20 Tooth movement and related problems
Darren, a 13-year-old boy, has been undergoing upper removable appliance therapy for 6 months to retract and align , following extraction of . Tooth movement has been very slow with no movement recorded at the last two visits.
The consequence of the spring activation is the setting up of pressure and tension zones within the periodontal ligament. Half of the periodontal ligament is stressed with maximum pressure created at the alveolar crest in the direction of movement and at the diagonally opposite apical area (Fig. 20.1).
The cellular response depends on whether a light or heavy force is applied. With a light sustained force, tooth movement occurs within a few seconds as periodontal ligament fluid is squeezed out and the vascular supply is compressed, setting off a complex biochemical response. Within 2 days, osteoclast invasion occurs and frontal resorption follows.
When a heavy sustained force is applied, the periodontal ligament is compressed to such a degree that the blood supply is cut off completely, producing an area of sterile necrosis (hyalinization). Small areas of hyalinization are inevitable even with light forces, but the area of hyalinization is extended with forces of greater magnitude. Osteoclast differentiation is impossible within the necrotic periodontal ligament space, but after several days osteoclasts appear adjacent to and within the adjacent cancellous spaces. From there they invade the bone adjacent to the hyalinized area and tooth movement eventually occurs by undermining resorption.
Following initial application of a light force, the blood vessels vasodilate and the periodontal ligament fibres are stretched, while fibroblast and preosteoblast proliferation occurs. The stretched fibres become embedded in osteoid, which later mineralizes. The normal periodontal ligament width is eventually regained by simultaneous collagen fibre remodelling.
Although the histological response to an applied orthodontic force has been investigated extensively, the mechanism by which a mechanical stimulus is transferred to a cellular response is complex and is at present unresolved. It is likely that vascular changes in the periodontal ligament in areas of pressure and tension, electrical signals in response to alveolar bone flexing following force application, as well as prostaglandins and cytokine release interact in the process.