PRELIMINARY INVESTIGATION OF A NOVEL CONTROLLED STIFFNESS PROXIMAL FEMORAL PROSTHESIS

Previous studies have suggested that a controlled stiffness prosthesis is required to address the conflicting requirements of minimizing str ess shielding and micromotion. The design for a controlled stiffness p rosthesis is proposed and a preliminary analytical investigation perfo rmed to assess its predicted performance before fabrication of a proto type component. The novel prosthesis consisted of a cobalt-chrome core and a flexible composite outer layer. Varying the composite layer thi ckness allowed the prosthesis stiffness to be controlled. Three varian ts of the controlled stiffness prosthesis were critically assessed usi ng the finite element method and their predicted performance compared with those of conventional prosthesis designs. The potential for stres s shielding was assessed by examining the periosteal strain energy and the potential for migration assessed by examining the endosteal minim um principal cancellous bone stresses. Both the conventional and contr olled stiffness implants performed poorly as press-fit prostheses. All the press-fit prostheses generated high cancellous bone stresses, sug gesting that excessive migration of these implants would be likely. Th e controlled stiffness implants performed better than the conventional implants when bonded to the surrounding bone. Although the controlled stiffness implants did not eliminate stress shielding of the calcar, they produced higher strain energies than the conventional designs. Th e findings of this study are that osseointegrated controlled stiffness implants may perform better than current osseointegrated cementless p rostheses and therefore it is worth while progressing to the next stag e, of prototyping an implant.