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ORIGINAL ARTICLE Factors affecting buccal bone changes of maxillary posterior teeth after rapid maxillary expansion Kitichai Rungcharassaeng,a Joseph M. Caruso,b Joseph Y. K. Kan,c Jay Kim,d and Guy Taylore Loma Linda, Calif Introduction: The purpose of this study was to use cone-beam computed tomography (CBCT) images to determine the factors that might affect buccal bone changes of maxillary posterior teeth after rapid maxillary expansion (RME). Methods: Thirty consecutive patients (17 boys, 13 girls; mean age, 13.8 ⫾ 1.7 years) who required RME as part of their orthodontic treatment and had the pre-RME (T1) and post-RME (T2) CBCT images available were included in the study. The T1 and T2 measurements of interdental distance, interdental angle (IA), buccal bone thickness (BBT), and buccal marginal bone levels (BMBL) of the first premolar (P1), the second premolar (P2), and the first molar (M1) were compared with the Friedman and the Wilcoxon signed rank tests. To determine which variables were associated with the changes in IA, BBT, and BMBL, the Spearman rank correlation analysis was performed (␣ ⫽ .05). Results and Conclusions: The results suggest that buccal crown tipping, and reduction of BBT and BMBL of the maxillary posterior teeth are the expected immediate effects of RME. There were no significant differences in dental expansion among P1, P2, and M1 (P ⬎.05). P2 had clinically more buccal crown tipping (P ⫽ .116) but statistically less reduction in BBT and BMBL (P ⬍.0001 and P ⫽ .001) than P1 and M1. Buccal bone changes and dental tipping on P2 were not affected by any other variables. Factors that showed significant correlation to buccal bone changes and dental tipping on P1 and M1 were age, appliance expansion, initial buccal bone thickness, and differential expansion (P ⬍.05), but rate of expansion and retention time had no significant association (P ⬎.05). (Am J Orthod Dentofacial Orthop 2007;132:428.e1-428.e8) R apid maxillary expansion (RME) is a commonly used method to correct maxillary constriction and arch length discrepancy. RME appliances are meant to produce orthopedic expansion; ie, the changes are produced primarily in the underlying skeletal structures rather than by movement of the teeth through alveolar bone.1-6 Orthopedic expansion is achieved through RME not only by separation of the midpalatal suture, but also by its effects on the circumzygomatic and circumaxillary sutural systems.7 An implant study on orthopedic expansion, however, showed that only 50% of skeletal movement was achieved in young children, and the rest of the expansion was attributed to dental movement. In addition, in adolescents, only 35% of the move- From the School of Dentistry, Loma Linda University, Loma Linda, Calif. a Associate professor, Department of Orthodontics. b Associate professor and chair, Department of Orthodontics. c Associate professor, Department of Restorative Dentistry. d Professor of statistics, Department of Dental Education Services. e Assistant professor, Department of Orthodontics. Reprint requests to: Kitichai Rungcharassaeng, Department of Orthodontics, Loma Linda University, School of Dentistry, Loma Linda, CA 92354; e-mail: [email protected]. Submitted, November 2006; revised and accepted, February 2007. 0889-5406/$32.00 Copyright © 2007 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2007.02.052 ment was skeletal, and 65% was dental.8 Therefore, as a patient grows older, dental tipping becomes more likely; this puts the teeth at higher risk of being moved through the envelope of the alveolar process. This can result in reduction of alveolar bone height, bone dehiscence, and gingival recession.8-15 Studies of RME generally measured the changes observed of the dental casts or the 2-dimensional cephalometric radiographs (lateral or posteroanterior radiographs) before and after treatment to evaluate the short- and long-term skeletal and dental effects of RME.15-18 Incidences of buccal marginal bone loss and cortical fenestrations have been reported in conjunction with gingival recession,19-21 but without quantitative data. With low-dose cone-beam computed tomography (CBCT) technology (NewTom 3G, AFP Imaging, Elmsford, NY), it is possible to obtain accurate radiographic images that allow clinicians and researchers to quantitatively evaluate hard-tissue changes in 3 dimensions.22-25 The purpose of this study was to use CBCT images to quantitatively evaluate buccal bone changes of the maxillary first premolar (P1), second premolar (P2), and first molar (M1) after RME and the variables associated with it. 428.e1 428.e2 Rungcharassaeng et al Fig 1. Occlusal view of 4-banded hyrax appliance. American Journal of Orthodontics and Dentofacial Orthopedics October 2007 Fig 2. Occlusal view of 2-banded hyrax appliance with expansion arms extended to P1. MATERIAL AND METHODS This study was approved by the Institutional Review Board of Loma Linda University, Loma Linda, Calif. Thirty consecutive patients, treated since January 2005 at the Graduate Orthodontic Clinic, Loma Linda School of Dentistry, who required RME with the hyrax appliance as part of their comprehensive orthodontic treatment and had before (T1) and after (T2) RME images made with CBCT available, were included in the study. The T1 images were obtained before orthodontic treatment, and the T2 images were obtained within 3 months after the end of activation. The hyrax appliances used were either 4-banded (supported by bilateral first premolars and first molars) (Fig 1) or 2-banded (supported by bilateral first molar with expansion arms and mesial rest bonded to first premolars) (Fig 2). General information about each patient was collected from the patient’s record and included sex, age at start of treatment, type of appliance, activation time (in weeks), and retention time (in weeks, from tie-off to the time of the CBCT). The CBCT data of each patient were reconstructed at 0.5 mm increments, and the DICOM (Digital Imaging and Communications in Medicine) images were assessed with software (Open-Source, OsiriX Medical Imaging Software, http://homepage.mac.com/rossetantoine/osirix/Index2.html). The following parameters were evaluated on P1, P2, and M1 and recorded. 1. Buccal marginal bone level and bone thickness. From the axial section of the T1 images, at the root level of the tooth of interest (P1, P2, or M1), an open-polygon cut was made buccolingually so that it bisected the root bilaterally. On the coronal image derived from the open-polygon cut, reference lines (RL) were constructed from the buccal cusp tips to Fig 3. T1 coronal image derived from the open-polygon cut. RL, Reference line; PL1, perpendicular line 1; PL2, perpendicular line 2; BMBL is depicted in yellow; BBT is depicted in pink; BTL is depicted in red. the buccal root tips bilaterally (Fig 3). A straight line connecting the buccal cusp tips was then drawn. A perpendicular line (PL1) to the RL was made at the most coronal point where the bone was in contact with the tooth. The buccal marginal bone level (BMBL) was defined as the distance on the RL from PL1 to the cusp tip. A second perpendicular line (PL2) was made at the level where the buccal bone deflected. The buccal bone thickness (BBT) was the distance from the root surface to the most buccal bone surface on PL 2. The distance on RL from PL2 to the cusp tip was the bone thickness level (BTL), where the BBT of the T2 image would be measured. The procedure was repeated for the American Journal of Orthodontics and Dentofacial Orthopedics Volume 132, Number 4 Fig 4. T2 image. Note significant ⌬BMBL (yellow) and the use of BTL (red) to determine the level where BBT would be measured. T2 measurements, except that PL2 on the T2 image was determined by the BTL at T1 (Fig 4). For M1, only BMBL and BBT of the mesiobuccal root were evaluated, because it is usually more prominent and likely to exhibit more changes than the distobuccal area.26 The change in BBT (⌬BBT) was obtained by subtracting the T1 value from the T2 value (BBTT2-BBTT1); whereas the change in BMBL (⌬BMBL) was derived by subtracting the T2 value from the T1 value (BMBLT1-BMBLT2). Negative ⌬BBT and ⌬BMBL values signified bone loss. 2. Interdental distance. From the axial section of the T1 images, at the crown level of the tooth of interest (P1, P2, or M1), an open-polygon cut was made buccolingually so that it passed through the central fossae bilaterally. On the coronal image derived from the open-polygon cut, an interfossal measurement was made and termed the interdental distance (ID) (Fig 5). The procedure was repeated for the T2 measurements, and their difference (IDT2-IDT1) was the amount of dental expansion (⌬ID). 3. Interdental angle. From the axial section of the T1 images, at the level of the cusp tips of the tooth of interest (P1, P2, or M1), an open-polygon cut was made buccolingually so that it passed through the buccal and lingual (for M1, mesiobuccal and mesiolingual) cusp tips bilaterally. On the coronal image derived from the open-polygon cut, lines were drawn across the buccal and lingual cusp tips of both left and right teeth. The interdental angle (IA) was the angle formed by their intersection Rungcharassaeng et al 428.e3 Fig 5. Coronal image derived from the open-polygon cut. Interfossal measurement was made (yellow line), which signified interdental distance (ID). Fig 6. Coronal image derived from the open-polygon cut. Interdental angle (IA), depicted in yellow, is the angle formed by the intersection of the lines drawn across the mesiobuccal and mesiolingual cusp tips of the first molars bilaterally. (Fig 6). The procedure was repeated for the T2 measurements, and their difference (IAT2-IAT1) signified the amount of dental tipping (⌬IA). A negative ⌬IA value indicated buccal crown tipping. 4. Appliance expansion. From the axial section of the T2 images, at the level of the hyrax appliance, an open-polygon cut was made, bisecting the appliance transversely. On the coronal image derived from the open-polygon cut, the separation distance of the appliance and the thickness of the middle portion of the appliance was measured (Fig 7). The difference was the amount of expansion activated to the appliance (AE). 428.e4 Rungcharassaeng et al American Journal of Orthodontics and Dentofacial Orthopedics October 2007 Table I. Means, standard deviations, and ranges of age, appliance expansion, activation time, rate of appliance expansion, and retention time Age (y): Overall (n ⫽ 30) Male (n ⫽ 17) Female (n ⫽ 13) Appliance expansion (mm) Activation time (wk) Rate of appliance expansion (mm per week) Retention time (wk) Fig 7. Coronal image derived from the open-polygon cut. AE is the difference between the separation distance of the appliance and the thickness of the middle portion of the appliance. 5. Rate of AE. The rate of AE was defined as the amount of AE divided by the activation time (mm per week). 6. Differential expansion (DifE) was defined as the difference between ⌬ID and AE (⌬ID-AE). Statistical analysis The intraexaminer reliability of the measurements was determined by using triple assessments of each parameter on M1 taken at least 2 weeks apart and expressed as the intraclass correlation coefficient. Means and standard deviations were calculated for each parameter. T1 and T2 data were analyzed by using the Friedman and the Wilcoxon signed rank tests. To determine which variables were associated with the changes in BMBL, BBT, and IA, the Spearman rank correlation analysis was performed. The significance level of ␣ ⫽ .05 was used for all statistical analyses. RESULTS Seventeen boys and 13 girls (mean age, 13.8 years; range, 10.3-16.8 years) were included in this study. Seventeen 4-banded and thirteen 2-banded appliances were used. The mean AE, activation time, rate of expansion, and retention time were 4.96 mm, 4.4 weeks, 0.83 mm per week, and 3.6 weeks, respectively (Table I). In 4 patients, bilateral P1 extractions were performed before the T2 images were taken, and, in 7 patients, the P2 teeth (4 bilateral and 3 unilateral) were not erupted at the time the T1 images were taken. Intraclass correlation coefficients for the measurements were higher than 0.90, indicating that the measurement methods were reliable and reproducible. Tables II through IV give the means and standard deviations of all T1 and T2 measured parameters, their differences, and the results of the statistical analyses. Mean ⫾ SD Range 13.8 ⫾ 1.7 14.1 ⫾ 1.8 13.3 ⫾ 1.5 4.96 ⫾ 1.88 7.8 ⫾ 4.4 10.3-16.8 10.3-16.8 10.3-15.8 1.93-10.6 2-18 0.83 ⫾ 0.56 3.6 ⫾ 4.1 0.22-2.43 0-12 Table II. Comparison of T1 and T2 measurements with Wilcoxon signed rank test (␣ ⫽ .05) ID P1 (mm) ID P2 (mm) ID M1 (mm) IA P1 (°) IA P2 (°) IA M1 (°) BBT P1 (mm) BBT P2 (mm) BBT M1 (mm) BMBL P1 (mm) BMBL P2 (mm) BMBL M1 (mm) T1 (mean ⫾ SD) T2 (mean ⫾ SD) P value 33.30 ⫾ 3.10 38.06 ⫾ 3.20 43.92 ⫾ 3.46 197.07 ⫾ 17.23 183.08 ⫾ 16.08 169.61 ⫾ 8.62 1.70 ⫾ 0.65 2.27 ⫾ 0.63 1.96 ⫾ 0.56 8.68 ⫾ 1.03 8.04 ⫾ 0.86 8.25 ⫾ 0.82 39.33 ⫾ 3.56 44.02 ⫾ 3.64 50.58 ⫾ 3.92 190.67 ⫾ 12.36 172.17 ⫾ 14.83 162.96 ⫾ 10.31 0.56 ⫾ 0.65 1.42 ⫾ 0.85 0.72 ⫾ 0.76 13.10 ⫾ 4.73 9.41 ⫾ 2.44 11.17 ⫾ 3.21 ⬍.0001 ⬍.0001 ⬍.0001 .009 ⬍.0001 .003 ⬍.0001 ⬍.0001 ⬍.0001 ⬍.0001 ⬍.0001 ⬍.0001 ID, Interdental distance; IA, interdental angle; BBT, buccal bone thickness; BMBL, buccal marginal bone level; P1, first premolar; P2, second premolar; M1, first molar. When comparing ID, IA, BBT, and BMBL at T1 and T2 on P1, P2, and M1 using the Wilcoxon signed rank test, we found statistically significant differences in all parameters (P ⬍.05) (Table II). When comparing the effect of RME on P1, P2, and M1 using the Friedman test, we found no statistically significant differences in the ⌬ID, the DifE, and dental tipping (P ⫽ .522, .215, and .116, respectively). However, the changes in BBT and BMBL were statistically significantly less in P2 than in P1 and M1 (P ⬍.0001 and P ⫽ .001, respectively). By using the Wilcoxon signed rank test, no statistically significant differences were found in the changes in BBT and BMBL between P1 and M1 (P ⫽ .219 and P ⫽ .094, respectively) (Table III). In addition, since there were no statistically significant differences in ⌬BBT and ⌬BMBL between the right and left P1, P2, and M1 (Table IV; P ⬎.05), only the measurements on the right side were used for correlation analyses. Rungcharassaeng et al 428.e5 American Journal of Orthodontics and Dentofacial Orthopedics Volume 132, Number 4 Table III. Comparison of RME effect on first premolar (P1), second premolar (P2), and first molar (M1) with Friedman and Wilcoxon signed rank tests (␣ ⫽ .05) P1 (mean ⫾ SD) P2 (mean ⫾ SD) M1 (mean ⫾ SD) P value 4.96 ⫾ 1.88 6.02 ⫾ 2.27 1.01 ⫾ 1.94 ⫺6.39 ⫾ 10.30 ⫺1.14 ⫾ 0.65 ⫺4.42 ⫾ 4.61 5.97 ⫾ 2.31 0.94 ⫾ 2.05 ⫺10.90 ⫾ 10.97 ⫺0.84 ⫾ 0.57* ⫺1.37 ⫾ 2.14* 6.66 ⫾ 2.69 1.71 ⫾ 1.41 ⫺6.64 ⫾ 9.49 ⫺1.24 ⫾ 0.56 ⫺2.92 ⫾ 3.11 .522 .215 .116 ⬍.0001* .001* AE (mm) ⌬ID (mm) DifE (mm) ⌬IA (°) ⌬BBT (mm) ⌬BMBL (mm) AE, Appliance expansion; ⌬ID, dental expansion; DifE, differential expansion; ⌬IA, dental tipping; ⌬BBT, change in buccal bone thickness; ⌬BMBL, change in buccal marginal bone level. *Statistically significant. Table IV. Comparison of buccal bone changes between right and left first premolar (P1), second premolar (P2), and first molar (M1) with Wilcoxon signed rank test (␣ ⫽ .05) P1 (mean ⫾ SD) Right ⌬BBT (mm) P value ⌬BMBL (mm) P value ⫺1.06 ⫾ 0.58 P2 (mean ⫾ SD) Left Right ⫺1.23 ⫾ 0.71 ⫺0.82 ⫾ 0.56 .127 ⫺4.10 ⫾ 4.46 M1 (mean ⫾ SD) Left Right ⫺0.86 ⫾ 0.58 ⫺1.27 ⫾ 0.62 .761 ⫺4.75 ⫾ 4.83 ⫺1.31 ⫾ 2.21 .151 Left ⫺1.21 ⫾ 0.49 .959 ⫺1.44 ⫾ 2.10 .749 ⫺3.27 ⫾ 3.34 ⫺2.56 ⫾ 2.87 .111 ⌬BBT, Change in buccal bone thickness; ⌬BMBL, change in buccal marginal bone level. For P1, ⌬BBT and initial BBT (BBTT1) showed strong correlations with ⌬BMBL (r ⫽ 0.43 and 0.39; P ⫽ .031 and .048, respectively). BBTT1 also showed a significant correlation with ⌬BBT (r ⫽ ⫺0.41; P ⫽ .036). No variable had a significant correlation with ⌬IA (P ⬎.05). For P2, a significant correlation was found between ⌬BBT and ⌬BMBL (r ⫽ 0.61; P ⫽ .001). Similar to P1, no variable had a significant correlation with ⌬IA (P ⬎.05). For M1, ⌬BBT, BBTT1, ⌬ID, and DifE demonstrated strong correlations with ⌬BMBL (r ⫽ 0.62, 0.50, ⫺0.38 and ⫺0.44; P ⫽ .000, .005, .037, and .015, respectively). Age and DifE also showed significant negative correlations with ⌬IA (r ⫽ ⫺0.42 and ⫺0.41; P ⫽ .023 and .026, respectively). DISCUSSION Although the effective radiation dose of CBCT (45-59 Sv) is about 4 to 7 times greater than that of the panoramic radiographs (6.3-13.3 Sv),27 it is comparable to that of film-based periapical radiographs (14-100 Sv).22,28 In addition, when compared with conventional computed tomography (CT) (314 Sv),27,29 CBCT produces a much lower effective radiation dose and can still produce clear images of highly contrasting structures.22,30,31 All CBCT units provide voxel resolutions that are isotropic— equal in all 3 dimensions; this pro- duces submillimetric resolution from 0.4 to 0.125 mm.22 Furthermore, CBCT images also have a substantially lower level of metal artifacts than conventional CT images.22,32 Therefore, with its ability to produce 3-dimensional data, which can be manipulated to generate images representing clinical situations, and comparatively low effective radiation doses, CBCT is an appealing alternative to conventional CT. CBCT has been used for 3-dimensional orthodontic diagnosis and assessment of treatment outcomes.23-25 In addition to dental expansion, the immediate effects of RME include dental tipping, reductions of alveolar bone height, and bone dehiscence.9-15,26 This study demonstrated similar results with significant dental expansion, buccal crown tipping, and loss in BMBL and BBT of maxillary posterior teeth in the short term (within 3 months) after RME (P ⬍.05; Table II). In this study, the mean ⌬ID values on P1 (6.02 mm), P2 (5.97 mm), and M1 (6.66 mm), although not significantly different from each other, were greater than the mean AE (4.96 mm) (Table II). These results were comparable with the findings of Chung and Font33 and Garib et al.34 They attributed DifE to the inherent buccal crown tipping effect of RME, which was also observed in this study. However, only in M1, and not in P1 and P2, was there a significant correlation between DifE and ⌬IA (the greater the DifE, the greater the buccal crown tipping; r ⫽ ⫺.41, P ⫽ .026). 428.e6 Rungcharassaeng et al Buccal crown tipping occurred in about 80% of the posterior teeth in this study; this closely corresponded to the 75% of the sample measured immediately after expansion reported by Thorne.35 The mean ⌬IA values observed in this study were ⫺6.39, ⫺10.90, and ⫺6.64 mm with ranges of ⫺24.79 to 7.78, ⫺39.22 to 5.53, and ⫺24.52 to 6.58 mm for P1, P2, and M1, respectively (Table II). Although there was no statistically significant difference in ⌬IA of P1, P2, and M1 due to high variability in the measurements (P ⫽ .116; Table III), the clinically greater mean buccal inclination observed on P2 when compared with P1 and M1 coincided with the finding of Garib et al.34 The most logical explanation would be the difference in how the teeth were attached to the appliance. P1 and M1 were rigidly fixated to the appliance with bands or bonded occlusal rests and therefore had more bodily movement during expansion. On the other hand, the expansion force applied to P2 via the lingual bar would most likely create a buccal moment that resulted in the buccal crown tipping of P2.34 Adkins et al36 also reported highly variable measurements in dental tipping after rapid palatal expansion and found no significant relationships of buccal crown tipping with age, initial palatal width, amount of expansion, and crossbite. They concluded that buccal tipping of anchor teeth is a factor to be considered as a part of the rapid palatal expansion procedure. Similar findings were observed in this study on P1 and P2; no significant correlations were found between ⌬IA and the other parameters recorded (P ⬎.05). Nevertheless, in addition to DifE, ⌬IA on M1 was also significantly correlated to the subject’s age (buccal crown tipping increases as age increases; r ⫽ ⫺0.42; P ⫽ .023). This result supports the idea that, with increased age, more dental expansion and less skeletal expansion can be expected during RME procedures.8,17 Significant buccal bone losses were observed in both horizontal (BBT) and vertical (BMBL) dimensions in all posterior teeth (P1, P2, and M1) after RME (Table II; P ⬍.0001). Although the bone losses (BBT, BMBL) around P1 (⫺1.14, ⫺4.42 mm) and M1 (⫺1.24, ⫺2.92 mm) were not significantly different from each other (P ⬎.05), they were statistically significantly greater than the bone losses observed around P2 (⫺0.84, ⫺1.37 mm) (Table III; P ⬍.0001 and P ⫽ .001). Garib et al26 reported similar results; they found significantly more buccal bone losses (BBT, BMBL) around P1 (⫺0.6, ⫺7.1 mm) and M1 (⫺0.7, ⫺3.8 mm) than P2 (⫺0.2, ⫺0.2 mm) (P ⬍.05). However, they did not find significant bone changes around P2 after RME (P ⬎.05). No significant differences were found in the amount of bone changes American Journal of Orthodontics and Dentofacial Orthopedics October 2007 between the left and right P1, P2, and M1 (Table IV; P ⬎.05), indicating that the effect of RME on buccal bone is relatively symmetrical. Significant correlations between changes in BBT and BMBL were observed in P1, P2, and M1 (P ⫽ .031, .001, and .000). These findings were anticipated because, when the tooth is being moved buccally, the loss in BMBL is usually the result of the loss in BBT. Nevertheless, only BBTT1 of P1 had a significant correlation with ⌬BBT (r ⫽ ⫺0.41, P ⫽ .036); this means that greater bone loss could be expected with greater BBTT1. This also can be interpreted as a higher probability for bone dehiscence around P1 than P2 and M1 after RME. This might be because P1 usually is located in an area where bone becomes narrower apically, and thus the root can perforate the bone more easily when there is bodily buccal movement.26 No other variables were associated with buccal bone changes in P2 (P ⬎.05). Regarding BBTT1, our results suggest that BMBL was better preserved with thicker BBTT1. Garib et al26 reported a similar significant correlation between BBTT1 and ⌬BMBL, and concluded that, in patients with initially thicker bone plates, RME did not have such negative effects on the buccal periodontium. Significant correlations between ⌬BMBL and the amounts of ⌬ID (r ⫽ ⫺0.38; P ⫽ .037) and DifE (r ⫽ ⫺0.44; P ⫽ .015) in M1 mean that greater loss in BMBL can be expected with greater ⌬ID and DifE. These variables had no bearing on ⌬BMBL of P1 and P2. The mean rate of appliance expansion in this study was 0.83 mm per week (range, 0.22-2.43 mm). This range encompasses the rates of both slow expansion (0.4-1.1 mm per week) and rapid expansion (0.2-0.5 mm per day) of the maxilla.37 RME is believed to result in maximum skeletal displacement and bodily tooth movement with minimum dental tipping.7,37 Slow expansion, on the other hand, is supposed to produce less tissue resistance in the circummaxillary structures and better bone formation in the intermaxillary suture; these help to minimize posterior relapse.37 Greater buccal tipping of the molars has been reported in patients treated with slow expanders such as the quad-helix or the nickel-titanium expander compared with those treated with RME appliances.38,39 In this study, there were no significant correlations between the rate of expansion and ⌬IA, ⌬BBT, and ⌬BMBL in P1, P2, and M1 after RME (P ⬎.05). These results suggest that the difference in force delivery system (hyrax delivers heavy [3-10 lb] interrupted force with cumulative residual force of 20 lb or more,40 and quad-helix delivers lighter [about 400 g] continuous force at 8 mm expansion),41 rather than the rate of expansion, influ- Rungcharassaeng et al 428.e7 American Journal of Orthodontics and Dentofacial Orthopedics Volume 132, Number 4 ences how teeth respond to maxillary expansion (bodily movement vs tipping). The long-term effect of the rate of expansion on relapse is beyond the scope of this study and will be reported when data are available. Starnbach et al7 demonstrated, in their histologic animal study, the incidence of buccal bone resorption of posterior teeth after 2 weeks of orthodontic expansion. Evidence of bone formation was apparent after 3 months of retention; it further improved to “near normal” 3 months later.7 Garib et al26 reported significant buccal bone reduction due to RME after 3 months of retention and attributed their finding to the absence of correspondent compensatory bone apposition under the buccal periosteum. Timms and Moss42 demonstrated evidence of a reversal line well within the buccal bone and also bone deposition 1 year after RME therapy. Greenbaum and Zachrisson43 evaluated the effect of rapid and slow palatal expansion therapy on periodontal supporting structures on first molars after orthodontic treatment. Although they found statistically significant differences in alveolar bone level and attachment level among the rapid expansion, slow expansion, and control groups (P ⬍.05), the mean differences were less than 0.5 mm and deemed insignificant clinically. They concluded that, regardless of the movement pattern of the dentition involved, the posttreatment response of the buccal tissues appears to have been minimal when compared with similar tissues not exposed to the forces of palatal expansion therapy.43 In our study, the retention time (0-12 weeks) did not seem to influence the amount of ⌬IA, ⌬BBT, and ⌬BMBL in P1, P2, and M1 after RME (P ⬎.05). Although the amount of bone loss observed (Table III) seemed alarming, these results were comparable to those in other studies and are considered the immediate effects of RME.7,26 Follow-up study is warranted to evaluate the postorthodontic tissue response of the buccal bone. This study involved data collection on consecutively treated patients, and, therefore, many variables were not controlled. However, this model allowed us to evaluate these variables because of regular clinical setting and incorporate them into the statistical analyses. Although useful information was found in this study, its limitation should be acknowledged. A larger sample size and a long-term follow-up will undoubtedly provide more insightful evidence on the effect of RME. CONCLUSIONS With the limits of this study, the following conclusions can be made. 1. Buccal crown tipping, and reduction of BBT and BMBL of P1, P2, and M1 are the expected immediate effects of RME. 2. The response of buccal bone to RME is relatively symmetrical bilaterally. 3. Dental expansion is uniform anteroposteriorly (among P1, P2, and M1). 4. P2 exhibited more buccal crown tipping than P1 and M1, although it was not statistically significant. 5. P2 experienced significantly less reduction in BBT and BMBL than P1 and M1. 6. Only ⌬IA on M1 (not P1 and P2) had a significant correlation with DifE and patient age. 7. ⌬BBT was significantly associated with ⌬BMBL in all teeth involved. Nevertheless, only in P1 was ⌬BBT significantly correlated to another variable (BBTT1). 8. 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