Class II malocclusion treatment changes with the Jones jig, Distal jet and First Class appliances

Abstract Objective Maxillary molar distalization with intraoral distalizer appliances is a non-extraction orthodontic treatment used to correct molar relationship in patients with Class II malocclusion presenting maxillary dentoalveolar protrusion and minor skeletal discrepancies. This study compares the changes caused by three distalizers with different force systems. Methodology 71 patients, divided into three groups, were included. The Jones jig group (JJG, n=30; 16 male, 14 female, 13.17 years mean age) was treated with the Jones jig for 0.8 years. The Distal jet group (DJG, n=25; 8 male, 17 female, 12.57 years mean age) was treated with the Distal jet for 1.06 years. The First Class group (FCG, n=16; 6 male, 10 female, 12.84 years mean age) was treated with the First Class for 0.69 years. Intergroup treatment changes were compared using one-way ANOVA, followed by post-hoc Tukey’s tests. Results Intergroup comparisons showed significantly greater maxillary incisor protrusion in DJG than in FCG (2.56±2.24 mm vs. 0.74±1.39mm, p=0.015). The maxillary first premolars showed progressive and significantly smaller mesial angulation in JJG, FCG and DJG, respectively (14.65±6.31º, 8.43±3.99º, 0.97±3.16º; p<0.001). They also showed greater mesialization in JJG than FCG (3.76±1.46 mm vs. 2.27±1.47 mm, p=0.010), and greater extrusion in DJG compared to JJG (0.90±0.77 mm vs 0.11±0.60 mm, p=0.004). The maxillary second premolars showed progressive and significantly smaller mesial angulation and mesialization in JJG, FCG and DJG, respectively (12.77±5.78º, 3.20±3.94º, -2.12±3.71º and 3.87±1.34 mm, 2.25±1.40 mm, 1.24±1.26 mm, respectively; p<0.001). DJG showed smaller distal angulation of maxillary first molars (-2.14±5.09º vs. -7.73±4.28º and -6.05±3.76º, for the JJG and FCG, respectively; p<0.001) and greater maxillary second molars extrusion (1.17±1.41 mm vs -0.02±1.16 mm and 0.16±1.40 mm, for the JJG and FCG, respectively; p=0.003). Overjet change was significantly larger in DJG compared to FCG (1.79±1.67 mm vs 0.68±0.84; p=0.046). Treatment time was smaller in FCG (0.69±0.22 years vs 0.81±0.33 years and 1.06±0.42 years, comparing it with the JJG and DJG, respectively; p=0.005). Conclusion The three appliances corrected the Class II molar relationship by dentoalveolar changes. The Distal jet produced smaller molar distal angulation than the Jones jig and First Class. The First Class appliance showed less anchorage loss, greater percentage of distalization and shorter treatment time than the Jones jig and Distal jet.


Introduction
Distalization of maxillary molars is indicated to treat Class II malocclusion without extractions in patients with maxillary dentoalveolar discrepancy and minor skeletal discrepancies. 1 Headgear 2 and Wilson maxillary bimetric distalizing arch system 3 have been widely used in the past, however these distalizing appliances require the patient's compliance to achieve molar distal movement. Protocols that require less patient cooperation are more effective and predictable. 4 Several fixed and intraoral appliances for maxillary molars distalization have been described as an option to reduce the need of patient compliance. Most of these appliances involve an anchorage unit, commonly an acrylic Nance button, and an active unit. The active components can be repelling magnets, 5 superelastic nickel-titanium (NiTi) archwires, 6 coil springs on continuous archwire or on sectional archwire, 7,8 springs in beta titanium alloy, 9 and vestibular screws associated with palatal NiTi coil springs. 10 These intraoral distalizers are practical resources to correct Class II molar relationship in a shorter time. 8,11 The amount of maxillary molar movement and subsequent side effects could be directly associated with the biomechanics and particularities of each appliance. The Jones jig is a buccal distalization appliance whereas the Distal jet applies a palatal distalization force. Some advantages of the Distal jet have been reported such as the ability to promote molar distalization with less angulation effects, because the distalizing force applied is closer to the molar center of resistance. 8 More recently, the First Class was proposed as an intraoral appliance with a palatal and buccal force system. 10 The dentoalveolar and skeletal changes of these appliances have been previously investigated. 4,7,8,10,12,13 However, no previous studies directly compared the changes among treatments. Therefore, this study cephalometrically compares the dentoalveolar, skeletal and soft tissue effects of three appliances with different force systems (Jones jig, Distal jet and First Class) used for maxillary molar distalization in Class II malocclusion patients.

Methodology
This retrospective study was approved by the Research Ethics Committee of Bauru School of Dentistry, University of São Paulo. Informed consent was signed by all patients' parents or legal guardians allowing their treatment and participation in the study.
Sample size was calculated considering a mean difference of 1.6 mm between groups for the amount of distal movement of maxillary molars in the sagittal plane, contemplated as the primary outcome, with a previously reported standard deviation of 1.5 mm, 10 using 80% test power, at 5% alpha level. Then, a minimum of 16 patients was necessary in each group. cemented on the second premolars, was used as anchorage ( Figure 1A).
The Distal jet group (DJG) consisted of 25 patients (8 male, 17 female) with 12.57±1.43 years initial mean age. In this appliance, the Nance button was cemented on the maxillary first premolars serving as anchorage ( Figure 1B). Different amounts of force (240g or 180g) were applied based on the clinical presence or absence of the second molars. The greatest force was used when second molars were erupted. 4 The device was reactivated once a month in the same manner.

Error study
In total, 42 cephalograms were randomly selected and retraced by the same examiner (L.V.) after a 1-month interval. The random errors were evaluated using Dahlberg's formula (S 2 = Σd 2 /2n), where S 2 is the error variance and d is the difference between two determinations of the same variable. The systematic errors were assessed with dependent t-tests at p<0.05. The random errors ranged between 0.50 mm (Mx1-PP) and 1.18 mm (LAFH) and between 0.52 (ANB) and 2.80 (NLA) degrees that were within acceptable limits, 13 and only one variable (A-PTV) demonstrated a significant systematic error.

Statistical analyses
Normal distributions were confirmed with Kolmogorov-Smirnov tests. Intergroup comparability regarding sex distribution, severity of Class II malocclusion and the number of erupted maxillary second molars were assessed with Chi-square tests.
Initial and final ages, treatment time, cephalometric statuses at pretreatment and treatment changes were compared between groups using one-way Analysis of Variance (ANOVA), followed by Tukey's tests.
Statistica software (Statistica for Windows, version 6.0, Statsoft, Tulsa, Oklahoma, USA) was used to perform all statistical analyses. Statistical significance was set at p<0.05.
Considering the anchorage loss of premolars and incisors, the effect of molar distalization in the total movement in the sagittal dimension, as reported by Kinzinger, et al. 15 (2008), were also calculated as percentages.

Results
The groups were comparable regarding sex and Class II malocclusion severity distributions, number of erupted maxillary second molars, initial and final mean ages (Table 1). However, the First Class group  Table 2.
At pretreatment, the First Class group had significantly greater skeletal Class II relationship,  Table 3). The first premolar mesial angulation was progressive and significantly smaller in the Jones jig, Distal jet and First Class groups, respectively.
During treatment, the maxillary incisors showed significantly greater protrusion in the Distal jet than in the First Class group ( Table 4).
The maxillary first premolars showed progressive and significantly smaller mesial angulation in the Jones jig, First Class and Distal jet groups, respectively. They also showed significantly greater mesialization in the Jones jig than in the First Class group, and significantly greater extrusion in the Distal jet than in the Jones jig group (Table 4).
The maxillary second premolars showed progressive and significantly smaller mesial angulation and mesialization in the Jones jig, First Class and Distal jet groups, respectively ( Table 4).
The maxillary first molar distal angulation was significantly smaller in the Distal jet than in the other groups. The extrusion of maxillary second molars was significantly greater in the Distal jet than in the other groups ( Table 3).
The overjet change was significantly larger in the Distal jet than in the First Class group ( Table 4).
The First Class group showed greater percentages of maxillary molar distalization considering the anchorage loss of premolars and incisors, followed by the Jones jig and the Distal jet (Tables 5 and 6).

Discussion
Previous clinical studies and systematic reviews have investigated the changes resulting from intraoral molar distalizers. However, inter-study comparisons are limited because of their heterogeneity. [15][16][17] This study is relevant since it evaluates three distalizing appliances with different force systems to directly compare their treatment effects. The sample size on each group was similar to other previous studies. 1,4,11,[18][19][20] Considering the number of variables used in this study, one could argue that Bonferroni corrections should be used. 21 Nevertheless, this would decrease the probability of detecting slight significant differences between groups, which are very important in these comparisons. Since the focus of this study was to investigate whether there is a minimum difference in the treatment changes between the three groups, Bonferroni corrections were not performed.
The groups were reasonably similar at T1 ( presented. Consequently, the maxillary incisor also presented greater protrusion in this group. The mesial angulation of the maxillary first premolars was progressive and significantly smaller in the Jones jig, Distal jet and First Class groups, respectively ( Table   3). However, these characteristics do not interfere with the comparison of results of the treatment changes since they do not affect the appliance performance.  (Table 4).
Commonly, the undesirable effects produced by these appliances include mesialization and mesial angulation of premolars and protrusion and labial Variables  T1  SD  T2  SD  T1  SD  T2  SD  T1  SD  T2   inclination of the anterior teeth, as reported by Kinzinger, et al. 15 (2008), and Antonarakis and  (Table 4). This difference could be explained by the greater anchorage unit used in the First Class group. Since the modified Nance button is attached to the maxillary first molars and second premolars, more teeth are included as anterior anchorage for molar distalization. Furthermore, the Nance button is also larger in this appliance. 12 Mesial angulation of maxillary first premolars was progressive and significantly smaller in the Jones jig, First Class and Distal jet groups, respectively.

Significantly greater first premolars mesial angulation
in the Jones jig group has been reported in previous studies as result of anchorage loss. 1,13,17,22,24 The maxillary first premolars showed significantly smaller   However, as the premolar bands were attached to the Nance button, this prevented them from excessive mesial tipping. 4,15 The significantly greater mesialization of the maxillary first premolars in the Jones jig than in the First Class could also be explained by the larger Nance button in the First Class, representing a greater anchorage unit. 12 The First Class results are in accordance with a previous study. 10 The Distal jet presented greater extrusion of maxillary first premolars than the Jones jig. This is probably because the first premolars are attached to the appliance. As the resulting mesial force on these teeth finds resistance to mesial movement by the anterior teeth and their tipping is restricted, there is a resultant vertical vector which causes extrusion of  Mesial angulation of maxillary second premolars were progressively smaller in the Jones jig and First Class, as expected because of the smaller and larger anchorage units, respectively. 1,13,17,22,24 In contrast to the these groups, the Distal jet showed distal angulation of the maxillary second premolar.

Variables JJ (Jones jig) Group (n=30) DJ (Distal jet) Group (n=25) FC (First Class) Group (n=16) P
Evaluation of dental casts in a previous study demonstrated similar results. 4 Differently from the other two appliances, the second premolars are not attached to the appliance. Therefore, as the molars distalize, the second premolars are pulled by the transeptal fibers and experience some distal tipping. 25 The Jones Jig group presented significantly greater mesialization of the second premolars than the other groups. This could be explained by the smaller Nance button used in this group. Moreover, the smallest mesial movement of the second premolars in the Distal jet group was expected since these teeth were not incorporated in the anchorage unit in this appliance.
The Distal jet presented smaller distal angulation of maxillary first molars than the other groups.
According to other studies, this could be explained by the appliance design. The force is applied on the palatal side, more cervically to the first molar crown, compared to the other appliances, producing forces parallel and closer to the center of resistance, resulting in greater bodily movement, 4,8,15 and smaller distal inclination, as mentioned by Antonarakis and Kiliaridis 16 (2008). Even with the small amount of distal movement of this group, distal angulation was observed. This reflects that Distal jet appliances might decrease the distal angulation effect, but it cannot neutralize the effect. 15,16 It seems that decreasing the forces for maxillary molar distalization has not been effective to reduce the molar distal angulation. In this study, the Jones jig  1,11,22 According to some authors, distal angulation of maxillary molars produces molar intrusion. 18,26 This could explain the greater, but not statistically  group, which probably resulted in greater amount of eruption of maxillary second molars at the end of the distalization phase.
The overjet increased significantly in the Distal jet than in the First Class. This probably occurs for the first premolars are included in the anchorage unit in the Distal jet and but not included in the First Class appliance. As mentioned, this increased overjet caused, as a consequence, the greatest and smallest incisor protrusions, in these appliances, respectively.
Since changes in maxillary incisor angulation were similar in all groups and only the maxillary incisor protrusion was significantly greater in the Distal jet compared to the First Class, the lack of statistically significant difference between groups regarding the nasolabial angle could be expected, as previously reported 27 (Table 4).
The greater percentage of molar distal movement, considering the anchorage loss, observed in the First Class group (Tables 5 and 6) could be expected since this group presented numerically but not statistically significant greater amount of maxillary molar distalization, in mm, than the other groups (Table 4).
Similar results were observed in previous studies. 10,12,15 When analyzing the percentages of distal movement between the Jones jig and the Distal jet, both had similar percentages of molar distal movement, as expected, because the amount of distalization were very close. This was also reported by Antonarakis and Kiliaridis 16 (2008) who compared buccal and palatal distalization appliances. Despite the similarity between buccal and palatal appliances, it is important to mention that the Distal jet presented smaller distal inclination of maxillary molars, as reported by Antonarakis and Kiliaridis 16 (2008), as well.
Independently of the amount of maxillary molar distalization and anchorage loss, Class II molar relationship correction was observed in all patients after distalization mechanics with the three appliances.
The results indicate that the type of anchorage used in the studied appliances is insufficient to counteract the distalization forces. 16,28,29 Side effects should be expected during maxillary molar distalization with conventional anchorage either in the distalized molar or in the anchorage unit. 15,16 Recently, alternative anchorage designs using devices with skeletal anchorage have been described as reducing the side effects of distalization, thus they seem to be efficient alternatives for maxillary molar distalization. 17,[29][30][31] Nonetheless, it is important to know the effects of the several distalization systems with and without skeletal anchorage to choose the ideal alternative, depending on the singular requirements of the patient.
After distalization, orthodontic mechanics must be complemented with fixed appliances to preserve the results of distalization and to correct its side effects. In general, maxillary molar distalization can be achieved with the three studied appliances. The device selection should depend on predictability, minimal undesirable side effects, cost-efficiency, and patient need.
Further studies with greater sample sizes should be performed to confirm our results. Moreover, long-term studies should be performed to evaluate treatment stability of these types of appliances. 16

Conclusions
The three appliances efficiently corrected the Class II molar relationship by dentoalveolar changes with some undesirable effects; The Distal jet presented significantly smaller molar distal angulation and smaller, but not statistically