Fracture of the outer metallic head of the bipolar hip prosthesis: an unusual bearing surface failure.

Fracture of the bearing surface is an infrequent cause of failure of a hip arthroplasty. Although well documented with ceramic heads, fracture of the metallic head is much rarer. We report a case of a fracture of the outer metallic head of a modular cemented bipolar hemiarthroplasty 2 years after the index procedure. Over time, the outer head lost its intended motion and assumed a vertical position. We hypothesized that this position caused asymmetrical loading with stress concentration at the poles, compounded by repeated impingement between the skirted inner cobalt-chromium (Cr-Co) head and the outer stainless steel head of this particular prosthesis. These were supported by the finite element studies. In addition, scanning electron microscopy and energy dispersive x-ray studies showed metallurgical defects that seemed to have initiated and/or accelerated the fracture. Although rare, this mode of failure calls for increased awareness, periodic follow-up, and quality control.