The human hip joint and its long-term load intake - how x-ray density distribution mirrors bone strength

The aim of this study was to gain information about the topographical distribution of the mechanical strength of the subchondral bone plate of the hip joint and evaluate the correlation to its density distribution. Our intention was to describe a method of visualising and monitoring the long-term load intake of the hip using conventional CT-data in a way which might be applied in clinical practice. We examined the lunate surface of the acetabulum (facies lunata) and femoral head in 25 cases, looking at the density of the subchondral bone plate by computed tomography osteoabsorptiometry (CT-OAM) and determined its mechanical strength by indentation testing using an osteo-penetrometer. The resulting distribution patterns were matched and statistically analysed, showing an inhomogeneous but regular and reproducible distribution of mineralisation and mechanical strength throughout the joint surface. Maximal density was found anterosuperiorly and near the rim of the facies lunata and in the superior area of the femoral head. For each specimen a correlation of density and strength (r2 = 0.77 - 0.97) was found (p>0.01). The density distribution pattern shown by CT-OAM allows conclusions to be drawn about the distribution of strength and therefore the long term load intake within the subchondral bone plate of the hip. Using conventional CT-data, the method can be used in the clinical setting for evaluation and monitoring.