Abstract:
A compressive force and compressive-shear force joint implant including a head defining at least one wear contact surface. At least the at least one wear contact surface is manufactured from a polycarbonate urethane material. The implant may further include a stem extending from the head opposite of the wear contact surface. The head may also be configured to define a second wear contact surface distinct from the first wear contact surface.

Description:
FIELD OF THE INVENTION 
       [0001]    This relates to the field of medical devices and more particular to a compressive-shear wear joint replacement. 
       BACKGROUND OF THE INVENTION 
       [0002]    Arthritis of the thumb basal joint (or alternatively refered to as the thumb carpometacarpal (CMC) joint) or the trapeziometacarpal joint (TMJ) joint is a disabling disorder of the thumb axis. Similarly, arthritis of the metatarsophalangeal joint (MTPJ) is a disabling disorder of the toe axis. Similarly, arthritis of the tarsometatarsal joints (TMT) is a disabling disorder of the feet. Similarly, arthritis and instability of the radiocapitellar joint is a disabling disorder of the elbow joint. 
         [0003]    Since the early 1960s, various solutions have been introduced for reconstruction of these joints to try to alievate the pain and discomfort. Silicone replacement arthroplasty of the thumb CMC was first advocated by Swanson in the early 1960s, however, such silicone joint replacements have essentially fell out of favor mainly because of the complications associated with wear of the silicone implant, and silicone synovitis. Silicone synovitis is essentially a recurrence of pain, swelling, and instability at the site of the original silicone replacement arthroplasty. It is characterized by bony destruction, and soft tissue swelling and inflammation.  FIG. 1  illustrates exemplary prior art silicone joint implants  10 ,  10 ′, with the implant  10  illustrating a condition prior to use and the implant  10 ′ showing wear to a head portion  12 ′ of the implant  10 ′ after use. Similarly,  FIG. 2  shows a silicone test implant  20  showing fragmentation wear after a wear test as described below. 
         [0004]    Another problem associated with silicone implants is silicone elastomer transfer wear which causes a spackling effect against the bone wherein pores of the bone are filled with the silicone.  FIG. 3  shows a scanning electron microscope picture of the surface of an artificial bone  30  counter face used in the wear test as described below. As seen therein, after repeated contact between the test implant  20  against the artificial bone  30 , a significant amount of silicone material  34  transferred to the artificial bone  30  and filled the pores  32  and formed ridges  36 . 
         [0005]    Subsequently various metallic, ceramic, absorbable polymeric, and pyro carbon implants have been introduced to serve either as spacers or hemiarthroplasty in order to provide for pain relief at the CMC, TMJ, MTPJ and radiocapitellar joints. 
         [0006]    Biomechanically, the prior art implants are either too stiff, or too soft to provide for a durable arthroplasty. For example, the stiffness of the trapezium generally is essentially similar to that of the scaphoid at approximately 150 Megapascals. The silicone implants initially advocated in the 1960s display a stiffness of less than 4 megapascals in vivo, where as the titanium implants are in general more than 100 Gigapascals. The cobalt chrome trapezial implants display a high stiffness at 200 GigaPascals while the zirconia ceramic implants are even stiffer at approximately 400 GigaPascals. The more recent pyrocarbon introduction is an attempt to use materials which are less stiff, however, the pyrocarbon stiffness nevertheless approaches that of cortical bone at approximately 15-20 GigaPascals (3 orders of magnitude more stiff than the native trapezium). Accordingly, these materials do not provide a biomechanically appropriate implant. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Looking at the CMC, for example, the ideal material for joint replacement arthroplasty would not only be mechanically and materially less stiff than the trapezium to provide for a stable spacer to prevent collapse of the thumb, but also would be less in stiffness to that of the cortico-cancellus bone of the thumb metacarpal medullary shaft in order to prevent thumb metacarpal subsidence over the implant. In addition, an ideal material would have superior wear qualities so that microscopic wear particles would not create polymeric synovitis. In short, material that is slightly stiffer than silicone elastomer yet resistant to in vivo degradation with superior wear properties would be an ideal candidate to serve as a sound CMC, TMJ, MTPJ or radiocapitellar joint implant. 
         [0008]    The inventor has recognized that polycarbonate urethanes (PCU), which are a class of thermoplastic polyurethanes (TPU), allow for desired elastomeric properties to be maintained in vivo, while at the same time provide for adequate protection against environmental stress cracking and breakdown in vivo. 
         [0009]    The present invention provides in at least one embodiment a compressive force and compressive-shear force joint implant including a head defining a wear contact surface and a stem extending from the head opposite of the wear contact surface. At least the wear contact surface is manufactured from a polycarbonate urethane material. 
         [0010]    In at least one embodiment, the present invention provides a compressive force and compressive-shear force joint implant including a head defining at least two wear contact surfaces with at least the wear contact surfaces manufactured from a polycarbonate urethane material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention. In the drawings: 
           [0012]      FIG. 1  is a photograph of prior art silicone implants, with one of the implants shown prior to use and the other shown after use in a patient. 
           [0013]      FIG. 2  is a photograph of test silicone implant after being subjected to a wear test. 
           [0014]      FIG. 3  is a scanning electron microscope picture of the surface of an artificial bone counter face used with the test silicone implant in the wear test. 
           [0015]      FIG. 4  is a schematic drawing of an exemplary implant of the invention positioned in a CMC arthroplasty. 
           [0016]      FIG. 5  is a schematic drawing of an exemplary implant of the invention positioned in a TMJ arthroplasty. 
           [0017]      FIG. 6  is a schematic drawing of exemplary implants of the invention positioned in a TMJ arthroplasty. 
           [0018]      FIG. 7  is a schematic drawing of an exemplary implant of the invention positioned in a MTPJ arthroplasty. 
           [0019]      FIG. 8  is a schematic drawing of an exemplary implant of the invention positioned in a radiocapitellar joint arthroplasty. 
           [0020]      FIG. 9  is an isometric view of an implant in accordance with a first exemplary embodiment of the invention. 
           [0021]      FIG. 10  is a cross-sectional view of an implant in accordance with another exemplary embodiment of the invention. 
           [0022]      FIGS. 11-19  are isometric views of implants in accordance with various other exemplary embodiments of the invention. 
           [0023]      FIG. 20  is a schematic drawing of another exemplary implant of the invention positioned in a CMC arthroplasty. 
           [0024]      FIG. 21  is a schematic drawing of anonther exemplary implant of the invention positioned in a CMC arthroplasty. 
           [0025]      FIG. 22  is a schematic view of a wear test assembly utilized to test the wear characteristics of an implant in accordance with an exemplary embodiment of the invention versus a prior art silicone test implant. 
           [0026]      FIG. 23  is a scanning electron microscope picture of the surface of an artificial bone counter face used with the implant in accordance with an exemplary embodiment of the invention in the wear test. 
           [0027]      FIG. 24  is a graph illustrating a dynamic mechanical analysis of the implant in accordance with an exemplary embodiment of the invention. 
           [0028]      FIG. 25  is a graph illustrating a dynamic mechanical analysis of a prior art silicone test implant. 
           [0029]      FIG. 26  is a schematic view of a compression test assembly utilized to test the compression fatigue characteristics of an implant in accordance with an exemplary embodiment of the invention versus a prior art silicone test implant. 
           [0030]      FIGS. 27-31  are graphs illustrating the cyclic compressive deformation of an implant in accordance with an exemplary embodiment of the invention under various testing conditions. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention. 
         [0032]    Referring to  FIG. 4 , a CMC arthroplasty is illustrated with an exemplary implant  50  positioned between the thumb metacarpal  40  and the remaining portion of the trapezium  42 . For context, the scaphoid  44 , trapezoid  46  and the next metacarpal  48  are illustrated. With reference also to  FIG. 9 , the exemplary implant  50  includes a cylindrical head  52  connected to a stem  54  via a collar  56 . The head  52  defines a wear contact surface  53  which is opposite the stem  54 . Upon implantation in a known manner, the stem  54  extends into a bore formed in the metacarpal  40  and the wear contact surface  53  bears against the portion of the trapezium  42  in compressive contact. The interaction between the wear contact surface  53  and the portion of the trapezium  42  allows for the normal multidirectional movement of the thumb. As used herein, the term wear contact surface refers to a surface of the implant configured to be placed in compressive contact with an opposed structure, e.g. bone or another implant member, with relative movement between the wear contact surface and the opposed structure. 
         [0033]    In the present embodiment, the head  52 , including the wear contact surface  53 , the stem  54  and the collar  56  are formed as a unitary structure of PCU material. While the present embodiment is illustrated as a unitary structure, the invention is not limited to such. For example, the implant  100  illustrated in  FIG. 14  includes a head  102  with a wear contact surface  103  and a separate stem  104  with a locking collar  106 . The stem  104  and collar  106  may be manufactured from, for example, a biocompatible metal or ceramic material while the head  102  is manufactured from PCU material. The head  102  may be overmolded about the collar  106 , snap-fit to the collar  106  or otherwise connected thereto. 
         [0034]    In the implant  50  of  FIG. 9 , the head  52  and the stem  54  are co-axial with a central axis CA extending through the center of each, however, the invention is not limited to such a configuration.  FIG. 10  illustrates an implant  60  with a head  62  defining a wear contact surface  63  on one side and a stem  64  extending from the opposite side of the head  62 . The stem  64  has an axis SA which is offset from the axis HA of the head  62 . The collar  66  is preferably configured to accommodate the offset. The offset allows the implant  60  to compensate for bone misalignments or allow use in alternative structures. Otherwise the implant  60  is as described with respect to implant  50  and includes a head  62  and wear contact surface  63  manufactured from PCU material. The implant  60  may be a unitary structure or a multipart structure as described above. 
         [0035]    The implant  50  of  FIG. 9  has a planar wear contact surface  53  which is substantially perpendicular to the central axis CA, however, the invention is not limited to such a configuration.  FIGS. 11 and 12  illustrate implants  70  and  80  each having a head  72 ,  82  defining a hemispherical wear contact surface  73 ,  83 . A stem  74 ,  84  extends from the opposite side of the head  72 ,  74  and is interconnected via a collar  76 ,  86 . The stem  74  and head  72  of the implant  70  are co-axial while the stem  84  and head  82  of the implant  80  are offset. Otherwise the implants  7 ,  800  are as described with respect to implant  50  and include a head  72 ,  82  and wear contact surface  73 ,  83  manufactured from PCU material. The implants  70 ,  80  may each have a unitary structure or a multipart structure as described above. 
         [0036]      FIG. 5  illustrates a TMJ arthroplasty with the trapezium completely removed and an exemplary implant  90  positioned between the thumb metacarpal  40  and the scaphoid  44 . The implant  90  is similar to the implant  50  and includes a cylindrical head  92  connected to a stem  94  via a collar  96 . The head  92  defines a wear contact surface  93  which is opposite the stem  94 . Upon implantation in a known manner, the stem  94  extends into a bore formed in the metacarpal  40  and the wear contact surface  93  bears against the scaphoid  44 . It is noted that the head  92  is longer than the head  52  to compensate for the larger distance between the metacarpal  40  and the scaphoid  44 . The interaction between the wear contact surface  93  and the scaphoid  44  allows for the normal multidirectional movement of the thumb. The implant  90  is similar to implant  50  and includes a head  92  and wear contact surface  93  manufactured from PCU material. The implant  90  may be a unitary structure or a multipart structure as described above and illustrated in  FIG. 14 . 
         [0037]      FIG. 15  illustrates an implant  90 ′ substantially the same as the implant  90 , however the implant  90 ′ includes a cross bore  98  extending through the head  92 ′ substantially perpendicular to the central axis CA. The cross bore  98  provides for tendon passage to secure the implant  90 ′. In all other respects, the implant  90 ′ is the same as the implant  90 . 
         [0038]      FIGS. 16-19  illustrate alternative exemplary implants  110 ,  120 ,  130  and  130 ′ which are similar to the implant  90 . The implant  110  of  FIG. 16  includes a cylindrical head  112  with a wear contact surface  113 , a stem  114  and a collar  116 . The implant  110  differs from implant  90  only in that the axis HA of the head  112  is offset from the axis SA of the stem  114 . 
         [0039]    The implant  120  of  FIG. 17  includes a cylindrical head  122  with a wear contact surface  123 , a stem  124  and a collar  126 . The implant  120  differs from implant  90  in that the head  122  includes an annular convex groove  127  and a cross bore  128  similar to implant  90 ′. The groove  127  and the cross bore  128  facilitate placement and securement of one or more tendons to the implant  120 . 
         [0040]    The implants  130 ,  130 ′ of  FIGS. 18 and 19  include a cylindrical head  132 ,  132 ′ with a wear contact surface  133 , a stem  134  and a collar  136 . The implants  130 ,  130 ′ differ from implant  90  in that the head  132 ,  132 ′ includes an annular rectangular groove  137  and the head  132 ′ of implant  130 ′ further includes a cross bore  138 . 
         [0041]    Similar to  FIG. 5 ,  FIG. 6  illustrates a TMJ arthroplasty with the trapezium completely removed, however, a pair of implants  60  and  70  are positioned between the thumb metacarpal  40  and the scaphoid  44 . The stem  74  of implant  70  is fixed in the metacarpal  40  while the stem  64  of implant  60  is fixed in the scaphoid  44 . The wear contact surfaces  63 ,  73  of the implants  60 ,  70  face one another and are in compressive contact. The interaction between the wear contact surfaces  63  and  73  allows for the normal multidirectional movement of the thumb. 
         [0042]    Referring to  FIG. 7 , an MTPJ arthroplasty is illustrated with an exemplary implant  50  positioned between the toe metatarsal  41  and the remaining portion of the proximal phalange  43 . For context, the distal phalange  45  is illustrated. Upon implantation in a known manner, the stem  54  extends into a bore formed in the proximal phalange  43  and the wear contact surface  53  bears against the metatarsal  41  in compressive contact. The interaction between the wear contact surface  53  and the metatarsal  41  allows for the normal multidirectional movement of the toe. While illustrated with respect to the MTPJ, the implant  50  may similarly be positioned between the metatarsal  41  and the cuneiform to provide TMT joint arthroplasty. 
         [0043]    Referring to  FIG. 8 , a radiocapitellar joint arthroplasty is illustrated with an exemplary implant  50  positioned between the radius  51  and the capitulum  57  of the humerus  55 . For context, the ulna  59  is illustrated. Upon implantation in a known manner, the stem  54  extends into a bore formed in the radius  51  and the wear contact surface  53  bears against the capitulum  57  in compressive contact. The interaction between the wear contact surface  53  and the capitulum  57  allows for the normal multidirectional movement of the elbow. 
         [0044]    Referring to  FIG. 20 , a CMC arthroplasty is illustrated with another exemplary implant  140  positioned between the thumb metacarpal  40  and the remaining portion of the trapezium  42 . In the present embodiment, the exemplary implant  140  includes a cylindrical head  142  which defines opposed wear contact surfaces  144  and  146 . The implant  140  does not include a stem and is configured to be positioned between and held in place by the existing bone structures  40  and  42 . The contact ends of the bone structures  40  and  42  may be shaped prior to positioning of the implant  140  such that the implant  140  is retained within a concave configuration of one or both bone structures  40 ,  42 . Upon implantation, the wear contact surface  144  bears against the portion of the metacarpal  40  in compressive contact and the wear contact surface  146  bears against the portion of the trapezium  42  in compressive contact. The interaction between the wear contact surfaces  144  and  146  and the metacarpal  40  and the portion of the trapezium  42 , respectively, allows for the normal multidirectional movement of the thumb. The head  142  may include a cross bore as described in conjunction with some of the prior embodiments. In a preferred embodiment, the entire head  142 , including the wear contact surfaces  144  and  146 , is manufactured from PCU material, however, the implant  140  may have other configurations, for example, a composite structure wherein only the wear contact surfaces  144  and  146  are manufactured from PCU material. 
         [0045]    Referring to  FIG. 21 , a CMC arthroplasty is illustrated with another exemplary implant  141  positioned between the thumb metacarpal  40  and the remaining portion of the trapezium  42 . In the present embodiment, the exemplary implant  141  includes a spherical head  143  which defines opposed wear contact surfaces  145  and  147 . The implant  141  does not include a stem and is configured to be positioned between and held in place by the existing bone structures  40  and  42 . The contact ends of the bone structures  40  and  42  may be shaped prior to positioning of the implant  141  such that the implant  141  is retained within a concave configuration of one or both bone structures  40 ,  42 . Upon implantation, the wear contact surface  145  bears against the portion of the metacarpal  40  in compressive contact and the wear contact surface  147  bears against the portion of the trapezium  42  in compressive contact. The interaction between the wear contact surfaces  145  and  147  and the metacarpal  40  and the portion of the trapezium  42 , respectively, allows for the normal multidirectional movement of the thumb. The head  143  may include a cross bore as described in conjunction with some of the prior embodiments. In a preferred embodiment, the entire head  143 , including the wear contact surfaces  145  and  147 , is manufactured from PCU material, however, the implant  141  may have other configurations, for example, a composite structure wherein only the wear contact surfaces  145  and  147  are manufactured from PCU material. 
         [0046]    While the present invention is described herein in relation to CMC, TMJ, MTPJ and radiocapitellar joint arthroplasty, the invention is not limited to such. Implants in accordance with the invention may be utilized in other applications wherein the implant wear contact surface is subject to compressive contact. Additionally, while various embodiments of the implant are described herein, the invention is not limited to such. The implants may have various configurations with a head having a wear contact surface manufactured from PCU material. As explained in more detail below, the use of such PCU material provides unexpected favorable results for a compressive implant having a head with a wear surface on one side and a stem extending from the opposite side. Such an implant meets the need for a reliable implant that has existed since the 1960s. 
         [0047]    To confirm the viability of the implants of the present invention, a wear test was performed on an exemplary PCU implant and a prior art silicone implant. In general, post reconstruction of the thumb basal joint, the maximum key pinch strength obtained is approximately 5±2.5 kilograms; activities of daily living require a pinch force no more than 2 kilograms. Therefore a normal force of 8 pounds was chosen to be applied to the prosthetic stem against synthetic bone # 40  (Pacific research labs) to study wear characteristics. 
         [0048]    Tests were performed on both silicone implants from Wright medical technology (flexspan) and the PCU implants of the present invention. Testing was performed utilizing a wear test assembly  150  as illustrated in  FIG. 22 . The specimens  160  were secured in a stainless steel rod  154  suspended from a load cell  152  over a fluid chamber  158 . The chamber  158  was filled with saline at 37° C. to simulate in vivo conditions. Each specimen  160  was equilibrated in the saline  159  for two days before the test. An artificial bone sample  30  was supported by a spring  156  extending from a support member  157 . The spring  156  urged the artificial bone sample  30  into contact with the sample  160  with the desired  8  pound normal force. An actuator  153  oscillated the artificial bone sample  30  relative to the specimen  160  to conduct the test. After 221,000 cycles, weight loss from the samples were recorded. 
         [0049]    Table 1 below provides a summary of the weight loss during the wear test results while Table 2 shows the normalized percentage of weight loss results of the test. As can be seen, there was significantly more weight loss in the silicone group when compared to the PCU implant group. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Wear Test Summary 
               
             
          
           
               
                   
                 Flexspan (Wright) 
                 PCU Implant 
               
             
          
           
               
                   
                 Weight Loss (mg) 
               
               
                   
                   
               
             
          
           
               
                   
                 Sample 
                 1 
                 26.0 
                 3.3 
               
               
                   
                 Number 
                 2 
                 10.6 
                 3.8 
               
               
                   
                   
                 3 
                 15.2 
                 3.0 
               
               
                   
                   
                 4 
                 17.5 
                 6.5 
               
               
                   
                   
                 5 
                 17.7 
                 9.3 
               
               
                   
                   
                 6 
                 16.9 
                 4.6 
               
             
          
           
               
                   
                 Mean 
                 17.3 
                 5.1 
               
               
                   
                 Std. Dev. 
                 5.0 
                 2.4 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Wear Test Summary 
               
             
          
           
               
                   
                 Flexspan (Wright) 
                 PCU Implant 
               
             
          
           
               
                   
                 Weight 
                 Weight 
                   
                   
                 Weight 
                 Weight 
                   
                   
               
               
                   
                 Before 
                 After 
                 Weight 
                 Coef. 
                 Before 
                 After 
                 Weight 
                 Coef. 
               
               
                   
                 Test 
                 Test 
                 Loss 
                 Of 
                 Test 
                 Test 
                 Loss 
                 Of 
               
               
                   
                 (mg) 
                 (mg) 
                 (%) 
                 Friction 
                 (mg) 
                 (mg) 
                 (%) 
                 Friction 
               
               
                   
                   
               
             
          
           
               
                 Sample 
                 1 
                 224.3 
                 198.3 
                 11.59 
                 0.41 
                 190.0 
                 186.7 
                 1.74 
                 0.66 
               
               
                 Number 
                 2 
                 196.7 
                 186.1 
                 5.39 
                 0.45 
                 152.6 
                 148.8 
                 2.49 
                 0.70 
               
               
                   
                 3 
                 173.7 
                 158.5 
                 8.75 
                 0.42 
                 177.7 
                 174.7 
                 1.69 
                 0.68 
               
               
                   
                 4 
                 221.2 
                 211.9 
                 9.3 
                 0.43 
                 169.2 
                 165.0 
                 2.48 
                 0.60 
               
               
                   
                 5 
                 183.5 
                 165.3 
                 9.92 
                 0.45 
                 196.8 
                 192.9 
                 1.96 
                 0.583 
               
               
                   
                 6 
                 181.6 
                 175.6 
                 5.95 
                 0.43 
                 200.9 
                 196.3 
                 2.30 
                 0.68 
               
             
          
           
               
                 Mean 
                 196.83 
                 182.62 
                 8.48 
                 0.43 
                 177.26 
                 173.62 
                 2.07 
                 0.64 
               
               
                 Std. Dev. 
                 21.42 
                 20.23 
                 2.38 
                 0.02 
                 17.45 
                 17.57 
                 0.39 
                 0.05 
               
               
                   
               
             
          
         
       
     
         [0050]    The above clearly demonstrates that PCU implants of the current invention are significantly more durable than silicone elastomer in conditions of abrasive wear against a rough counter face which is the expected situation in vivo. More specifically, as shown in Table 2, the current silicone specimens wear 4 times more than the PCU implant specimens under uniform testing conditions for both groups. 
         [0051]    Furthermore,  FIG. 23  shows a scanning electron microscope picture of the surface of an artificial bone  30  counter face that was pressed against the PCU implants, similar to  FIG. 3  which shows the artificial bone  30  counter face that was pressed against the silicone implants. As seen in  FIG. 23 , the PCU implants did not have significant material transfer like the silicone and the pores  32  remain clear and there are no ridges formed. 
         [0052]    It was clear from the wear tests that the PCU implant showed significantly less wear against an artificial bone counter face. Volumetric wear is significantly less and is demonstrated by significantly less weight loss from the PCU implant sample when compared to that of the silicone elastomer implant. 
         [0053]    In light of the fact that there is less volumetric wear of the PCU implants, and no electron microscopic evidence evidence for transfer wear as demonstrated by the scanning electron microscopy, it is believed that particulate synovitis can be avoided with the use of a more biomechanically and biomaterially sound elastomeric implant material of the present invention. 
         [0054]    To further confirm the viability of the implants of the present invention, a thermal dynamic mechanical analysis of the silicone elastomer and the PCU implant samples were carried out at 37° C. and the results are charted in  FIGS. 24 and 25 . The results show that the PCU implant samples are about 5 times more stiff in compression than silicone elastomer in vivo. The stiffness of the silicone samples at 37° C. under dynamic compression at 0.5% strain is approximately 4 Megapascals, whereas on the other hand the stiffness of the PCU implant samples are at approximately 20 megapascals. 
         [0055]    As a further confirmation, the PCU implants specimens were subjected to a cyclic compressive fatigue test using a fatigue testing assembly  170  as shown in  FIG. 26 . The assembly  170  was an Instron testing machine (Model of machine—8500.) with a small capacity load cell (3 Kip)  172  with a stainless steel rod  174  depending therefrom.. The specimen  180  was supported beneath the rod  174  in an implant holder  177  which was submerged in a saline  179  at 37° C. within chamber  178 . The specimen  180  was equilibrated in the saline  179  for two days prior to the fatigue cyclic compression test. 
         [0056]    The assembly  170  was on the LOAD control, half sine wave form (sine wave, only compression force−half sine). For example—the system was run from minus 0.5 Kg to minus 60 Kg. Frequency was set at 10 Hz. For stability of the wave form and force we used a special mode of amplitude control. Five different loads were tested at 10 kg, 15 kg, 25 kg, 50 kg, and 60 kg. At each load the testing took approximately 14 days to achieve 10 million cycles of compressive fatigue. As shown in  FIGS. 27-31 , the PCU implant remained structurally stable to 10 million cycles at all five loads tested.