Offset acetabular components introduce torsion on the implant and may increase the risk of fixation failure

Loading of an offset acetabular component causes torsion on the implant around the center of the reamed acetabular bed. A mechanical analysis was performed to determine this torsional moment. A 70 kg person walking normally on a well-positioned, 4-mm offset acetabular component will produce torsion on the prosthesis of approximately 3.86 Nm. A 10-mm offset increases the applied torsion to 9.65 Nm. Vertical placement of the cup further increases this torsion. These torsional moments are comparable to moments shown to cause failure of the initial interference fit of cementless acetabular components in vitro. If using an offset cementless acetabular component, one should initially limit weight bearing and consider using pegs or screws to augment fixation and minimize the risk of interference fit failure.