Mechanisms of failure of modular prostheses

The expectations of wear and longevity of total hip components are based in large part on Charnley's early work. The evolution of the total hip from the one-piece, all-polyethylene acetabular component and fixed-head femoral component to the myriad of parts that comprise many of today's total hip designs has brought with it an array of potential mechanisms for failure that were not present in the earlier design. The risk/benefit ratio of these new designs may need to be reevaluated based on the additional mechanisms for failure that they provide. One hundred eleven acetabular hip prostheses and 139 femoral prostheses, all of modular configuration, retrieved by surgeons in the field, and sent for histologic examination, were analyzed for this study. A number of component characteristics were found to be correlated to early failure. These included acetabular designs with thin polyethylene bearings, poor fixation of the polyethylene to the metal shell, and geometries that permitted a moment to be applied to the bearing insert, tending to cause it to rotate in the metal shell. Modular femoral components were observed to be susceptible to corrosion, with titanium-alloy stems mated to cast cobalt-alloy heads at greatest risk attributable to a galvanic effect. All modular connections of femoral and acetabular components are at risk for disassociation and fretting; therefore, clever design and precision machining are necessary to produce prostheses in which the benefits of modularity exceed the risks.