Design and testing of a flexible hip prosthesis

A prosthesis with flexibility and optimal proximal fit was constructed in titanium alloy. The aim was to create the same elasticity as in the femoral shaft. Distally it was cylindrical with slots in the stem and a spacer made of polymer was placed in the slots. The proximal part of the prosthesis was wedge-shaped in both planes and a collar with a conical contact surface to bone was used. Pure titanium was plasma sprayed on the upper part as well as underneath the collar allowing bony ingrowth. A test jig was constructed and 10 million cycles with 4000 N loading was planned. The prosthesis broke after 600 000 cycles and it was a typical fatigue failure. A commercially available flexible prosthesis was tested as a reference and broke after 29 000 cycles. It is concluded that construction of a flexible hip prosthesis using metal with reasonable security against fatigue failure breakage is difficult, and we are somewhat doubtful if it is possible.