A study of transposed canines in a sample of orthodontic patients.

Tooth transposition is a positional interchange of two adjacent teeth. The most commonly transposed tooth is the permanent canine with either the first premolar or lateral incisor. The records of 54 subjects with transposed canines, both maxillary and mandibular, were collected. Pretreatment study models of these subjects were matched with a similar number of models from unaffected individuals. Bucco-lingual and mesio-distal tooth widths, arch depth and arch width were measured on each model. Thirty-four subjects (63 per cent) were female. Thirty-seven (68.5 per cent) of the cases involved the maxillary arch and thirty-three (89.2 per cent) of these upper arch transpositions were of the canine and first premolar. In cases involving the lower arch the canine was invariably transposed with the lateral incisor. Peg-shaped lateral incisors, supernumerary and/or congenitally absent teeth occurred in 19 subjects. There were some small, but significant differences in the dimensions of some teeth, however there were no statistically significant differences in arch depths, arch widths and most tooth dimensions in subjects with and without transposed canines. These factors do not appear to be related to the development of canine transposition.

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.