BONE-MINERAL DENSITY AND TURNOVER FOLLOWING FORELIMB IMMOBILIZATION AND RECOVERY IN YOUNG-ADULT DOGS

The purpose of this experiment was to study changes in bone mass, stru cture, and turnover in the canine forelimb after unilateral immobiliza tion and recovery, The right forelimbs of 14 adult mongrel dogs were i mmobilized for 16 weeks. Six dogs served as controls. Seven immobilize d and three control dogs were euthanized at the end of the immobilizat ion period, Recovery consisted of 16 weeks of kennel confinement follo wed by 16 weeks of treadmill exercise. Seven once-immobilized and thre e control dogs were euthanized at the end of the recovery period. Bone mineral density of both the proximal (PBMD) and central (CBMD) radius was determined by dual X-ray absorptiometry. Standard histomorphometr ic endpoints for bone mass and turnover were determined in the cancell ous bone of the proximal radius. After immobilization, PBMD, CBMD, and trabecular thickness were lower in the immobilized limb than in eithe r the contralateral or control limbs (P < 0.05). Only CBMD remained si gnificantly lower (P < 0.05) after recovery. At the end of immobilizat ion, bone formation endpoints were significantly higher in the immobil ized limb than both the contralateral and control limbs. Bone turnover was also significantly lower in the contralateral limb than in the im mobilized and control limbs. After recovery, all differences in bone t urnover had resolved. Immobilization of 16 weeks duration caused an el evation in cancellous bone formation rate and reduced bone density in both cortical and cancellous bone, After 32 weeks of recovery, turnove r abnormalities disappeared, cancellous bone normalized, but cortical bone mass remained low. Recovery of cortical bone from immobilization takes longer than recovery of cancellous bone.