Three-dimensional surface models of the normal hemipelvis derived from volumetric CT data on 42 patients were used to determine the radius, depth and orientation of the native acetabulum. A sphere fitted to the lunate surface and a plane matched to the acetabular rim were used to calculate the radius, depth and anatomical orientation of the acetabulum. For the 22 females the mean acetabular abduction, anteversion, radius and normalised depth were 57.1° (50.7° to 66.8°), 24.1° (14.0° to 33.3°), 25 mm (21.7 to 30.3) and 0.79 mm (0.56 to 1.04), respectively. The same parameters for the 20 males were 55.5° (47.7° to 65.9°), 19.3° (8.5° to 32.3°), 26.7 mm (24.5 to 28.7) and 0.85 mm (0.65 to 0.99), respectively.

The orientation of the native acetabulum did not match the safe zone for acetabular component placement described by Lewinnek. During total hip replacement surgeons should be aware that the average abduction angle of the native acetabulum exceeds that of the safe zone angle. If the concept of the safe zone angle is followed, abduction of the acetabular component should be less than the abduction of the native acetabulum by approximately 10°.

Introduction and Aims: A recently reported study and reports in the German lay press have indicated that there may be an increased incidence of limp following Robodoc THR. This paper presents the detailed functional aspects from the first randomised US FDA multicentre trial.

Method: The controlled randomised trial was conducted from 1994–98, with 136 hip replacements performed on 119 patients at three centres. The implants were either AML (Depuy) or Osteoloc (Howmedica). Twenty-eight patients were lost to follow-up at two years. Twenty-four month Harris hip score surveys for the remaining 108 patients – 53 Robodoc and 55 Control – were reviewed and relevant scores were tabulated for limp and use of support aids.

Results: In the Robodoc group, 45 patients had no limp and eight patients had slight to moderate limp. In the control group, 44 patients had no limp, and 11 patients had slight to moderate limp. The average Harris limp score was 10.49 for the Robodoc group and 10.24 for the control group (NS). In the use of supportive aids and canes, at 24 months, there was essentially no difference between the average of the scores. In the Robodoc group,47 patients required no aids, and six required the use of a cane. In the control group, 49 patients required no supportive aids, and six patients required the use of a cane.

Conclusions: In this randomised controlled study no statistically significant difference was found between the Robodoc and the manual method in the incidence of limp and use of supportive aids, following primary Total Hip Arthroplasty. This data is also consistent with other published studies using Robodoc and is in contrast to a recent study from one centre.