Photoelastic stress analysis of upper molars intrusion by SAS

Objective: Recently, it has become possible to move a tooth toward a direction which had so far been deemed to be impossible with the aid of a newly devised tooth movement approach using skeletal anchorage system (SAS). Though the intrusion of maxillary posterior arch segment, which was quite difficult to perform in the past, has also become possible, its mechanism is still unclear. In the present study was performed in order to elucidate the biomechanical influence of upper molars intrusion by SAS.

Methods: A photoelastic model of an adult human maxilla was fabricated using birefringent materials, and the molar segment was attached with sectional arch. Assumed the implant site to be around zygomatic arch, the direction of traction was set so as to cross at right angles to the occlusal plane from the center of 1st molar bracket to the stress direction. By setting the loading condition at 100gf, 150gf, 200gf and 250gf respectively. Stresses that developed in the supporting structure were monitored photoelastically and recorded photographically.

Results: At all loading conditions, it was observed that a predominant stress concentrated at the 1st molar's root apex. On the other hand, no such a predominantly concentrated stress was noted at the 2nd molar site.

conclusion: The traction force to upper molars intrusion was revealed to be limited to the 1st molar at the initial loading despite that the molar region was served as interconnected firmly by sectional arch. Concerning the changes in the stress concentration in response to the increased traction force, the change of stress in other teeth was all minimal, although the stress at the 1st molar roots was augmented. Namely, a potential risk of adding an excess orthodontic force on the 1st molar alone was suggested with the increased traction force under upper molars intrusion by SAS.