Orthodontic Movement and Return of the Root Apex Through the Cortical Plate in the Macaque Speciosa Monkey

Abstract

During orthodontic therapy, the apices of teeth are sometimes moved or tipped against and even through the cortical plate of bone. This may be a final tooth position or the root may then be brought back into cancellous bone. The purpose of this study was to observe histologically the effects of these movements and of retaining the root in these positions. Four Macaque Speciosa monkeys were used. Their ages corresponded to late adolescence in the human. The force system used to achieve the desired tooth movement on the animals' teeth was calculated from a force system which from clinical experience is considered to be optimal in root movement in humans. This force system was kept constant. Procion Brilliant Red H-8BS was used as a vital marking agent and was administered once to each animal prior to the last stage of movement in each case. As the root apex penetrated the cortical plate, the buccal surface was the only root surface that became devoid of bone. There was considerable proliferation of cortical plate which "followed" the root and maintained its relationship to bone on the remaining root surfaces. During a four month retention period there was continued osteogenesis of the buccal surface of the cortical plate, but the root apex was not completely covered. Bone apposition also occurred on the lingual surface of the buccal plate and there was remodeling of the osteophytes. As the root was moved back and retained, there was complete repair of the perforation site with further slight thickening of the cortical plate. Root resorption was present on the buccal surface when it was under pressure, and the lingual surface on reversal of the force system. The resorption was increasingly severe towards the root apex. Although the tooth movement was 50 percent faster on the reversal movement, the severity of resorption was comparable on buccal and lingual surfaces. Cellular cemental repair was considerable and was most marked at sites of greatest recent root resorption and on the tension side of the tooth in movement. Less cemental deposition occurred on the mesial and distal root surfaces. Since the appliances were reactivated monthly, areas of hyalinization were rare. There was no inflammation due to tooth movement and the pulp tissue was normal in all cases.