Abstract:
A stemless hip prosthesis uses one or more cables which wrap around the prosthesis and the femur to mount the prosthesis in place thereby avoiding the need to provide the prosthesis with a stem located in the medullary canal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon provisional application Ser. No. 60/140,437, filed Jun. 23, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     Total hip arthroplasty (THA) using metallic hip prostheses has been successfully performed since the early 1960&#39;s and is now a routine procedure. The use of cement to fill the space between the bone and the prosthesis, and bony ingrowth, where the bone grows into a porous prosthesis, have been the two primary methods of fixation used. For the structure of the femur before THA, the load distribution can be essentially resolved into an axial component, two bending moments and a torsional moment which depend upon the leg stance. The distribution of these load components is changed after THA. The current methods of fixation allow for transfer of axial loads to the bone mainly through shear stresses at the bone-implant interface. (The muscles attached to the femur transfer load and moments as before THA.) The bending moment is effectively transferred to the bone, primarily through contact between the prosthesis and bone in two or more localized regions. In addition, the great disparity in the stiffness of a metallic prosthesis and the surrounding bone reduces bending displacements changing the bending moment distribution in the surrounding bone. The current procedure although very successful in the older population has a relatively lower rate of success in the younger population. 
     A major cause of failure of the prosthesis using the current design methodology is associated with the resorption of the bone which can be the result of stress shielding of the bone caused by the use of a stiff prosthesis as well as the increase in shear stresses at the bone-prosthesis interface. The effective transfer of loads will depend on the stiffness of the prosthesis and the bone-prosthesis interface as well as physiological loading of the proximal end. 
     The conventional approach for providing a hip prosthesis incorporates the use of a stem mounted in the femur. It would be desirable if a hip prosthesis could be provided which is stemless. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a stemless hip prosthesis. 
     A further object of this invention is to provide such a stemless hip prosthesis which can easily be applied while still maintaining its effectiveness. 
     In accordance with this invention the prosthesis is mounted to the femur through the use of cables. Preferably the cable or cables are wrapped around a portion of the femur and a portion of the prosthesis. 
    
    
     THE DRAWINGS 
     FIG. 1 is a side elevational view of a stemless hip prosthesis in accordance with this invention mounted to a femur which is shown in phantom; and 
     FIGS. 2-4 are views similar to FIG. 1 of alternative practices of the invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is a new prosthetic device to be used in total hip replacement surgery. It involves a conceptual change from the current design methodology which will enable a long term use of the prosthesis in a relatively younger and active population. The focus of this new approach is to apply the load at the proximal end of the femur rather than transferring the load along the prosthesis stem length. The bending moment is applied across the entire cross section of the femur including the greater trochanter. This approach required the development of a new method of fixation which results in a reduction in interface shear stresses, relative torsion and stress shielding. 
     The problems identified with the conventional design are as follows: The bending moment is applied through an intermedullary stem resulting in stress concentrations at the proximal, medial and the distal lateral ends of the prosthesis. The axial loads and the torsional moments are transferred to the bone across the bone prosthesis interface resulting in high interface shear stresses. Due to the high stiffness of the prosthesis, there is a reduction in bending displacements, resulting in stress shielding. This disparity in stiffness also contributes to interface shear stresses. 
     The new design eliminates interface shear stresses by using cables as a means of fixation. Cables support axial loads, but not bending moments. Thus, they do not increase the bending stiffness of the bone. The short flexible stem-bolt arrangement  12  in the medial calcar region extends through the prosthesis body  13  into the femur  14  and provides torsional support and an anchor while increasing interface shear stresses locally. The bending stiffness of the lateral region is effectively utilized by the later described use of the metal clip-cable arrangement  30 . This also provides an anchor and some torsional support. The cables contact the tendon, resulting in only a marginal increase in local stresses. This arrangement allows a more “natural” stress distribution across the proximal femur cross section. Stress shielding is effectively minimized, since there is no increase in the effective bending stiffness of the femur. 
     An important aspect of this design is the relative ease of revision (if required) which would essentially involve tightening of the cables. In the extreme, revision surgery would be the use of a conventional prosthesis. In this case the new design has a distinct advantage that a very small portion of the bone is initially removed. 
     The invention could be applied in various manners. In general, a cable  30  which could be in the form of a known surgical high strength steel cable is applied around a portion of the prosthesis  10  and a portion of the femur  14 . Such known surgical cable includes a clamping device  32  which would be used to tighten the cable and dispose the cable in its anchored condition. Any excess cable could then be removed. The cable would be sufficiently flexible to conform to the prosthesis and femur around which the cable is wrapped. 
     FIG. 1 illustrates one practice of the invention wherein the prosthesis  10  includes the generally spherical joint  16  extending from prosthesis body  13  and includes a mounting plate  18  with its downward extension  20 . Extension  20  would be located within the bone. A conventional fixation screw  12  would pass through prosthesis  10  into femur  14  at an angle in the known manner. The cable  30  is illustrated as being wrapped around the prosthesis body  13  directly above the plate  18  and around the femur. The cable is then sufficiently tightened by any suitable clamp device  32  to firmly anchor the prosthesis  10  on the femur  14 . Such clamp device could be the type of device generally used with surgical high strength cables. 
     FIG. 2 illustrates a variation wherein either a plurality of cables  30 ,  30 A are wrapped around the prosthesis  10  and the femur  14  or a single cable is wound a multiple number of times around the prosthesis  10  and the femur  14 . 
     FIG. 3 shows a variation wherein a medial lateral hole  34  is drilled in the femur at the angle of cable  30 . The cable  30  is threaded through the hole  34  and then wrapped around the femur and prosthesis. 
     FIG. 4 shows a variation of the invention in which a channel or slot  36  is formed in prosthesis  10  and the cable  30  is located in the channel  36  as well as being wrapped around the femur  14 . 
     As shown in FIG. 4 the slot  36  thus creates a shoulder against which the cable would be located to hold the cable in place. Similarly, as shown in FIG. 1 the mounting plate  18  extends outwardly beyond the peripheral surface of the prosthetic device body. The cable  30  is located against the shoulder so that the shoulder holds the cable  30  in place. 
     It is thus to be understood that the cables are used in combination with a fixture screw to firmly and effectively anchor the prosthesis in place without the need for a stem extending downwardly from the prosthesis into the medullary canal. The number and location of the cables may vary provided that the cables are wrapped around both the prosthesis and the femur to firmly anchor the prosthesis in place.