Abstract:
A prosthesis includes a trunnion having a first end defining a uniform cross-section and a second end adapted to be fixed to a prosthesis stem. A ceramic head has a first bearing surface on which a second prosthesis fixed to a second bone can articulate. The ceramic head defines a recess. The first end of the trunnion is fixedly received within the recess. The second end of the trunnion is releasably connectable to the prosthesis stem by a taper connection.

Description:
FIELD 
     This invention relates to prostheses and particularly but not exclusively relates to femoral hip prostheses. 
     INTRODUCTION 
     Total hip replacement surgery is a common procedure involving the removal of defective bone tissue on both sides of the hip joint and the subsequent implantation of acetabular and femoral prosthesis components. Some of the most commonly used femoral components comprise a metallic stem, about 150 mm in length, which is secured into the inermedullary cavity of the femur, extending along the femoral axis. A neck may be provided on the upper portion of the stem, in order to support a prosthetic femoral head. The superior portion of the neck forms a trunnion, allowing connection with the substantially spherical femoral head by means of a Morse taper. This type of connection permits the use of femoral heads formed from a range of different materials. One such material is alumina ceramic, preferred for its superior wear characteristics when articulating against polyethylene and ceramic acetabular prosthesis components. 
     A known technique for strengthening ceramic femoral heads includes the insertion of a metal bush into the tapered recess in the head. The Morse taper connection may be strengthened by the inclusion of the metal bush, however, significant hoop stresses may be generated within the ceramic component. 
     It is also known to construct a modular femoral prosthesis component, whereby a metal adaptor is used in place of the conventional femoral neck. The adaptor may be secured to the femoral head and to the supporting stem by means of standard Morse taper connections. This type of construction allows both the femoral head and the metal adaptor to be replaced during revision surgery without the need to disturb the implanted femoral stem. 
     SUMMARY 
     A prosthesis system can include a trunnion having a first end defining a substantially uniform cross-section and a second end adapted to be fixed to a prosthesis stem. A ceramic head has a first bearing surface on which a second prosthesis fixed to a second bone can articulate. The ceramic head can define a recess. The first end of the trunnion can be fixedly received within the recess. The second end of the trunnion can be releasably connectable to the prosthesis stem by a taper connection. 
     According to additional features, the second end of the trunnion can define a non symmetrical cross section. The trunnion includes an annular shoulder which defines a second bearing surface in a plane substantially perpendicular to a longitudinal axis of the first end of the trunnion. The head can further comprise an annular bearing surface and the first end of the trunnion can be permanently fixed within the recess of the head such that the second bearing surface on the trunnion abuts the annular bearing surface on the head. In one example the trunnion defines an arcuate contour along its longitudinal axis. 
     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 preferred embodiment 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 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  shows two components of an exemplary hip prosthesis; 
         FIG. 2  shows the hip prosthesis components of  FIG. 1  in an assembled position; 
         FIG. 3  shows a hip prosthesis according to another embodiment; 
         FIG. 4  shows an exemplary stem for use with the hip prosthesis of  FIG. 1 ; and 
         FIG. 5  shows the stem and prosthesis of  FIG. 4  in an assembled position. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     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 same may be directed to an implant for any portion of the body. 
       FIG. 1  shows an exemplary embodiment of a hip prosthesis  2  comprising a head  4  and a trunnion  6 . The head  4  may be made from a ceramic material in the form of a partial sphere, which has a single planar surface  16 . A cylindrical recess  14  extends into the head  4 , from the planar surface  16  in a central position. The trunnion  6  may be made from metal and has a first end  8 , a second end  10  and a midsection  12 . The first end  8  of the trunnion  6  is a right circular cylinder. An annular shoulder  18  projects from the midsection  12 , supporting a planar bearing surface  20 . The second end  10  of the trunnion  6  may be tapered and has a non-rotationally symmetrical cross section XX. A pin  22  projects from the lower surface  24  of the second end  10 . Alternatively, the second end  10  of the trunnion  6  may include a recess in the form of a blind bore. 
     In  FIG. 2 , the two components of the hip prosthesis are shown in their assembled condition. The first end  8  of the trunnion  6  is received within the recess  14 . The first end  8  may be permanently bonded in place any suitable method such as by adhesive or by brazing, which may include active alloy brazing. The dimensions of the recess  14  and the first end  8  of the trunnion  6  are such that the surface  16  of the head  4  abuts the surface  20  of the annular shoulder  18 . 
     With reference to  FIGS. 2 and 3 , the midsection  12  of the trunnion  6  may be cylindrical and define a length L, which may be varied according to the particular needs of the patient. In another example, the midsection  12  may be curved, as shown in  FIG. 3 . The angle of curvature of the midsection  12  may also be varied according to the needs of the patient. The length and curvature of the midsection may be selected to provide the desired antiversion angle in the prosthetic joint. 
     The assembled components of the prosthesis  2  are suitable for use in conjunction with a metallic stem  24 , as shown in  FIG. 4 . The stem  24  has a linear portion  26  and a flared portion  28 . A recess  30 , in the form of a first blind bore, extends into the flared portion  28 . A second recess  32 , in the form of a second blind bore, extends from the base of recess  30 . The recess  30  may be tapered or it may be cylindrical. Alternatively, a pin (not specifically shown) may protrude from the flared portion  28  of the stem  24 . 
     In use, the stem  24  may be implanted into the intermedullary cavity of the femur such that the flared portion  28  emerges at the level of a resected femoral neck. The prosthesis  2  may be secured to the stem  24 , as shown in  FIG. 5 , by means of a taper connection between the second end  10  of the trunnion  6  and the recess  30 . The taper connection may alternatively be formed between the recess formed in the second end  10  of the trunnion  6  and the pin protruding from the flared end  28  of the stem  24 . In an assembled position, the pin  22  is received within the second recess  32  and acts as a centering device. The head  4  of the prosthesis  2  articulates within the shell of a corresponding acetabular implant (not shown). The surfaces  16  and  20  act as bearing surfaces for the transmission of forces generated in the joint. 
     The prosthesis components described may be supplied as a prosthesis kit. Each kit may be selected from a plurality of components including: femoral heads of varying diameters, trunnions of varying lengths and angles of curvature, and stems of varying sizes. The surgeon may then select the most appropriate combination of components for each individual patient in order to achieve a prosthesis that resembles the patient&#39;s original hip joint. Further, should problems be encountered during the positioning of the stem, misalignments can be corrected through the appropriate choice of trunnion. 
     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.