Purpose: The distribution of weight bearing area within the acetabulum is of importance in addressing trauma to the acetabulum, hip joint deformities and causes of osteoarthritis. According to Wolf’s law, bone density can indicate loading patterns experienced. The objective of this study was to characterize distributions of acetabular bone density patterns by regions in the normal population.

Method: CT scans of 22 subjects, mean age 70.6 with no evidence to hip joint pathologies were analysed. Bone density distribution maps were generated within AmiraDEV4.1 image analysis software using custom written plugins (Visage Imaging, Carlsbad, USA). Acetabular cup surfaces were semi-automatically segmented from the reconstructed CT volumes with an atlas-based approach. The acetabular cups were expanded 2.5 mm into the acetabular bone, and surface bone densities were calculated as the average bone density within ±2.5mm. The distribution maps were analysed using zones to spatially classify areas of high and low bone density in a healthy population. The acetabular cups were aligned using the acetabular rim plane that was landmarked, and by rotating the cups, such that a 900 abduction angle and a 00 anteversion angle were achieved. The grid used was divided to quadrants, and subdivided into radial thirds of the average rim radius. The correspondence of left and right density maps was investigated by comparing the average bone density in corresponding zones and across the population.

Results: High bone densities were found around the roof of the acetabulum aligning with the femoral mechanical axis during standing. The highest average bone density were found to be the superior and posterior walls of the acetabulum, corresponding to regions 8, 9, and 12 compared to other regions of the acetabuli (P< 0.01). A strong correlation was found between left and right sides within subjects (R=0.91, P< 0.05); and weaker correlation was also found for overall average bone density, (R=0.77, P< 0.05).

Conclusion: The location of the zones with the highest average bone density agrees with cadaveral studies of the maximum contact stress in the acetabulum (zones 9 and 12). [1,2]. It may explain why trauma to these areas carries a higher risk for early arthritic changes.