The temporomandibular joint (TMJ) is composed of the mandibular condyle, glenoid fossa, articular eminence, articular disc, and associated ligaments and muscles. The articular eminence serves as a guide for the condyle-disc complex during mandibular movement [1]. At birth, the articular eminence is flat, but it rapidly grows with dentoskeletal growth, creating the slope for the path of the condylar movement [2-4]. The development of the articular eminence is considered a structural adaptation of TMJ to functions such as chewing and speaking which become more prominent with the eruption of permanent teeth [5]. Throughout adulthood, the TMJ continues to undergo morphological changes, with the depth of the fossa decreasing due to tooth wear or the loss of posterior teeth [6].

For many years, researchers have investigated the relationship between the morphology of the articular eminence and dental function. The condylar path follows the posterior slope of the articular eminence, while the anterior discluding path depends on the inclination of the lingual surface of the maxillary incisors. It has been suggested that the condylar path or posterior guidance by the articular eminence should harmonize with the anterior path or anterior guidance by the anterior teeth. Kinesiological analysis has shown a positive correlation (r=0.32) between the inclination of the condylar path and the incisal path [7]. Furthermore, a strong positive correlation between the articular eminence angle and the lingual surface angle of the maxillary central incisors has been reported. Bell and Harris [8] found that the lingual surface angle was, on average, 6.1° steeper than the articular eminence angle. In a more recent study using cone-beam computed tomography (CBCT), it was confirmed that anterior guidance showed slightly higher inclination values than condylar guidance [9].

Several orthodontic studies have explored the association between malocclusion [10], skeletal pattern [11-14], orthopedic treatment [15-17], orthodontic treatment [18-21], and articular eminence or condylar position. Gianelly et al. [18] evaluated pre- and post-treatment condylar positions in 111 patients treated with fixed orthodontic appliances and found no significant differences in condylar position or joint spaces between extraction and non-extraction groups. However, they observed condylar retropositioning in nine of the 222 joints on an individual basis. Similarly, Major et al. [19] reported no significant differences in pre- and post-treatment joint spaces between extraction and non-extraction groups, but they found a significant increase in anterior joint space in the non-extraction group following orthodontic treatment. More recently, Alhammadi et al. [20] reported that posterior positioning of the condyle occurred after orthodontic treatment with maxillary premolar extraction and incisor retraction in patients with Class II malocclusion. Lin et al. [21] also found changes in the articular eminence angle after orthodontic treatment in Class II patients.

Although previous studies have investigated the relationship between articular eminence and condylar position after orthodontic treatment, they focused on a specific treatment method or tooth extractions. The diversity of malocclusion among their samples means that changes in the inclination of the maxillary incisors or the occlusion of the anterior teeth might be varied. However, to date, no studies have examined whether the inclination of the articular eminence and condylar position are affected by changes in the inclination of the maxillary incisors and/or anterior occlusion after orthodontic treatment. Therefore, this study aimed to evaluate the relationship between changes in the inclination of the maxillary incisors and anterior occlusion after orthodontic treatment and changes in the inclination of the articular eminence and the condylar position.

Samples

Pretreatment and posttreatment CBCT images of 95 adult patients (21 males [22.8±3.6 years old] and 74 females [23.3±7.8 years old]) were collected for this retrospective study from the patient archives of the Department of Orthodontics, Gangneung-Wonju National University Dental Hospital. This study was approved by the Ethics Committee at Gangneung-Wonju National University Dental Hospital (IRB 2016-006). The patients had received comprehensive orthodontic treatment using fixed orthodontic appliances and achieved normal occlusion after treatment between January 2011 and May 2016. Patients with signs and symptoms associated with the temporomandibular disorders, orthognathic surgery, or previous orthodontic treatment were excluded from this study. The molar relationship, the anterior overjet and overbite, the orthodontic tooth extraction, and the type of orthodontic brackets used in treatment were not taken into consideration during the sample selection process.

The patients presented with a diverse range of malocclusion, including Class I, II, and III before treatment. Additionally, the inclinations of the maxillary incisors, anterior overjet and overbite were also found to be varied (Table 1). However, following the orthodontic treatment, all patients exhibited a Class I molar relationship and achieved normal overjet and overbite, indicating successful correction of their initial malocclusion. Sixty-eight patients underwent orthodontic treatment with extraction of the premolars, and the other 27 underwent non-extraction orthodontic treatment. The majority of patients were treated with the MBT bracket prescription, while some patients received treatment with the Damon bracket system. The treatment duration was 2.3±0.7 (mean±standard deviation) years.

Orientation of CBCT images and measurements

CBCT images were obtained with habitual occlusion using an Alphard-3030 (Asahi Roentgen Industries Co., Kyoto, Japan). The field of view of the CBCT image was 200×179 mm, and the voxel size was 0.39 mm. The images were reconstructed and oriented using the OnDemand 3D software (Cybermed, Seoul, Korea). The horizontal plane was constructed by the plane passing the nasion and parallel to the Frankfurt horizontal plane (FH plane: right porion and bilateral orbitales). The plane passing through the nasion and basion perpendicular to the horizontal plane constituted the midsagittal plane. The coronal plane was defined by the plane passing through the nasion and perpendicular to the horizontal and midsagittal planes (Fig. 1).

Fig. 1. Three reference planes used in this study. FH plane, Frankfurt horizontal plane.

In the present study, we modified some reference points and parameters from previous studies (Table 2) [2,9,20-24]. Articular angles were measured using two methods. Articular angle α was measured within the posterior slope of the articular eminence, and articular angle β was measured from the glenoid fossa to the top of the articular eminence (Fig. 2A). Condylar position was evaluated by measuring the superior, anterior, and posterior joint spaces (Fig. 2B). The condylar axis angle was also evaluated using the angle between the condylar axis and the midsagittal plane (Fig. 2C). The inclination of the lingual surface of the maxillary incisor was measured at the mesiodistal midpoint of the maxillary central incisor (Fig. 2D).

Fig. 2. Angular and linear measurements on the CBCT. (A) Inclination of the articular eminence, (B) temporomandibular joint spaces, (C) condylar axis angle, and (D) anterior guiding angle. CBCT, cone-beam computed tomography; FH plane, Frankfurt horizontal plane.

Method errors and statistical analysis

Ten out of the 95 CBCT images were selected arbitrarily and measured repeatedly after one week. The method error was calculated using Dahlberg’s formula. The mean method error was 0.5° for the articular eminence angle and 0.1 mm for the joint space, which is considered clinically acceptable.

The normality of variables was confirmed using the Shapiro–Wilk test. Pearson correlation analysis was conducted to assess the correlation between changes in occlusion and TMJ structure following orthodontic treatment. Independent variables included U1 to FH, L1 to mandibular plane (MP), overjet, overbite, and anterior guiding angle. Dependent variables consisted of articular angle α, articular angle β, anterior joint space, posterior joint space, superior joint space, and condylar axis angle, respectively. Multiple regression analyses were performed using these variables to investigate whether post-orthodontic treatment changes in inclination of the incisors and anterior occlusion affect the posterior inclination of the articular eminence and the condylar position. Statistical analysis was performed using SPSS 25.0 (IBM Co., Armonk, NY, USA).

The mean and standard deviation of the inclination of the maxillary incisor to FH plane (U1 to FH) and the mandibular incisor to MP (L1 to MP) were 115.8±7.3° and 96.2±7.6° at pretreatment, and 112.6±5.9° and 91.8±5.4° at posttreatment, respectively. The mean anterior overjet and overbite were 4.0±2.6 mm and 1.3±2.1 mm at pretreatment, and 3.0±0.6 mm and 1.7±0.5 mm at posttreatment, respectively.

In the Pearson correlation analysis, a significant relationship was observed between changes in the inclination of the incisors and anterior occlusion after orthodontic treatment and changes in the inclination of the articular eminence and the condylar position (Table 3). In multiple regression analyses, certain occlusal factors demonstrated a significant relationship with changes in the articular eminence and condylar position. The change in anterior overbite was a factor exhibiting positive correlation with the changes in the articular angle α (Table 4), articular angle β (Table 5), and anterior joint space (Table 6). The inclination of the maxillary incisors, which was measured in ‘U1 to FH’, showed a negative correlation to changes in the posterior joint space (Table 7). Changes in the superior joint space and in the condylar axis angle did not show any significant correlation with changes in anterior occlusion.

The relationship between the changes in anterior occlusion and those in the articular angle and condylar position can be displayed using a scattergram (Fig. 3). Changes in anterior overbite showed a positive correlation with changes of articular angle α, articular angle β, and anterior joint space, and those in ‘U1 to FH’ showed a negative correlation with the posterior joint space.

Fig. 3. Scattergram and regression line showing the relationship between the anterior occlusal changes and the articular angle α (A), articular angle β (B), anterior joint space (C), and posterior joint space (D) after orthodontic treatment. FH plane, Frankfurt horizontal plane.

Nonetheless, the articular angle and condylar position did not exhibit noticeable changes in any of the patients. Approximately 90% of patients exhibited very mild or no changes in the articular angle after orthodontic treatment (Fig. 4). Only five patients (5.3%) exhibited an increase or decrease in the articular angle α by more than 2°. In the anterior joint space, most patients (71 patients, 74.7%) showed minimal or no changes (≤0.5 mm). However, 23 patients (24.2%) showed moderate changes (0.5–1.0 mm), and one patient showed a great change (>1.0 mm). Similarly, the posterior joint space was generally stable (≤0.5 mm) in most patients (79 patients, 83.2%), while moderate changes (0.5–1.0 mm) and great changes (>1.0 mm) were observed in 12 patients (12.6%) and 4 patients (4.2%), respectively.

Fig. 4. A histogram showing the prevalence according to the amount of change after treatment. (A) Articular angle α, (B) articular angle β, (C) condylar axis angle, (D) superior joint space, (E) anterior joint space, and (F) posterior joint space.

As a result of a post hoc test for the regression analysis of this study with an alpha significance level of 0.05 and a beta power of 0.8, the minimum required sample sizes for each effect size of articular angle α, articular angle β, anterior joint space, and posterior joint space were 77, 74, 88, and 82, respectively. Therefore, it seems that 95 patients in the present study were not insufficient.

While there may be differences in the size of teeth and the TMJ between males and females, these differences are very subtle when compared to the variability among individuals. In clinical orthodontic practice, treatment methodologies and orthodontic appliances do not differ between genders. Therefore, the unequal distribution of male and female patients might not have any significant influence on the research findings.

During the growth period, the articular eminence grows rapidly until the completion of deciduous dentition, attaining more or less 45% of its adult value, and it completes 70%–72% of its growth by the age of 10 years, and 90%–94% by the age of 20 years [23]. Then, inclination of the articular eminence slowly decreases due to attrition or loss of the posterior teeth through adult life [6]. A previous study on the articular eminence in the Korean population (age 5.9–19.7 years) demonstrated that the height and inclination of the articular eminence does not change significantly after the age of 18 in males and 16 in females [4]. Considering that the patients in this study had a mean age of 23.3 years and underwent orthodontic treatment for 2.3 years on average, it is likely that any potential changes in the articular eminence angle due to growth or tooth wear may not be substantial.

The substantial variation in the inclination of incisors, as well as the anterior overjet and overbite before treatment, significantly decreased after treatment. This reduction in variability indicates that the initial anterior occlusion exhibited considerable diversity but gradually transitioned toward a normal occlusion due to the successful orthodontic treatment.

Previous studies reported that individuals with shorter faces, who often present with an anterior deep bite, have a steeper articular angle and higher articular eminence [12,25]. Chen et al. [26] also reported larger articular eminence inclination in deep overbite group than control and open bite group. Our results of regression analysis indicate that the changes in anterior overbite may have an impact on the articular eminence inclination. Additional research is needed to determine whether the inclination of the articular eminence decreases in patients with a deep bite and increases in patients with an open bite after orthodontic treatment.

Our study found that changes in anterior teeth inclination do not seem to affect the changes in the articular eminence angle. This finding is not consistent with previous reports that found a strong positive correlation between the articular eminence angle and the lingual surface angle of the maxillary incisors [7-9]. Overall, our results suggest that changes in the vertical overlap of the maxillary incisors over the mandibular incisors, rather than the inclination of the maxillary incisors, may have a greater impact on the articular eminence angle during orthodontic treatment.

The position of the mandibular condyle has been studied in relation to orthopedic treatment [16,17,27], and orthognathic surgery [22,28]. Some studies have also assessed the condylar position in patients who have undergone comprehensive orthodontic treatment [18-20], with the primary aim of investigating whether tooth extraction for orthodontic treatment affects the condylar position. However, their findings were conflicting. While Gianelly et al. [18] reported no significant differences in the condylar position and joint spaces between the extraction and non-extraction groups, Major et al. [19] and Alhammadi et al. [20] reported contrasting results. In the present study, we aimed to investigate the alterations in inclination of the maxillary anterior teeth and anterior occlusion, without considering tooth extraction for orthodontic treatment. Our results revealed a significant positive correlation between changes in overbite and changes in the anterior joint space. Alyafrusee et al. [29] observed significant posterior condylar positioning in severe deep overbite group, and they concluded that the deep overbite as a vertical factor is more critical than the overjet as a horizontal factor in affecting the TMJ morphology.

The R² values obtained from our regression analysis were notably low, indicating that relying on a regression equation for predicting TMJ changes after orthodontic treatment based on our findings is not advisable. Nevertheless, our study did reveal a statistically significant correlation between changes in anterior occlusion and TMJ, even though this correlation is relatively low. These changes in the inclination of the articular eminence, resulting from bone remodeling, might result from functional adaptations such as mastication, swallowing, and speech adjustments following orthodontic treatment or stem form changes in mandibular position. Subtle changes can help maintain good articular fit and function, whereas more significant changes may indicate a potential risk for TMJ disease in susceptible individuals. However, as depicted in Fig. 4, most of our patients showed only mild or no changes in the articular eminence angle and condylar position after orthodontic treatment. It is crucial to interpret our results with caution, since our study observed changes over a two-year period. Furthermore, the CBCT used in the present study had a voxel size of 0.39 mm, which has limitations in distinguishing subtle changes in the TMJ. Further investigations may be necessary to evaluate the long-term effects of orthodontic treatment on the TMJ using a CBCT with smaller voxel sizes.

This study assessed the articular eminence and condylar position after orthodontic treatment using fixed orthodontic appliances. Our results suggest that changes in anterior occlusion, especially anterior overbite, after orthodontic treatment may produce slight adjustments in the posterior inclination of the articular eminence and condyle position.

None.

The authors declare that they have no conflicts of interests.