Abbreviations
Air-V
upper airway volume
AJS
anterior joint space
CBCT
cone-beam computed tomography
CoP-L
position of the left condylar head inside the glenoid fossa
CoP-R
position of the right condylar head inside the glenoid fossa
CoR-CoL
intercondylar distance
CoV-Avg
average condylar volume
CoV-L
left condylar volume
CoV-R
right condylar volume
MaCA
least constricted cross-sectional area of the upper airway
MiCA
most constricted cross-sectional area of the upper airway
PJS
posterior joint space
TMJ
temporomandibular joint
TWB
Twin Block
INTRODUCTION
Skeletal Class II malocclusion can manifest as mandibular retrognathism, maxillary prognathism, or a combination of both. Class II Division 1 malocclusion is a sub-phenotype characterized by proclined upper incisors with increased overjet and overbite [1]. The retrognathic mandible relative to the maxilla is the predominant feature of this malocclusion, necessitating mandibular advancement or redirection of growth toward a more favorable trajectory [2]. For growing patients, the use of functional appliances has become a cornerstone in addressing the skeletal component of jaw discrepancies. Over recent decades, the Twin Block (TWB) appliance, developed by Clark in 1982 [3], has been widely adopted as a treatment of choice for Class II Division 1 malocclusion in young patients. Functional appliances are believed to promote mandibular growth while restricting the downward and forward movement of the maxilla, thereby achieving sagittal correction of the jaw discrepancy [4]. Current evidence from human clinical studies indicates that although treatment effects are primarily dentoalveolar, more pronounced skeletal changes are observed with the TWB appliance [5].
Mandibular retrognathia is associated with a posterior displacement of the tongue and hyoid bone, which narrows the upper airway and can contribute to the development of obstructive sleep apnea [6]. This airway narrowing has been observed in Class II patients [7] and tends to improve with functional appliance therapy [8]. Additionally, condylar changes occur during TWB treatment. Several researchers have suggested that part of the sagittal discrepancy correction results from alterations in the condyle’s volume, dimensions, and growth pattern [9,10]. However, available data on condylar changes remain limited due to lateral cephalometry’s inherent inability to assess the transverse dimension or volumetric changes in the condylar region. Moreover, technical difficulties arise from the superimposition of multiple anatomic structures in the condylar area when using lateral cephalometry. The advent of cone-beam computed tomography (CBCT) has enabled a more accurate assessment of the condyles. A recent systematic review highlighted the limited number of CBCT studies on this subject, although those available indicated favorable condylar growth changes following functional appliance treatment [11].
To our knowledge, no study has yet investigated the association between temporomandibular joint (TMJ) and upper airway changes following orthodontic treatment with the TWB appliance using a CBCT imaging technique. In addition, limitations in commonly used software programs hinder an accurate, clinically realistic assessment of airway changes due to technical constraints.
This study documented the changes induced by the TWB appliance in both the upper airway and the condyles, as well as the correlation between these parameters. A detailed comparative analysis of pretreatment and posttreatment CBCT images was performed on growing patients diagnosed with Class II Division 1 malocclusion. Volumetric, cross-sectional, and linear measurements were employed to evaluate alterations in hard tissue, soft tissue, and the upper airway. The null hypothesis stated that no statistically significant differences exist in the upper airway or TMJ in growing patients with Class II Division 1 malocclusion treated with the TWB appliance before and after treatment.
METHODS
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).
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].
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).
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):
• 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].
Assessment of the condylar position
The position of the condylar head inside the glenoid fossa (CoP) can be determined by the formula [17]:
This calculation was performed independently for the right (CoP-R) and left (CoP-L) sides. The mean condylar position for each patient (CoP-Avg) was determined by averaging the right and left values. The results were interpreted as follows: a CoP between −12% and 12% indicates a concentric condylar position; a value less than −12% indicates a posterior position; and a value greater than 12% indicates an anterior position [18].
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.).
RESULTS
In total, 12 patients (six females and six males) were included in this retrospective study. Table 1 displays the demographic and cephalometric characteristics of the sample. The active treatment time with the TWB appliance was 10.4±2.0 months. Upper airway measurements demonstrated a statistically significant increase following treatment with the TWB appliance. A significant increase was found for Air-V and MiCA (p<0.05) (Table 2). Regarding the TMJs, all variables showed a statistically significant difference (p<0.05). Except for the AJS on both the right and left sides, which decreased bilaterally, all other measurements increased significantly (Table 2).
However, no statistically significant correlation was observed between the TMJ measurements and the upper Air-V at T0 or T1, or for the difference (T1–T0) (Table 3). Similarly, no significant correlation was found between the changes in Air-Volume and CoP-Avg after TWB therapy (r=0.29, p>0.05). Accordingly, the net increase in airway volume showed no correlation with the degree of anterior repositioning of the condyles within the glenoid fossa.
DISCUSSION
The goal of TWB therapy is to correct the Class II skeletal discrepancy by stimulating the growth of the jaws in a favorable direction [21]. The use of CBCT has enabled a more comprehensive analysis of treatment effects on both the airway and the TMJ. The primary aim of the current study was to assess TMJ and upper airway changes in growing patients with Class II Division 1 malocclusion treated with the TWB appliance. Secondary aims included investigating the association between Air-V and TMJ characteristics, as well as the net changes before and after TWB therapy. Several studies have been conducted on patients with Class II Division 1 malocclusion treated with functional appliances [8]. However, most of them relied on two-dimensional imaging techniques. Our study utilized CBCT images, which not only allowed for consideration of the third dimension but also enabled volumetric measurements of the upper airway. In addition, cross-sectional measurements were performed following the natural curvature of the airway tube. Consequently, the findings provide a more realistic representation of the upper airway and its changes. To our knowledge, no other studies based on CBCT images have employed this technique, primarily due to technical limitations of CBCT image processing software.
Patients were included if they were in the CS3 or CS4 cervical vertebral maturation stage to ensure active growth, which is essential for achieving an optimal skeletal response with TWB therapy. Patients in the late mixed or permanent dentition phase were selected to prevent eruption-related changes from confounding treatment outcomes. An overjet greater than 6 mm was required, as it represents a clinically significant Class II discrepancy warranting functional appliance intervention. Lastly, full-time TWB wear was mandated to standardize compliance levels and treatment response. Conversely, patients with craniofacial anomalies or prior orthodontic treatment were excluded to eliminate confounding variables that could impact airway and condylar measurements. Non-compliant patients or those with medical conditions affecting growth or respiratory health were also excluded to ensure reliable comparisons.
The threshold controlling the software’s ability to differentiate areas based on grayscale resolution was standardized at 500 units to optimally recognize the airway and calculate its volume in cubic centimeters. Unlike most previous studies [22,23] that made cross-sectional measurements using lines parallel to the axial plane, the current study determined the axial plane by using a reference line perpendicular to the upper airway’s midline. This approach was facilitated by the in-built functionality of the Romexis 6.2.1 software (Planmeca), which provides a more accurate cross-sectional area measurement in the curved segment of the upper airway. Using this novel functionality, we measured the airway volume, the minimum and maximum airway cross-sectional areas, as well as the cross-section at the levels of the epiglottis and the posterior nasal spine.
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.
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