Changes of Crown/Root Ratio during Orthodontic Tooth Movement and the Analysis of Its Risk Factors
|Keywords||orthodontic treatment crown/root ratio apical root resorption alveolar crest loss|
Objectives:To observe the changes of crown/root ratio during orthodontic tooth movement, analyze its risk factors and discuss their relativity.Method:Choose 10 patients who have had accepted an orthodontic treatment with straight wire appliance (SWA) and bilateral maxillary bicuspids extracted. Measure their external apical root resorption (EARR) and alveolar crest loss of upper anterior teeth on the before/after treatment panoramic-radiographs. All the 60 teeth were divided into 3 groups, which are the central incisor group, lateral incisor grouop and cuspid group. Calculate the crown/root ratio of the teeth before and after treatment. The changes of crown/root ratio, and the relativity between EARR and alveolar crest loss were evaluated.Result:After the orthodontic treatment, statistically significant differences were found in the crown/root ratio of upper anterior teeth. Before the treatment, the crown/root ratio of central incisor group, lateral incisor group and cuspid group were 0.69±0.10mm,0.65±0.08mm, 0.56±0.07mm, respectively. While after the treatment, it changed to 0.85±0.17mm, 0.80±0.11mm,0.69±0.10mm, which increased by 24%,23%,22%, respectively. The ratio of EARR and alveolar crest loss of the 3 group were 4:1,3.5:1,3:1, respectively. EARR is more serious than alveolar crest loss, but no significant differences were found among the 3 groups.Conclusion:The crown/root ratio of upper anterior teeth was found significant increase after orthodontic treatment, but it is still acceptable clinically. The increase of crown/root ratio was mainly induced by EARR and alveolar crest loss, which lead to shorter clinical root and longer clinical crown. The amount of EARR is larger than that of alveolar crest loss. Tooth type showed no significant impact on the change of crown/root ratio.