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.