Abstract:
A prosthesis includes a body portion adapted to engage a bone. A rod is adapted to pass through the bone. A fixing element includes a bone engaging plate and a locking piece which engages the rod and is adapted to articulate with the bone engaging plate.

Description:
FIELD  
       [0001]     This invention relates to prostheses and particularly but not exclusively relates to femoral hip prostheses.  
       INTRODUCTION  
       [0002]     Total hip replacement has been used in the treatment of arthritic hips since the 1960&#39;s. Femoral and acetabular prosthesis components have been used to replace defective bone tissue. Some of the most commonly used femoral components comprise a metallic stem, about 150 mm in length, which may be inserted into the intramedullary cavity, extending along the femoral axis. While this type of prosthesis has been used with success among elderly patients, the success rate among younger, more active patients tends to be lower.  
         [0003]     As a result, a more conservative type of prosthesis has been developed, allowing a greater portion of the patient&#39;s natural bone and associated soft tissue to be preserved. In one example, during surgery, the bone may be resected on the femoral neck, just below the spherical head and the prosthesis may be mounted in the upper region of the femur, aligned with the axis of the femoral neck.  
         [0004]     Despite the increase in success associated with conservative femoral components, attachment of the prosthesis to the bone remains a problem. In some conventional systems, bone cement, HA or press-fit designs may be used to secure the prosthesis in place. It is also known in another example to use a bolt or screw in conjunction with a fixing plate to provide greater stability, however, it has been suggested that the rigidity of these devices may cause weakening of the bone and render it less able to withstand the stresses that may be imposed when the joint is under load.  
       SUMMARY OF THE INVENTION  
       [0005]     A prosthesis includes a body portion adapted to engage a bone. A rod is adapted to pass through the bone. A fixing element includes a bone engaging plate and a locking piece which engages the rod and is adapted to articulate with the bone engaging plate.  
         [0006]     According to various embodiments the locking piece defines a first bearing surface that is rotatable about a second bearing surface defined on the bone engaging plate. The rotation is about an axis substantially perpendicular to a longitudinal axis of the rod. The articulation of the locking piece with the bone engaging plate is spherical. The bone engaging plate includes attachment members adapted to secure the bone engaging plate to the bone. The attachment members may include screws or spikes.  
         [0007]     A method of implanting a prosthesis into bone includes resecting a neck portion of the bone and defining a resected surface thereat. A stem is inserted into a bone tunnel formed in the bone. A collar disposed on a first end of the stem is abutted onto the resected surface. A bone engaging plate disposed on a second end of the stem is located against an outer surface of the bone. A bolt operatively coupled between the stem and the bone engaging plate is advanced relative the stem whereby spikes defined on the bone engaging plate drive into the bone surface.  
         [0008]     According to various embodiments advancing the bolt compresses the stem and the collar into the bone. Locating the bone engaging plate against the outer surface includes locating the bone engaging plate against a lateral cortex of a femur. Advancing the bolt includes rotating a first bearing surface defined on the locking plate about a second bearing surface defined on the bone engaging plate. The rotation is about an axis substantially perpendicular to a longitudinal axis of the bolt.  
         [0009]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0011]      FIG. 1  shows an exemplary embodiment of hip prosthesis;  
         [0012]      FIG. 2  shows the positioning of the hip prosthesis of  FIG. 1  within the femur;  
         [0013]      FIG. 3  shows a hip prosthesis according to another embodiment; and  
         [0014]      FIG. 4  shows a hip prosthesis according to another embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     The following description of the embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Moreover, while the description below is directed to a hip prosthesis, the invention and implant may be directed to any portion of the body.  
         [0016]      FIG. 1  shows an exemplary embodiment of hip prosthesis  2  comprising a stem  4 , a bolt  6  having a head  8  and a proximal end  9 , and a bone engaging plate  10 . The stem  4  is tapered towards its distal end  5  and may be provided with an internally threaded bore  12  into which the end  9  of the bolt  6  can be screwed. The head  8  of the bolt  6  has a spherical bearing surface  7 , which engages in a spherical opening  11  formed through the bone engaging plate  10 . The bone engaging plate  10  may be free to track over the spherical bearing surface  7 , so that a desired angle between a plane X-X of the bone engaging plate  10  and a longitudinal axis Y-Y of the bolt  6  can be accommodated, as shown in  FIG. 2 . The bearing surface  7  of the head  8  and the corresponding bearing surface of the spherical opening  11  of the bone engaging plate  10  may be formed from cobalt chrome alloy although other materials may be used.  
         [0017]     The bone engaging plate  10  may be fitted with spikes  14 , which project towards the stem  4 . In an alternative embodiment, not shown, the bone engaging plate  10  could be provided with holes which receive bone screws. The outer surface of the bone engaging plate  10  may be smooth to allow free movement of soft tissues over the plate.  
         [0018]     The stem  4  has a rounded, trapezoid shape and may be fitted with a collar  16  for abutment on a resected surface of the femoral neck. Mounted on the collar is a neck  18 , comprising a support  20  and a mount  22 . The mount  22  may be tapered to receive an artificial femoral head in a manner well known in the art. Those surfaces of the collar  16 , stem  4  and bone engaging plate  10  which are adjacent to the bone may be provided with a porous or HA coating to promote bone ingrowth.  
         [0019]     Referring to  FIG. 3 , the embodiment described above is shown in position within the outline of a human femur, the removed bone tissue being indicated by a dashed line. The stem  4  may be inserted into the bone, aligned with the original femoral neck  26 . The collar  16  abuts the resected surface  28  of the femoral neck. The bone engaging plate  10  may be positioned on the lateral cortex  24  of the bone and the rod  6  may be passed through the lateral cortex  24 . Tightening of the bolt  6  within the stem  4  drives the spikes  14  on the bone engaging plate  10  into the lateral cortex  24  and also compresses the stem  4  and collar  16  into the bone. In this manner the prosthesis may be held securely within the bone. The spherical articulation of the head  8  of the bolt  6  with the bone engaging plate  10  enables the bone engaging plate to align accurately with the lateral cortex and also enables the securely held prosthesis to maintain a certain freedom of movement, thus avoiding the disadvantages associated with the fully rigid prostheses known in the art.  
         [0020]     Referring to  FIG. 4 , in an alternative embodiment, the bolt may be replaced by a rod  130  which may be attached to, or formed integrally with, the stem  104 . The rod  130  has a threaded end  132 , which may be adapted to receive a nut  134 . The nut  134  may be substantially spherical and performs in a like manner to the head  8  of the bolt  6  in the embodiment described above. The bone engaging plate  110  is thus able to articulate on the nut  134 , and thereby align accurately with the lateral cortex. The prosthesis may also be provided with a collar  116 , a neck  118  and attachment spikes  114  in a manner consistent with the above description.  
         [0021]     While the invention has been described in the specification and illustrated in the drawings with reference to various embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. Furthermore, the mixing and matching of features, elements and/or functions between various embodiments is expressly contemplated herein so that one of ordinary skill in the art would appreciate from this disclosure that features, elements and/or functions of one embodiment may be incorporated into another embodiment as appropriate, unless described otherwise above. Moreover, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.