Biomechanics of bipolar hip endoprostheses.

The bipolar endoprosthesis was designed to alleviate problems encountered with conventional hip endoprostheses. Recent designs have advocated the use of offset centers of rotation between the inner and outer joint-bearing surfaces to generate a valgus-producing moment on loading (antivarus head dynamics). This study was undertaken to justify this biomechanical feature by performing (a) cyclic loading of the bipolar head in the varus position, (b) analysis of motion between the inner and outer bearing surfaces, and (c) evaluation of antivarus head dynamic function. Four endoprosthetic designs were studied: the Bateman UPF, Osteonics UHR, Giliberty II, and Bi-Centric. Significant plastic deformation occurred in the UPF and UHR prostheses when loaded in an extreme varus position. Theoretically, motion is most likely to occur at the inner bearing if the frictional coefficients are equal at both surfaces. The UHR prosthesis with eccentric offset of bearing centers can provide the antivarus effect in laboratory conditions. Whether the antivarus effect and/or the dominant inner bearing motion will occur in vivo awaits careful clinical study.