Orthodontics. Part 11: orthodontic tooth movement.

Orthodontic tooth movement is dependent on efficient remodelling of bone. The cell-cell interactions are now more fully understood and the links between osteoblasts and osteoclasts appear to be governed by the production and responses of osteoprotegerin ligand. The theories of orthodontic tooth movement remain speculative but the histological documentation is unequivocal. A periodontal ligament placed under pressure will result in bone resorption whereas a periodontal ligament under tension results in bone formation. This phenomenon may be applicable to the generation of new bone in relation to limb lengthening and cranial-suture distraction. It must be remembered that orthodontic tooth movement will result in root resorption at the microscopic level in every case. Usually this repairs but some root characteristics apparent on radiographs before treatment begins may be indicative of likely root resorption. Some orthodontic procedures (such as fixed appliances) are also known to cause root resorption.