The aim of this study was to verify less resorption of incompletely developed roots compared with those that were fully developed during the same orthodontic treatment and to test the value of the amount of external apical root resorption for predicting tooth development.
A sample of 524 patients aged 10-15 years was selected following the inclusion criteria. For each subject, pretreatment and posttreatment digital panoramic and lateral radiographs were collected, and tooth development was determined from each radiograph. Through calculations, the amount of root resorption was assessed by a created and scientific approach for large-scale application using radiographs with only 8 measurement indexes for each patient. Other basic information and treatment parameters regarded as possible risk factors were also collected from standardized recordings or radiographs. The root length between the groups or in the single group were compared with t tests and correlation analyses. Linear univariate and multivariate regression analyses were used to test identify predictors for root resorption and to develop a prediction model.
There was a statistically significant difference in the amount of root resorption with tooth development before correction ( P <0.001) as well as after correction ( P = 0.002). There was a statistically significant correlation ( P <0.001) but no difference between pretreatment and posttreatment root length in the immature tooth group because of less root resorption. In the multivariate analyses, tooth development ( P <0.001), treatment duration, apex horizontal movements, apex vertical movements, and previous orthodontic treatment were included in the final model as risk factors, and tooth development had the highest beta value.
There is an association between root resorption and tooth development, and tooth development is an important predictor of root resorption. Patients with immature teeth are at a much lower risk of apical root resorption.
A method to assess external apical root resorption with digital panoramic and lateral radiographs was created.
There is an association between root resorption and tooth development.
Patients with immature teeth are at a lower risk of apical root resorption.
External apical root resorption (EARR) is an inevitable side effect of orthodontic treatment. , Whereas external root resorption occurs from all directions, EARR of teeth shows only apical resorption, which is characterized by root shortening. EARR is estimated to occur in 80%-90% of orthodontic patients. , Although tooth survival and normal function are not affected in most cases of mild EARR, orthodontic treatment should be suspended in some cases of severe EARR, and it is important for the practitioner to identify the dominant factors for the development of EARR to be able to adjust treatment to manage this adverse effect. According to our previous study, EARR was a multifactorial problem, and several potential genetic and clinical risk factors were reported. The common consensus is that the type and the amount of orthodontic force were considered as the main causes of EARR in treatment factors. , As for patient characteristics, genotype, sex, nutrition, amount of apical displacement, and malocclusion type , can also initiate and induce root resorption.
Previous studies also found that increased ages coincided with more root resorption , indicating older people are more vulnerable. However, physiological age, which many studies used, was unable to accurately reflect root development, which was more likely to associate with root resorption. Thus, considering whether the tooth was fully developed as inclusion and exclusion criteria can be simple for observers to increase repeatability and reproducibility. The stages of dental maturity can be assessed by methods such as Demirjian’s and Haavikko’s. , In addition, open or closed apical foramina may play a vital role in root resorption. It has been suggested that there is a contact between the root and the alveolar bone causing EARR and a relationship between EARR and pulp, , and the apical foramina connect the pulp and alveolar bone. Because the pulp vessels are the branches of alveolar bone vessels, orthodontic forces can stretch or damage apical vessels, decrease blood flow, and cause pulpal disturbances. An unclosed apical foramina, indicating more free space and adequate blood supply, may help relieve pressure and maintain balance.
To reliably assess the amount of EARR in a patient is the major obstacle in such clinical studies. It had been widely reported that root length change could be considered as evidence of EARR, and 2-dimensional images were mainly used for measuring root shortening, but undesirable errors may occur when aligning the images taken at different times. , Thus, both digital panoramic radiographs and lateral radiographs were involved in this assessment, and the quantitative measurements that have small changes in the images taken at different times were selected. Through calculations, the reduced root length can be obtained and compared before and after orthodontic treatment.
Therefore, the purpose of this investigation was to verify the following hypothesis during clinical experience: There is an association between root resorption and tooth development, and patients with immature teeth will undergo less root resorption.
Material and methods
This longitudinal retrospective study was approved by the ethical board of West China Hospital of Stomatology involving all subjects who accepted orthodontic treatment from 2015 to 2018. Patients were selected according to the following inclusion criteria: aged 10-15 years during the entire treatment, both digital panoramic and lateral radiographs before and after treatment, the Frankfort plane of the radiographs parallel to the floor, good occlusal finishing, and periapical radiographs. All patients were treated the standard fixed–straight wire appliances with 0.022 × 0.028-in slots and general archwire sequences of 0.016-in nickel-titanium to 0.019 × 0.025-in stainless steel. All radiographs were taken and scanned on the same machine at West China Hospital of Stomatology with the same setting used. Radiographs of insufficient quality, films not displaying the entire root, impacted teeth, and incomplete orthodontic records were excluded.
The tooth development of each radiograph was determined by 2 observers independently according to Moorrees and Kent, and Moorrees et al. The observers were blinded to sample information, and they repeated the procedures twice in a 3-week interval. The within-observer repeatability and reproducibility of the 2 observers were evaluated using intraclass correlation coefficients (ICCs). Then the included cases were further divided into an immature tooth (IT) group and a mature tooth (MT) group. Because the maxillary incisors usually showed more EARR than any other teeth in several studies, , all maxillary central incisors of which the apexes had not completely formed both before and after treatment were viewed as IT cases, and those of which the apexes had completely formed both before and after treatment were viewed as MT cases ( Fig 1 ).
The basic information and treatment parameters of each included patient were collected, including initial age, sex, history of trauma, history of earlier orthodontic treatment or endodontic treatment, extraction decision, and treatment period, which were possible factors for root resorption.
Then several lengths and angulations were measured using the same program on the pretreatment and posttreatment radiographs for each patient. The reference points and lines for measurements are shown in Figure 2 . Before treatment, the lengths of the crowns and roots on the lateral radiographs were C 1L ′ and R 1L ′, and the lengths of the crowns and roots on the panoramic radiographs were C 1P ′ and R 1P ′. The magnifications of the lateral and panoramic radiographs were V L and V p . Respectively, the real lengths of the crowns and roots on the lateral radiographs were C 1L and R 1L , and the real lengths of the crowns and roots on the panoramic radiographs were C 1P and R 1P . The minimum scale of the rules on the lateral radiographs were d 1 , and the real minimum scale was d (d = 10 mm). The angulations of the maxillary central incisors on the lateral radiographs were θ 1 . Similarly, the lengths and angulations on the posttreatment radiographs were C 2L ′, R 2L ′, C 2P ′, R 2P ′, V L , V p , C 2L , R 2L , C 2P , R 2P , d 2 , d, and θ 2 . Attention should be paid to the crown and root lengths that were measured differently compared with those that were calculated by measuring the distance from the maxillary central incisal edge to the midpoint of the line. In the panoramic radiographs, the highest point of cementoenamel junction (CEJ) was determined, and the crown lengths were the distance from the incisal edge to the highest point, whereas the root lengths were the distance from the highest point to the root apices.
Through the following calculations, the quantitative measurements that had small changes in the images taken at different times were selected. R 1L can be presented as
where V p can be presented as