Determination of the Effects of Removable Orthodontic Appliances Used in Orthodontic Treatment on the Axis Angles of Molars

Introduction

Differences arising in the dentofacial system due to various factors can negatively affect the development of the jaws, leading to anomalies in the sagittal, vertical, and transverse directions [1,2]. These anomalies result in different types of relationships between the teeth. These inter-dental relationship anomalies can originate from dental causes [3,4] or from jaw development issues [5]. Dental anomalies can affect a single tooth or a group of teeth [2]. Inter-dental relationship anomalies are seen in 8% to 16% of the population [6]. It is known that many factors play a role in their etiology, including genetics, mouth breathing, abnormal pressure habits, early occlusal contacts, and faulty treatments [7]. Dental anomalies can influence both the relationships between the teeth and the relationships between the jaws. The position of the teeth on the arch and their angulation with the arch can affect this condition. Variability in tooth morphology plays an important role in achieving optimal occlusion [8]. Malocclusions are influenced by tooth morphology, the angulations of the teeth, and the relationships between the teeth [9]. Root morphology also has critical importance for achieving functional and stable occlusion and for the favorable prognosis of orthodontic treatment [10]. Therefore, many studies have been conducted on tooth morphology. However, little attention has been paid to root morphology or the relationship between the crown and the root. Differences in the anatomical features of molar teeth can affect orthodontic treatment [9]. For this reason, the morphological features and body angulations of teeth are being investigated [10]. The term “molar base angle” (MBA), the angle of molar teeth relative to the jaw axis, has been used to describe the angle between the jaw axes and the root axis. MBA is the angle between the line passing through the cusps of the molar teeth and the straight line drawn from the lower borders of the mandible and maxillary jaws. Some previous studies have used lateral cephalometric radiographs or panoramic radiographs to measure the angles of maxillary molar teeth [11]. Dental panoramic radiographs show the positions of the teeth in the mandible and maxilla and their relationships to each other as a whole. Several studies have been conducted on the MBA of molar teeth in different types of malocclusion [12]. According to most studies, the average tooth angles of the Class II malocclusion group were significantly lower than those of the Class III groups [13].

In the treatment of different malocclusions, crowding of the teeth in early ages is managed with rapid maxillary expansion appliances that are either tooth-supported or tooth-tissue-supported [14]. In the treatment of jaw-originated malocclusions, different approaches are used. The removable functional orthodontic appliance (RFA) is one of these treatment approaches [5,6]. RFAs are frequently used to enhance the cellular activity in the circummaxillary sutures to improve the response of the maxilla to protraction forces [15,16]. During protraction, rigid intraoral support units are needed to resist the heavy orthopedic forces applied. For this purpose, fixed mechanics [17], removable plates [18], Nance appliances, labiolingual arches [19], and various types of expansion appliances are used. Various RFAs can be used to modify the direction and amount of growth in cases of mandibular and maxillary developmental deficiency [5]. Maxillary expansion is a dentofacial orthopedic treatment performed by separating the midpalatal suture using orthodontic appliances [6]. Monoblock and twin-block appliances are designed as a single-piece acrylic splint by combining the lower and upper bite blocks on the occlusal plane to form the monoblock appliance [15]. The twin-block appliance is introduced as a 2-part monoblock. The appliance consists of lower and upper bite blocks, which lock together at a 70° angle, with inclined planes on the front parts that position the mandible downward and forward [16]. The strong pulling forces transmitted to the maxilla via face masks help normalize the relationship with the mandibular base in a short period. Chin-cup treatment changes the direction of mandibular growth by rotating the chin downward and backward [6,7].

The dental arch form is defined as a shape resulting from the interaction between environmental factors, such as the 3-dimensional relationships of the teeth with one another, jaw shapes and positions, pressure forces of the soft tissues, tooth morphologies, and eruption patterns and genetics. The axial angles of teeth are important in dentofacial treatment because they are used as references in diagnosis and treatment. Achieving esthetic, functional, and stable axial angles of teeth is one of the main objectives of orthodontists.

Within this context, the aim of our study is to investigate the effects of RFAs used in the treatment of orthodontic anomalies during the pubertal growth spurt period on MBA values.

Material and Methods

PARTICIPANTS:

This retrospective study evaluated pre-treatment and post-treatment panoramic radiographs of individuals who met the criteria for RFA therapy. A total of 40 patients were included based on the following criteria: availability of panoramic radiographs at the start of treatment, absence of pathology in the examined area, and no presence of congenital or acquired anomalies. Patients must be aged between 130 and 150 months, have Class I, Class II, and Class III characteristics, and must not have received orthodontic treatment before. Patients with skeletal Class I malocclusion had A point-Nasion-B point angle (ANB) values in the range of 0° ≤ANB ≤4°. In the Class II malocclusion group, patients were characterized by Class II molar-canine relationship and convex profile, by the ANB angle being greater than 4, and by the presence of a normodivergent growth model. In the creation of the Class III malocclusion group, attention was paid to the fact that the patients were characterized by Class III molar-canine relationship, had a concave or flat profile, the ANB angle was less than 0, and there was a normodivergent growth model. Sella-Nasion to Gonion-Gnathion angle (SN/GoGn) angles were considered for individuals to be classified as normodivergent. Care was taken to keep this angle between 26 ≤SN/GoGn ≤38. The skeletal development of the patients included in the study was evaluated separately on hand-wrist radiographs, and this evaluation was made according to the Greulich-Pyle atlas standards.

Patients were categorized into 4 groups according to the type of RFA used: group A, maxillary expansion appliance; group B, twin-block/monoblock appliances; group C, face mask appliances; and group D, chin cup appliances.

ETHICS APPROVAL:

After all patients provided written approval, their medical and dental histories were taken. Each patient read the Helsinki Declaration before their involvement in the study. The study protocols and methodologies were approved by the Izmir Katip Celebi University Human Ethics Research Committee.

ANGLE OF MOLAR TEETH RELATIVE TO JAW AXIS, MBA:

Radiographs were acquired using a standardized X-ray unit, ensuring high image quality by excluding radiographs with issues such as distortion, low contrast, or blurring. All images were taken in or near the natural head position. Calibration was performed using the built-in software of the X-ray machine to maintain a 1: 1 scale. Further digital calibration and measurement of sinus dimensions were conducted using ImageJ software (Wayne Rasband, National Institutes of Health, Bethesda, MD, USA). To ensure the reliability of the study measurements, the measurements were performed by a single person. Furthermore, the radiography device was identical. The radiological images were calibrated and measured (Figure 1).

STATISTICAL ANALYSES:

The data obtained in this study were analyzed using the SPSS 27.0 statistical software (IBM Corp, Armonk, NY, USA). Categorical variables were expressed as numbers (n) and percentages (%), while continuous variables were presented as mean±standard deviation (SD). The normality of the data distribution was assessed using the Kolmogorov-Smirnov and Shapiro-Wilk tests. Parametric tests were used for variables with a normal distribution, while non-parametric tests were applied for those not normally distributed. For comparisons between 2 independent groups, the independent samples t test was used when the distribution was normal, and the Mann-Whitney U test was used when it was not. For comparisons involving 3 or more groups, one-way analysis of variance (ANOVA) and the post-hoc Tukey test were used for normally distributed data, whereas the Kruskal-Wallis test and the Dunn test were used for non-normally distributed data. Comparisons of pre-treatment and post-treatment panoramic radiographs within groups were performed using the paired samples t test for normally distributed data and the Wilcoxon signed-rank test for non-normally distributed data. Pearson and Spearman correlation analyses were conducted to examine the relationships between clinical parameters and biomarkers. A P value of <0.05 was considered statistically significant in all analyses.

Results

ANGLE OF MOLAR TEETH RELATIVE TO JAW AXIS, MBA MEASUREMENTS IN GROUP A:

The average age of the 10 patients in group A was 138.11±12 months. Pre-treatment measurements in group A were as follows: Tooth No. 17: 19.40±4.2; Tooth No. 16: 11.45±7.6; Tooth No. 26: 19.34±9.7; Tooth No. 27: 26.51±4.3; Tooth No. 37: 18.09±10.2; Tooth No. 36: 17.65±10.9; Tooth No. 46: 16.45±10.7; and Tooth No. 47: 17.01±8.5. Post-treatment measurements in group A were as follows: Tooth No. 17: 21.03±12.4; Tooth No. 16: 13.28±6.1; Tooth No. 26: 16.53±7.5; Tooth No. 27: 26.72±12.1; Tooth No. 37: 14.70±7.3; Tooth No. 36: 13.98±9.01; Tooth No. 46: 12.88±8.3; and Tooth No. 47: 14.47±6.9 (Tables 1, 2; Figures 2, 3). Comparing pre- and post-treatment MBA values, a decrease was observed in the angles of the teeth in the lower jaw. An increase was observed in Teeth Nos. 17, 16 and 27 in the upper jaw, and a decrease was observed in the others (P=0.031; Table 1).

ANGLE OF MOLAR TEETH RELATIVE TO JAW AXIS, MBA MEASUREMENTS IN GROUP B:

The average age of the 10 patients in group B was 129.02±9 months. Pre-treatment measurements in group B were as follows: Tooth No. 17: 23.9680±3.1; Tooth No. 16: 12.67±6.1; Tooth No. 26: 14.32±7.2; Tooth No. 27: 23.99±2.0; Tooth No. 37: 18.96±7.5; Tooth No. 36: 15.64±8.02; Tooth No. 46: 16.07±5.8; and Tooth No. 47: 18.61±6.9. Post-treatment measurements in group B were as follows: Tooth No. 17: 24.49±10.7; Tooth No. 16: 12.09±3.5; Tooth 26: 15.73±5.8; Tooth No. 27: 30.52±10.2; Tooth No. 37: 15.92±7.6; Tooth No. 36: 12.49±4.7; Tooth No. 46: 14.75±4.9; and Tooth No. 47: 19.67±3.4 (Tables 1, 2; Figures 2, 3). In comparing pre- and post-treatment MBA values, a decrease was observed in the angles of the teeth in the lower jaw (except 47). An increase was observed in the angles of the teeth in the upper jaw (P=0.01; Table 1).

ANGLE OF MOLAR TEETH RELATIVE TO JAW AXIS, MBA MEASUREMENTS IN GROUP C:

The average of the 10 patients in group C was 122.42±28 months. Pre-treatment measurements in group C were as follows: Tooth No. 17: 19.43±4.1; Tooth No. 16: 12.39±3.6; Tooth No. 26: 16.70±6.7; Tooth No. 27: 22.71±4.8; Tooth No. 37: 16.87±12.5; Tooth No. 36: 15.10±11.1; Tooth No. 46: 17.88±7.1; and Tooth No. 47: 18.39±7.3. Post-treatment measurements in group C were as follows: Tooth No. 17: 26.38±12.1; Tooth No. 16: 14.09±8.4; Tooth26: 14.14±5.1; Tooth No. 27: 25.20±5.2; Tooth No. 37: 24.95±9.7; Tooth No. 36: 20.55±11.8; Tooth No. 46: 18.97±9.2; and Tooth No. 47: 23.51±7.3 (Tables 1, 2; Figures 2, 3). In comparing pre- and post-treatment MBA values, an increase was observed in the angles of the teeth in the lower jaw. An increase was observed in the angles of the teeth in the upper jaw (except 26) (P=0.008; Table 1).

ANGLE OF MOLAR TEETH RELATIVE TO JAW AXIS, MBA MEASUREMENTS IN GROUP D:

The average of the 10 patients in group D was 132.23±23 months. Pre-treatment measurements in group D were as follows: Tooth No. 17: 6.15±1.1; Tooth No. 16: 6.15±1.1; Tooth No. 26: 14.40±5.7; Tooth No. 27: 14.40±4.7; Tooth No. 37: 17.22±2.8; Tooth No. 36: 17.22±2.8; Tooth No. 46: 18.12±2.05; and Tooth No. 47: 18.12±2.05. Post-treatment measurements in group D were as follows: Tooth No. 17: 17.25±2.1; Tooth No. 16: 11.84±3.7; Tooth No. 26: 10.48±2.2; Tooth No. 27: 18.52±11.1; Tooth No. 37: 19.41±1; Tooth No. 36: 17.84±2.0; Tooth No. 46: 16.13±5.7; and Tooth No. 47: 22.15±1 (Table 1; Figures 2, 3). In comparing pre- and post-treatment MBA values, an increase was observed in the angles of the teeth in the lower jaw (except 46). An increase was observed in the angles of the teeth in the upper jaw (except 26) (P=0.01; Table 2).

When the MBA changes in the groups were examined, a decrease in the post-treatment MBA data in the lower jaw and an increase in the upper jaw MBA data were observed in group A: maxillary expansion appliance group (P<0.05). An increase in the post-treatment MBA data in the upper jaw and a decrease in the lower jaw were observed in group B: twin-block/monoblock appliance group (P<0.05), and an increase in the MBA values in the lower and upper jaws was observed in groups C and D: chin cup and face mask groups (P=0.024) (Table 2).

Discussion

Tooth morphology is a critical factor influencing orthodontic treatment, and the relationship of teeth with the arch, particularly in molars, is considered one of the fundamental components of this morphology. Anatomical studies have shown that the angles formed between teeth and the arch tend to be lower in cases of deep overbite. Most previous studies have examined the MBA using extracted teeth or 2-dimensional cephalometric radiographs [20,21]. These studies have reported various relationships between the angles of teeth. However, to date, the angles of these teeth in relation to the dental arch and the mandibular and maxillary structures have not been evaluated. The present study is the first to assess the MBA in individuals with different RFAs.

Our findings revealed that in groups A, B, C, and D, there was a statistically significant increase in MBA values in the maxillary teeth (P<0.05). In groups A and B, MBA values in the mandibular teeth decreased, while in groups C and D, they increased. These changes were statistically significant (P<0.05). In this study, we classified cases solely based on sagittal skeletal relationships. However, future research should also consider other factors that can affect the MBA, such as root development, eruption patterns, crowding due to a constricted apical base, and spatial relationships between tooth germs. This is the first study comparing MBA data across 4 different removable orthodontic model treatments.

Many studies have investigated the relationship between the MBA and different types of orthodontic treatment models, using 2-dimensional radiographs [8,13]. These studies often report lower MBA values in cases of Class II, Division 2 malocclusion, compared with that in other groups. Since teeth are generally in balance with surrounding structures, increased upper lip pressure could explain this morphological change [13,21]. Backlund [22] suggested that a higher lower lip line in patients with Class II, Division 2 malocclusion could cause bending of the maxillary incisors during eruption. Differences in MBA values among different types of orthodontic treatment model groups have important clinical implications. In camouflage treatment of non-extraction, the maxillary molars often become more proclined, which increases the risk of their roots contacting the palatal cortical plate. Bryant et al [8] reported similar clinical challenges in different types of orthodontic treatment models. McIntyre and Millett [13] recommended the use of prediction templates to assess whether the expected tooth movements are feasible or whether the incisor apex would contact the palatal bone. In considering our results, the orthodontic treatment applied in groups A and B decreased the inclination of the lower jaw teeth and increase the inclination of the upper jaw teeth. It can be concluded that the removable appliances used in groups A and B tend to force the upper jaw teeth into greater angulation while aiming to correct the lower jaw teeth. In groups C and D, on the other hand, it is thought that the removable appliances used in treatment exert an effect that increases the angle of inclination on the molar teeth in both the upper and lower jaws. The etiology of the bending phenomenon observed in maxillary-mandibular molars is not yet clear. While some studies emphasize environmental factors, such as lip and tongue posture and the relationship with overbite [22,23], other researchers, including Logan [24], suggest that genetic factors can also play a significant role.

Previous studies evaluating MBA using 2-dimensional radiographs [1,15] have inherent limitations, such as image distortion, tooth overlap, and difficulties in accurately identifying tooth contours [25,26]. This study is the first to investigate the MBA in maxillary and mandibular molar teeth using standardized orientation with reference lines, thereby offering higher accuracy.

This study had some limitations. Environmental factors that could influence MBA were not analyzed. Furthermore, the sample size was limited. Possible distortion and magnification differences are inherent in panoramic radiographs, and there was a lack of 3-dimensional imaging data (eg, cone beam computed tomography), which could offer more accurate MBA assessment. Future research should evaluate the influence of various environmental factors and incorporate sufficiently large sample sizes, including patients undergoing orthodontic treatment with varying RFA measurements in molar teeth.

Conclusions

In groups A and B, a decrease in mandibular MBA and an increase in maxillary MBA were observed, likely due to the specific characteristics of the appliances used. In contrast, groups C and D showed an overall increase in MBA, resulting in slight mesial angular changes of the teeth. These angular inclinations can negatively impact orthodontic treatment outcomes. Therefore, angular changes in the teeth during treatment with these appliances should not be overlooked. When all the data were considered, we determined that, in the maxillary expansion appliance group (group A) and the twin-block/monoblock appliances group (group B), there was a tendency for a decrease in the angular eruption of the mandibular molar teeth prior to treatment, and an increasing tendency in the angular inclination of the maxillary molar teeth. This effect appears to be related to the characteristics of the appliances; therefore, it can be concluded that angular changes in the teeth during treatments with these appliances deserve careful consideration. In the face mask appliances group (group C) and chin cup appliances group (group D), a general increase in angular inclination of both the upper and lower jaw molar teeth prior to orthodontic treatment was observed. This effect causes even a slight angular change of the teeth in the mesial direction. Such angular inclination can negatively affect orthodontic treatment. These results should be carefully considered when selecting these appliances.