Photoelastic analysis of stresses produced by different acetabular cups

Porous-coated acetabular components can provide long-term biologic fixation to bone. However, the periacetabular stress patterns and mechanisms by whi ch different types of cementless acetabular cups obtain initial stability i s not clear. In the current study, periacetabular stresses produced by diff erent cementless acetabular cup geometries were quantitated using a three-d imensional photoelastic model. The cup geometries consisted of trispiked, f inned, hemispherical, and nonhemispherical (wider than a hemisphere at the periphery) geometries. The cup models were loaded incrementally in the phot oelastic material to simulate periacetabular stress distributions at the ti me of implantation during surgery rather than under physiologic weightbeari ng loads. The peripheral stress distributions and their magnitudes induced by the trispiked and oversized hemispherical cups were similar, but the tri spiked cup induced localized high stress regions where the spikes penetrate the bone model, The fins separated the periacetabular material into quadra nts, which was associated with decreased peripheral stresses. A nonhemisphe rical geometry with a wider diameter at the rim than a hemisphere increased peripheral stresses more than an oversized hemispherical geometry and requ ired less force to seat the implant. Although various cementless acetabular cups can perform well clinically, they produce different periacetabular st resses and appear to obtain initial fixation by different mechanisms.