Structural adaptation and intracortical bone turnover in an ovine model of osteoporosis.

Compact bone makes up approximately 80% of the human skeletal mass. This study examines the effect of estrogen deficiency on compact bone turnover and associated geometrical structural adaptation over a 31-month period in a large animal model. Twenty-seven skeletally mature sheep were divided into control (n = 16) and ovariectomy group (OVX, n = 11). Animals were administered five different fluorochrome dyes to label intracortical bone turnover, and sacrificed at 31 months. Compact bone samples were analyzed for cortical geometry, intracortical turnover at five time points, resorption cavities, porosity, and compressive strength. Intracortical bone turnover was significantly increased in OVX, which demonstrated seasonal variation. Cross-sectional area in OVX was significantly greater than control and was associated with an increased section modulus. Intracortical porosity was significantly increased in OVX, however, there was no significant difference in ultimate compressive strength between the groups. Our results demonstrate increased intracortical bone turnover, resportion spaces, and porosity in OVX, without adversely affecting compressive strength. Our results also support the hypothesis of geometrical adaptation of compact bone in response to estrogen deficiency. These results suggest an early structural compensatory response in compact bone, despite increased intracortical turnover.