Cephalometric analysis based on lateral cephalograms has been a mature and widely used tool for the studies of craniofacial anatomical structures15. Cephalogram is one of the routine films requested for the orthodontic patients. Despite of the inherent defect of the 2D images, valuable information for the orthodontic treatment planning could be obtained at a low radiation dose. CBCT can be a more accurate way to measure the morphology of the alveolar bone 16. However, the studies based on CBCT usually were conducted using a small sample size. Because no consensus has been achieved that CBCT could be taken as a routine x-ray examination before the orthodontic treatment due to the radiation issue. Therefore, in this respective study, a large sample consisted of pre- and post- treatment lateral cephalograms from the existed data base was used to evaluate the change in alveolar bone morphology (width and height) and investigate whether the position of points A and B would be affected by bone remodeling related to incisor retraction. The large sample size and comprehensive measurements lead to a greater chance for this study to reflect the most possible changes occurred incisor retraction in the orthodontic treatment.
The anterior alveolar bone defines the boundary for the retraction of the anterior teeth in orthodontic treatment. Though theoretically bone remodeling occurs during tooth movement, it remains controversial whether the changes in the anterior alveolar bone always follow the direction and quantity of tooth movement. De Angelis 3 described the bending capacity of alveolar bone, which suggested that the alveolar bone retained its structural characteristics and size as it moves with coordinated apposition and resorption. However, this bending capacity wasn’t verified by other studies. The apposition and resorption of bone are in a dynamic situation during tooth movement. Melsen 17 indicated that most resorption activity occurs at sites that undergo compression, and reduced activity occurs in the tension zone. Bimstein et al. 18suggested that the amount (mainly the height) of buccal alveolar bone might increase as a result of orthodontic treatment that involves lingual positioning of procumbent mandibular permanent central incisors without intrusion. In contrast, Sarikaya et al. 19reported bone width in the mandible and in the lingual side of the maxilla was significantly decreased after orthodontic treatment, whereas maxillary bone thickness labial to the incisors remained unchanged. Similar results were found by Vardimon et al.20, Ahn et al.21, and Thongudomporn et al.22. One study found that upper incisor inclination and intrusion changes may increased the degree of alveolar bone loss23. In the present study, we found that the labial alveolar bone width showed a significant decrease in the maxilla and an insignificant increase in the mandible; the lingual side of the alveolar bone showed a significant decrease in both maxilla and mandible. Our results are consistent with the studies which showed the alveolar bone width decrease after retraction of the anterior teeth and suggested that the bone apposition process was slower than the resorption process. Significant increase of ABH was found on the labial side of the maxilla. A possible reason for this change might be the extrusion of the upper incisor during the retraction. Decrease of ABH was found on the palatal side of maxilla and both sides of mandible, especially the lingual side (P<0.001). Similar result was found by Lund et al.24, who reported alveolar bone height decrease of 1.1-3.3mm in premolar extraction cases and the most significant decrease found on the palatal/lingual side. This may due to the narrower width of the alveolar bone in the mandible, which would be more sensitive to the stress concentration around the cervical area from the controlled tipping movement of the lower incisor. Both the alveolar bone width and height decreased the most on the lingual side of the lower incisor suggested that more attention should be given to this area to prevent excessive bone resorption in the treatment.
The upper incisor and lower incisor were found to move in different types in this study. UIE and UIR of the maxillary incisor moved lingually by 4.98mm and 1.27mm, respectively, whereas UIA moved labially by 0.86mm. This result indicated that the incisor movement during retraction in the maxilla was mainly tipping, which meant that the edge and apex of the incisor moved in the opposite direction and the center of rotation located between the center of resistance and the apex (Fig.5a). The upper incisor became more upright during the retraction. In the mandible, LIE, LIR and LIA were found to move lingually by 2.04mm、1.39m and 0.55mm, which indicated the controlled tipping of lower incisors(Fig.5b). This finding is consistent with the results of Sarikaya et al.19 and Vardimon et al20. who observed that in patients undergoing retraction with torque, the result was combined movement with some tipping rather than pure translation.
Root resorption is a common side-effect of orthodontic treatment, especially with extensive tooth movement. In this study, the distance between UIE and UIA in maxilla and between LIE and LIA in mandible were measured as the length for upper and lower incisor respectively. The results showed that the length of the incisor decreased by 0.79mm and 1.03mm in maxilla and mandible respectively, which was close to the results of the meta-analysis conducted by Samandara et al. In their study, the average amount for root resorption in anterior teeth was found to be 0.9mm 25. Kaley and Phillips 26 indicated that the contact between the root and the cortical bone is an important cause for root resorption. Horiuchi et al.27 reported that apex approximation to the palatal cortical plate due to incisor retraction was one of the critical factors for root resorption. In addition, insufﬁciency of the maxillary width during tooth movement could be a risk associated with root resorption27. One study observed the relationship between contact the incisive canal of upper central incisors and root resorption28. The results showed that contact between upper incisors and the cortical plate of the incisive canal cause significantly more apical root resorption. The result of this study also showed the decrease of the alveolar bone width after incisor retraction. Therefore, the alveolar width should be carefully evaluated before the treatment to prevent excessive incisor retraction which may lead to significant root resorption.
A few previous studies focused on the effect of incisal inclination changes on points A and B and did not consider changes caused by the sagittal and vertical movement of the incisor12-14, 29, 30. Al-Nimri et al. 30 stated that, in Class II division 2 malocclusion, the movement of point A, affected by local bone remodeling, occurred in a backward direction. An earlier study by Al-Abdwani 29 stated that each 10° proclination of the upper incisors resulted in a significant average change in point A of 0.4 mm in the horizontal plane. Moreover, each 10°proclination of the lower incisors resulted in a borderline significant average change in point B of 0.3 mm in the horizontal plane. Cangialosi and Meistrell 11 studied the effect of lingual root torque on the sagittal position of point A, and showed that the posterior movement of the apex of the maxillary incisors resulted in a 1.7-mm posterior movement of point A. Hassan et al. 13 reported that there was no evidence of significant horizontal and vertical displacement of point B due to lower incisor inclination changes. In the present study, we found that point A moved backward of 0.45mm (P<0.001) and downward of 0.24mm (P＜0.001) with the retraction of upper incisor. A positive correlation was observed between the position change of point A and the displacements of points UIE, UIR and UIA. In the mandible, point B didn’t show significant backward movement in the sagittal direction, which was in consistent with previous study13. However, a positive correlation was found between the sagittal position of point B and the horizontal position changes of points LIE, LIR and LIA (r=0.604 ,0.693 and 0.637, respectively). The vertical displacement of point B and the displacements of points of lower incisors also showed a positive correlation (r= 0.486,0.618 and 0.594, respectively). The result of Pearson correlation analysis indicated that the backward movement of points A and B increase with the extent of incisor retraction.