Effect of long-term nontraumatic immobilization on metaphyseal spongiosa in young adult and old beagle dogs.

A radiographic and histomorphometric change in volume in the metaphyseal spongiosa in response to long-term immobilization was investigated in young adult and old beagle dogs and compared with changes in diaphyseal compacta of the same bones. Sixty weeks of nontraumatic immobilization of young dogs produced a 66% loss of metaphyseal spongiosa in the third metacarpus, a 50% loss in the radius, and a 25% loss in the humerus. In older animals, the loss amounted to 54%, 46%, and 37%, respectively. In both age groups, the loss of the spongiosa exceeded that of the compacta in the same bones, and, as in the compacta, it was greatest in the small peripheral bones. Although maximal loss occurs during the first 24 to 32 weeks, it may continue to a lesser degree up to 60 weeks and perhaps beyond 60 weeks of immobilization. Since bone loss can occur only on bone surfaces, the earlier and greater loss from the spongiosa may be explained in part by the higher endosteal surface-to-volume ratio of spongy bone. Given a similar bone structure and physiology in man and dog, the observations made in this study may apply to man as well.