External root resorption evaluated by CBCT 3D models superimposition

ABSTRACT Introduction: The literature reports the association of external root resorption (ERR) with orthodontic movement. In cases of premolars extractions, orthodontic movement of anterior teeth is usually quite expressive, which are precisely the most susceptible teeth to suffer from ERR. Objective: The aim of this study was to assess the root morphology of maxillary canines and incisors in patients submitted to four premolar extraction and orthodontic retraction of the anterior teeth, by means of 3D surface models superimposition and mapping. Methods: The sample consisted of six adult patients, five female and one male, with a mean age of 23.5 ± 6.5 years, who underwent orthodontic treatment. All patients presented bimaxillary dental protrusion, with indication of maxillary and mandibular first premolar extractions, followed by the retraction of anterior teeth and space closure. Cone beam CT scans were performed before the beginning of the treatment (T0) and right after space closure (T1). 3D models were built at both times and superimposed to identify the root changes for the given period. Results: All average differences were close to zero and, even when evaluating the extreme values, the observed changes were always smaller than the accuracy of the CBCT. Conclusion: A mild resorption trend was observed, although it was not clinically significant, with values lower than the tomography accuracy.


INTRODUCTION
External root resorption (ERR) is characterized by the permanent shortening of the tooth root, which is a common clinical complication of orthodontic treatment. Although ERR may occur in any or all teeth, it most often involves the maxillary incisors. 1 ERR is a sterile inflammatory process that is extremely complex and involves various components such as forces, tooth roots, bone, cells, surrounding matrix, and certain known biologic messengers. 2 ERR is undesirable because it can affect the long-term viability of the dentition. The etiologic factors are complex and multifactorial, including individual biologic variability, genetic predisposition, and the effect of mechanical factors. 3 ERR can also depend on the orthodontic technique, tooth and jaw morphologies, and presence of root resorption before treatment. 4 Some general dentists believe that ERR is avoidable and blame orthodontists when it occurs during orthodontic treatment. 5 The patient/parents must be informed about the risk of root resorption as a consequence of orthodontic treatment. For precaution, after six months of treatment, periapical radiographs of the teeth should be obtained and, when ERR is detected, treatment should be halted for two to three months, with passive archwires. 2 After treatment, if severe ERR is shown on the final radiographs, follow-up radiographic examinations can be Pereira ABN, Almeida R, Artese F, Dardengo C, Quintão C, Carvalho F External root resorption evaluated by CBCT 3D models superimposition 5 recommended until the resorption has been stabilized, which usually occurs after appliance removal. If it continues, sequential endodontic treatment with calcium hydroxide may be considered 6 , although this treatment is controversial. 7 The use of conventional two-dimensional (2D) radiograph is not accurate for the detection of mild resorption. 8 Furthermore, 2D radiograph may not represent the resorption lesions and their dimensions, which depend on the severity of the root resorption. 9,10 Clinically, radiographs are an important diagnostic tool in detecting ERR, but the varying degrees of magnification and the limitation of 2D measurement of a 3-dimensional phenomenon make the quantitative value of radiographs questionable and geometrically inaccurate. 11,12 Savoldi et al. 13 have proposed a trigonometric correction for the use of panoramic radiographs to try to overcome these issues, but it still presents the limitations of a two-dimensional exam.
Recent studies suggest that CBCT is a more sensitive imaging modality for detecting root resorption than conventional radiograph. 14 However, further studies are needed to assess the safety and cost effectiveness of CBCT in the management of orthodontic patients with ERR. 9,15 Pereira ABN, Almeida R, Artese F, Dardengo C, Quintão C, Carvalho F External root resorption evaluated by CBCT 3D models superimposition 6 Various three-dimensional (3D) superimposition methods are used for clinical diagnosis and treatment evaluation purposes in orthodontic treatment and craniofacial surgeries, but each method has valuable benefits and some limitations. 16 CBCT images provide both crown and root information, which makes it possible to reconstruct a complete tooth model. With the digital model, orthodontists perform diagnosis and treatment planning through manipulating the tooth model in a graphical user interface, thereby realizing digital, efficient, and accurate orthodontic treatment. 17 For these factors, CBCT was the imaging method chosen in this study, and segmentation of 3D models would be useful to allow the mapping, localization and quantification of root resorption in 3D virtual models.
Thus, the aim of this study was to assess the root morphology of maxillary canines and incisors in patients submitted to four premolar extraction and orthodontic retraction of the anterior teeth, by means of 3D surface models superimposition and mapping.

MATERIAL AND METHODS
This preliminary prospective study, of experimental character, evaluated six adult patients, five female and one male, with a mean age of 23.5 ± 6.5 years, who underwent orthodontic treatment at the orthodontic clinic of Universidade do Estado do Rio de Janeiro Pereira ABN, Almeida R, Artese F, Dardengo C, Quintão C, Carvalho F External root resorption evaluated by CBCT 3D models superimposition 9 Afterwards, the CBCT postprocessing was as follows: (1) The 3D models of anatomical structures of interest (dental elements: #13, #12, #11, #21, #22 and #23) were built with the aid of the open source software ITK-SNAP 3.6 18 , which uses a semi-automatic segmentation method (Fig 1).
(2) Each tooth was individually exported to a STL (Standard Triangulation Language) file.
(3) All models of T 0 and T 1 teeth were imported by the Geomagic (4) The T 0 was considered as reference (fixed model), and the T 1 was superimposed to the T 0 using a best-fit alignment algorithm.
As the purpose of the superimposition was to evaluate the root changes, only the coronal portion of the teeth above the base of the bracket was considered for the best-fit alignment (Fig 2).     Table 2). The Levene test verified the variance homogeneity (p = 0.405). To evaluate whether the difference between T 0 and T 1 was statistically different from zero, the one-sample t-test was used. The alpha level considered for all analyzes was 0.05. The evaluation of each tooth was carried out as independent subjects, so it was possible to observe how specific anatomical regions of interest would behave and this would also avoid an artificial sample size increase.

RESULTS
For the evaluation of the mean values of distances and root mean square (RMS) between the surfaces of the roots of T 1 in relation to T 0 , they were compared to each other by means of colored maps. The descriptive statistics (maximum and minimum values, mean, standard-deviation, and one sample t-test) were calculated and are described in Table 1.
The RMS value was used to evaluate the differences between the roots. The RMS, in fact, corresponds to the absolute mean of the distances in a normalized form, useful when there are large variations of values, both positive and negative. 19 RMS for a collection of "n" values {X 1 , X 2 , ..., X n } is given by the formula below:   Considering the normal distribution of the sample, the one sample t-test was performed to evaluate if the values observed were statistically different from zero. As all values were above 0.05, no statistically significant differences between the mean and the desired error value were identified (Fig 4).

DISCUSSION
The CBCT was used to evaluate the morphological changes caused by the retraction of the maxillary anterior teeth. CBCT is a powerful complementary diagnostic method to assess apical root resorption during orthodontic treatment, and it can be better than conventional radiograph, which underestimates root resorption. 21 The introduction of CBCT technology can still be considered quite recent, and the literature still shows few research dedicated to studying its accuracy and specificity for the diagnosis of ERR. Due to the lack of evidences, the present study aimed to assess the root morphology of maxillary canines and incisors in patients submitted to four premolar extraction and orthodontic retraction of anterior teeth. The choice of these teeth in particular was due to the fact that the maxillary incisors are the teeth most frequently affected by root resorption. The degree of root resorption can be correlated to the magnitude of apex displacement and the length (treatment time) of the orthodontic treatment. 22 Sameshima and Sinclair 23 reported that root resorption mostly occurred in the anterior teeth rather than in the posterior teeth of the maxilla in 868 orthodontic patients.
Even though two different space closure techniques were used in this study, previous paper has shown that there was no difference in the root resorption associated with en masse or two-stage closure. 24 The present sample size does not allow any statement on this topic, but we also did not observe any difference between the two techniques.
Although the CT is an imaging method superior to other radiographic methods for visualizing bone tissue, the accuracy of CT scanning in visualizing tooth root resorption is not wellknown. Artifacts may affect the diagnostic reliability, such as beam-hardening effects, linear and nonlinear partial volume effects, edge gradient effects, and metal artifacts. 20 Regarding the quality of CBCT, when root resorption can be identified in a Pereira ABN, Almeida R, Artese F, Dardengo C, Quintão C, Carvalho F External root resorption evaluated by CBCT 3D models superimposition 18 CBCT scan with low resolution, it can mean that the resorption is present. However, if a CBCT scan did not show resorption in a highly suspicious case, then we can indicate a scan with high-resolution. 4 Liedke et al. 25 assessed the effect of CBCT resolution on the accuracy of root resorption measurements, and demonstrated that the CBCT approach was a reliable tool for assessing root resorption, and the 0.3-mm voxel resolution is the best configuration, because it associates great diagnostic performance with lower patient exposure to radiation.
Spatial resolution is the minimum distance required to distinguish two objects of similar density in a tomographic image, and may be incorrectly assumed to be equal to the scan's reported resolution of a scan or voxel size. Spatial resolution defines the ability of the CBCT to separate two close objects, which can be improved by decreasing voxel size and increasing scan time.
However, this can be detrimental due to increased radiation exposure and possible patient movement. Factors such as partial volume averaging, artifacts, and noise make it impossible to have a spatial resolution equivalent to the smallest voxel size. 26,27 Spatial resolution and its contributing factors should be considered during the design or interpretation of CBCT studies.
In the study of Ballrick et al, 26 the authors found that a 0.2-mm voxel scan had an average spatial resolution of 0.4 mm.
The two most common voxel sizes used for orthodontic scans, It can be said that the voxel dimension used to obtain the image is directly related to the dose of radiation to which the patient will be submitted during the procedure. Therefore, before selecting the image acquisition protocol, it is necessary to know its cost-benefit relationship, following the ALARA (As Low As Reasonably Achievable) principle. In other words, the professional should choose the exam protocol that presents the lowest radiation dose possible, but at the same time, is sufficiently sharp to identify the structures that need to be evaluated. 29 There is no evidence that the detection of moderate to severe ERR differs between 2D and CBCT radiography or that its discovery during treatment leads to a different treatment decision in both techniques. However, identifying lingual or buccal root resorption could contribute to treatment decisions, as it would be detectable only by a CBCT. 30 When compared to conventional CT scanners, CBCT machines are considered a less expensive and smaller equipment that exposes the patient to approximately 20% of the radiation of a helical CT, which is equivalent to the dose from a full-mouth periapical series. 31 Even at the highest settings of the CBCT, the radiation dose is very below conservative limits recommended by the National Council on Radiation Protection and Measurements. 32 There are two main reasons for using CBCT: in research, it increases our knowledge of root resorption; and in clinics, CBCT images may help to monitor the risk of developing root resorption during orthodontic tooth movement, in patients with agenesis or syndromes. 21 Consideration of spatial resolution in orthodontic diagnosis is inexorable in the evaluation of small anatomical regions or when the objective is to identify small differences between the evaluated regions. Therefore, the evaluation of ERR is quite challenging, as well as the evaluation of bone plates.

CONCLUSION
Based on the methodology applied and the results obtained, it is possible to present the following conclusion: There was no trend of ERR associated with orthodontic retraction of maxillary anterior teeth in the evaluated sample.