Early periosteal changes in translation-induced bone modelling.

This primarily ultrastructural study examines the effects of strain induced in the periosteum using an in vivo translation model with minimal internal bone strain. Caudal vertebrae (CV 7, 8, 9) from 4 d rats were threaded onto the arms of prestressed helical torsion springs and transplanted subcutaneously into 50 g hosts of the same inbred strain. After 7 d the appliances were activated in the experimental rats causing the bones to translate, i.e. to move through the soft tissues. Tissues for histology were obtained at this time (0) and at 1, 3, 5, 7, 10 and 14 d; for electron microscopy, experimental tissues were obtained at 0 time, 30 min, 1, 2, 6, 12, 18 and 24 h and at 0 time and 12 h for the controls. As the arms of the appliance move apart, traction on the enveloping soft tissues produces compression of the periosteum on the leading side and tension on the trailing side with resultant eccentric remodelling of the bones, generally opposite to the direction of movement. A rapid and differential structural response occurs, characterised by accelerated formation on the trailing side with the reverse on the leading, where changes are not as marked initially. Long thin trabeculae oriented in the line of tension form on the trailing side whereas the shaft on the leading side becomes thinner and flatter. Ultrastructural examination of the early stages shows that the fibrous periosteum is first affected, with alterations in collagen packing preceding cellular changes. The midzone shows the greatest change and events here presage those which finally occur at the bone surface and are reflected in altered osteoblastic activity. This study shows that translation-induced stress produces rapid morphological changes in the periosteum which, by acting as an integrated unit, has the capacity to modulate the adaptive bone modelling response.