Root resorption is one of the common complications of orthodontics, and it is also the focus of orthodontic clinical research. Previous studies have shown that the degree of root resorption is lower in patients treated with self-ligating brackets than in those treated with traditional brackets . Many studies have shown that there is no difference in root resorption between self-ligating brackets and traditional brackets[19–21]. However, the self-ligating bracket has many advantages, such as a light force and a low friction force. They can reduce the operating time in the chair and bring comfort to the patients. They are widely used in orthodontics. With the development of self-ligating brackets, brackets with different torque angles have been introduced to meet the needs of clinical treatment. This experiment was designed to detect the effect of high torque brackets and standard torque brackets on root resorption in bimaxillary protrusion patients.
With the wide application of CBCT, an increasing number of studies have been performed to evaluate root resorption by CBCT[22–24]. CBCT can accurately measure tooth and root resorption volumes, and it is a more accurate and reliable 3D measuring method for root resorption. CBCT measures root resorption more often than on X-rays. At present, most of the research on root resorption relies on length measurements. However, root resorption is not just a two-dimensional variation in length. It includes changes in the three-dimensional direction of the buccal tongue and the proximal middle, and the absorption on each side is slightly different. Therefore, taking the volume of the tooth used to describe the amount of root absorption is more appropriate. It can more accurately reflect root resorption [26–29]. However, research in this area is still relatively limited. To better assess the degree of root resorption, this study also used the method of measuring the tooth volume to measure the degree of root resorption before and after treatment.
Patients with extracted teeth are more likely to develop root resorption than those who do not receive orthodontics[30–32]. In this study, all of the patients had the first premolar removed before treatment. The same orthodontist provided the same treatment, including a similar arch wire replacement throughout the entire treatment process. Mini-screw implants were inserted into the buccal region between the second premolar and first molar.
Excessive force is one of the factors involved in severe root resorption . In this study, to avoid excessive force causing root resorption, 150 g of force was applied to retract the upper and lower anterior teeth to close the extraction space on each side. All of the patients underwent CBCT by the same radiologist under the same parameters before and after treatment, which ensured good comparability between the two groups. In addition, we selected more patients (32,192 teeth) to reduce the study error caused by a small sample size.
In this study, we only considered the maxillary anterior teeth. On the one hand, the torque of the brackets in the two groups was different for the maxillary anterior teeth. On the other hand, the maxillary anterior teeth are the most prone to root resorption in orthodontic treatment [35–37]. Although the torque on the mandibular teeth was different from the standard torque, it was not included in this study. For patients with bimaxillary protrusion, a large number of anterior teeth were moved to improve the degree of protrusion. Therefore, it is necessary to control the torque of the anterior teeth to acquire the desired tooth position . The maxillary anterior teeth show the movement tendency of the coronal lip and the root tongue when using the high torque bracket, which is helpful to prevent torque loss in the anterior teeth. The stress expression value of the perimembrane in the high torque bracket was obviously higher than that of the standard torque bracket. Case et al. reported that the effect of torque force on root resorption showed that the greater the force, the greater the root resorption scope.
In this study, the average root absorption of the high torque bracket group and the standard torque bracket group were 23.15 mm3 and 20.34 mm3, respectively. Compared with the standard torque bracket, the root resorption of the high torque self-ligating bracket was slightly higher than that of the standard torque bracket. However, the difference between them was not statistically significant. In the relatively severe root resorption between the two groups, the number and quantity of moderate and severe root resorption in the high torque group were both greater than those in the standard torque group. Yangxue et al. also showed that more torque control in the maxillary anterior teeth of the high torque group led to overall and partial root control and higher root resorption.
In this study, we found that there was no statistically significant difference in the treatment time between the groups. However, a longer treatment time led to more root resorption, which is consistent with previous studies[20, 43, 44]. Treatment duration is a risk factor for the development of severe EARR. However, some authors disagree[2, 45].
Some of the shortcomings of this study need attention. First, although we strictly matched the treatment group and the control group when choosing the cases, it may be difficult to avoid the effects of confounding factors on the results. It is best to compare the root resorption of the two types of brackets by random selection in the future. Second, although the detection process has increased the sample size compared with previous studies, the sample size is still insufficient. In future studies, we will continue to increase the sample size to correct for the effect of the sample size on the results. Finally, we still need to include more patients of different races to verify whether the two brackets affect root resorption.