Experimental stress-induced changes in growing long bones.

Animal experiments were carried out to determine what changes result from the controlled overloading of a bone. Forty-four guinea-pigs were used and a partial resection of the right ulna was done so that subsequently the radius would be overloaded. Exercise was carried out daily on a treadmill. The animals were divided into four groups and were sacrificed at varying intervals from 15 to 60 days. Morphometric studies indicated that there was an immediate stress-induced growth in thickness of the diaphysis, which also occurred in the metaphysis after some delay. Calculations were made to determine the stress-induced changes in the bone. The results showed that the geometry of the metaphysis and diaphysis was able to accommodate to sudden pressure, bending and torsional stresses. Autoradiography did not demonstrate increased mitoses in the overloaded bone, but the density of osteocytes was reduced compared to normal. This, with a simultaneous increase in bone weight, indicates that there is an increased production of intercellular substance by the existing osteocytes which give rise to hypertrophy.