A histological study on the effect of different periods of orthodontic force on the innervation and dimensions of the cat periodontal ligament.

Using light microscopy, the numbers of myelinated axons in the periodontal ligament of the cat mandibular canine were counted after orthodontic forces had been applied for either 3 days or 12 weeks, and also 8 weeks after removal of a force that had been applied for the previous 12 weeks. After 3 days no significant changes in axon number were recorded, but after 12 weeks there were significantly fewer myelinated axons in the periodontal ligament of the experimental teeth than in the control teeth. This reduction was not reversed after an 8 week recovery period. The width of the ligament was measured in an attempt to correlate regions of nerve fibre degeneration with areas of compression or tension, and the tooth root circumference was measured to assess the extent of any root resorption. It appeared that axon degeneration occurred both in areas of compression and tension. Root resorption had occurred after 12 weeks of applied force and there was no significant change after the 8 week recovery period. These findings support the view that nerve injury occurs as a result of orthodontic force and may possibly be permanent.