Changes in mineralization and biomechanics of tibial metaphyses in splinted rats

The effect of 3 wk of splintage of a single hindlimb on the midarea and mineral content of both tibial metaphyses was assessed immediately after splint removal and after 1 mo of mobilization in 12-wk-old Sprague-Dawley rats. Immobilization reduced tibial metaphyseal bone mineral density (BMD) in immobilized limbs compared with "free" limbs of splinted animals and with controls. These changes persisted and were accentuated by relatively greater increases in tibial metaphyseal BMDs of unsplinted (control) animals after 7 wk. Immediately after splintage, tibial metaphyseal areas and total mineral contents of both hindlimbs of splinted animals were reduced compared with those of unsplinted animals. However, the relationship between mineralization and area differed between the free and immobilized limbs of splinted animals. The breaking strain and the breaking energy of immobilized and free femurs of splinted animals were impaired 4 wk after the removal of the splint. This impairment was correlated with an effect of splintage on femoral size with some additional local effect from immobilization. Thus osteoporotic changes consequent on immobilization include both local effects on mineralization and general effects on growth, which may separately influence the elastic properties of bone.