Bone-mineral content in the lower limb: Relationship to cross-sectional geometry

For this study, bone-mineral content and bone-mineral width were measured using photon absorptiometry at eleven locations in forty excised femora and tibiae obtained from an archaeological sample. An additional seventy-nine femora were scanned at the middle of the shaft and through the femoral neck. After scanning, the bones were sectioned at each location and cross-sectional areas and other geometrical properties were determined directly from section tracings. The results indicated that locational, sex, and age-related differences in bone-mineral content were largely determined by variation in cortical area. Due to differences in bone geometry, variation in bone width does not reflect variation in cortical area, and as a consequence the use of bone width to standardize for volumetric or bone 'size' differences produces misleading results in sex and age comparisons. In this study, decreases with age in the bone-mineral content and bone-mineral content:bone width ratio were similar to those observed in living populations. However, the bone-mineral content:cortical area ratio showed no significant decline with age for any cross section. Thus, the age-related changes in compact cortical bone appeared to be mainly volumetric, not densitometric. Clinical Relevance: Photon absorptiometry has been shown to be of limited usefulness in distinguishing between patients who are at high or low risk for fracture on an individual basis. The presence of significant individual variations in bone geometry indicates the need to measure geometrical parameters in clinical evaluations that use photon absorptiometry, both to provide better estimates of bone volume and density differences and to assess additional structural properties that are important in determining bone strength.