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 th e femoral shaft. Distally it was cylindrical with slots in the stem an d 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 spr ayed 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 pros thesis was tested as a reference and broke after 29 000 cycles. It is concluded that construction of a flexible hip prosthesis using metal w ith reasonable security against fatigue failure breakage is difficult, and we are somewhat doubtful if it is possible.