Up to now, there is no definite conclusion about the limit of tooth movement. Some hold the view that tooth movement takes place in coordination with bone resorption and apposition, that is tooth remains in the alveolar housing[20, 21]. But in fact, unlimited tooth movement is not possible during retraction of the incisors, especially the mandibular incisors restricted by the symphyseal bone. The main object of this study was to explore the limit of tooth movement.
The results of Control group confirmed the mandibular forward growth with age. As previously reported, the alveolar bone apposition took place at the labial side and the resorption took place at the lingual side. By comparison of posttreatment alveolar bone measurements with pretreatment in Tip and Torque group respectively, we found that the labial and lingual alveolar bone thickness at S1 reduced significantly. This result agreed with the results of Picanco and Sarikaya, who observed a reduction of lingual alveolar bone thickness due to lingual movement of maxillary and mandibular incisors[6, 12]. Moreover, Ahn also observed that alveolar bone area on the palatal side significantly decreased after retraction of maxillary anterior teeth[8]. But our finding for decrease of alveolar bone thickness in the direction of tooth movement was contrary to De Angelis, who claimed that alveolar bone had a bending capacity to coordinate bone apposition and resorption, thus the alveolar bone retained its structural characteristics and size as it moved[20]. Also, these results did not agree with the hypothesis of Hadelman regarding the limitation of tooth movement by alveolar cortical plates, showing that alveolar bone modeling is possible during tooth movement induced by biological forces[22, 23]. Moreover, the buccolingual ABT at three levels in both Tip and Torque groups decreased significantly due to combined results of growth effect and tooth movement. This finding was in consistence with the hypothesis of Sarikaya that suggested the apposition process in the inner cortical plate was somewhat slower than the resorption process in the outer cortical plate[6]. In other words, alveolar bone resorption was more than apposition during tooth movement.
When growth effect was excluded, changes of buccolingual ABT at S3 in Tip and Torque group, as well as labial ABT at S3 in Torque group became insignificant. We suggested the reason was whether by tipping or controlled tipping movement, the retraction force applied to the incisors was concentrated at the alveolar crest, leading to greater accumulation of pressure in the marginal region while less in the apical region[6]. In addition, labial ABT at S2 of Tip group and labial ABT at S2 and S3 of Torque group also decreased significantly, which agreed with the results of Vardimon that the labial cortical bone traced the tooth movement in a resorptive model[7]. With regard to the modeling of cortical plate, Edwards proposed the supporting alveolar bone did not model to the root’s position and concluded that the apical zone is the limit for orthodontic tooth movement. On the contrary, our results demonstrated that although apical ABT did not change significantly, the alveolar cortical plate modeled to the same direction with tooth movement. Considering the results aforementioned, we suggest that not only the apical region, but the entire alveolar housing should be the therapeutic limitations for orthodontics.
Intergroup comparison indicated that lingual movement of incisors in Torque group was greater than that in Tip group. Although changes of ABT did not show any significant differences between two groups, the cortical bone modeling varied significantly. The cortical plate at S1 modeled toward lingual side in both groups, but the amount of modeling was smaller in Tip group. The cortical plate at S2 and S3 modeled toward labial side in Tip group, whereas toward lingual side in Torque group. These results indicated that the alveolar bone modeled to the same direction as tooth movement and the amount of modeling is proportional to the distance of tooth movement.
Furthermore, we confirmed the conclusion about relationship between vertical facial type and alveolar bone, which agreed with previous studies, that high-angle patients had the thinnest ABT while low-angle patients had the thickest[24-26]. It is loading exerted by muscles that altered cortical bone thickness, not only at the site of muscle insertion but also in the alveolar bone of tooth-bearing region[27, 28]. However, do patients with different vertical types have different B/T ratios? We finally found that B/T ratio of high-angle patients was less than average- and low-angle patients. Due to lower bone density, high-angle patients are more sensitive to orthodontic force and have more tooth movement, which leads to smaller B/T ratio[29, 30].
In this study, lateral cephalogram instead of cone-beam computed tomography (CBCT) was used for measurement and analysis for the following reasons. First, based on European and American clinical practice guides, CBCT has not been recommended as a standard diagnostic and treatment planning method in the field of orthodontics, because of the ethical problem of radiation exposure[31-33]. And considering samples involved in this research were adolescents, who were more susceptible to harmful effects of radiation, CBCT was not recommended for this study. Although there actually were some studies using CBCT to detect alveolar bone changes, their sample size were so small that would limit the power of statistical test and lead to false-negative conclusions[6, 8, 11-13]. In addition, CBCT also has the risk of overestimating fenestration and dehiscence because thin bone below 1mm is difficult to be detected[34, 35].
However, there are some limitations in this study. First, the control group selection was limited by ethical problems of radiation exposure for patients without requirement for orthodontic treatment. Second, the accuracy of lateral cephalograms was not good enough. Moreover, due to lack of follow-up radiographs, we cannot estimate the repair of alveolar loss during retention period. Therefore, further investigations on changes of alveolar bone during retention period are needed, and widely application of CBCT in the future, with advances of technology and economy, will surely provide more comprehensive understanding of alveolar bone modeling in orthodontic treatment.