The Importance of Force Levels in Relation to Tooth Movement

Orthodontic literature is dominated by the pressure-tension theory with regard to the tissue reaction following mechanical loading of teeth. The present article discusses the reaction of the alveolar bone to the perturbation generated by different orthodontic force levels. Orthodontists generally relate the pressure zone to resorption and the tension zone to apposition. Conversely, orthopedic surgeons know that unloading generates a negative balance of the ongoing bone remodeling and that loading will result in a positive balance leading to increased bone density. Research based on histological analysis of monkey teeth, which were loaded with different force levels, demonstrated that direct resorption could be a response to a relatively lower force level. Undermining resorption, on the other hand, is a repair mechanism attempting to remove the alveolar bone underlying the ischemic periodontal ligament (PDL). The osteocyte lacunae in this bone appear empty as a sign of apoptosis or necrosis of the osteocytes. Finite element analysis (FEA) based on microcomputed tomography (μCT) and using material properties reflecting the physiological values of the PDL and alveolar bone demonstrated that the classical pressure-tension theory could not be corroborated. However, the studies did confirm that the reaction of the alveolar wall can be caused by the changes in strain occurring during various types of loading. The FEA also demonstrated that the localization of the center of resistance depends not only on the anatomical relationship of the tooth, PDL, and surrounding bone, but also on the difference in microanatomy at the different levels of the alveolus as well as the force magnitude that is applied.