Reversibility of nontraumatic disuse osteoporosis during its active phase.

The potential for the recovery of bone lost during the active phase of disuse osteoporosis, both in the diaphyseal compacta and metaphyseal spongiosa was tested in young adult and old Beagle dogs. Immobilization for up to 60 weeks was achieved by placing the forelimb in a spica cast and remobilization by removing it. Bone volume was estimated in the third metacarpus, radius, ulna and humerus at the mid-diaphysis and at the level of distal metaphyseal spongiosa in both forelimbs by radiography and histomorphometry. Measurements carried out on animals remobilized showed considerable recovery of the original bone loss. In both age groups, the residual deficits increased, however, with the duration of immobilization and were similar in the metaphyseal spongiosa and in the diaphyseal compacta. The old dogs which began the study with 10% less bone than the younger dogs, showed smaller proportional losses than the younger dogs but greater residual deficits, most evident in the diaphysis. In both age groups the distal, weight-bearing bones tended to show greater losses and also greater recovery both in diaphyseal compacta and the metaphyseal spongiosa. Thus, 28 weeks after cast removal following 32 weeks of immobilization the following findings were noted: In the third metacarpal diaphyseal compacta in the younger dogs, a 53.6% loss (mostly from the periosteal envelope) decreased to 16.3% (a 70% recovery) while in the older dogs a 37.6% loss (mostly from the endosteal envelope) decreased to 23% (a 40% recovery).(ABSTRACT TRUNCATED AT 250 WORDS)