Abstract:
A prosthesis for partial replacement of a long bone in humans or animals, the prosthesis including: a proximal or distal end component forming one component of a prosthetic joint; a shaft on or assembled with the distal or proximal end component in use; a stem for engagement in a resected bone; and an initially separate collar assembled to the shaft or an extension thereof in use, the collar being dimensioned to abut the resected end face of the bone into which the prosthesis is to be fitted, wherein the end face of the collar facing the resected face of the bone is sized and shaped to correspond to the resected end face of the bone.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 11/981,394 filed Oct. 31, 2007 which is a continuation-in-part of application Ser. No. 11/433,953 filed May 15, 2006 now U.S. Pat. No. 7,662,189, which is a continuation of application Ser. No. 10/450,559, filed Jun. 12, 2003, now U.S. Pat. No. 7,044,976. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to prostheses for partial replacement of a long bone in human or animal bodies. 
     BACKGROUND TO THE INVENTION 
     In the case where limbs are badly damaged or large amounts of bone must be surgically removed, e.g. in the case of treatment of bone cancer, there is a need for prostheses which replace a large part of the natural bone and which can be assembled by the surgeon to meet a wide range of conditions. The present invention provides a modular system and components thereof for constructing effectively customised prostheses to meet a range of such surgical requirements. Typical requirements may be, for example, to replace the proximal or distal femur. Existing modular prostheses for the purpose such as, for example, those in U.S. Pat. No. 4,623,349, U.S. Pat. No. 4,963,155 and U.S. Pat. No. 6,048,365 generally allow customisation but generally do not accommodate for significant differences between patients and do not allow for effective, cost-effective and efficient tailoring of the prosthesis to the needs of a patient. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a prosthesis for partial replacement of a long bone in humans or animals, said prosthesis comprising a proximal or distal end component forming one component of a prosthetic joint, a shaft assembled with the distal or proximal end and a stem for engagement in a resected bone, including a collar which may be assembled to the shaft or extension thereof, the collar being dimensioned to abut the resected face of bone into which the prosthesis is to be fitted, wherein the end face of the collar facing the resected face of the bone is sized and shaped to correspond to the resected end face of the bone. 
     Preferably the collar has a portion, proximate the resected face in use, having a surface treatment adapted to encourage bone growth thereover. Suitably that portion of the collar is adapted to taper towards the resected face. 
     It is very important that the end of the prosthesis that abuts the resected bone of the patient is sized and shaped to approximate the resected bone. In the femur the proximal and distal segments of the bone are generally ovoid (distally with the greater dimension being medial to lateral, proximally with the greater dimension being anterior to posterior). The central section of bone is generally round. Furthermore there is significant variation in the overall dimensions of the bone between gracial female femorae and the largest robust male femorae. 
     The prosthesis end should not be undersized since in a weight bearing environment the prosthesis shaft may then unduly stress the bone and in certain situations may even telescope into it. Conversely it is important that the abutment end not be oversized relative to the bone since the soft tissues—muscles, tendons and ligaments—of the limb should suitably be substantially unhindered at the interface between the prosthesis and the bone to allow those soft tissues to glide freely back and forth over the bone surface and not rubbing against a raised step/shoulder at the interface leading to wear and/or tissue inflammation. By designing a separate collar defining the end face of the prosthesis that abuts the resected end face of the bone, the surgeon can be provided with a cost effective and efficient kit having multiple different collars to select amongst them the one that best suits the dimensional requirements of a given patient addressing the above problems. Furthermore, by optimally matching the abutment end of the prosthesis to the resected bone of the patient this helps to optimise osseointegration of that part of the prosthesis with the bone when that part is treated for osseointegration and thereby significantly reduces risk of aseptic loosening. 
     Preferably the shaft, stem and the distal or proximal end components are connected by male and female tapers, each male taper including a projection which extends longitudinally of the prosthesis but is offset from its axis and engages in a corresponding recess in the female taper. The projection maintains adjacent components in the desired angular relationship. Advantageously a cross-hole is provided for access transversely to the region of the end of the male taper by a disassembly tool. 
     In the case of proximal femoral replacement prostheses, the prosthesis preferably includes a trochanter replacement component assembled with a shaft and stem, the trochanter replacement component being shaped to correspond approximately with an anatomical trochanter and including a femoral neck for receiving a femoral ball and a generally flat face opposite the femoral neck for connection to residual bone or soft tissue. 
     According to another aspect of the invention there is provided a collar for a prosthesis for partial replacement of a long bone in humans or animals, said prosthesis comprising: a proximal or distal end component forming one component of a prosthetic joint; a shaft on or assembled with the distal or proximal end component; a stem for engagement in a resected bone; and an initially separate collar assembled to the shaft or an extension thereof in use, the collar being dimensioned to abut the resected end face of the bone into which the prosthesis is to be fitted, wherein the end face of the collar facing the resected face of the bone is sized and shaped to correspond to the resected end face of the bone. 
     Preferably the end face of the collar facing the resected end face of the bone is substantially circular cylindrical or ovoid cylindrical and has an outside diameter that is selected to correspond substantially to the outside diameter of the resected end face of the bone. Preferably the end face of the collar terminates inboard of the external dimension of the resected end face of the bone. 
     Preferably at least a portion of the collar has a surface treatment designed to encourage bone growth. Said portion of the collar may be formed with an externally stippled surface and/or coated with hydroxyapatite or other bone growth promoting material. 
     Preferably at least a portion of the collar is adapted to taper towards the resected end face of the bone. Preferably the collar has at least one lug or ear which is adapted to co-operatively engage with a corresponding at least one recess on said shaft of the prosthesis to counter relative rotation therebetween. 
     According to a further aspect of the invention there is provided a kit for a prosthesis for partial replacement of a long bone in humans or animals, said prosthesis comprising: a proximal or distal end component forming one component of a prosthetic joint; a shaft on or assembled with the distal or proximal end component; a stem for engagement in a resected bone; and an initially separate collar assembled to the shaft or an extension thereof in use, the collar being dimensioned to abut the resected end face of the bone into which the prosthesis is to be fitted, the kit comprising two or more said collars of different sizes and/or shapes. 
     The kit preferably comprises a greater number of collars than number of stems. The kit may comprise only one stem and a plurality of differing collars. Preferably the kit comprises at least one collar that is substantially circular cylindrical and one that is ovoid cylindrical. Preferably the kit comprises at least one collar that is surface treated for osseo-integration and one that is not. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Detailed features of prostheses in accordance with the invention will become apparent from the accompanying drawings, in which: 
         FIG. 1  is a side elevation of assembled components of a proximal femoral prosthesis; 
         FIG. 2  is a side elevation of a distal femoral prosthesis in accordance with the invention; 
         FIGS. 3A and 3B  show various views of a trochanter replacement component in accordance with the invention; 
         FIGS. 4A and 4B  show various views of a modular shaft for use in the present invention; 
         FIG. 5  shows various views of a femoral stem for use in the present invention; 
         FIG. 6  shows various views of a collar for use in the prosthesis in accordance with the invention; 
         FIG. 7  shows several views of a modular extension shaft for use in the present invention; 
         FIGS. 8A to 8C  show side elevation views of three variants of collar having differing shapes—distally tapered, untapered and proximally tapered; 
         FIG. 9  is a side elevation view of a variant of collar without stippling being suitable for use in palliative limb reconstruction where there is no long term fixation requirement; 
         FIGS. 10A and 10B  are side and distal end elevation views of two variants of the collar that are oval in end shape; and 
         FIGS. 11A and 11B  are side and distal end elevation views of two further variants of the collar that are roughly circular cylindrical in end shape but with flats to correspond to the resected bone end face. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring firstly to  FIG. 1 , this drawing shows diagrammatically an assembled, proximal femoral prosthesis comprising a trochanter component  1  having a femoral neck  2 , which is tapered to receive a femoral ball in a conventional fashion. The trochanter component is shown in more detail in the views shown in  FIG. 3  and it may be seen that it corresponds roughly to the anatomical shape and has a generally flat face  3  for attachment to any residual bone or to soft tissues. The trochanter component  1  has an internal female taper, best seen in  FIG. 3 , and is assembled onto a male taper carried by a shaft  5 . The detailed construction of shaft  5  are apparent from the views in  FIG. 4  and it will be seen that shaft  5  has a male taper  33  at one end for engagement into the female taper of the trochanter component  1  and at its other end has a female taper  7  for engagement with one end of a stem  8  (shown in more detail in  FIG. 5 ). 
     Stem  8  is fitted into a resected femur  9  either by press-fit or by using bone cement. Surrounding the distal end of the shaft  5 , is a collar  10  which is shown in more detail in  FIG. 6 . Collar  10  has a distal lower end  11  which abuts cancellous bone forming the face of resected bone  9 . The dimensions of collar  10  are selected so that the end face of collar  10  approximates to the dimensions of the resected end face of the resected bone. As illustrated it terminates in board of the external dimension of the bone  9 , ie is somewhat smaller in outside diameter than the outside diameter of the resected end face of the bone. In this first embodiment the collar  10  is formed on its external surface with stipples and coated with a bone growth stimulating material such as hydroxyapatite. These measures encourage cancellous bone to grow over the surface of collar  10  and help to more firmly lock the prosthesis into the residual resected bone  9 . 
     It is often possible when removing damaged bone or tumour to retain the residual portion  12  of the trochanter. This residual piece of bone may have ligaments or other soft tissue attached to it and it is useful to use this to assist in stabilising the trochanter replacement component. The residual bone  12  can be attached to the trochanter component by clamping a plate  13  to the face  3  of the trochanter component. 
       FIG. 2  shows a view of an assembled distal prosthesis comprising a condylar head component  21  forming one part of the knee joint which is connected to a shaft  5  in an analogous way to the way in which the trochanter component  1  is connected to the shaft  5  in  FIG. 1 . Similarly, shaft  5  is connected to stem  8  in a similar way to that previously described in connection with  FIG. 1  and collar  10  is slid over the tapered lower portion of shaft member  5  just as described above in connection with  FIG. 1  and has a distal tapered portion  11  which abuts the resected face of the femur  9 . In a similar way, this encourages bone growth over the surface of the collar  10 . 
     Details of the trochanter component  1  are shown in the views of  FIG. 3 . It would be seen that the flat face  3  of the trochanter component  1  are formed in a pattern of spikes  30 . This pattern of spikes is provided to ensure a firm connection with any residual bone  12  (see  FIG. 1 ) and serves to further stabilise the prosthesis. It would be seen that the trochanter component has a female taper terminating internally with an offset recess  31 . Recess  31  is shaped to be engaged with a projection  32  formed on the male taper  33  of the shaft  5 . (See  FIG. 4 ). The trochanter component is provided with a transverse hole  34  into which a disassembly tool can be inserted to force the trochanter component and the shaft apart where disassembly is required. As can be seen in  FIG. 4 , the shaft  5  also has a female taper  7  and is formed with a similar recess  35  to the recess  31  in the trochanter component. Shaft  5  also includes a transverse hole  36  into which a disassembly tool can be inserted to force apart the shaft and the stem component which is designed to fit into the taper  7 . 
     Referring further to  FIG. 3 , it will be seen that the trochanter component also incorporates holes  36 . These are intended for suturing the trochanter component to soft tissue such as tendons in the event that there is no residual bone, to which tendons are attached, which could be clamped to the stippled face  3 . The trochanter component is intended to be provided as a standard component of a modular system although there would be right and left handed versions. There may also be alternative trochanter components in which the degree of offset of the femoral neck differs to suit a particular patient. 
     As can be seen from  FIG. 4 , the femoral shaft, although standardised so far as the tapers are concerned, is supplied in lengths of 15 mm intervals. The femoral shaft is also provided with notches  37  designed to engage with corresponding lugs or ears  38  which are integral with the collar  10 . The engagement of the collar on the shaft  5  and also the ears  38  in the notches  37  enables the collar to be fitted securely on the shaft. The lugs or ears  38  provide stability of the prosthesis against rotation. Suitably the collar has a pair of diametrically opposed ears  38  but in some embodiments may have only one such ear  38 . Suitably the femoral shaft has more notches than the number of corresponding ears  38  and at a range of different spacings around the circumference to give the surgeon choice in the orientation of mounting of the collar, suitably at two different radial orientations spaced at 90 degrees to each other. This is particularly useful where the collar is ovoid cylindrical at its end face rather than circular cylindrical. 
       FIG. 5  shows various views of the stem component  8 . As can be seen from the cross-sectional view, the stem is formed with flutes for engaging in cortical bone and may be straight or curved to accommodate any curvature in the bone canal. The proximal end of the shaft has a taper  42  which corresponds with the internal taper  7  of the shaft component. 
     The shaft also includes a projection  43  which corresponds with the recess  35  in the shaft component. The shaft component  5  includes a transverse hole  36  for disassembly purposes whereby a tool inserted through the hole will press apart the stem and the shaft by bearing on the corresponding end faces of the stem and shaft. As can be seen from  FIG. 5 , the stem is available in a number of diameters and lengths and may be curved or straight to accommodate the various surgical problems which might be encountered in fitting such a prosthesis. 
     As can be seen from  FIG. 6 , collar  10  has an internal taper  39  adapted to fit on a corresponding taper  40  at one end of the shaft  5 . As can be seen from  FIGS. 6 and 8 to 11 , the collar is supplied in a number of different forms, with different diameters, D 1  and D 2  as well as in a range of different shapes. The variants of the collar may be supplied individually or as a kit to enable the surgeon to select the best match in size and shape for the resected end face of the resected bone. In  FIG. 6  the external cylindrical form of the collar is inwardly tapered towards the distal end facing toward the resected end face of the bone. The surface that is treated for osseointegration by stippling and hyrdoxyapatite (HA) coating is shown as extending over this tapered end portion and extending up the outer cylindrical surface too. In  FIG. 8B  the collar has no external taper and in  FIG. 8C  the collar is proximally tapered instead. In  FIG. 9  a variant is shown having no stippling or HA coating and which is suitable for use in situations where no permanent fixation is required. In  FIGS. 10A, 10B, 11A and 11B  further variants are shown with ovoid or not wholly circular cylindrical distal or proximal ends. 
       FIG. 7  shows a modular extension shaft which enables the total assemblies as shown in  FIGS. 1 and 2  to be extended. Modular extension shaft  50  has male and female tapes at opposite ends which correspond respectively with taper  7  of the modular shaft and taper  42  of the modular stem. Of course, modular extension shaft  50  may be used in conjunction with a standard shaft or alone to connect the trochanter component  1  or condular member  21  with the stems  8 . 
     Although the prosthesis shown in  FIGS. 1 and 2  is a femoral replacement prosthesis, analogously formed components could be employed to replace another long bone such as the humerus. 
     As described above, the components of the prosthesis can be readily dismantled by using a disassembly tool inserted in the transverse walls in the components at each tapered join either after a trial assembly or in a revision operation. 
     The material preferably used is a titanium alloy throughout except for the femoral heads where cobalt chromium or ceramic heads are preferably used.