Aim: Total hip arthroplasty implant inventories are based on anatomic studies on populations with a different ethnic mix from New Zealand. The purpose of this study was to:

Compare the proximal femoral geometry with published data from the American population.

Compare the proximal femoral geometry with implant sizes currently being used in New Zealand.

Hypothesis: That New Zealanders have a significantly greater offset when compared with published American data.

Method: Fifty-seven femora were obtained from medical school cadavers. Using a standardised technique the femora (stripped of soft-tissue) were x-rayed and surface and endosteal measurements were taken. These measurements were compared with previously published data from an American population. The implants used in 200 consecutive primary total hip arthroplasties were measured to compare with them the New Zealand population.

Results: Twelve of the 14 measurements were significantly different when compared with the American population. In particular, femoral head offset averaged 46.7mm compared with 43mm in the American study. Femoral head diameter was also significantly larger (49.2mm vs 46.1mm).

Conclusion: If the goal of implant sizing is to reproduce the rotational axis and offset of the native hip joint, NZ Surgeons may be frequently under sizing implants with regard to offset.