External Apical Root Resorption after Six and 12 months of Non-Extraction Orthodontic Treatment

Objectives: The aim of the present study was to test the hypothesis that external apical root resorption (EARR) after six months of orthodontic treatment could be an incidence indicator of EARR after 12 months of treatment in non extraction orthodontic cases. A comparison of EARR between different types of root morphology was also performed. Material and Methods: Periapical radiographs of the upper incisors were obtained prior to treatment (T1) as well as at six months (T2) and 12 months (T3) of non-extraction orthodontic treatment among 47 patients aged 11 years or older. The roots were classified based on anatomic shape. Triangular, pipette-shaped, bent and/or short roots were classified as having a tendency toward EARR, whereas those with a rhomboidal and rectangular shape were classified as having no tendency toward EARR. Results: At 12 months of orthodontic treatment EARR ranged from 0 to 12.1% of total tooth length (mean: 3.5%; SD: 3.03), which meant 0 to 2.7mm of EARR. There was significant correlation between EARR at six months and EARR at 12 months (r=0.7606; p<0.0001). There was no correlation between root shape and EARR. Conclusions: EARR after the first six months of orthodontic treatment was a good incidence indicator of EARR after 12 months of treatment (r = 0.8). Root shape did not show significant influence in root resorption level in non extraction orthodontic cases. *Corresponding author: Dr. Paula Cabrini Scheibel, Dental office, Av. Luiz Teixeira, 2266, CEP: 87010-370, Maringá, Paraná, Brazil, Tel: (05544) 30262613; E-mail: paulascheibel@msn.com Received July 20, 2011; Accepted August 12, 2011; Published August 23, 2011 Citation: Scheibel PC, Micheletti KR, Ramos AL (2011) External Apical Root Resorption after Six and 12 months of Non-Extraction Orthodontic Treatment. Dentistry 1:102. doi:10.4172/2161-1122.1000102 Copyright: © Scheibel PC, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

The aim of the present study was to test the hypothesis that EARR after six months of orthodontic treatment may give an indication of the incidence of some EARR after 12 months of treatment in non extraction orthodontic cases. A comparison of EARR between different types of root morphology was also performed.

Materials and Methods
The sample in the present prospective study was made up of 91 upper central incisors of 47 patients aged 11 years and older, who had their complete fixed orthodontic appliance installed (straightwire technique) by orthodontic graduate students from July 2008 to April 2009. Signed informed consent was the primary condition for the inclusion of each patient. The following were the other inclusion criteria: no past history of fixed orthodontic treatment; no past history of dentoalveolar trauma in the region of the upper incisors; upper incisors with either intact crown or only proximal restorations; nonextraction orthodontic treatment plan. The study received approval from the Ethics Committee on Research Involving Human Subjects of the Universidade Estadual de Maringá (Brazil) (#190/2008).
Periapical radiographs were taken of the upper incisor region in each participant on three occasions: initial (T 1 , immediately prior to or immediately after placement of the braces), at six months (T 2 ) and 12 months (T 3 ) of orthodontic treatment. The radiographic equipment used was either the Pro 70-Intra (Prodental, RibeirãoPreto, São Paulo, Brazil) or RX Timex 70 Col (Gnatus, RibeirãoPreto, São Paulo, Brazil), with a 0.25-second exposure time. The film was processed in a standardized time/temperature method. The images were then digitalized on a scanner (ArtixScan 18000F, Microtek) with a resolution of 400 ppi for subsequent computerized measurement of the amount of apical root resorption (CorelDRAW X4 program).

Radiographic analysis
Measurement of external apical root resorption: The length of the upper central incisors (teeth 11 and 12) and respective crowns was measured on the three occasions (DT 1 , DT 2 , DT 3 and CT 1 , CT 2 , CT 3 , respectively) to a precision of 0.1 mm with the aid of the CorelDRAW X4 program [23,24]. These measurements respectively corresponded to the distance from the incisal border to the root apex and the greatest distance between the incisal border and cementum-enamel junction, using the long axis of the tooth as reference (Figure 1). In order to compensate for possible variations in the inclination of the radiographic takes on the different occasions, supposing that the crown measurement remains unaltered throughout treatment, the expected tooth length at T 2 (expected DT 2 ) was calculated using the following equation [23,24]: expected DT 2 = (CT 2 .DT 1 ) / CT 1 . The amount of EARR was determined by subtracting the expected tooth length at T 2 from the tooth length measured at T 2 : EARR at T 2 = expected DT 2 -DT 2 . The same procedure was used to determine EARR at T 3.
The amount of root resorption was calculated in millimeters and then expressed in percentage values in relation to the initial tooth size. Teeth with resorption percentage of zero were classified as having undergone no resorption, whereas those with 1 to 4% resorption were classified as having apical rounding. Resorption between 4% and 8% was considered mild and resportion between 8% and 12% was considered moderate.

Determination of root morphology
Levander and Malmgren [14] classified roots as normal, short, blunt, with apical bent and pipette shape, and Consolaro [5] classified roots as triangular, rhomboid and rectangular based on the shape of the apical third. These root anatomies are considered in the judgment of the susceptibility to apical resorption (morphologic risk). Thus, the roots in the initial radiographic images were classified as (1) with a tendency toward EARR and (2) without a tendency toward EARR. Triangular, pipette shape, apical bent and short roots were grouped as with a tendency toward EARR ( Figure 2) and rhomboid and rectangular roots were grouped as without a tendency toward EARR ( Figure 3).
Two examiners classified root morphology (Kappa = 0.96). A consensus was obtained in cases of divergence. Intra-examiner reliability regarding EARR was statistically analyzed by the difference between duplicate measurements on the radiographic images of 25 randomly selected patients at T 1 , T 2 and T 3 , with a two-week interval between assessments. The error of the method was calculated using Dahlberg's formula: in which d is the difference between pairs of measurements and n is the number of pairs of measurements [25]. Spearman's correlation coefficient (r) was also employed. Although there were no statistically significant differences between the first and second measurements, the mean of each region measured was used in the subsequent statistical texts in order to minimize the random error.
Examiner took tooth measurements without knowing group identification. Also professionals who treated sample patients did not know study groups.

Statistical analysis
EARR at T 2 and T 3 did not exhibit normal distribution (Lilliefors test). Therefore, the non-parametric Spearman correlation test was performed. The Mann-Whitney test was used for the comparison of the amount of EARR between groups with and without a tendency toward resorption. Simple linear regression analysis was used to formulate an equation for estimating the amount of resorption after 12 months in relation to resorption after six months of treatment. The level of significance was set at 5% for all statistical tests.
There were no significant differences in EARR between the groups of roots with and without a tendency toward resorption (morphologic risk) after either six (P=0.151) or 12 (P=0.079) months of treatment. Among the 91 incisors analyzed, 26 had root morphology with a tendency toward EARR and 65 had root morphology without a tendency toward EARR (Table 3).
Descriptive statistics of the evaluated upper central incisors (n=91) in relation to determination of root morphology are presented in Table  4.
There was a significant correlation between EARR at T 2 and at T 3 (r=0.7606; P=0.000). The simple linear regression analysis revealed that the amount of EARR at T 2 was associated to EARR at T 3 (r 2 =0.64, P=0.000). The percentage of EARR at T 3 was estimated by the following formulas: EARR T 3 = 1.436+ 0.9957 (EARR T 2 ).

Discussion
Previous reports have suggested a correlation between root morphology and EARR during orthodontic movement, with pipette shape, triangular and bent roots associated to resorption [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. The likely explanation for the greater tendency toward EARR is the possibility of a greater concentration of forces in thinner apical root shapes and, consequently, greater harm to the cementum [5]. While this may be a plausible reason, the association between the dissipation of apical force and EARR remains unclear. Contrary to previously raised hypothesis, there were no statistically significant differences in the present study regarding the percentage of root resorption between root groups with and without a supposed tendency toward EARR (Table 3). A number of recent studies has also reported a weak or no correlation between root shape and EARR [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Moreover, Smale [4] report that, while narrow and bent roots may exhibit an increased risk of EARR in the early stages of treatment, the explanation of the variance of these risk factors is less than 25%.
In the present study, the amount of root resorption was assessed in millimeters and then expressed in percentage. From the clinical standpoint, even mild resorption in short roots may be more important than the same absolute amount in long roots. Thus, although short roots may not be more prone to resorption, care must be taken with this particular root shape. For example, the incisor #38 had a 27mm total length before treatment and after 12 months it presented 2.0mm of EARR, which meant 7.3% of tooth reduction. While a shorter incisor (#42) with 18.7mm total length prior to orthodontic treatment presented after 12 months almost the same level of EARR    (2.1mm), but reflected in 11.2% of tooth reduction. Furthermore, it must be noted that if root length only is considered, instead of total dental length, percentage of reduction almost doubles, more affecting alveolar insertion. Prognosis of tooth depends upon the surface area of periodontal ligament attachment not necessarily root length and the apical surface corresponds to the smallest part of periodontal support [26]. Root apex loss of 3mm equals alveolar crest bone loss of 1mm from cervical margin in a normal tooth [27].
It has been reported that cases involving premolar extractions are more subject to greater degrees of EARR [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. In a retrospective study of 1049 cases, Marques [6] found a high prevalence of EARR (14.5%) at the end of treatment, with an odds ratio of 6.38 for cases treated with the extraction of first premolars. It is suggested that the greater apical movement in cases of extraction, especially in the anteroposterior direction, is the real risk factor for EARR . According to some authors, this may be related to the approximation of the roots of the maxillary incisors to the palatine cortical bone, which has greater density, thereby providing a greater concentration of force at the apical region and possibly greater EARR [7][8][9]. In the present sample, no extraction cases only were included. Furthermore, the 12-month duration of the study can explain the low degree of EARR (0 to 12% or 0 to 2.7mm).
It has been reported that patients with resorption greater than 1 mm in the first six months of orthodontic treatment have a three times risk of severe resorption (greater than 5 mm) at the end of treatment. Those with more than 2 mm resorption at six months have a 15 times greater chance of exhibiting severe resorption at the end of treatment 16 . In the present study, 18 patients (38%) had at least one incisor with mild EARR (> 4 and ≤ 8% or > 1 and ≤ 1.7 mm) and one patient had moderate EARR (> 8 and ≤ 12% or > 1.7 and ≤ 2.7 mm) after six months of treatment. At 12 months, however, 45% of the patients had either no resorption or only apical rounding, 38% had mild EARR and 17% exhibited moderate resorption (Table 2). This corroborates the findings of most studies that the risk of severe resorption is generally low, and confirms EARR progression during treatment [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].
As risk factors of EARR are not adequate predictive factors, a number of authors have sought to identify patients in the early stages of orthodontic treatment who will exhibit severe EARR at the end of treatment [14][15][16]. Årtun [16] found a significant correlation between EARR at six and twelve months with EARR at the end of active treatment, reporting that EARR at six months explained 46% (P<0.001; r 2 =0.46) of EARR at the end of treatment and EARR at 12 months explained 64% (P<0.001; r 2 =0.64) [16]. The regression formula in the present sample explained 64% of the cases of resorption at 12 months (Figures 4 and 5). The data corroborate the significant incidence indicator power that periapical radiography after six months of treatment can have regarding the risk of EARR.

Conclusion
The amount of EARR after the first six months of orthodontic treatment does give an indication of the incidence of some external apical root resorption 6 months into treatment, but could still vary in this sample depending on the internal (genetic) and external (mechanics) present in this sample.
In non extraction orthodontic cases, no significant difference was found in the amount of root resorption between roots classified as having a tendency toward EARR (triangular, bent, pipette shape and short) and those classified as not having this morphologic risk (rhomboid and rectangular).