Stem fractures of extra-heavy Cobra femoral hip prostheses. Report of two cases.

Stem fractures of three extra-heavy, large Charnley serrated Cobra femoral components were observed in two young, heavy, and active male patients. The average time from implantation to stem failure was 68 months (43, 76, and 86 months). Two fractures extended from the lateral surface toward the lowest serration, which apparently acted as a stress-riser despite the caliber of the stem. Making a prosthesis thicker by using the same material does not solve the problem of fatigue fracture if the material, design, or cement interface is at fault.