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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. 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 Sao 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. 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. 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.

Background: The periodontal ligament is a soft connective tissue which joins the tooth root to the alveolus and thus provides for anchorage of the tooth in the alveolar bone. Due to its composition of elastic and viscous components, this tissue displays viscoelastic material properties. In a previous study [4], in vitro experiments revealed typical viscoelastic material properties of the periodontal ligament in samples from pig mandibles. These properties included force relaxation, hysteresis, and dependence on loading history. Material and Methods: Based on those experiments, a dependence of tooth displacement on loading velocity was registered in the present study and the stress-strain behavior of the periodontal ligament was examined until the tissue ruptured. For this purpose, segments of the periodontal ligament taken from anterior teeth from the pig mandible were tested in a purpose-developed clamping fixture in a uniaxial tensional experiment. Result: It was found that the initial phase of the stress-strain curve in particular was dependent on loading velocity and that the shape of the hysteresis curve was subject to a variation in loading velocity. The stress-strain behavior of the periodontal ligament was characterized, divided into several phases, and the elastic modulus of the initial and the linear phase of the curve was determined at different loading velocities. Conclusion: Knowledge of the material properties of the periodontal ligament is fundamental to an understanding of orthodontic tooth movement and thus to selection of an optimal force system for orthodontic treatment.

PHYSIOLOGY OF THE PERIODONTAL LIGAMENT: The periodontal ligament is a soft biological tissue that controls tooth movement under physiological loads by joining tooth and alveolar bone. Its various components differ in their material properties. Their spatial configuration and interaction are responsible for the reaction of the tissue in a loading situation. Due to the combination of fluid and elastic elements the periodontal ligament shows a viscoelastic behavior typical of soft biological tissues. It is characterized by non-linearity and time dependency, and additionally depends on loading history.In orthodontics, external loads are applied to the tooth crowns using orthodontic appliances. Since stresses and strains in the periodontal tissue, caused by the initial tooth movement, stimulate alveolar bone remodeling and thus orthodontic tooth movement, knowledge of the material properties of the periodontal ligament is fundamental to selection of an optimal force system for targeted tooth movement.For this reason, typical properties of the viscoelastic material behavior of the periodontal ligament were tested experimentally in the present study, using samples from pig mandibles. This enabled the properties of force relaxation and hysteresis of this tissue, both of which depend on loading history, to be verified.The experimental results allow characterization of the tissue and thus contribute to an understanding of the biomechanics of tooth displacement under externally applied loads.