Acetabular morphology and resurfacing design

The bony surfaces of 18 archaeological hemipelves were scanned using a 3D laser surface scanner and CyDir™ software on a Silicon Graphics workstation. The acetabular area was selected and point data from the approximately spherical bone surface saved. These data were input to a MATLAB routine that calculated the radius and centre of the best-fit sphere. The goodness of fit was estimated using the mean and standard deviation of the distance of the bone surface points from the sphere surface. Eight points, at approximately equal distances around the acetabular rim, were selected with reference to bony landmarks. A plane containing three of these points served as an orientation reference plane. The vectors joining the eight rim points to the centre of the best-fit sphere were found. The angles between these vectors and the normal to the reference plane were calculated. Paired angles were summed to give the angle subtended by the acetabular rim in four directions. The overall mean angle was 158° (range of mean angles 145°–173°). The largest individual angles, some exceeding 180°, were in the superior–inferior direction, while the mean angle in the anterior–posterior direction, i.e. that controlling flexion-extension, was 152°. Males had larger subtended angles than females, although the difference was not statistically significant. Simulated reaming increased all angles by approximately 10°. The subtended angles are important parameters in the design of the acetabular component of a hip replacement and particularly important in resurfacing hip replacement when the volume available is tightly constrained.