Assessment of upper airway and temporomandibular joint changes in growing patients with Class II Division 1 malocclusion, treated with the Twin Block appliance: a retrospective cone-beam computed tomography study

Study population

This cross-sectional, descriptive, and retrospective study analyzed CBCT images from 12 children with skeletal Class II relationships treated with the TWB appliance, retrieved from the database of the University Dental Hospital of Sharjah. Ethical approval was granted by the Research Ethics Committee of the University of Sharjah (Ethical approval number: REC-23-05-08-01-PG). The inclusion criteria were as follows: (1) patients at CS3 or CS4 cervical vertebrae maturation stages; (2) patients in the late mixed or permanent dentition phase; (3) patients with an overjet greater than 6 mm; and (4) patients adhering to a full-time TWB wear protocol. Exclusion criteria comprised: (1) patients who underwent fixed orthodontic treatment or expansion before or during TWB therapy; (2) patients with pathologies affecting the upper airway or a history of adenotonsillectomy; (3) patients with a history of respiratory problems; and (4) poor quality CBCT images.

The TWB appliance comprises two removable bite planes that interlock at a 70° angle, maintaining the mandible in a forward position relative to the maxilla with an incisor edge-to-edge relationship (one-step mandibular advancement). Retentive components included 0.7-mm Adams clasps on the permanent upper first molars and primary lower second molars, along with 0.9-mm ball clasps positioned in the embrasures of the mandibular incisors. A passive maxillary labial bow was also employed to control the incisors and enhance appliance retention (Fig. 1).

Fig. 1

Twin Block appliance used in the study.

CBCT machine and settings

CBCT images were acquired using a Planmeca ProMax CBCT unit. Subjects stood during image acquisition as the x-ray tube and acquisition screen rotated around their head, with each patient positioned in their natural head orientation. Vertical and horizontal laser-positioning guides ensured proper alignment, and scanning was performed with the patient in centric occlusion. The scanner settings were 120 kVp at 5 mA, with an exposure time of 4 milliseconds per frame and a field of view measuring 23×17.3 cm. The Digital Imaging and Communications in Medicine files were reconstructed using Romexis 6.2.1 imaging software (Planmeca).

Measurement of upper airway volume

Fig. 2 outlines the measurement of the upper airway volume (Air-V). The superior limit was defined by a line drawn perpendicular to the airway’s midline passing through the posterior nasal spine. The inferior limit was defined by a line drawn perpendicular to the curvature midline of the airway at the level of the epiglottis tip. The posterior boundary was determined by the posterior pharyngeal wall, while the anterior boundary was primarily defined by the tongue and soft palate [12].

Fig. 2

Colored area: outline of the upper airway space.

Cross-sectional measurements of the airway

The upper airway was segmented by drawing a reference line perpendicular to its midline. Measurements were taken at the level of the posterior nasal spine and the epiglottis, as well as for the minimal cross-sectional area (the most constricted cross-sectional area of the upper airway, MiCA) and the maximal cross-sectional area (the least constricted cross-sectional area of the upper airway, MaCA) of the airway (Fig. 3).

Fig. 3

Cross-sectional measurements of the airway. (A) Upper airway segmentation following the curvature of the upper airway. (B) Axial line perpendicular to the midline of the airway curvature used in cross-sectional measurements.

Measurement of the condylar volume

The following measurements were obtained: (1) condylar boundaries–the left and right condyles were delineated using a reference plane drawn through the lowest points of the mandibular sigmoid notches, parallel to the Frankfort horizontal plane, which served as the boundary line [13]; (2) condylar volume–the right and left condyles were segmented from the three-dimensional mandibular image to calculate their respective volumes (CoV-R and CoV-L). The average condylar volume (CoV-Avg) was determined by averaging these two values; (3) joint space (Fig. 4): a tangent line was drawn from the highest point of the glenoid fossa to the anterior and posterior aspects of the joint, allowing measurement of the anterior joint space (AJS) and posterior joint space (PJS) [14]; and (4) linear condylar measurements (Fig. 5):

Fig. 4

Measurement of the anterior joint space (AJS) and posterior joint space (PJS). A tangent line is drawn from the most superior point of the glenoid fossa (point S) to the most anterior (point A) and most posterior (point B) parts of the condyle. The shortest distances from the anterior and posterior tangent points to the glenoid fossa were the AJS and PJS, respectively.

Fig. 5

Linear condylar measurements. (A) Condylar width measurement: linear distance between the most medial (MCo) and the most lateral point of the condyle (LCo), (B) condylar length measurement: linear distance between the most anterior (ACo) and most posterior point of the condyle (PCo), (C) intercondylar width measurement (CoR-CoL): distance between the most lateral point of the left and the right condyle.

• • The mediolateral condylar width, measured as the linear distance between the most medial and the most lateral point of the condyle in the coronal view.

• • The sagittal condylar length, measured as the linear distance between the most anterior and most posterior point of the condyle head in the sagittal view [15].

• • Intercondylar distance (CoR-CoL), measured as the distance between the most lateral point of the left and the right condyle in the coronal view [16].

Intra-examiner and inter-examiner reliability

To minimize measurement variability, all assessments were performed by a single trained orthodontist. Intra-observer error was determined by repeating the measurements 1 month later, and inter-observer error was assessed by having a second examiner perform the measurements. The intraclass correlation coefficient was used to evaluate both intra- and inter-observer reliability.

Sample size calculation

Sample size was calculated based on the study by Rizk et al. [19], which evaluated oropharyngeal airway changes in Class II patients treated with a mandibular anterior repositioning appliance. Their study reported a pretreatment mean airway volume of 9.081 mm³ (standard deviation=3,406 mm³) and a posttreatment mean of 14.691 mm³ (standard deviation=5,534 mm³) for the experimental group. Using G Power software, a sample size of 12 subjects was determined to achieve 95% power to reject the null hypothesis of equal means at a significance level (alpha) of 0.05.

Statistical analysis

The normality of data distribution was assessed using the Jarque-Bera test. The paired t-test was used to determine whether the differences between means (T₁–T₀) were statistically significant (p<0.05). Pearson correlation coefficient analysis was employed to evaluate the correlation between airway volume and temporomandibular measurements, as described in previous studies [20]. The dependent variable was the Air-V measured at T₀, T₁, and the difference between these time points. The independent variables included CoV-R, CoV-L, CoV-Avg, CoP-R, CoP-L, CoP-Avg, and CoR-CoL, measured before (T₀) and after (T₁) TWB therapy, as well as the net changes due to treatment. Statistical analyses were conducted using SPSS software (IBM Corp.).

Airway changes after mandibular advancement

Our results demonstrated a statistically significant increase in Air-V following treatment with the TWB functional appliance. This finding is consistent with most published studies [8,22,24,25]. It indicates a strong and well-documented effect of the TWB appliance on Air-V. Mandibular advancement appears to reorient the anatomical structures of the upper airway, thereby producing a positive effect on total airway volume—a benefit for retrognathic patients who commonly suffer from a constricted upper airway.

An important finding of the current research was the change observed in the MiCA of the airway. A significant increase in MiCA was noted, indicating that the most constricted regions of the airway were substantially widened. Our results regarding the MiCA are in agreement with previous studies [22,23]. An increase in the MiCA represents an important clinical outcome favoring the TWB appliance, as it suggests that breathing quality could be notably improved. In contrast, the MaCA showed no statistically significant increase based on our measurements. This might indicate that the areas of the upper airway that were already wider were not significantly affected by TWB therapy. Such a discrepancy may be attributed to the dynamic nature of the upper airway, which does not respond uniformly to mandibular advancement. To our knowledge, no other study has evaluated the maximum cross-sectional area of the upper airway.

The cross-sectional area at the level of the epiglottis showed no change, implying that the effect of the TWB on the lower portions of the upper airway is minimal. This observation is consistent with a systematic review and meta-analysis [8]. In contrast, the cross-sectional area at the level of the posterior nasal spine increased significantly by 40.81 mm², suggesting that the nasopharynx was positively affected and that respiratory function may be enhanced. This finding aligns with other CBCT studies [23,24]. Conversely, the systematic review by Xiang et al. [8] concluded that mandibular advancement did not significantly increase the nasopharynx, as two out of five studies reported no increase. It is important to note that this systematic review included studies performed on 2D radiographs.

TMJ changes after mandibular advancement

Our results indicate that the condyles were initially positioned anteriorly in the Class II subjects, consistent with the findings of Elfeky et al. [26]. The initial forward positioning of the condylar head may be viewed as a natural compensatory mechanism for mandibular retrognathism. Conversely, other studies have reported a concentric position of the condyles prior to treatment [17,27].

Following mandibular advancement with the TWB appliance, both right and left condylar heads shifted further anteriorly at T₁ compared to their initial positions. This suggests that functional appliance therapy induces a forward shift of the condyles within the glenoid fossa. This finding was similarly reported by Chavan et al. [28] in their magnetic resonance imaging study and contrasts with the results of Ruf and Pancherz [29], who found no statistically significant anterior repositioning of the condylar head following mandibular advancement. It would be valuable to evaluate the forward repositioning of the condyles during the posttreatment period to investigate the long-term stability of TWB therapy.

In addition, condylar dimensions have been the subject of investigation by several researchers. Mandibular advancement alters both the linear dimensions of the condyles and the total condylar volume. According to Pancherz et al. [9], the condylar head was three times larger at the posttreatment stage compared to its initial size. This increase was primarily attributed to the Herbst appliance stimulating remodeling in the TMJ in a sagittal direction, predominantly backward. Because two-dimensional lateral cephalometry does not allow assessment of the transverse dimension, its influence was not examined. In our study, using a three-dimensional imaging technique, both condylar length and width increased significantly after TWB therapy by an average of 1.35 mm and 1.3 mm, respectively. These findings agree with those of Elfeky et al. [26]. Conversely, Kinzinger et al. [30] reported no statistically significant changes in any of the three dimensions of the condylar heads.

Regarding changes in condylar volume, our study showed a statistically significant increase in condylar head volume following TWB therapy, as well as an average increase of 3.79 mm in intercondylar distance. This observation is corroborated by other studies [10,27]. It supports the hypothesis that condylar growth is stimulated by mandibular advancement. The right and left condylar heads increased by 0.16 cm³ and 0.17 cm³, respectively. However, Xiang et al. [8] also found a significant increase in condylar volume in a control group receiving no treatment, although the absolute value was lower. The absence of a control group in our study limits the interpretation of these condylar changes.

Correlation of airway and TMJ changes

Our results revealed no correlation between Air-V and condylar measurements at T₀ and T₁. This finding partially disagrees with Truong et al. [31], who reported a statistically significant correlation between Air-V and condylar position. This discrepancy may be attributed to the fact that in our sample of Class II growing subjects, no posteriorly seated condyles were observed. In their study, concentric and anteriorly seated condyles showed no correlation with Air-V—a finding also observed in our study.

Furthermore, the net increase in airway volume did not correlate with either the degree of anterior repositioning of the condyles or the increase in condylar volumes. Based on these findings, the increase in airway volume might be attributed to dentoalveolar changes secondary to functional appliance therapy, such as lower arch lengthening resulting from proclination of the lower incisors [32]. Another possible explanation is the advancement of the mandibular dentition, which could lower tongue posture [21].

Patients used the TWB appliance full time, and the active treatment duration was 10.4±2 months. This period falls within the typical range for TWB therapy and is sufficient to induce significant skeletal adaptations in growing patients. The observed increase in Air-V suggests that sustained functional mandibular advancement over this period contributed to airway expansion through remodeling of the surrounding structures. Additionally, the significant increases in condylar volume, width, and length indicate that continued mandibular repositioning stimulates condylar growth and adaptation within the glenoid fossa. The anterior repositioning of the condyle further supports the occurrence of skeletal and positional changes during this timeframe, reinforcing the importance of early intervention in growing patients. Despite these structural changes, the lack of correlation between TMJ and airway measurements suggests that while TWB therapy influences both regions, their adaptations occur independently over this period.

Future studies with longer follow-up periods could explore whether extended treatment duration leads to additional airway or TMJ adaptations beyond those observed in the current study. Moreover, further research is needed to investigate the long-term retention of upper airway and TMJ changes induced by functional appliances. Additional studies should also examine the correlation between upper airway and TMJ changes in Class II Division 1 patients treated with functional appliances.

This study demonstrates the significant impact of functional appliance therapy on both airway dimensions and condylar adaptations in growing patients with Class II Division 1 malocclusion. TWB therapy was associated with a measurable expansion in Air-V, suggesting its potential role in enhancing respiratory function during skeletal development. Concurrently, mandibular advancement promoted condylar remodeling, resulting in increased condylar dimensions and a more anterior positioning within the glenoid fossa—indicative of favorable skeletal adaptations. The observed increase in intercondylar distance further supports the concept of bilateral condylar remodeling in response to functional treatment.

Despite these structural changes, no direct relationship was found between condylar modifications and Air-V, underscoring the complexity of craniofacial growth dynamics. These findings contribute to a deeper understanding of the orthopedic effects of TWB therapy, not only in improving jaw relationships but also in its potential implications for airway volume.