Undesirable sequelae of orthodontic force
A mild pulpitis following initial force application is common, but has no effect long term. Where the apical blood vessels are severed by the use of heavy continuous force or by injudicious root movement through the alveolar plate, pulp death is likely, although this is usually associated with previous trauma.
Areas of cementum resorbed during tooth movement are usually repaired. Some permanent loss of root length is found, however, on nearly all teeth following bodily movement over long distances. This occurs primarily in
the apical or apicolateral regions of maxillary incisors and mandibular first permanent molars. Fortunately, in most instances, loss of 0.5-1.0 mm of root length is of no long-term significance. The risk is increased where
• root resorption is present before treatment
• there is a history of previous trauma irrespective of whether endodontic treatment was undertaken
• the root is pipette shaped, blunt or demonstrates a marked apical curvature.
Torquing movements and apical contact with cortical bone are also significant risk factors.
Loss of alveolar bone height
With fixed appliance treatment, 0.5 to 1 mm loss of crestal alveolar height is common, with the greatest loss occurring at extraction sites. In the presence of good oral hygiene, this appears of little concern.
Pain and mobility
Even with appropriate force magnitude, ischaemic areas develop in the periodontal ligament after activation of an orthodontic appliance, leading to mild discomfort and pressure sensitivity. These usually last for 2–4 days and return when the appliance is reactivated. Some increase in mobility is common, as the periodontal ligament space widens and the fibres reorganise in response to the applied force. With heavy orthodontic forces, however, the likelihood of almost immediate onset of pain and marked mobility is increased, as the periodontal ligament is crushed and further undermining resorption occurs.