Age-dependent external root resorption during tooth movement in rats.

OBJECTIVE To investigate the effect of age on root resorption and distribution along different parts of the root during prolonged light force application. MATERIAL AND METHODS Orthodontic appliances were placed in two groups of 30 rats (one group 6 weeks old, the other 9-12 months old), with contralateral sides as controls. Groups of animals were killed at 1, 2, 4, 8, and 12 weeks. At the study site, incidence was counted as either 1 (with resorption) or 0 (without); severity was measured as the summed length of all resorption lacunae as a percentage of study total root length. RESULTS Young and adult rats had the same incidence of root resorption in the early phase (<4 weeks), and both increased in the late phase (4-12 weeks) to the same level. Severity of resorption increased with prolonged tooth movement only in adult rats. However, there was no age-related difference in either the early or the late phase. In both groups, the middle part of the root had the highest incidence of resorption; the most severe resorption occurred exclusively at this part. CONCLUSION Orthodontic intervention even with light forces increased both the incidence and severity of root resorption, the more so in the middle part of the root. Adult rats had increased incidence and severity with prolonged tooth movement.