Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance

ABSTRACT Introduction: Maxillary molar distalization is a common approach for correcting dental Class II malocclusions. Objective: This study aimed at comparing the outcomes of maxillary first molar distalization using the Carriere appliance before and after second molar eruption. Methods: Two groups of patients with dental Class II malocclusions were treated with Carriere distalizer appliance with heavy rectangular mandibular wire and lingual arch for anchorage. Patients of the first group presented unerupted maxillary second molars during the distalization period. In the second group, maxillary second molars were in occlusion on treatment onset. Cone beam computed tomography images were taken at the beginning of treatment and after finishing molar distalization, to compare both groups regarding first molar distalization, intrusion, mesiodistal tipping, buccolingual torquing and rotation, anchorage loss and skeletal changes. Also, the treatment durations were compared. Results: The mean first molar distalization period in the first group (19.2 ± 1.6 weeks) was significantly smaller than the second group (23.3 ± 2.3 weeks). The amount of maxillary first molar distalization was significantly greater, while the amount of rotation was significantly smaller in the first group. No statistically significant differences in the amounts of maxillary first molar intrusion, mesiodistal tipping and buccolingual torquing between both groups was found. Mandibular incisor labiolingual torquing and mandibular first molar mesialization and mesiodistal tipping were significantly greater in the second group. Conclusions: Maxillary first molar distalization before maxillary second molar eruption is more efficient, with less anchorage loss than after second molar eruption.


INTRODUCTION
Dental Class II molar relationship is a frequent malocclusion that can be successfully resolved by means of extractions in at least one arch, 1,2 using intermaxillary elastics 2,3 or maxillary molar distalization. [4][5][6] Molar distalization has become more prevalent because Class I molar relationship is achieved, a certain amount of space is gained, and tooth extractions can be prevented. 6 Different types of appliances can be used to distalize maxillary molars including pendulum, 4 distal jet, 5 headgear 7 and miniscrews. 8 The Carriere distalizer (Henry Schein Inc., New York, NY) is a simple fixed appliance used for nonextraction Class II correction, by moving the Class II buccal segment as a block unit into a Class I occlusion. 9 It was designed to use anchorage from the mandibular arch to create Class I molar and canine relationships. 9,10 The distalization phase with the Carriere distalizer appliance commonly precedes full Edgewise appliances bonding, thus increasing adolescent patient's comfort and general experience. 11,12 The following fixed appliance therapy may be combined with orthodontic or orthopedic maxillary expansion, to refine and detail the occlusion. Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance 5 The type of first molar movement and the treatment timing (before or after second molar eruption) are two factors affecting not only the success, but also the efficiency of molar distalization. 13 An unerupted maxillary second molar can perform as a fulcrum, resulting in much more evident first molar tipping than when both molars are distalized together. Accordingly, the eruption level of the second molar can have an essential influence on the first molar distalization. 14 On the contrary, distalization of maxillary first molar alone can result in greater amount of distalization, higher movement rate and less anchorage loss than when both first and second molars are distalized concurrently. 15 The treatment duration for maxillary first molar distalization increases if the second molar is erupted. 16,17 Accordingly, the ideal age for maxillary first molar distalization is supposed to be prior to second molar eruption. [15][16][17] Other studies concluded that the change of the first molar position and the amount of anchorage preservation are not changed significantly whether the second molar is erupted or not. 4,18,19 The belief that the unerupted second molar represents a fulcrum, increasing the distal tipping of the distalized first molar, is unsupported. 13 Dental Press J Orthod. 2021;26(4):e2119146 Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance 6 Most of the previous studies explaining the effect of maxillary second molar eruption on maxillary first molar distalization used distalizers depending on the upper arch for anchorage, and relied on two-dimensional lateral cephalometric radiographs. 13,14,17,19 Shortcomings of these two-dimensional radiographs included magnification, geometric distortion, superimposition of anatomical structures and inconsistent head position. 20 There were no studies, to the best of our knowledge, that used cone beam computed tomography (CBCT) to compare maxillary first molar distalization with Carriere distalizer appliance before and after second molar eruption.
Using CBCT to measure various skeletal and dental changes in the present study could offer the distinct advantage of one-toone geometry, and provide the potential for utilizing additional anatomical landmarks not detectable in the two-dimensional cephalograms. 21,22 Moreover, distinct views could be obtained for both right and left sides, allowing to increase the efficiency of image utilization, by omitting the superimposition of structures that were unrelated to the required landmark determination, and three-dimensional measurements. 22  Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance 7 Accordingly, the aim of this study was to compare the outcomes of maxillary first molar distalization using the Carriere distalizer appliance before and after second molar eruption. The null hypothesis was that the results of maxillary first molar distalization - including three-dimensional maxillary first molar movements, anchorage loss, amount of Class II correction and treatment duration -were not affected whether maxillary second molar was erupted or not.

MATERIAL AND METHODS
This prospective study included two groups of patients indicated for maxillary first molar distalization (thirty patients for each group). In the first group (19 females and 11 males, mean age of 11.6± 0.9 years), the treatment was accomplished prior to the eruption of the maxillary second molar, with the follicles of the second molars placed directly toward the cervical third of the first molar root. In the second group (21 females and 9 males, mean age of 14.3 ± 1.4 years), distalization started when both maxillary first and second molars erupted.
Patients in both groups fulfilled the following inclusion criteria: 1. More than half-cusp bilateral Angle's Class II molar relation.

Skeletal Class I malocclusion, with ANB angle less than 4°
and YEN angle between 117° and 123°. 24   Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance Every patient was instructed to fill-in a schedule, for self-reporting the duration of wearing Class II elastics every day. Follow-up visits were scheduled every two weeks, to report any problems and to check the compliance of the patients. Reports from parents were required to overcome the social desirability bias during filling-in the schedule. Patients were also instructed to fill in the report every hour, helped by memory aids to overcome the recall bias. 26 One CBCT image (Scanora3D, Sorredex-Finland) was taken for each patient before distalization, and another one when a bilateral Class I molar relationship was attained, in the same stan- The three-dimensional images were subsequently reoriented to the Frankfort horizontal reference plane. The sagittal reference plane was set perpendicular to the horizontal reference plane, and connecting the nasion and the right porion points.
The frontal plane was extended from the nasion, and normal to the horizontal and sagittal planes. Identification of landmarks Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance was determined by using the generated multiplanar projections. The selected points were then assessed in the three-dimensional image. Measurements were taken to compare both groups regarding the amounts of maxillary first molar distalization, mesiodistal tipping, bucccolingual torquing and rotation, in addition to anchorage loss and skeletal changes. Moreover, the treatment durations were compared. Figure 3 and Tables 1,   2 and 3 show the landmarks, planes and measurements used in this study. Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance Table 1: Three-dimensional cephalometric reference landmarks. Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance 14 Table 2: Three-dimensional cephalometric reference lines and planes.  Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance 15 Before starting distalization in both groups, the severity of the In all patients of both groups, bilateral Class I molar relationship was achieved. All patients in both groups properly tolerated the appliance. No distalizer debonding occurred in any subject from any group during the treatment period.

STATISTICAL METHOD
The collected data were statistically analyzed using SPSS Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance 16 All variables were subjected to Shapiro-Wilk test, which revealed normal distribution for all of them (p ˃ 0.05 for all variables).
Analyses between both groups for parametric quantitative data were done using independent samples t-test, and for qualitative data, using Chi-square test (expected number per cell > 5).
The level of significance was defined at p value < 0.05.
Correlations between pretreatment Class II severity and other variables were analyzed using Pearson's correlation coefficient.
Differences with less than 5% probabilities were considered statistically significant.

ERROR OF THE METHOD
All reference landmarks, planes and measurements were relocated and measured again by three different operators.

Reliability of measurements was estimated by Cronbach's
Alpha and Inter-Class Correlation.

RESULTS
The method reliability was excellent, with Cronbach's Alpha and Inter-Class Correlation of more than 0.9 for all measurements in both groups (Table 4). For all variables included in the study, no significant differences were found between boys and girls in both groups (Table 5). Accordingly, for both groups, the results for both boys and girls were analyzed together. Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance and 20.9 ± 1.5 hours per day, respectively). Also, no significant difference was found (p = 0.32) in the amount of force produced by Class II elastics between both groups (194 ± 26g and 201 ± 31 g, respectively).
The mean first molar distalization period in the first group was 19.2 ± 1.6 weeks. It was significantly smaller (p = 0.001) than the mean distalization period in the second group, which was *: Significant level at P value < 0.05.  Quantitative data expressed as mean ± SD.
Chi square test for qualitative data between both groups.
Independent samples t-test for quantitative data between both groups.
Significant level at p-value < 0.05.

± 2.3 weeks. No significant differences between both groups
were observed in all skeletal measurements. No significant correlations between the pretreatment Class II severity and other variables included in the study were observed (Table 6).
Maxillary first molar distalization constituted 67.4 ± 15.1% from the total Class II correction in the first group, which was significantly greater than in the second group, that was 58.5 ± 13% (p = 0.022). Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance 19 The amount of maxillary first molar distalization was significantly greater (p = 0.001) in the first than the second group (3.9 ± 0.8 and 3 ± 0.6 mm, respectively). No statistically significant differences were found between both groups regarding the quantities of maxillary first molar intrusion, mesiodistal tipping and buccolingual torquing.
Regarding maxillary first molar rotation, distalizing both first and second molars together resulted in more significant first molar rotation than distalizing the first molar alone (p ˂ 0.001). Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance 20 The mandibular incisor labiolingual torquing and the mandibular first molar mesialization and mesiodistal tipping were significantly increased in the second group, indicating more anchorage loss. All these results are summarized in Table 7.  Table 7: Changes in three-dimensional cephalometric measurements, elastics wearing time and treatment duration after maxillary first molar distalization, in both groups.
Quantitative data expressed as mean ± SD while qualitative data expressed by frequency and percentage.
Chi-square test for qualitative data between both groups.
Independent samples t-test for quantitative data between the two groups. Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance

DISCUSSION
Attaining a Class I molar relationship is a fundamental component of appropriate balanced occlusion and facial esthetics. 27 There are controversies regarding the influence of second molar eruption upon various aspects of maxillary first molar distalization. The results of this study did not show any statistically significant differences in different skeletal measurements whether second molar had erupted or not.
The treatment time was significantly shorter, the quantity of maxillary first molar distalization was significantly larger and the amount of rotation was significantly smaller in the group with unerupted maxillary second molar. The differences between both groups were not significant regarding the amount of first molar buccolingual torquing.
Also, the amount of anchorage loss (indicated by mandibular first molar mesial migration and mandibular incisor labiolingual inclination) was significantly greater when the second molar was erupted.
The main outcomes of this study corroborate the results of two lateral cephalometric studies utilizing intra-arch NiTi coil spring with Nance appliance 15 and molar distalizing bow. 28 This approach is efficient to distalize maxillary first molar prior to second molar eruption, attaining the advantages of Hashem AS Effect of second molar eruption on efficiency of maxillary first molar distalization using Carriere distalizer appliance 22 more efficient first molar distalization and less anchorage loss.
Continuing maxillary first molar distalization following maxillary second molar eruption slows down the rate of distalization, that becomes equivalent to starting first molar distalization after second molar eruption. 17 However, according to two other studies using the XBow appliance 13 and the Pendulum appliance, 29 lateral cephalometric measurements did not show significant differences in the quantity of maxillary first molar distalization and anchorage loss whether the second molar was erupted or not, suggesting that second molar eruption has negligible influence on first molar distalization.
Also, the results of this study support the concept that it is more hazardous to the anchorage if both first and second molars are distalized together, as combined teeth have larger root surface area than a single tooth. Anchorage is less compromised when the first molar is distalized before second molar eruption, resulting in less time-consuming correction of the anchorage loss. 15