A study of the relationship between incisor intrusion and root shortening

Abstract

Apical root shortening is one of the most common complications of orthodontic treatment. Force magnitude has been suggested as an important factor. Studies on the occurrence of root resorption show equivocal results. The aim of the present study was to evaluate the relationship between intrusion with low forces (25 gm) using utility arches in the bioprogressive technique and root shortening. Age, sex, facial type, treatment time, extraction versus nonextraction therapy, width of the symphysis, and the angle of the incisors to skeletal reference planes also were studied for their relationship to intrusion and root shortening. Root shortening was found to average 1.84 mm for maxillary incisors and 0.61 mm for mandibular incisors subjected to intrusive force. Intrusion of incisors in a population exhibiting growth was found to be one of “holding against growth” and in the upper arch to a change in angulation of the maxillary incisors. Furthermore, when extraction was a part of the orthodontic treatment, it was related to intrusion of maxillary incisors but not to intrusion of mandibular incisors. No relationship was found between the amount of root shortening and degree of intrusion achieved. However, a long treatment time was significantly correlated to root shortening. None of the other characteristics studied were related to either intrusion or root shortening. In the present study, it was found that intrusion with the utility arch type of technique is not related to amount of root shortening. The degree of root shortening was markedly higher in the maxilla than the mandible. In general, treatment time was the most significant factor for occurrence of root shortening. A review of cases exhibiting the most severe root shortening indicated that there are persons with high resorptive potential in whom root shortening occurs in both the mandible and the maxilla. In these patients the intrusion achieved was significantly related to the amount of root shortening observed. It may be concluded that control of treatment time is of importance especially when intrusion in the maxilla is performed. Furthermore, there are patients with a high resorptive potential in both the maxilla and mandible who need to be carefully monitored during intrusion.