Aim: This study was undertaken to calculate the stress in the tooth, surrounding periodontal ligament, and in the alveolar bone when a lower first premolar is subjected to intrusion or torque movement using a constant moment. Root resorptions occur even when very low forces and moments are used in orthodontic therapy. It is therefore of great interest to determine and measure the stress that occurs under particular treatment conditions in the periodontal ligament. Material and Methods: In this study, three finite element calculations were carried out with a realistic 3D model developed by CT data that consisted of a lower premolar, the surrounding periodontal ligament and alveolar bone. In close reference to the in-vivo experiments carried out by Faltin et al. [3, 5, 6] in São Paulo, Brazil, our model was subjected to an intrusive force on the premolar of 0.5 N and a lingual root torque of 3 Nmm. Results: The three main stress directions and hydrostatic stress were quantified in all the surrounding tissues, revealing that the hydrostatic stress profile in the periodontal ligament correlated closely with resorption findings in Faltin et al.'s patients. Resorption occurred in the experimental study in Brazil when the hydrostatic stress exceeded capillary blood pressure in the periodontal ligament. Conclusion: We maintain that hydrostatic stress represents a suitable indicator for potential root resorptions caused by higher forces and moments, making it a helpful tool in the development of new orthodontic appliances. We must of course mention that there are many factors other than forces that are responsible for resorptions. But at the moment, only the force can be influenced by the orthodontist. © Urban & Vogel München 2005.
