Comparative study of transverse dental changes induced by palatally-buccally applied 2K-loop appliance and palatally applied pendulum appliance

Introduction

Class II malocclusion patients represent about 35% of cases in European and American populations [1]. For patients with class II molar relationship, distalization of the upper molar is frequently chosen as a treatment alter­native to extraction [2].
Intra-oral non-cooperated intra-maxillary distali­za­tion appliances rely on various kinds of anchorage means, such as palatally anchorage, mini-plate anchorage, and palatal bone screw anchorage. Temporary anchorage devices (TADs) are not recommended before ages of 12 to 13, because of inappropriate bone density [3, 4]. Therefore, there is a need for distalization appliances that use traditional palatally anchoring (Nance palatal arch) in patients under the age of 12 years [3]. Although palatally anchorage is preferred at this age, there are several contraindications to the use of this anchorage method, such as patients with increased overjet with proclination of maxillary incisors, increased mandibular plane angle, and skeletal or dental open bites [5].
According to the literature, in patients with class II molar relationship, 85% of maxillary first molars rotate mesially [6]. Derotation of maxillary first molars is the first step in class II treatment, and merely correcting mesial rotation of molars changes in molar relationship in a class I direction [3].
Many studies examine the changes in the appliance design in terms of the side of force application and their impact on dimensions of dental arch at the end of distalization. Several studies [7-9] investigated the side of force application or the presence of active components, such as buccally, palatally, or palatally-buccally (PB), as the factor impacting the effectiveness, quality, and duration of molar distalization using intra-oral non-cooperated molar distalization appliances. In the literature, there are studies that evaluated the nature and magnitude of transverse changes in the dental arch during distalization [10-12]. According to our knowledge, there is only one study by Bellini-Pereira [13], who compared the transverse dental changes induced by palatally and PB acting molar distalization appliances. Therefore, the aim of the current study was to compare the transverse dental changes resulting from the molar distalization of 2K-loop appliance [14] not studied in a controlled clinical study that used two active components to apply force. The first was buccally and the other was palatally applied force, using molar distalization pendulum appliance [15] with only one palatally active component.

Objectives

The current study aimed to evaluate transverse dental changes caused by using both 2K-loop and pendulum appliances, and to compare different effects of both appliances using transverse dental measurements.

Material and methods

This study was a two-arm, parallel, prospective clini­cal trial, conducted between August 2020 and April 2022. Ethical approval was obtained from Scientific Research Council of Tishreen University (approval number: 1787; dated on May 5, 2020). A written consent was taken from patients’ guardians, since all participants were underaged.

Sample size calculation

Sample size was calculated using G*Power software, version 3.1.9.6 (Franz Faul, University of Kiel, Germany), with the following assumptions: significance level of 0.05, power of 95%, and based on measurements of the amount of rotation of the right maxillary first molars that were obtained by conducting a pilot study among 12 patients. The power analysis showed that 8 patients in each group were required to conduct two-sample t-tests.

Participants and eligibility criteria

The study sample included 16 participants, who were randomly divided into two groups of eight each: 2K-loop group (5 girls, 3 boys; mean age, 10.8 ± 1.2 years) treated with PB applied 2K-loop appliance, and pendulum group (5 girls, 3 boys; mean age, 11.1 ± 1.1 years) treated with palatally applied pendulum appliance. Duration of the study was approximately six months. Inclusion criteria were skeletal class I malocclusion (ANB angle, 1-5º), bilateral class II or end on molar relationship, low to moderate mandibular plane angle (SN/Go GN, ≤ 37º), late mixed dentition with upper second molars that were not emerging to the functional occlusal level, eruption upper first premolars to functional occlusal level, no fractures or severe wear of molars and premolar cusps, overjet less than 5 mm, treatment plan without extraction. Exclusion criteria were congenital syndromes, such as cleft lip/palate, degenerative temporo-mandibular joint disease, congenitally missing teeth, periodontal disease, prosthodontics rehabilitation of maxillary molars, endodontic treatments of maxillary molars.

Appliances used in the study

A Nance button supported by 0.043-inch stainless steel wire welded to the palatal side of bands of the upper first premolars was applied for anchorage control in both appliances.

PB acting 2K-loop appliance

The appliance consisted of four identical active springs, two on the palatal side and the other two on the buccal side, and it was made from a 0.017 × 0.025-inch titanium-molybdenum alloy (TMA) wire. Springs were designed as described by Kaltra [16], with each leg of 8 mm long and 1.5 mm wide, and bent 20º down to help counteract torques generated by horizontal forces of the appliance. Buccal loop was inserted between the buccal bracket, with 0.022 × 0.028-inch of the first upper premolars and main tube of 0.022 × 0.028-inch of the first upper molar bands, while the anterior end of the palatal loop was inserted into an acrylic pad of Nance button, with its posterior free-end inserted into palatal sheaths of the molar bands (Figures 1A and 1B).
Regarding device activation, the wire was marked distally of the premolar bracket and mesially of the molar tube after that (1.5 mm). High step bends were bent into the wire (1 mm) distally to the distal mark and (1 mm) mesially to the mesial mark. This allowed 2 mm of activation (Figures 1D and 1E), and the appliance was re-activated (2 mm) after 6 to 8 weeks. The buccal loop was re-activated extra-orally, while palatal, intra-orally. This was sufficient to include the molars into super class I relationship after 5 to 6 months from the start. Trans-palatal bar with Nance button was used for retention of distalized molars within 24 hours of removing distalization appliance (Figure 1C).

Palatally acting pendulum appliance

The appliance consisted of two active springs located palatally. Each spring was made from a 0.032-inch (TMA) round wire. Its anterior end was inserted into the acrylic Nance button, and its posterior end remained free. After activation, it was re-curved at the end and fit into palatal sheaths of the upper first molar bands (Figure 2A). Two pre-activation bends were made of spring: 1. A toe-in bend (10-15º) at the transverse level, as described by Kinzingeret et al. [17], to reduce molar rotation during molar distalization by producing distal rotation of the upper first molars (Figure 2D). 2. An up-righting bend (10-15º) at the anterior-posterior level, as described by Byloff et al. [18], to reduce molar distal tipping during molar distalization (Figure 2B); the springs were activated at 45º in the center of helices, with an initial force of 200 g (Figure 2C).
Activation was repeated according to the amount of molar distalization, and super class I molar relationship was achieved after 6 months. To obtain identical springs in all appliances, all appliance springs were manu­fac­tured using an acrylic guide with a groove of the desired spring shape (Figures 3A and 3B).

Study models measurements

Study models taken before and after treatment were used to evaluate transverse dental changes. Eight points and three lines were used to determine the median pala­tal suture and position of the maxillary first molars. Model’s cast landmarks applied in this study are shown in Figure 4. Transverse dental changes of upper first premolars were not evaluated because the bands of upper first premolars were welded to Nance palatal arch, and second premolars did not erupt in all patients.
A line was used as the midline reference plane. Transverse movements of the maxillary molars (distance, UR6F–UL6F) were calculated by measuring the distance between UR6F and UL6F. The amount of rotation of the maxillary first molars per degree was determined by measuring the angles between R line, L line, and A line. All calculations were derived from measurements directly made on the model casts (Figure 4).
Scanned dental casts of two patients before and after treatments are shown in Figure 5, one was treated with the 2K-loop appliance and another with the pendulum appliance.
Ten randomly selected models were marked, and measurements were recorded by another orthodontist. A method error and intra-observer reliability were determined with Dahlberg’s formula [19] and paired samples t-test.

Statistical analysis

SPSS version 26 (SPSS Inc., Chicago, IL, USA) was applied to execute all statistical analyses. To check for data normality, Shapiro-Wilk test was applied. Because data was normally distributed, paired samples t-test was used to evaluate the mean changes during treatment in each group, and compared measurement differences between the two groups.

Results

The method error did not surpass 0.2 mm for linear measurements and 0.6° for angular measurements of the variables investigated, and the duplicated measurements were not significantly different (p > 0.05).
Measurements taken before the initiation of treatment and Shapiro-Wilk test results are presented in Table 1. The assessment of the maxilla casts before and after molar distalization showed the following transverse dental changes in each variable in the two groups and intra-group and inter-group comparisons of these changes (Table 2).
In the 2K-loop group, a significant increase was observed in the intermolar distance of the first molars (distance, UR6F–UL6F; p < 0.01). Furthermore, significant distal rotation was observed for the bilateral maxillary first molars (A–L and A–R angles, p < 0.001).
In the pendulum group, a non-significant increase was observed in the inter-molar distance of the first molars (distance, UR6F–UL6F; p > 0.05). In addition, there were a significant increase in the maxillary left and right first molar angles (A–L and A–R angles; p < 0.01 and p < 0.05, respectively), which indicated mesial rotation.
The increase in the distances between the central fossa of the upper first molars was significantly greater in the 2K-loop group than in the pendulum group (p < 0.05). Furthermore, the decrease in the maxillary first molar angles (A–L and A–R angles) induced by PB acting forces in the 2K-loop group was significantly less than the increase induced by palatal force in the pendulum group (p < 0.001).

Discussion

Several studies [7-9] concluded that different sides of force application (buccally, palatally, or PB) in intra-oral non-compliance intra-maxillary distalization appliances had an important effect on the antero-posterior dimensions of the dental arch. However, very few studies [10, 11, 20] compared the transverse changes of the dental arch resulting from the difference in the design of these appliances. But no study evaluated transverse dental changes caused by using both PB acting 2K-loop and palatally acting pendulum appliances, and compared different effects of both appliances with the type and amount of the maxillary first molars rotation.
Regarding the rotation of the upper first molars, in the case of the pendulum, where the force was acting from the palatal side, palatally from the center of resistance, there was a significant mesial rotation of 4.83 ± 3.97º on the right and 4.83 ± 2.71º on the left. Our findings were consistent with those of Kircelli et al. [21], who observed a mesial rotation of the first molars of 9.0 ± 4.1º with pendulum, and Uzuner et al. [10], who used palatally acting frog appliance and induced mesial rotation ranging from 4.4 to 5.9°. Our results differed from those of Hourfar et al. [11], who used frog appliance, and observed a distal rotation when they added a toe-in bending to the springs of their appliances, as in the present study. This was probably because Hourfar et al. compared the casts before treatment with the casts after completing the alignment, and leveling stage by fixed appliances.
Derotation of the maxillary first molars is the first step in class II treatment of almost every type [3]. In the case of 2K-loop, where the force is acting from the palatal and buccal sides, there was a significant distal rotation of 7.83 ± 2.92º on the right and 8.66 ± 2.73º on the left. Our findings were consistent with those of Acar et al. [22], who used a pendulum appliance supported with a K-loop buccally, and induced distal rotation ranging from 2.0 to 2.5° as well as Bellini-Pereira et al. [13], who observed a distal rotation of the first molars of 1.76º with PB acting first class appliance.
The distal rotation in molars that was observed in the case of 2K-loop, although equal forces were applied on both sides, was probably caused by a greater thickness of the bone supporting the palatal root compared with the vestibular roots of the upper first molars [23]. This makes the palatal root act as a rotational axis during distalization. Regarding transverse movements of the maxillary first molars, both the groups achieved an increase in posterior width, but the increase was significant in the 2K-loop group with 4.33 ± 0.81 mm, and not significant in the pendulum group, with 2.19 ± 2.00 mm. Oberti et al. [24] reported a significant increase in inter- molar width of 4.7 ± 2.0 mm with PB acting dual- force distalizer. Also, Papadopoulos et al. [25] observed a significant increase of 2.74 mm with PB acting first class appliance (FCA). This increase was probably caused by the new position of the first molars on the natural V-shape of the arch form. Uzuner et al. [10] suggested that a greater amount of rotation per mil­limeter of the first molar produces a more buccal position of the molar, regardless of the rotation direction. In the present study, this was observed in the 2K-loop group, where the distal rotation per millimeter was greater than the mesial rotation in the pendulum group.
One important limitation of this study was the lack of measurements of sagittal and vertical dental changes, which can be obtained by analysis of lateral cephalograms. Another limitation is a short-term evaluation period of about 22 weeks. Therefore, future research should focus on long-term evaluation of post-retention period.

Conclusions

The current study results support that the PB acting 2K-loop appliance causes an increase in the inter- molar distance and distal rotation of maxillary first molars. Palatally acting pendulum appliance cause mesial rotation of maxillary first molars. In cases, where maxillary molars need to be derotated, and in case of need to increase inter-molar distance, PB acting appliance, such as the 2K-loop is a better choice than palatally acting appliance, such as a pendulum appliance.

Conflict of interests

The authors declare no potential conflicts of interest concerning the research, authorship, and/or publication of this article.

References