CHANGES IN CANINE CORTICAL AND CANCELLOUS BONE MECHANICAL-PROPERTIES FOLLOWING IMMOBILIZATION AND REMOBILIZATION WITH EXERCISE

The purpose of this study was to assess cortical and cancellous bone r esponses to unilateral limb immobilization and, subsequently, to remob ilization with exercise, in a young adult canine model. Right forelimb s of 14 1-2-year old mongrel dogs were immobilized in a non-weight-bea ring position by a bandage for 16 weeks. Six control dogs were untreat ed. At 16 weeks, seven immobilized and three control dogs were euthani zed. The remaining seven immobilized dogs began a recovery protocol co nsisting of 16 weeks of kennel confinement (without the right forelimb bandaged) followed by 16 weeks of treadmill exercise conducted three times per week. These seven dogs and three control does were euthanize d at 48 weeks. Bone mineral density of the proximal radii was determin ed with dual-energy X-ray absorptiometry and humeral middiaphyseal cro ss-sectional areas were determined with computed tomography. Humeri we re tested in craniocaudal three-point bending to failure. Cancellous b one cores from the lateral humeral condyles had wet apparent density d etermined and were tested to failure in compression. Mechanical proper ties, bone density, and cross-sectional areas were compared between im mobilized (right forelimb), contralateral weight bearing (left forelim b), and control forelimbs with Kruskal-Wallis and post hoc tests. At 1 6 weeks, bone mineral density, cortical load, yield, and stiffness as well as cancellous bone failure stress, yield stress, and modulus were significantly lower (p < 0.02) for immobilized limbs than control lim bs. Immobilized limb cancellous bone mechanical properties were 28%-74 % of control values, and cortical bone mechanical properties were 71%- 98% of control values. After 32 weeks of remobilization, cortical and cancellous bone mechanical properties were not different from control values except that cortical bone failure stress and modulus were signi ficantly higher (p < 0.01) between remobilized and control limbs. In s ummary, 16 weeks of forelimb immobilization was associated with signif icantly lower mechanical properties, and with greater differences in c ancellous than cortical bone properties. Mechanical properties were no t different from control values after 32 weeks of recovery that includ ed 16 weeks of treadmill exercise. (C) 1997 by Elsevier Science Inc. A ll rights reserved.