Growth of the humerus after denervation. An experimental study in the rat.

The purpose of the study was, firstly, to determine if muscle activity in the rat has any effect on the growth in length of the humerus and on the migration of the deltoid tuberosity and, secondly, to evaluate the effect of muscle activity on the size and shape of the deltoid tuberosity. Three groups of 25 days old male rats were used. In the first group the brachial plexus innervating one forelimb was resected (the treatment side) and the opposite side was unoperated (the non-treatment side). The second group was sham-operated on one side and the third group, the control group, was unoperated. All animals were killed at 53 days of age and the dried humeri measured and compared. For all dimensions the experimental group varied significantly from the remaining two groups in the magnitude of the differences between the treatment and non-treatment sides. In the experimental group the deltoid tuberosities were smaller, less curved, and closer to the proximal end of the bone in humeri from the treatment sides compared to humeri from the non-treatment sides. Humeri from the treatment sides were significantly shorter and were narrower opposite the deltoid tuberosity. A prominent nutrient artery canal was significantly closer to the proximal end of the bone in humeri from the treatment side. These results indicate that after denervation more growth occurred distally and less growth occurred proximally. The differences were, however, small. The present study provides evidence of the contribution made by muscle activity to the form and growth in length of a long bone, and provides indirect evidence of the interaction between the activity of the growth plates and periosteal tension. It is postulated that muscle pull affects periosteal tension and consequently bone form and growth in length.