Effect of restoration of combined offset on stability of large head THA.

Improvements in bearing technology have resulted in a reduction in wear and larger femoral heads to reduce dislocation rates. We retrospectively reviewed 668 primary THAs (580 patients) inserted with 32 mm or 36 mm femoral heads (1/2003-6/2007) performed through a posterior approach with posterior repair. Patients received a questionnaire assessing instability and dislocations. We compared clinical and radiographic data between patients with and without dislocations. Follow-up averaged 3.4 years (range 2-6.6 years). The dislocation rate was 1.3% (9/668). Decreasing the combined postoperative offset (lateral offset + medial offset measurements) and using acetabular cups larger than 58 mm in outer diameter increased the risk of dislocation. A majority of the hips that dislocated had negative combined offset and abduction angles in the upper and lower quartiles of all hips studied. Appropriate positioning should be used to minimise dislocation risk in THAs inserted with large femoral heads.