Structural adaptation to changing skeletal load in the progression toward hip fragility: The study of osteoporotic fractures

Longitudinal dual-energy X-ray absorptiometry (DXA) hip data from 4187 most ly white, elderly women from the Study of Osteoporotic Fractures were studi ed with a structural analysis program. Cross-sectional geometry and bone mi neral density (BMD) were measured in narrow regions across the femoral neck and proximal shaft. We hypothesized that altered: skeletal load should sti mulate adaptive increases or decreases in the section modulus (bending stre ngth index) and that dimensional details would provide insight into hip fra gility. Weight change in the similar to3.5 years between scan time points w as used as the primary indicator of altered skeletal load. "Static") weight was defined as within 5% of baseline weight, whereas "gain" and "loss") we re those who gained or lost >5%, respectively. In addition, we used a frail ty index to better identify those subjects undergoing changing in skeletal loading. Subjects were classified as frail if unable to rise from a chair f ive times without using arm support. Subjects who were both frail and lost weight (reduced loading) were compared with those who were not frail and ei ther maintained weight (unchanged loading) or gained weight (increased load ing), Sixty percent of subjects (n = 2559),vith unchanged loads lost BMD at the neck but not at the shaft, while section moduli increased slightly at both regions, Subjects with increasing load (n = 580) lost neck BMD but gai ned shaft BMD; section moduli increased markedly at both locations. Those w ith declining skeletal loads (n = 105) showed the greatest loss of BMD at b oth neck and shaft; loss at the neck was caused by both increased loss of b one mass and greater Subperiosteal expansion; loss in shaft BMD decline was only caused by greater loss of bone mass, This group also showed significa nt declines in section modulus at both sites, These results support the con tention that mechanical homeostasis in the hip is evident in section moduli but not in bone mass or density, The adaptive response to declining skelet al loads, with greater rates of subperiosteal expansion and cortical thinni ng, may increase fragility beyond that expected from the reduction in secti on modulus or bone mass alone.