Root resorptions associated with canine retraction treatment


The hypothesis of this study was that multiple factors are dominant in causing external apical root resorption (EARR). The objective of this investigation was to better understand the clinical factors that may lead to EARR.


Maxillary cone-beam computed tomography scans of 18 subjects who were treated with bilateral canine retractions during orthodontics were used to calculate EARR. The subjects were treated using well-calibrated segmental T-loops for delivering a 124-cN retraction force and the moment-to-force ratio suitable for moving the canine under either translation or controlled tipping. The subjects’ age, sex, treatment duration, and genotype were collected.


Six subjects of the 18 showed definite EARR, meaning that load was not the only causing factor. All 5 subjects with the genotype identified had GG genotype of IL-1β rs11143634, indicating that people with this genotype may be at high risk. Longer treatment duration, female sex, and older age may also contribute to EARR, although the findings were not statistically significant.


EARR appears to be related to multiple factors. The orthodontic load and the genotype should be the focuses for future studies.


  • Canine retraction under a retraction force of 124 cN may result in EARR.

  • The teeth with no orthodontic load do not have EARR.

  • Orthodontic force alone is not sufficient to cause EARR.

  • High-risk factors for EARR may be older age, female, longer treatment time, and high moment-to-forceratio.

  • Patients with GG genotype of IL-1β rs1143634 might also be at higher risk for EARR.

External apical root resorption (EARR) is a side effect that occurs during orthodontic treatment. It is characterized by root shortening or shrinking. EARR occurs only in certain patients. It is important to identify the dominant factors causing EARR so that clinicians may adjust treatment to prevent it. EARR is a multifactorial issue. Level of orthodontic load sensed by the tooth, treatment type, duration, genotype, and age are considered potential contributing factors. The common consensus is that the elevated stress due to the orthodontic load on a tooth, especially in the periodontal ligament, causes EARR ; longer treatment also increases the chance of EARR. Al-Qawasmi et al found that the interleukin IL-1β gene contributes to EARR. Previous studies also indicated that older people are more vulnerable. Most systematic studies were on animals. Clinical data are critical to validate the findings. The major obstacles to validate these clinically had been the ability to control the orthodontic loading and to reliably assess the EARR.

Clinical orthodontic load systems reported in the literature were normally qualitative. Typically, the loads provided by the actuators (eg, segmental wire or spring) were reported, which were not the loads sensed by the teeth. The load on a tooth is difficult to quantify in vivo. Root length change had been widely reported as evidence of EARR. Two-dimensional (2D) images were primarily used for measuring root shortening. Errors due to difficulty to align the images taken at different times could have occurred. Studies on premolar extraction patients proved the existence of EARR. Limited information is available for understanding the clinical effects of orthodontic treatment on EARR. A clinical study with well-controlled orthodontic load and reliable tooth lengths and volume measurements will help us to understand the dominant factors for EARR.

The hypothesis of this study was that multiple factors are dominant in causing EARR. The objective of this study was to investigate EARR associated with well-controlled canine retraction treatment. The evaluated factors include treatment strategies, level of orthodontic force, genotype, age, sex, and treatment duration.

Material and methods

Eighteen subjects (7 male, 11 female) who needed bilateral maxillary canine retractions were involved in this study. The study was approved by the Institutional Review Board of Indiana University. The inclusion criteria included necessity of extraction of both maxillary first premolars and maxillary canine retraction as a part of the orthodontic treatment. The average age of the subjects was 19 ± 9 years old. Their ages ranged from 12 to 47 years old. One subject was 47 years old, one was 35 years old, and the other 16 subjects were between 12 and 22 years old. They all had canine retractions. The cone-beam computed tomography (CBCT) scans of the 18 subjects were used for assessing EARRs.

Before the study, the right and left first premolars were extracted, and the maxillary dental arch was leveled and aligned with a 0.019 × 0.025-in stainless steel archwire engaged in 0.022 × 0.028-in slot brackets. The maxillary second molars were included in the archwire and coligated to the maxillary second premolar and first molar with a 0.010-in stainless steel wire on each side that served as anchorage. The bilateral first molars were connected with a transpalatal arch for anchorage reinforcement. Segmental T-loops designed for providing 124 cN of initial retraction force and the desired moment-to-force (M/F) ratios were attached to the corresponding first molar and the canine by clinicians. The M/F ratio, which was based on the location of the center of resistance obtained with the finite element method, was accomplished by adjusting the gable bends. The controlled tipping load had relatively lower M/F ratios than did translation. The load system delivered was quantified by an orthodontic force tester. Details of the experimental protocol, the specially designed T-loops, and the repeatability testing results were reported previously. For each subject, customized segmental T-loops were randomly assigned to the right or left canines to implement either translation or controlled tipping force. The treatment period varied depending on the size of initial space, appointment, and intersubject variations. The average was 4.9 months. The age and treatment duration for each subject were recorded. To maintain the desired force and the M/F ratio, the T-loops were replaced with new loops when a canine displaced by more than 1 mm.

All CBCT scans were performed on the same i-CAT imaging system (Imaging Sciences International, Hatfield, Pa) at the Indiana University School of Dentistry. The voxel size was 0.25 mm, and the scan time was 26.9 seconds. The scans of each subject were taken immediately before and after the canine retraction. The same setting was used for all scans.

The EARR was quantified by using the CBCT scans obtained immediately before and after canine retraction. The CBCT images were processed with Mimics software (version 13.0; Materialise, Leuven, Belgium). The canine was segmented first, and the tooth length was easily measured by using the 3-dimensional (3D) length measuring function in Mimics. The tooth length was defined as the distance between crown tip and root tip. ( Fig , A ) The length reduction measured from the pretreatment and posttreatment scans was used to quantify EARR.

A, Tooth length measurement by using Mimics software; B, root volume was measured by the tooth volume minus the crown volume enclosed in the sphere with a 10-mm diameter.

The metal bracket causes reflection blur at the crown area in the CBCT images; this leads to unreliable contour recognition. The crown portion was removed because the EARR primarily occurs at the apical portion of the root. To make a consistent cut for all teeth, a sphere with a diameter of 10 mm and centered at the crown tip was created, and then the sphere part including the entire crown was cut from the tooth. The volume of the remaining part of the tooth was considered the root volume ( Fig , B ).

If the difference in tooth length between the pretreatment and posttreatment canine on the CBCTs was more than 0.5 mm, this was considered EARR. A difference less than 0.5 mm was considered uncertain because of the 0.25-mm voxel size of the CBCT image; thus, it was not counted as EARR. For volume change, because the average volume of the layer of the root surface with 1 voxel thickness was 73 ± 11 mm 3 , only a volume change greater than that was considered as a definite volume change.

The maxillary canines and incisors were evaluated for root resorption. During the canine retraction using the segmental T-loops, only the canines were under orthodontic force; the incisors were not. Therefore, it was expected that there should be no root resorption on the incisors.

Subjects’ saliva samples were collected using the DNA Collection Kit (OG-100, DNA Genotek, Ottawa, Ontario, Canada), and genomic DNA was extracted using Oragene Purifier (OG-L2P, DNA Genotek) and quantified spectrophotometrically using the default OD 260 nm absorbance algorithm, and then stored at −20°C until use. Automated polymerase chain reaction were performed on PTC-100 Programmable Thermal Controller (MJ Research, St Bruno, Quebec, Canada), and allelic discrimination were done using the 7300 Sequence Detection System (Applied Biosystems, Foster City, Calif), TaqMan polymerase probes and primers using the method provided by DNA Genotek (TaqMan SNP Genotyping Assays Protocol). Rs1143643 (Applied Biosystems TaqMan C_1839949_10), rs1143634 (Applied Biosystems TaqMan C_9546517_10), rs1143629 (Applied Biosystems TaqMan C_1839945_1_) for IL-1β, and rs1794065 (Applied Biosystems TaqMan C_3133518_10), rs315952 (Applied Biosystems TaqMan C_1151247_10), and rs315951 (Applied Biosystems TaqMan C_948691_1_) for IL-1RA were genotyped.

Statistical analysis

Associations of sex and the distribution of each genotype with EARR were analyzed using Mantel-Haenszel chi-square tests, and age and treatment duration were evaluated using 2-sample t tests.


Some maxillary canines showed definite EARR, whereas all incisors evaluated had no definite EARR. Table I shows the calculated root length and volume changes of the canine on the translation and controlled tipping sides as well as the apical tooth displacements as functions of the subject’s sex, age, treatment duration, and genotype. The incisor root shortenings were all less than 0.5 mm and were not included in Table I . The apical tooth displacements were reported previously. A root reduction equal to or greater than 0.5 mm was marked and considered to be definitely EARR. The volume changes were less than 75 mm, the resolution due to the voxel size of the CBCT images, and thus were considered not definite. The P values of these factors are shown in Tables II and III .

Table I
Results from the 18 patients, including age, sex, treatment duration, EARR on the CT and TR sides, volume change on the CT and TR sides, and patient genotypes
Age Sex Duration (d) Length change Volume change Subject genotype Apical movement intrusion (+)
(mm 3 )
(mm 3 )
IL-1 β rs1143629 IL-1 β rs1143634 IL-1 β rs1143643 IL-1 RA rs315951 IL-1 RA rs315952 IL-1 RA rs1794065 CT (mm) TR (mm)
P1 20 F 292 −0.9† −0.6† −17 −51 0.73 0.35
P2 18 F 176 −0.1 −0.2 26 3 AA GA CC CC TT AG −0.16 −0.17
P3 22 F 175 −0.3 0.3 −6 −11 GG GA CC CC TT GG −0.28 −0.73
P4 15 M 203 −0.5† −0.7† −23 −11 AG GG CT CC TT AA −0.35 −1.03
P5 47 F 132 −0.3 −0.7† −15 −25 GG GG CC CC TT AG 0.07 −0.41
P6 15 M 184 0.3 0.1 −1 5 AA GA CT CC TT GG 0.11 −0.04
P7 35 F 357 0.3 −0.5† −3 5 AG GG CC CG CT GG 1.03 −0.06
P8 14 F 196 −0.3 0.1 10 12 AG GA CC CG CT GG −1.18 −1.84
P9 18 M 98 0.1 0.4 −24 −12 AG GA CT CG CT AG −0.86 −0.42
P10 17 M 68 −0.2 −0.3 23 27 −0.57 0.16
P11 16 F 120 −0.3 −0.2 −2 −17 −0.06 −0.62
P12 19 F 107 −0.3 −0.5† −23 −1 AG GG CT CG CT GG −2.38 −3.53
P13 14 F 58 0.3 0.4 6 5 AG GG CT CC TT AA 0.00 0.35
P14 15 M 98 −0.1 −0.3 −2 9 −0.76 −1.00
P15 12 F 134 −2 −2.1† −12 −58 AA GG CC GG CC GG 1.70 −2.12
P16 15 M 62 −0.2 0 54 55 AG GA CC CG CT GG 0.03 0.30
P17 14 M 65 0.1 0 15 4 AA GG CT CC TT GG −0.82 −0.63
P18 22 F 99 0.2 0.3 12 9 AG GG CT CC TT AG 1.43 −0.34
RR , Root resorption; CT , controlled tipping side; TR , translation side; RRV , root volume change; P , patient; F , female; M , male.

Subjects with definite EARR; †length changes exceeding 0.5 mm; negative length values indicate root shortening.

Table II
Genotype distribution and allele frequencies for IL-1β gene cluster SNPs (rs1143629, rs1143634, and rs1143643) and IL-1RA gene cluster SNPs (rs315951, rs315952, and rs1794065) in patients with high and low levels of EARR
Group Genotype, n Allele, n
IL-1β rs1143629 High EARR (n = 5) 1 (20.0%) 3 (60.0%) 1 (20.0%) 5 (50.0%) 5 (50.0%)
( P = 0.460) Low EARR (n = 12) 4 (33.3%) 7 (58.3%) 1 (8.3%) 15 (62.5%) 9 (37.5%)
IL-1β rs1143634 High EARR (n = 5) 5 (100.0%) 0 (0.0%) 0 (0.0%) 10 (100.0%) 0 (0.0%)
( P = 0.031) Low EARR (n = 12) 5 (41.7%) 7 (58.3%) 0 (0.0%) 17 (70.8%) 7 (29.2%)
IL-1β rs1143643 High EARR (n = 5) 3 (60.0%) 2 (40.0%) 0 (0.0%) 8 (80.0%) 2 (20.0%)
( P = 0.503) Low EARR (n = 12) 5 (41.7%) 7 (58.3%) 0 (0.0%) 17 (70.8%) 7 (29.2%)
IL-1RA rs315951 High EARR (n = 5) 2 (40.0%) 2 (40.0%) 1 (20.0%) 6 (60.0%) 4 (40.0%)
( P = 0.160) Low resorption (n = 12) 8 (66.7%) 4 (33.3%) 0 (0.0%) 20 (83.3%) 4 (16.7%)
IL-1RA rs315952 High EARR (n = 5) 2 (40.0%) 2 (40.0%) 1 (20.0%) 6 (60.0%) 4 (40.0%)
( P = 0.160) Low EARR (n = 12) 8 (66.7%) 4 (33.3%) 0 (0.0%) 20 (83.3%) 4 (16.7%)
IL-1RA rs1794065 High EARR (n = 5) 1 (20.0%) 1 (20.0%) 3 (60.0%) 3 (30.0%) 7 (70.0%)
( P = 0.631) Low EARR (n = 12) 1 (8.3%) 3 (12.5%) 8 (66.7%) 5 (20.8%) 19 (79.2%)
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Dec 19, 2018 | Posted by in Orthodontics | Comments Off on Root resorptions associated with canine retraction treatment
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