Disuse osteoporosis: physiological aspects.

Immobilization in plaster, bed rest, and the weightless state all result in calcium loss which if continued for a few months will result in detectable demineralization of the lower extremities. The upper extremities are "different" bones (presumably differently programmed genetically), for they have not been seen to develop X-ray-detectable demineralization except after several months of severe paralysis. The substantial losses of calcium in inactivity are accompanied by sizeable losses of nitrogen, reflective of muscle atrophy. Hence, we do not know how much of the bone loss in disuse is mediated by diminished direct physical forces on bone, how much by decreased muscle pull on periosteum, and how much perhaps by circulatory or other changes.