Cortical aging differences and fracture implications for the human femoral neck

Clinical imaging techniques cannot consistently identify individuals at risk for hip fracture. Individual differences in falling likelihood partly account for these inconsistencies, but it is also thought that microscopic bone changes may play a role. In this study, subcapital, mid-neck, and trochanteric sites from eight young adult (26 +/- 7 years) and nine older (63 +/- 3 years) males were studied using backscattered electron imaging to identify age-related microscopic structural and mineral changes around the cortex. Cortical bone volume (BV(Ct)/TV), cortical void volume (Vd.V(Ct)/TV), hypermineralized bone volume (BV(H-min)/TV), the number of osteons/mm2 (N.On/B.Ar), lacunae/mm2 (N.Lc/B.Ar(Ct)) in the cortex, lacunae/mm2 (N.Lc/B.Ar(H-min)) in the hypermineralized phase, and cortical thickness (Ct.Th) were measured at subcapital, mid-neck, and trochanteric levels. Cortical void volume showed no differences (P = 0.26) between levels in the younger group, but differences (P < 0.05) were observed in the older group, indicating locational osteopenic differences. Cortical thickness differences were greater at the subcapital (27.7%) and mid-neck (25.2%) levels than at the trochanteric level (10.5%). Both age (P = 0.0022) and level-location interaction (P < 0.0001) influenced the hypermineralized bone volume present, with larger hypermineralized regions generally occurring at the thinner superior locations. Significant (P < 0.05) lacunar differences with aging in the hypermineralized phase suggest a necrotic origin. Artifactual cracks occurred preferentially within the hypermineralized phase, indicating localized reductions in fracture toughness, which may provide a site for crack initiation following an impact.