Remodelling of bone and bones: effects of translation and strain on transplants.

Tail segments, from 4-day-old Sprague-Dawley rats, consisting of caudal vertebrae (CV) approximately 7-9 were impaled on 0.23-mm diameter Elgiloy wire and transplanted subcutaneously into 50-70 g male hosts to study the effects on transplants of (a) impaling (b) strain and (c) translation. The CV were impaled onto straight lengths of wire to serve as controls (a); onto a wire curved to form a loop and exert a bending force (b) and onto the arms of a spring which moved bones through the surrounding tissues, i.e. translation (c). Tissue changes were studied up to 28 days by radiographic and histological techniques. Control bones grow relatively normally along the straight wire. The CV subjected to strain bend initially and then grow in an arc along the curve of the wire. The outer bone shaft usually becomes straighter while the inner one becomes concave and rarefied. In the translated bones remodelling occurs in a direction generally opposite to the direction of movement but this is modified by the influence of soft tissue tension and pressure. Bone resorbs on the outer leading side under continuous pressure and forms on the inner trailing side under continuous tension. The process is essentially the same as that seen in 'cortical drift'; however, since translation is rapid there is an alteration in the shape of the translated bones as formation on the trailing side is faster than resorption on the leading side.(ABSTRACT TRUNCATED AT 250 WORDS)