Abstract:
A universal shaft component capable of insertion as an anchorage in skeletal bone including a proximal humerus, phalange, distal or proximal tibia, distal or proximal femur, or thumb wherein the shaft is insertable axially within an internal bone cavity such that the outer surface of the shaft engages inner walls of said cavity, characterized in that the shaft has a proximal end and a distal end and on said outer surface of said shaft between said ends, at least one thread such that when the shaft is screwed into said bone cavity the at least one thread induces an axial compression force in said bone and distributes that compression force evenly along the bone over the length of the at least one thread.

Description:
BACKGROUND  
         [0001]    The present invention relates to surgical prostheses and more particularly relates to a universal component adapted for use in various sites in a skeletal frame. More particularly the invention relates to a shaft component which is adapted for insertion in bone including skeletal joints in the human or animal body and which upon insertion induces a compression in the bone to enhance fixation. More particularly the invention relates to a universal shaft component for insertion in bone and which includes at least one thread whose pitch and angle are constant or vary along the length of the threaded region/s of the shaft. The shaft is particularly applicable to skeletal joints such as but not limited to the shoulder, ankle, thumb, knee and finger where repair or replacement is required. Joint replacements may be required as a result of trauma or disease such as arthritis. Degenerative joints due to such conditions can be extremely disabling and painful often rendering surgery the only treatment for pain relief.  
           [0002]    Whilst the present invention will be described principally with reference to its insertion in a selection of joints including the shoulder, knee ankle and finger , it Will be appreciated that the prosthesis component is capable of insertion in other bone sites such as a jaw bone with proportionate reduction or enlargement in size to accommodate the size of the joint the component will replace.  
           [0003]    More particularly the present invention provides a universal shaft component for insertion in a bone cavity of human and animal skeletal bone wherein an outer bone engaging surface of the component includes at least one thread having a variable or the same pitch along an axial length of the component which induces and even compression distribution in the bone along the length of the at least one thread.  
         PRIOR ART  
         [0004]    There are a wide variety of hip prostheses available for repair and replacement of various joints of the human body. Most if not all of these are joint specific and cannot be easily adapted for insertion into other joints of the body due to their purpose built geometry or manner of fixation.  
           [0005]    Hip replacements, for example are a common orthopaedic surgical procedure and are usually necessitated by degenerative disease of the hip joint, hip trauma or disease of the hip creating later hip degeneration. There are in existence a number of hip prostheses which have been used to replace the femoral head. Whilst many of the prior art femoral head prostheses have enjoyed widespread use with varying degrees of success, each have suffered from certain attendant disadvantages. The generally known and widely used prostheses typically comprise an arcuate distal shaft having a gradual taper along its full length and terminating proximally in a neck which mates with the head of the prosthesis. The shaft is inserted into the intra medullary cavity of the femur.  
           [0006]    This prosthesis is fitted after the surgeon has reamed out the medullary cavity to an extent conducive to the production of tight interfitting between bone and prosthesis when the prosthesis is hammered into position. In practice, the reaming followed by sizing with the prosthesis may be carried out a number of times ie, reaming followed by inserting the prosthesis until there is a small distance of travel of the shaft left near the neck of the femur to enable final hammering into position to thereby create tight interfitting between prosthesis and bone. Femoral explosion both during insertion and extraction is one major drawback when using certain prior art prostheses. However, explosion during insertion is largely due to poor surgical technique or poor prosthesis design which develop high stress forces within bone.  
           [0007]    In the past, cementing of the prosthesis has also been employed, however, problems have existed with the use of cement. Failures in hip prostheses have occurred due to loosening at the cement bone interface and at the prosthesis bone interface. In some patients, a rotational failure of the prosthesis can be generated when a patient moves from a seating to a standing position. Also, artificial hips may loosen and fail due to repetitive movement of the distal shaft induced by the locomotion of a wearer. This may eventually lead to a prosthesis failure and possibly unwanted axial dislocation;  
           [0008]    One feature of existing hip prostheses is a series of indentations which have been moulded into the distal shaft in order to encourage and stimulate bone growth therein. This bone ingrowth assists in holding the prosthesis firmly in position and also provides a keying and locking effect thereby lessening the possibility of rotational failure and/or unwanted axial subsidence of the prosthesis.  
           [0009]    A further problem which exists with this type of prior art prosthesis and in particular with the shaft design is the difficulty in removal from the medullary cavity of a failed prosthesis. A revision hip replacement, necessitates full extraction of the failed prosthesis from the medullary cavity. Where the prosthesis has been held in position by bone growth into the aforesaid recesses of the distal shaft, extraction of the prosthesis can sometimes be extremely difficult, and in some unfortunate instances, may necessitate total longitudinal division of the femur into at least two pieces. Even after division of the femur in this way, a particularly recalcitrant prosthesis firmly affixed to one half of the bone may, in order to effect removal thereof, necessitate further undesirable femoral destruction. Whilst hip prostheses of this type have been in use for some time and have met with considerable field success, the attendant disadvantages of the device are so significant that improvements were necessitated.  
           [0010]    Other prosthesis designs are also used having screw threads on the distal shaft however, these suffer from the major disadvantage that it is very difficult for the surgeon to achieve, co-incidence between the correct orientation of the prosthesis at full screw tightness and proper alignment or anteversion between the prosthesis head and the acetabulum. This requires considerable skill on the part of the surgeon with very little margin for error due to the critical alignment and screw tight ness requirements. For this reason surgeons have not utilised the screw prostheses as much as the previously described cemented prosthesis. A further disadvantage of the existing screw prosthesis is its poor resistance to rotational effects which can result in unwanted reverse rotational withdrawal from the femoral medullary cavity. Prior art prostheses employing single screw threads of the same pitch along the length of the shaft have thus been quite unsatisfactory resulting in their limited use. Known hip prosthesis with screw thread is disclosed in the following patents which are incorporated herein by reference; U.S. pat. No. 4,693,724, French patent 2 295729 and European patent 0010527. A hip prosthesis including a spaced apart thread on an axial shaft has been described in international application PCT/AU91/00244 to Sekel which teaches a shaft for insertion and fixation in a femur and in which an axially disposed. compression force is induced in a bone to which it is fixed.  
           [0011]    Many types of artificial joints are available for replacement not only in hips but also in joints such as the finger thumb, ankle and major shoulder joint. In shoulder arthroplasty the damaged joint is surgically removed and it is replaced with an artificial joint made of preferably titanium, other metal or both plastic and metal. One prosthesis used in the shoulder replacement is the Neer shoulder system. Whilst the Neer prosthesis has been used successfully in shoulder joint replacement over a number of years there has been no previous use of a shaft which is capable of use in multiple and which induces axial compression in a bone upon and/or during insertion. Also, the prior art does not teach a universal shaft which is insertable in multiple bone sites and joints and which induces a compression in the bone in which the shaft is inserted to enhance fixation. The known bone and particularly joint prostheses have to date been purpose designed for a particular site or more particularly joint in the skeletal frame and are not intended for potential use in multiple sites with the same or substantially the same geometry.  
         INVENTION  
         [0012]    The present invention is directed to a universal shaft component capable of use in bone and also in skeletal joint replacement in a variety of human (and animal) joints including the hip, shoulder, ankle, finger, knee and thumb. More particularly the invention relates to a universal shaft component capable of use in a plurality of bone sites including the aforesaid joints and including a threaded outer surface wherein the geometry of the thread allows an even compression force to be induced in a bone at the site in which the shaft is inserted. The shaft is preferably adapted to detachably receive a mating component which completes or partially completes the prosthesis for a particular site.  
           [0013]    Due to the fundamental form (or geometry) of the shaft, and particularly its absence of curvature which typifies many joint prostheses including hip and shoulder prostheses it is universally applicable to joints and in places in bone where an anchorage post is required such as in a jaw bone.  
           [0014]    In its broadest form the present invention comprises:  
           [0015]    a universal shaft component capable of fixation in multiple sites in a skeleton; wherein the shaft is insertable axially within an internal bone cavity such that the outer surface of the shaft engages inner walls of said cavity, characterised in that the shaft has a proximal end and distal end; and on said outer surface of said shaft a continuous thread having at least one thread which includes a region of variable pitch such that when the shaft is screwed into said bone cavity the variable pitch of each said at least one thread induces an axial compression force in said bone and distributes that compression force evenly along the bone for the length of the at least one thread. Preferably the at least one thread varies in angle relative to a vertical or horizontal axis along the length of the shaft. Preferably, the shaft includes a first flared end and a second end narrower than the first end, wherein the first end is capable of receiving a mating component.  
           [0016]    Preferably the universal shaft is insertable in bone and joints such as but not limited to a hip, shoulder, knee, finger, ankle, thumb.  
           [0017]    In another broad form the present invention comprises:  
           [0018]    a universal shaft component capable of insertion in a skeletal joint including a hip, shoulder, finger, ankle, thumb wherein the shaft is insertable axially within an internal bone cavity formed in the joint such that the outer surface of the shaft engages inner walls of said cavity, characterised in that the shaft has a proximal end and a distal end and on said outer surface of said shaft between said ends, at least one thread having the same or a variable pitch and angle relative to a horizontal or vertical axis such that when the shaft is screwed into said bone cavity the thread pitch induces an axial compression force in said bone and distributes that compression force evenly along the bone over the length of the thread.  
           [0019]    Preferably, the helical thread is continuous along the length of the shaft with a gradual variation from a slow thread at the proximal end to a fast thread towards the distal end. The thread travels in the same direction but due to a regular variation in pitch along the length of the thread from a fast to slow thread an even axial compression distribution is induced in the bone in which the shaft inserted.  
           [0020]    An advantage of the gradually varying thread along the shaft is that there is no local compression concentration as the compression forces are distributed evenly.  
           [0021]    A universal shaft component capable of fixation in multiple sites in a bone skeleton; wherein the shaft is insertable axially within an internal cavity in bone such that the outer surface of the shaft engages an inner walls of said cavity; wherein the shaft has first and seconds ends wherein one said ends includes a formation which receives and retains a joining component, the outer surface of said shaft including at least one thread having a pitch geometry which when the shaft is screwed into said bone cavity induces an axial compression force in said bone. Preferably, the axial compression force is evenly distributed in the bone along the length of said at least one thread. The Universal shaft includes a flared tapered region with the second end is narrower than the first end. Longitudinal ridges may be provided along the shaft as a key.  
           [0022]    According to one embodiment, the shaft has disposed between said first and second ends one helical thread, wherein the pitch of the thread varies as the thread travels axially along the shaft. According to one embodiment , the angle of repose of the thread relative to a vertical or horizontal axis varies along the thread as the thread travels axially along the shaft.  
           [0023]    The thread undergoes a variation in pitch from a slow thread near the flared end to a fast thread towards the second end wherein, the thread has a gradual but regular variation in pitch along the length of the thread from a fast to slow thread.  
           [0024]    An even compression force is induced in the bone due to differences in travel rates induced on insertion of the shaft by the thread. A profile part at the flared end receives and retains a detachable joining component by male female or female male interfitting . According to one embodiment, the flared end includes a female recess which receives a male end of a joining component.  
           [0025]    The shaft is capable of insertion in joints which include glenohumeral joint of the shoulder, a distal end of a femur as a partial knee component, a proximal end of a tibia as a partial knee component, in a proximal phalange to form a part finger joint component, in a distal end of a tibia, in a talus, in a proximal femur.  
           [0026]    In another broad form the present invention comprises:  
           [0027]    a universal shaft component capable of fixation as an anchorage in multiple sites in a bone skeleton; wherein the shaft is insertable axially within an internal cavity in bone such that the outer surface of the shaft engages an inner wall of said cavity; wherein the shaft has first and seconds ends wherein one said ends includes a formation which receives and retains a joining component, the outer surface of said shaft including first and second spaced apart helical threads; wherein the threads having a pitch geometry which when the shaft is screwed into said bone cavity induces an axial compression force evenly distributed in the bone along the length of said at least one thread. Each thread preferably has a different pitch. Preferably, the angle of repose of a first of the threads relative to a vertical or horizontal axis of the shaft is different from the angle of repose of the second shaft.  
           [0028]    According to one embodiment, the first thread is disposed in a region of the shaft near the flared tapered region and the second thread is disposed in a region approximating a longitudinal center of the shaft. The first thread is a slow thread and the second thread is a fast thread wherein the first thread causes a slower axial travel of the shaft than the second thread upon screwing the shaft into a bone cavity. The even compression force is induced in the bone due to differences in travel rates induced on insertion of the shaft by the thread. A profile part at the flared end receives and retains a detachable joining component. The flared end includes a tapered female recess which receives the joining component. One of the threads may have constant pitch or a gradual but regular variation in pitch along the length of the thread from a fast to slow thread.  
           [0029]    The double threaded shaft is also capable of insertion in joints which include glenohumeral joint of the shoulder, a distal end of a femur as a partial knee component, a proximal end of a tibia as a partial knee component, in a proximal phalange to form a part finger joint component, in a distal end of a tibia, in a talus, in a proximal femur.  
           [0030]    In another broad form the present invention comprises:  
           [0031]    A universal shaft component capable of insertion as an anchorage in skeletal bone including a proximal humerus, phalange, distal or proximal tibia, distal or proximal femur, or thumb wherein the shaft is insertable axially within an internal bone cavity such that the outer surface of the shaft engages inner walls of said cavity, characterised in that the shaft has a proximal end and a distal end and on said outer surface of said shaft between said ends, at least one thread such that when the shaft is screwed into said bone cavity the at least one thread induces an axial compression force in said bone and distributes that compression force evenly along the bone over the length of the at least one thread.  
           [0032]    In another broad form the present invention comprises:  
           [0033]    a universal shaft component for use as an anchorage in a bone and which is capable of forming at least part of a joint replacement in an ankle, hip, finger, thumb, shoulder or knee; the shaft comprising a threaded outer surface which is profiled to induce a compression force in the bone in which it is inserted to enhance fixation and further comprising a flared end and a narrow end, the flared end having a formation which receives and retains a joining member; 
       
    
    
     DETAILED DESCRIPTION  
       [0034]    The present invention will now be described in more detail according to preferred but non limiting embodiment and with reference to the accompanying illustrations wherein:  
         [0035]    [0035]FIG. 1 shows a shaft according to one embodiment of the invention with double spaced apart threads.  
         [0036]    [0036]FIG. 2 shows a shaft according to a preferred embodiment, inserted in a glenohumeral shoulder joint;  
         [0037]    [0037]FIG. 3 shows the shaft inserted in a distal end of a femur and a proximal end of a tibia to form anchorage for a knee replacement;  
         [0038]    [0038]FIG. 4 shows the shaft inserted as a finger joint replacement.  
         [0039]    [0039]FIG. 5 shows the shaft of FIG. 1 inserted in an ankle joint.  
         [0040]    [0040]FIG. 6 shows an enlarged view of a shaft inserted in a talus. ankle bone;  
         [0041]    [0041]FIG. 7 shows the ankle replacement joint assembly of FIGS. 5 and 6 incorporating two universal shafts.  
         [0042]    [0042]FIG. 8 shows a universal shaft according to an alternative embodiment with continuous single thread of varying pitch and repose. 
     
    
       [0043]    [0043]FIG. 1 shows a shaft according to a preferred embodiment of the invention.  
         [0044]    Universal shaft  1  comprises a shaft body  2  having first and second ends  3  and  4 . Intermediate said ends are threads  5  and  6 . Threads  5  and  6  are respectively slow and fast threads and due to the difference in axial travel rates induced by the slow and fast threads an even compression is induced in the bone along the length of the thread. First end  3  comprises a flared tapered region  7  and narrow region  8  at second end  4 . The prior art teaches the use of prostheses which are purpose built for particular joints. The universal shaft according to the invention has a geometry which enables it to be inserted as a joint component in a wide variety of joints and which is anchored by means of threads which induce a local compression in the bone site upon insertion.  
         [0045]    [0045]FIG. 2 shows a shaft according to a preferred embodiment, inserted in a glenohumeral shoulder joint;  
         [0046]    Referring to FIG. 2 there is shown a simplified view of the glenohumeral joint (right side). This constitutes the major shoulder joint and essentially comprises the humerus  10  which terminates in humerus head  11  which locates in depression  12  in scapularis  13  . The anatomical name of the depression  3  is the glenoid fossa. This joint is held together by extensive muscle and ligament attachments which are not shown. Due to the nature of this joint it is susceptible to arthritis and generally wear over time which can lead to pain in the joint requiring surgical attention. In extreme cases the joint may require replacement. Many surgeons choose to use a Neer prosthesis for replacement of shoulder joints which suffer from osteo arthritis, rheumatoid arthritis, old fractures or fracture dislocations with traumatic arthritis. The shoulder may be totally or partially replaced known as a total or hemi shoulder arthroplasty respectively. There are numerous humeral components used at the present time with choices in respect of head thickness, distal shaft sizes and type, stem length and surface finishes. Prostheses are very often a matter of the surgeon&#39;s choice and may also be dictated by the needs of the patient.  
         [0047]    [0047]FIG. 2 shows a glenohumeral joint replaced with a shaft component  14  according to one embodiment of the present invention. In FIG. 2 the distal shaft is shown connected to an elbow  15  which is in turn connected to a head component  16  which engages the scapular  13 . Shaft  14  comprises a recess  17  which receives male taper  18 .  
         [0048]    In order to insert the prosthesis, the surgeon reams out the cavity of the humerus according to the size of the chosen shaft. Reaming is done approximately to accommodate thread, depth, shaft width and taper. A humerus cavity is reamed to approximately the width of the prosthesis and along the length of a humerus according to the length of the prosthesis shaft taking into account the ultimate alignment between the head of the prosthesis and the glenoid fossa. The Reaming may be done with a tool having a closely made configuration to that of the prosthesis. Shaft  14  is screwed into the medullary cavity if necessary with bone graft supplementation to ensure a strong prosthesis bone bond. Threads  19  and  20  impart an advantage to shaft  14  as they co operate to induce compression that cementing or precoating of the prosthesis is rendered non essential. Nevertheless at the surgeons choice, the prosthesis shaft  14  may be coated with bone growth promotion compounds such as hydroxyapetite. In order to insert the shaft in the humerus  10  the surgeon typically reams out the medullary cavity in order to accommodate shaft  14 . Where humerus head  11  is to be replaced this is surgically removed by the surgeon. Shaft  14  is then inserted in the medullary cavity by means of an alien key or with the assistance of a torque wrench. Shaft  14  includes fast thread  14  and slower thread  15  which advance axially at different rates upon rotation of the shaft. This induces a compression in humerus  10  and therefore adequate fixation of shaft  11 . As an alternative to spaced apart threads  19  and  20 , shaft  11  may be adapted with a single thread along at least part of the length of the shaft (see FIG. 8) having variable pitch thereby inducing an even axial compression in the humerus along the length of the threaded region.  
         [0049]    Recess  17  of shaft  14  into which is placed elbow component  15  is tapered so engagement is effected by means of a mutual taper in that recess  17  of shaft  14  is tapered outwardly whereas the mating taper on that end of elbow  15  which engages recess  17  tapered inwardly such that it is narrowest at its extremity. Similarly opposite end  15   a  of elbow  15  tapers as it extends into the shaft and this engages recess  21  in cup  16 . Cup  16  is adapted to move within recess  12  of scapula  13 .  
         [0050]    The aforesaid describes the present invention with reference to its insertion in a shoulder joint but it will be appreciated that the prosthesis can be universally inserted in other joints in the human body such as but not limited to the ankle, thumb, finger knee and hip.  
         [0051]    [0051]FIG. 3: shows the shaft inserted in a distal end of a femur and a proximal end of a tibia to form anchorage for a knee replacement;  
         [0052]    Referring to FIG. 3 there is shown a simplified view of a knee joint (right side) with opposing shafts. This joint essentially comprises the femur  30  and tibia  31 . Due to the nature of this joint it like the shoulder is susceptible to arthritis, injury and generally wear over time which can lead to pain in the joint requiring surgical attention. In extreme cases the joint may require replacement. There are numerous knee components available for use at the present time with choices including size, type, material and surface finishes. The selection will usually be dictated by the needs of the patient.  
         [0053]    [0053]FIG. 3 shows a knee joint  32  including a shaft component  33  located distally in femur  30 . The joint further includes opposing shaft component  34  located in tibia  31 . Components  33  and  34  are respectively inserted in cavities  35  and  36  prepared respectively in the in the medullary cavity of the femur  30  and tibia  31 .  
         [0054]    In order to insert shaft  33  the surgeon reams out the cavity  35  in femur  30  according to the size of the chosen shaft component. Reaming is done approximately to accommodate thread, depth, shaft width and taper so cavity  35  is a close fit to the outer contour of shaft  33 . Cavity  35  is reamed to approximately the width of the prosthesis allowing for taper and along the length of femur  30  according to the length of shaft  33  taking into account the ultimate knee alignment required. As with insertion of the shaft in the shoulder and other joints the reaming may be done with a reaming tool having a configuration close to that of the shaft. Shaft  33  is screwed into the medullary cavity  35  if necessary with bone graft supplementation to ensure a strong prosthesis bone bond. Threads  37  and  38  co operate to induce a compression force in the distal region of femur  30 . Cementing or precoating of the prosthesis is rendered non essential. Nevertheless at the surgeons choice, the shaft  33  may be coated with bone growth promotion compounds such as hydroxyapetite. Shaft  33  is then inserted in the medullary cavity  35  by means of an alien key or with the assistance of a torque wrench. Threads  37  and  38  advance axially at different rates upon rotation of the shaft, thereby inducing a compression in femur  30  enhancing fixation. As an alternative to spaced apart threads  37  and  38 , shaft  33  may be adapted with a single thread along at least part of the length of the shaft (see FIG. 8) having variable pitch thereby inducing an even axial compression in femur  30  along the length of the threaded region.  
         [0055]    Shaft  33  includes a female recess  39  into which is placed a male tapered stem  40  of liner  41  which completes the femoral component of knee  32 . This engagement is effected by means of a mutual taper in that recess  39  of shaft  33  is tapered outwardly whereas the mating tapered stem  40  which engages recess  39  is tapered inwardly such that it is narrowest at its free end.  
         [0056]    Shaft  36  is inserted in tibia  31  in a similar manner to that described for the insertion of shaft  33 . Shaft  36  is screwed into the medullary cavity  42 . Threads  43  and  44  co operate to induce a compression force in the proximal region of Tibia  31 . The shaft  36  may be coated with bone growth promotion compounds such as hydroxyapetite.  
         [0057]    Threads  43  and  44  advance axially at different rates upon rotation of shaft  36 , thereby inducing a compression in Tibia  31  enhancing fixation. As an alternative to spaced apart threads  43  and  44 , shaft  36  may be adapted with a single thread along at least part of the length of the shaft (see FIG. 8) having variable pitch thereby inducing an even axial compression in Tibia  31  along the length of the threaded region.  
         [0058]    Shaft  36  includes a female recess  45  into which is placed a male tapered stem  48  of knee platform liner  47  which completes the Tibial component of knee  32 . This engagement is effected by means of a mutual taper in that recess  45  of shaft  36  is tapered outwardly whereas the mating tapered stem  48  which engages recess  45  is tapered inwardly such that it is narrowest at its free end.  
         [0059]    The tapered connections formed by engagement of stems  40  and  48  with respective recesses  39  and  45  allow for some rotational alignment prior to driving home the stems.  
         [0060]    [0060]FIG. 4 shows the shaft inserted as a finger joint replacement.  
         [0061]    Referring to FIG. 4 there is shown a simplified view of a finger joint  50  with opposing shafts. Due to the nature of this joint it is susceptible to arthritis, injury and generally wear over time which can lead to pain in the joint requiring surgical attention. In extreme cases the joint may require replacement.  
         [0062]    [0062]FIG. 4 shows a proximal phalange  51  including a shaft component  52  located proximally. The joint further includes opposing shaft component  53  located in bone  54 . Components  52  and  53  are respectively inserted in cavities  55  and  56 .  
         [0063]    In order to insert shaft  52  and  53 , the surgeon reams out the cavity  55  and  56  according to the size of the chosen shaft component. Reaming is done approximately to accommodate thread, depth, shaft width and taper so cavities  55  and  56  are a close fit for components  52  and  53 . As with insertion of the shaft in the shoulder and other previously described joints the reaming is done with a reaming tool having a configuration close to that of the shaft. Also if required bone graft supplementation may be employed to ensure a strong prosthesis bone bond. As in the previously described joint applications threads  57  and  58  of shaft  52  co operate to induce a compression force in the phalange  51  as a result of thread  57  inducing faster axial travel on rotation of shaft  52  than is induced by slow thread  58 . Thus threads  57  and  58  advance axially at different rates upon rotation of the shaft, thereby inducing a compression enhancing fixation.  
         [0064]    As an alternative to spaced apart threads  57  and  58 , shaft  52  may be adapted with a single thread along at least part of the length of the shaft (see FIG. 8) having variable pitch thereby inducing an even axial compression in phalange  51  along the length of the threaded region.  
         [0065]    Shaft  52  includes a female recess  59  into which is placed a male tapered stem  60  of articulating platform  61  which completes the proximal phalange component of finger joint  50 . This engagement is effected by means of a mutual taper in that recess  59  of shaft  52  is tapered outwardly whereas the mating tapered stem  60  which engages recess  59  is tapered inwardly such that it is narrowest at its free end.  
         [0066]    Shaft  53  is inserted in bone  54  in a similar manner to that described for the insertion of shaft  52 . Shaft  53  is screwed into the medullary cavity  56  and threads  62  and  63  co operate to induce a compression force. Threads  62  and  63  advance axially at different rates upon rotation of shaft  53 , thereby inducing a compression enhancing fixation. As an alternative to spaced apart threads  62  and  63  of shaft  53  may be adapted with a single thread along at least part of the length of the shaft (see FIG. 8) having variable pitch thereby inducing an even axial compression along the length of the threaded region.  
         [0067]    Shaft  53  includes a female recess  64  into which is placed a male tapered stem  65  of articulating platform liner  66  which completes the artificial finger joint  50 . This engagement is effected by means of a mutual taper in that recess  64  of shaft  53  is tapered outwardly whereas the mating tapered stem  65  which engages recess  64  is tapered inwardly such that it is narrowest at its free end.  
         [0068]    The tapered connections formed by engagement of stems  60  and  65  with respective recesses  59  and  64  allow for some rotational alignment prior to driving home the stems.  
         [0069]    [0069]FIG. 5 shows the shaft inserted in an ankle joint including a distal tibia  71  and talus  72 . Shaft component  73  is located proximally in tibia  71 . The joint  70  further includes opposing shaft component  74  which is abbreviated to accommodate the limited space available in talus  72 . Components  71  and  74  are respectively inserted in preformed cavities  75  and  76 .  
         [0070]    In order to insert shaft components  73  and  74 , as previously described the surgeon reams out the cavities  75  and  76  according to the size of the chosen shaft component. Reaming is done approximately to accommodate thread, depth, shaft width and taper so cavities  75  and  76  provide a close fit for components  73  and  74 . As with insertion of the shaft in the shoulder and other previously described joints the reaming is done with a reaming tool having a configuration close to that of the shaft. Also if required bone graft supplementation may be employed to ensure a strong prosthesis bone bond. Threads  77  and  78  of shaft  73  co operate to induce a compression force in tibia  71  as a result of thread  77  inducing faster axial travel on rotation of shaft  73  than is induced by slow thread  78 . Thus threads  77  and  78  advance axially at different rates upon rotation of the shaft, thereby inducing a compression enhancing fixation in tibia  71 .  
         [0071]    As an alternative to spaced apart threads  77  and  78 , shaft  73  may be adapted with a single thread along at least part of the length of the shaft (see FIG. 8), having variable pitch thereby inducing an even axial compression in tibia  71  along the length of the threaded region.  
         [0072]    Shaft  73  includes a female recess  79  into which is placed a male tapered stem  80  of articulating platform  81  which completes the proximal tibial component of ankle joint  70 . This engagement is effected by means of a mutual taper in that recess  79  of shaft  73  is tapered outwardly whereas the mating tapered stem  80  which engages recess  79  is tapered inwardly such that it is narrowest at its free end.  
         [0073]    [0073]FIG. 6 shows an enlarged view of a shaft  74  inserted in a talus ankle bone  72 . Shaft  74  is inserted in talus  72  in a similar manner to that described for the insertion of shaft  73 . Shaft  74  is screwed into cavity  76  and threads  82   a  and  82  co operate to induce a compression force in talus  72  enhancing fixation of shaft  74 .  
         [0074]    Shaft  74  includes a female recess  83  into which is placed a male tapered stem  84 . Stem  84  has at its opposite end a male taper  85  which receives and retains thereon articulating platform  86  which completes the talus component of joint  70 . The engagement is effected by means of a mutual taper in that recess  83  of shaft  74  is tapered outwardly whereas the mating tapered stem  65  which engages recess  64  is tapered inwardly such that it is narrowest at its free end.  
         [0075]    The tapered connections formed by engagement of stems  80  and  84  (see FIG. 6) with respective recesses  79  and  83  allow for some rotational alignment prior to driving home the stems.  
         [0076]    [0076]FIG. 7 shows the ankle replacement joint assembly of FIGS. 5 and 6 incorporating two universal shafts detached from ankle joint  70 . FIG. 7 has corresponding numbering as for the components described in FIGS. 5 and 6.  
         [0077]    [0077]FIG. 8 shows a universal shaft  90  according to an alternative embodiment with continuous single thread of varying pitch and repose. Threads with the steepest angle are preferably parallel.  
         [0078]    In each of the above examples of insertion of the universal joint prosthesis the double threads may be disposed at the same or different pitch or the same or different angle of repose relative to a vertical or horizontal shaft axis. This will influence whether the shaft moves axially at the same rate along the length of the shaft upon insertion. It will also influence the level of compression force in the joint.  
         [0079]    The prosthesis can be universally inserted in other joints in the human body such as but not limited to the ankle, thumb, finger knee and hip. It will therefore be recognised by persons skilled in the art that numerous variations and modifications may be made to the invention broadly described herein without departing from the overall spirit and scope of the invention.