INTRODUCTION

Examination of the alveolar bone response to orthodontic force is a topic of interest as the quantity of alveolar bone surrounding a tooth after orthodontic movement is associated with the safety of the treatment. The concept that orthodontic force induces alveolar bone resorption on the pressure side and bone apposition on the tension side is well accepted.¹ However, a histological study in dogs showed alveolar bone remodeling occurred not only on the periodontal ligament side but also on the periosteal side.² Clinical studies also revealed cortical bone remodeling occurred following orthodontic tooth movement.^3–6

Studies conducted in the 1960s reported that when orthodontic tooth movement occurred, the bone around the alveolar socket remodeled at a 1:1 bone remodeling/tooth movement (B/T) ratio.^7,8 More recent cone-beam computed tomography (CBCT) studies demonstrated that the rate of alveolar bone remodeling may not be consistent with maxillary incisor retraction^4,9 or maxillary incisor proclination.^5,6 These CBCT studies also reported significant relationships between alveolar bone change and inclination change,^4,5 the amount of intrusion,⁴ and the rate of tooth movement.⁴ However, the tooth axis was used as a reference line, which is advantageous in terms of evaluation of the safety of mechanotherapy and the amount of surrounding alveolar bone support after tooth movement.⁵ However, this method cannot be used to explore the actual response of alveolar bone to orthodontic tooth movement because the points of bone being measured also change as the tooth axis changes. In addition, if a tooth is moved vertically, the same points of bone are not measured before and after treatment (Figure 1).

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To assess the extent of alveolar bone change after orthodontic tooth movement accurately, external reference lines constructed from stable skeletal structures were used in the current study to define changes in the alveolar bone position and tooth movement. Using this method of measurement, the B/T ratio can be defined at different root levels. Therefore, the aim of this study was to evaluate the alveolar bone change in relation to dental position change after maxillary incisor retraction using stable skeletal structures as a reference. The parameters that correlated with alveolar bone change were also examined. The null hypothesis of the study was that there was no significant change in the alveolar bone position following maxillary incisor retraction and that the B/T ratio was not equal to 1:1.

MATERIALS AND METHODS

This prospective clinical trial was approved by the Ethics Committee on Human Research of the Faculty of Dentistry, Prince of Songkla University (EC6012-42-P-LR). The sample size was calculated by G*Power (version 3.1)¹⁰ using parameters from a previous study of Sarikaya et al. (the difference of means = 0.35 mm, the difference of standard deviations = 0.49, significance level = .05, power = 0.80).⁹ A sample size of 16 was required. A total of 17 female patients (18 to 30 years old) were included in the study on the basis of the following inclusion criteria: requiring bilateral extraction of the maxillary first premolars and ≥2 mm maxillary incisor retraction, no significant medical history, no long-term use of anti-inflammatory drugs or bisphosphonates for at least 12 months before or during orthodontic treatment, no evidence of periodontal tissue destruction at the beginning of or during orthodontic treatment, and no history of trauma to the maxillary anterior teeth. All of the patients were informed of the purpose of the study and provided signed, written informed consent before participating in the study.

The patients were treated with preadjusted bidimensional edgewise fixed appliances (Roth prescription; 0.018” × 0.025” brackets on incisors, 0.022” × 0.028” brackets on canines, and posterior teeth and buccal tubes on second molars). The maxillary teeth were bonded and aligned using sequential 0.012”, 0.014”, 0.016”, and 0.016” × 0.016” nickel–titanium and 0.016” × 0.016” stainless-steel archwires, and then the maxillary canines were distalized. Preretraction CBCT images (CT0) were taken after the completion of canine distalization. Incisor retraction was performed using 0.016” × 0.022” titanium–molybdenum alloy retraction T-loop (8-mm loop length and height) continuous archwires with an accentuated curve. Each loop was activated by 1.5 mm per month until the incisors were completely retracted. The maxillary arch was passively maintained with 0.016” × 0.022” stainless-steel archwires for 3 months to facilitate completion of the alveolar bone remodeling process.¹¹ Postretraction CBCT images (CT1) were taken at the end of the 3-month period.

CBCT Imaging

All CBCT images were taken with the same machine (Veraviewepocs 3D F80; J. Morita, Irvine, Calif) with the same patient positioning protocol. CBCT images covering the area of the anterior nasal spine (ANS)–posterior nasal spine (PNS) were taken at 70 kV, 51 mAs, 0.125-mm voxel resolution and 80 × 80 mm field of view. One Volume Viewer software (version 11.0; J Morita, Chatsworth, CA, USA) was used to measure all parameters in this study. The reference planes of each CBCT image were oriented according to a previous study.¹² The palatal plane (ANS-PNS) on the sagittal section, mid-palatine suture on the axial section, and the most inferior level of the nasal floor on the coronal section were parallel or perpendicular to the image border (Figure 2). Later, two reference lines were constructed on the sagittal image: (1) a horizontal reference line (H-line) connecting the ANS to the PNS and (2) a vertical reference line (V-line) perpendicular to the H-line halfway between ANS and PNS determined by coordinates read from the software (Figure 2A).

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The investigator was blinded to the subject's identity in each image. The sagittal slice passing through the center of the mesio-distal width of each tooth was used to assess the changes in each tooth and the corresponding bone. The dental crown measurement variables were (1) vertical dental position, the perpendicular distance between the maxillary incisor edge and the H-line; (2) horizontal dental position, the perpendicular distance between the maxillary incisor edge and V-line; and (3) dental inclination, the angle formed between the tooth axis and H-line (Figure 3). The difference in the vertical dental position between CT0 and CT1 was defined as vertical dental change; the difference in the horizontal dental position between CT0 and CT1 was defined as retraction distance. The rate of tooth movement was calculated as the retraction distance divided by the retraction time. All parameters were calculated and averaged for the upper incisors in each individual.

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For the measurements of labial (L) and palatal (P) root (R) and bone (B) positions on the CT0 images, each tooth was measured at the crestal (S1), mid-root (S2), and apical (S3) levels apical to the labial cemento-enamel junction every 2 mm along the V-line. The root and bone positions were defined as the distances between each point (LR1-3, PR1-3, LB1-3, PB1-3) and the V-line measured parallel to the H-line (Figure 4).

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To assess horizontal root and bone changes, H-line, V-line, and horizontal lines parallel to the H-line at the level of S1, S2, and S3 from the sagittal slice of the CT0 image (Figure 4) were transferred to and superimposed on the H-line and V-line of the sagittal slice of the CT1 image. Changes were defined as the horizontal distances between LR1-3, PR1-3, LB1-3, and PB1-3 points of the two images on the S1, S2, and S3 lines.

RESULTS

Dahlberg's error for each parameter was less than 0.5 mm for linear variables and less than 0.5° for angular variables. Intraobserver reliability was higher than 0.90 for all measurements.

The mean retraction distance, vertical dental change, inclination change, retraction time, and rate of tooth movement were 3.3 ± 1.1 mm, 0.6 ± 1.2 mm, 6.4 ± 5.2°, 4.2 ± 1.4 months, and 0.8 ± 0.2 mm per month, respectively (Table 1). Wilcoxon matched-pairs signed-rank tests demonstrated the retraction distance and inclination change (both P < .001), but not the vertical dental change (P > .05) were significantly different between CT0 and CT1.

Table 1. Mean, SD, and Range of Age, Retraction Distance, Vertical Dental Change, Inclination Change, Retraction Time, and Rate of Tooth Movement^a

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The Kruskal-Wallis test demonstrated that mean alveolar bone change and root change were not significantly different at any root level between teeth (P > .05; Tables 2 and 3). Therefore, the data from all four incisors in each patient were pooled and averaged for further analysis.

Table 2. Alveolar Bone Changes at Different Levels of the Maxillary Incisor Roots

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Table 3. Root Changes at Different Levels of the Maxillary Incisor Roots

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Wilcoxon matched-pairs signed-rank tests demonstrated labial alveolar bone significantly decreased between CT0 and CT1 at all root levels (P < .05). However, the palatal alveolar bone was not significantly different between CT0 and CT1 at any root level (P > .05). The labial and palatal root position relative to the V-line significantly decreased at all root levels (P < .05). The B/T ratio was calculated from labial alveolar bone change and dental change. As a result of the noncontinuous characteristics of the ratio data, the medians and interquartile ranges are presented. The labial B/T ratio was nearly 1:1 at all root levels. In contrast, the B/T ratio on the palatal side ranged from 0.2 to 0.4 (Table 4).

Table 4. Alveolar Bone and Root Positions at CT0 and CT1 and Changes at Different Levels of the Maxillary Incisor Roots and B/T Ratio^a

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Bivariate correlations were examined between the variables that changed significantly between CT0 and CT1. Spearman rank correlation tests demonstrated that the labial alveolar bone changes at the crestal and mid-root level correlated significantly with the inclination change (r = 0.8 and 0.6; P < .05), but were not significantly related to the retraction distance (Table 5).

Table 5. Correlations Between Alveolar Bone Change, Inclination Change, and Retraction Distance

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DISCUSSION

Using stable skeletal structures as a reference, the present study revealed that alveolar bone on the labial side remodeled concomitantly with root movement during orthodontic retraction. The B/T ratio of nearly 1:1 observed for all root levels was in agreement with previous CBCT studies that used the tooth axis as a reference.^9,14 The small degrees of inclination change may explain why the results were similar between studies, regardless of the reference used. The inclination changes in previous studies ranged from 7.8 to 10.4°,^9,14 whereas the average inclination change in this study was 6.4°. Thus, either an external reference or the tooth axis could be used to study the B/T ratio if the inclination change is within the range of 10° or less.

On the contrary, the results disagreed with a previous cephalometric study¹⁵ that found a 1:2 B/T ratio on the labial side after maxillary incisor retraction. The difference in results may be because the amount of inclination change of this previous cephalometric study was about 12.3°, signifying a larger degree of tipping than the present study. Also, the nasion–pogonion line was used as a reference line to measure the positions of the incisor root apex and the bony point A. The inclination of this reference line is not stable throughout the treatment as it could be changed because of mandibular rotation or relocation as a result of orthodontic mechanics.

The nonsignificant change in the palatal alveolar bone position during incisor retraction confirms other investigations that suggested the preretraction palatal bone position represented the boundary for maximal retraction of the maxillary incisors.^9,14,16 Handelman et al.¹⁷ advocated that tooth movement could alter the distance between the alveolar cortical plate and the roots of orthodontically moved teeth, stating “the alveolar cortical plate behaved like the orthodontic wall.” Current results for the palatal side, but not the labial side, were in agreement with those of Handelman et al.¹⁷ The stability of the palatal cortical plate, regardless of the amount of maxillary incisor retraction, indicated that caution should be exercised to not overretract the maxillary incisors palatally, especially in cases with thin alveolar bone housing. Uncontrolled mechanics may predispose patients to alveolar bone loss, fenestrations, dehiscence, or root resorption if the amount of tooth retraction required is greater than the pretreatment position of the palatal cortical plate.^3,14,17,18 Previous studies reported that recovery of a well-defined dense cortical plate would not occur if the cortical plate had been penetrated by the root.¹⁹

Bivariate correlation analysis revealed that the inclination change influenced alveolar bone remodeling, in agreement with previous studies.^4,5 However, the correlation between alveolar bone change at the labio-apical level (LB3) and inclination change was not statistically significant. This may be explained by two factors. First, incisor movement was definitely not bodily, as the apical part of the root moved less than the cervical part. The small amount of movement for the apical part of the root may not be sufficient to signal the corresponding alveolar bone to remodel concomitantly with root movement. Second, type II error may have occurred as the sample size was relatively small.

This study provided a method to evaluate alveolar bone change after maxillary incisor retraction that was not confounded by dental inclination change. The use of CBCT is advantageous as it allows the bone surrounding all four incisors to be investigated. However, because the vertical dental changes were not significant, it may not be possible to extrapolate the results of this study to cases in which the incisors are intruded or extruded during retraction. Using three-dimensional superimposition on stable structures such as the palate may provide more accurate and reliable results. However, specific software is needed to apply that methodology. The study of three-dimensional bone volume changes and the changes in cortical bone thickness after incisor retraction could provide additional insight into alveolar bone remodeling attributed to orthodontic force.

The results of this study highlight the importance of assessing the distance between the root surface and palatal cortical plate in cases undergoing maxillary incisor retraction. Excessive retraction beyond this distance may cause the root to hit the cortical plate and possibly lead to several complications such as root resorption and bony defects. Safer treatment options, such as corticotomy with a bone graft or orthognathic surgery, should be considered for cases with a narrow distance between the root and palatal bone.

CONCLUSIONS

Using stable skeletal structures as a reference, after maxillary incisor retraction with T-loop mechanics:

• the labial alveolar bone remodeled in accordance with tooth movement in a 1:1 B/T ratio;

• the palatal alveolar bone remained unchanged, with the distance between the root and palatal cortical plate decreasing; and

• inclination change was the only factor associated with alveolar bone change.