Bearing box  70  is designed for placement between condyler bearing surfaces  28 ,  30  of femoral component  22  as illustrated, e.g., in  FIG. 1 . Bearing box  70  is further illustrated in  FIGS. 10-12  and includes affixing protrusions  72 , hinge pin aperture  62 , hyperextension stop  66 , and anti-rotation surface  78 . As illustrated in  FIG. 2 , femoral component  22  includes affixing protrusion apertures  74  sized to receive affixing protrusions  72 .  FIG. 1  illustrates bearing box  70  operably positioned on femoral component  22 , with anti-rotation surface  78  flush with bearing box wall  76  of femoral component  22 , and affixing protrusions  72  received in affixing protrusion apertures  74 . The abutting relationship of anti-rotation surface  78  with bearing box wall  76  discourages rotation of bearing box  70  about the longitudinal axis of affixing protrusions  72 . When bearing box  70  is positioned on femoral component  22 , hinge pin apertures  62  of bearing box  70  align with threaded hinge pin aperture  56  and hinge pin aperture  58  of femoral component  22 . Bearing box  70  can be formed of any suitable plastic, such as, e.g., UHMWPE. 
     Hinge post  40  is rotatably connected to femoral component  22  via hinge pin  34 . Hinge post  40  is placed between opposing walls of bearing box  70  and is positioned so that hinge pin aperture  52  is aligned with apertures  56 ,  58 , and  62 . The opposing walls of bearing box  70  thus act as a bearing surface between hinge post  40  and internal side walls  80  of femoral component  22 . Prior to placement of hinge post  40  between opposing walls of bearing box  70 , hinge pin sleeve  36  is operably positioned within hinge pin aperture  52  of hinge post  40 . Hinge post  40  is formed from a cobalt-chromium alloy, while hinge pin sleeve  36  is formed from a suitable plastic, such as, e.g., UHMWPE. Hinge pin sleeve  36  acts as a bearing between hinge pin aperture  52  of hinge post  40  and hinge pin  34 . Accordingly, hinge pin sleeve  36  includes hinge pin aperture  50  sized to accommodate hinge pin  34 . After positioning of hinge post  40  between the opposing walls of bearing box  70 , hinge pin  34  is positioned through apertures  56 ,  62 ,  50 , and  58 . Hinge pin threads  60  are thereafter threadedly engaged in the threads of threaded hinge pin aperture  56  until the head of hinge pin  34  is flush with external side wall  82 . 
     As illustrated in  FIG. 1 , hinge pin plug  120  is positioned within the hexagonal indentation of hinge pin  34  after installation of hinge pin  34  as described above. When positioned within the hexagonal indentation of hinge pin  34 , hinge pin plug  120  is flush with the head of hinge pin  34 . In use, hinge pin plug  120  substantially prohibits the entry of foreign materials into the hexagonal indentation of hinge pin  34 . For example, hinge pin plug  120  substantially prohibits bone growth into the hexagonal indentation of hinge pin  34 , as well as prohibiting positioning of bone cement therein. The above-described connection of hinge post  40  to femoral component  22  is performed prior to implantation of femoral component  22 . Femoral component  22  is packaged and sold with bearing box  70 , hinge post  40 , hinge pin sleeve  36 , hinge pin  34 , and hinge pin plug  120  preassembled as described above, with the assembly preferably occurring in the manufacturing environment. 
     Pre-assembly of hinge post  40  to femoral component  22  eliminates a number of meticulous assembly steps (many of which were performed during implantation) which were required with constrained knees of the prior art. Furthermore, the assembly of hinge post  40  and femoral component  22  as described above facilitates replacement of various portions of knee prosthesis  20 . Specifically, the threaded connection of hinge pin  34  to femoral component  22  allows for removal and replacement of various components of knee prosthesis including, e.g., bearing box  70 , hinge pin sleeve  36 , and hinge post  40 . 
     In use, femoral bone stock may abut external side walls  82  of femoral component  22  and extend to the underside of condyler bearing surfaces  28 ,  30 . To remove hinge pin  34 , a hole saw is utilized to remove a relatively small portion of femoral bone stock to provide access to hinge pin  34 . Advantageously, femoral component  22  does not require extensive removal of femoral bone stock for implantation thereof (since bone stock can extend to the underside of condylar bearing surfaces  28 ,  30 ), and, furthermore, does not require removal of femoral component  22  to effect replacement of, e.g., hinge post  40 , bearing box  70 , or hinge pin sleeve  36 . Upon accessing hinge pin  34  (e.g., utilizing a hole saw as described above), hinge pin plug  120  is removed, e.g., with a scalpel and forceps to provide access to the hexagonal indentation of hinge pin  34  so that a hexagonal wrench may be inserted therein to unthread hinge pin  34  from femoral component  22 . 
     Knee prosthesis  20  includes a pair of hyperextension stop mechanisms. The first hyperextension stop comprises a portion of condylar bearing surfaces  28 ,  30  of increased radius of curvature as compared to the remaining condylar bearing surface. At three degrees of hyperextension this portion of increased radius of curvature will contact meniscal component  26  and act to retard further hyperextension. If hyperextension continues, the area of increased radius of curvature will cause femoral component  22  to lift away from meniscal component  26 . The second hyperextension stop mechanism functions at four degrees of hyperextension to prohibit further hyperextension of knee prosthesis  20 . The second hyperextension stop mechanism comprises hyperextension stop surface  66  of hinge post  40  and hyperextension stop  68  of bearing box  70 . Hyperextension stop surface  66  comprises the concave back wall of cannulated hinge post  40  as illustrated, e.g., in  FIGS. 2 and 3 . Hyperextension stop  68  of bearing box  70  comprises a protrusion extending from the back wall of bearing box  70  opposite anti-rotation surface  78 . Hyperextension stop  68  includes a convex outer surface as illustrated, e.g., in  FIG. 12 . Hyperextension stop surface  66  of hinge post  40  cooperates with hyperextension stop  68  of bearing box  70  to provide a hyperextension stop for knee prosthesis  20 . Concave hyperextension stop surface  66  becomes flush with the convex outer surface of hyperextension stop  68  of bearing box  70  at four degrees of hyperextension to prevent further hyperextension of knee prosthesis  20 . 
     Tibial component  24  is depicted in  FIGS. 1-5 ,  8 ,  13 , and  14 . As illustrated, e.g., in  FIG. 2 , tibial component  24  includes tibial tray  98  connected to tibial stem  92 . Stabilizing ribs  94  stabilize tibial tray  98  relative to tibial stem  92  and impede rotation of tibial component  24  in tibia  118  (see, e.g.,  FIG. 3 ). In one exemplary embodiment, tibial component  24  is formed from a cobalt-chromium alloy. Tibial component  24  further includes tibial bushing  64  positioned within hinge post extension aperture  110 . Tibial bushing  64  is formed of plastic, such as, e.g., UHMWPE and provides a bearing surface between hinge post extension  42  and hinge post extension aperture  110  of tibial component  24 . As described above, meniscal component  26  comprises a rotating bearing, and, thus, hinge post extension  42  will rotate relative to tibial component  24 . Tibial bushing  64  facilitates this rotation of hinge post extension  42 . 
     Tibial component  24  further includes rotation protrusion  96 . As illustrated, e.g., in  FIG. 3 , rotation protrusion  96  protrudes upwardly from tibial tray  98  of tibial component  24  and further extends in a plane substantially parallel to tibial tray  98 . Rotation protrusion  96  cooperates with cutout  90  of meniscal component  26  to guide rotation of meniscal component  26  about hinge post extension  42 , as further described hereinbelow. 
     One embodiment of meniscal component  26  is illustrated in  FIGS. 1-7 ,  13 , and  14 . Meniscal component  26  is formed from a suitable plastic such as, e.g., UHMWPE and provides a rotating bearing surface between femoral component  22  and tibial component  24 . Meniscal component  26  includes bearing surfaces  86 ,  88  which contact condylar bearing surfaces  28 ,  30  of femoral component  22  during movement of knee prosthesis  20 . As described above, meniscal component  26  further includes hinge post aperture  114  accommodating passage of hinge post  40  and, consequently, hinge post extension  42  therethrough. Meniscal component  26  is operable to rotate about the longitudinal axis of hinge post extension  42  to form a rotating bearing. 
     Meniscal components of varying heights may be constructed in accordance with the present invention. In one advantageous aspect of the present invention, meniscal component  26  is packaged for sale and use together with hinge post extension  42  to facilitate component choice and, in one embodiment, to ensure proper extension of hinge post extension  42  into tibial component  24 . The extension of hinge post extension  42  into tibial component  24  functions to prevent separation of knee prosthesis  20  after implantation thereof. As is known in the art, the femoral component of a knee prosthesis may, in some situations, move relative to and away from the tibial component in a direction parallel to the longitudinal axis of the hinge post extension. With this in mind, hinge post extension  42  is made to be of sufficient length to be retained within tibial component  24  even in situations in which femoral component  22  moves as described immediately supra. In one exemplary embodiment, hinge post extension  42  extends four centimeters into hinge post extension aperture  110  in tibial component  24 . 
     Meniscal component  26  includes cutout  90  which cooperates with rotation protrusion  96  of tibial component  24  to guide rotation of meniscal component  26  and to resist lifting of meniscal component  26  from tibial tray  98  of tibial component  24 . As illustrated, e.g., in  FIG. 3 , cutout  90  accommodates the portion (i.e., lip) of rotation protrusion  96  extending in a plane substantially parallel to the plane containing tibial tray  98 , with a portion (i.e., lip) of meniscal component  26  being positioned between rotation protrusion  96  and tibial tray  98  in a direction substantially perpendicular to the plane containing tibial tray  98 . This configuration functions to discourage displacement of meniscal component  26  away from tibial tray  98  in a direction parallel to the longitudinal axis of hinge post extension  42 . Furthermore, rotation protrusion  96  acts against the back of cutout  90  to limit rotation of meniscal component  26  about the longitudinal axis of hinge post extension  42 . 
     As illustrated in  FIG. 5 , meniscal component  26  may be slid out from between tibial component  24  and femoral component  22  when the hinge post extension  42  has been removed from knee prosthesis  20 . As illustrated, hinge post aperture  114  is sized to allow rotation of hinge post  40  so that meniscal component  26  may be slid out from its position between femoral component  22  and tibial component  24 . This configuration allows for replacement of an implanted meniscal component  26  without requiring removal of hinge post  40 .  FIG. 5  illustrates removal of hinge post extension  42  to accommodate replacement of meniscal component  26 . As illustrated, hinge plug wrench  102  engages hinge plug  38  for removal thereof. After removal of hinge plug  38 , slap hammer  104  is threadedly engaged with threaded aperture  44  in hinge post extension  42 . Slap hammer  104  may then be utilized to unlock the engagement of locking taper  46  in elongate hinge post extension aperture  112  so that hinge post extension  42  may be removed. 
       FIGS. 13 and 14  illustrate an alternative embodiment of the knee prosthesis of the current invention. This alternative embodiment utilizes hinge post extension  42 a having locking taper  46 a, cylindrical extension  48 a, and flange  106 . In this embodiment, a locking instrument may be utilized to apply force atop hinge post extension  42 a so that locking taper  46 a is seated in elongate hinge post extension aperture  112  and locked therein. Flange  106  may be utilized to facilitate removal of hinge post extension  42 a. As illustrated in  FIG. 13 , set screw  108  may be utilized as a secondary lock for hinge post extension  42 a. In all other respects, the knee prosthesis illustrated in  FIGS. 13 and 14  is constructed as described above with respect to the first embodiment of the knee prosthesis in accordance with the present invention. 
       FIGS. 15 ,  16  and  17  illustrate an alternative embodiment of the hinge post extension and tibial bushing of the present invention. In this embodiment, tibial component  24 a includes annular tibial bushing expansion groove  122  formed in hinge post extension aperture  110 . Tibial bushing  64 a includes retaining flange  130  positioned within annular tibial bushing expansion groove  122 .  FIG. 15  illustrates insertion of cylindrical extension  48 b of the hinge post extension into tibial bushing  64 a positioned within tibial component  24 a. As cylindrical extension  48 b proceeds into tibial bushing  64 a, bevel  126  contacts annular locking protrusion  128  of tibial bushing  64 a and causes outward movement of retaining flange  130  to allow cylindrical extension  48 b to proceed to its seated position as illustrated in  FIG. 17 . Annular tibial bushing expansion groove  122  is sized to allow radial expansion of retaining flange  130  to accommodate placement of cylindrical extension  48 b within tibial bushing  64 a. In the fully seated position ( FIG. 17 ) cylindrical extension  48 b is locked in place by the engagement of annular locking protrusion  128  in annular locking groove  124 . Furthermore, retaining flange  130  cooperates with annular tibial bushing expansion groove  122  to prohibit axial displacement of tibial bushing  64 a and, consequently, cylindrical extension  48 b. In this embodiment, the femoral component is retained in abutting relationship to the meniscal component and lift off of the femoral component is substantially prohibited. Tibial bushing  64 a is, in one exemplary embodiment, formed of UHMWPE 
       FIGS. 18 and 19  illustrate another alternative embodiment of the knee prosthesis of the current invention. In this embodiment, locking clip  134  is utilized to retain the position of hinge post  40 b within hinge post aperture  114  of meniscal component  26 a. Hinge post  40 b is rotatably attached to femoral component  22  utilizing hinge pin  34  as described above. In this embodiment, hinge post  40 b includes locking clip grooves  132 , and meniscal component  26 a includes locking clip apertures  136 . Upon positioning of hinge post  40 b within hinge post aperture  114 , locking clip  134  is positioned as illustrated in  FIG. 19  with each prong of locking clip  134  being inserted into locking clip apertures  136  of meniscal component  26 a. As illustrated in  FIG. 19 , locking clip  134  engages locking clip grooves  132  to retain hinge post  40 b within hinge post aperture  114  of meniscal component  26 a. In this embodiment, lift off of femoral component  22  is prohibited by the engagement of hinge post  40 b with meniscal component  26 a. This embodiment of the knee prosthesis of the current invention may further utilize a meniscal component cutout together with a rotation protrusion on the tibial component to resist lifting of the meniscal component from the tibial tray as described above. 
       FIG. 20  illustrates a further alternative embodiment of the hinge post of the present invention. Hinge post  40 c illustrated in  FIG. 20  includes reinforcing material  138  to strengthen hinge post  40 c. 
     While this invention has been described as having exemplary designs, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.