[Histomorphometric and biological data on osteoporosis due to immobilization]

The effect on the bones of immobilization was studied by means of quantitative histological methods on 34 biopsies of the iliac crest after different periods of immobilization in 28 patients, 22 of whom were immobilized by medullary lesions. At the same time a biochemical study was carried out on 68 immobilized patients over a 52-week period. This histomorphometric study included measurements of the absolute volume of trabecular bone, the volume of the osteoid matrix, the osteoclastic resorption surfaces, the size of the osteocyte lacunae, the thickness of the iliac cortical zones and the medullary adipose volume. 19 subjects received in addition double labeling with tetracycline with the object of carrying out a histodynamic study of the osteoblastic activity. The decrease in the absolute volume of the trabecular bone reached 33 percent of its original value and continued up to the 25th week, and was followed by stabilization at a value slightly higher than the vertebral fracture limit. Immobilization also led to an increase in the osteoclastic resorption surfaces and, secondarily, to an increase in the periosteocyte lacunae, a thinning of the iliac cortical zones, and a decrease in the volume of the osteoid matrix. Calciura increased as did total hydroxyprolinuria in parallel with an increase in the resorption surfaces. The histological and biochemical changes suggest a histodynamic hypothesis, according to which the longevity of the basic multicellular bone unit increases, leading to a new equilibrium characterized by very slow renewal of the bone.