Remodelling of bone and bones: effects of altered mechanical stress on caudal vertebrae.

Sprague-Dawley rats weighing 50 g were divided into two groups: (i) control, (ii) rats with tails bent in situ incorporating 7, 5 and 3 caudal vertebrae in the loop. Tails were radiographed weekly up to six weeks and a microradiographic and histological study undertaken on selected specimens. Results showed that the bones in the apex of the loop of the bent tail moved through their investing soft tissues towards the outer side of the bend, the joints became V-shaped and in tails bent acutely the epiphyses and metaphyses tilted. By six weeks the bones appeared bent with a thinner straight to convex shaft on the outer side and a thicker, more concave one on the inner side. The changes observed can be explained by taking into account (i) strain within the bone, (ii) altered growth and (iii) the translation of bones through their investing soft tissues. The results are consistent with the supposition that, on application of a continuous moderate stress, tension induces formation and pressure resorption of bone.