Introduction
This study aimed to determine the volumetric effects on the upper airways of growing patients with Class II malocclusion treated with the Herbst appliance (HA).
Methods
Volumetric measurements of the upper airways of 42 skeletal Class II malocclusion patients (mean age: 13.8 ± 1.2 years; ranging from 12.0 to 16.9 years) were assessed using cone-beam computed tomography scans acquired before treatment (T0) and approximately 1 year later (T1). The sample comprised a Herbst appliance group (HA group [HAG]; n = 24), and a comparison group (comparison group [CG]; n = 18) of orthodontic patients who had received dental treatments other than mandibular advancement with dentofacial orthopedics.
Results
In CG, nasopharynx and oropharynx volumes decreased slightly during the observation period (9% and 3%, respectively), whereas the nasal cavity volume increased significantly (12%; P = 0.046). In HAG, there was an increase in the volume of all regions (nasal cavity, 5.5%; nasopharynx, 11.7%; and oropharynx, 29.7%). However, only the oropharynx showed a statistically significant increase ( P = 0.003), presenting significant volumetric changes along the time (T1-T0) in HAG.
Conclusion
Mandibular advancement with the HA significantly increased the volume of the oropharynx, but no significant volumetric modifications were observed in the nasal cavity and nasopharynx.
Highlights
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Oropharynx volume increased significantly after Herbst appliance therapy.
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The oropharynx volume of comparison patients with Class II malocclusion was stable in the observation period.
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Nasal cavity and nasopharynx volumes were stable after Herbst appliance therapy.
In growing patients, mandibular retrusion often is observed as a variation of normal craniofacial growth. However, in patients with severe Class II malocclusion, excessive mandibular retrusion can be both an esthetic and a functional concern, in part associated with a smaller volume of the lower part of the airway in comparison with patients with Class I malocclusion.
Dentofacial orthopedics is a treatment option for growing patients with Class II mandibular retrusion, and the Herbst appliance (HA) is one of the most widely fixed functional appliances worldwide. In the United States, HA is the most frequently used appliance for mandibular advancement in Class II.
Although some specific treatment effects of functional appliances remain controversial, , fixed and removable appliances used for the orthopedic correction of mandibular retrognathism seem to increase airway space in adolescents in the short term, at least from a 2-dimensional (2D) cephalometric perspective. However, lateral cephalograms do not capture the transverse dimension nor accurately determine the volumetric changes of these areas. Therefore, 3-dimensional (3D) measurements should be obtained to investigate this issue.
The present study was performed to evaluate the 3D effects of the treatment of Class II malocclusion with the Herbst appliance on the upper airway of growing patients with mandibular deficiency.
Material and methods
The sample size calculation was based on the average standard deviation value of 2500 mm 3 reported by El and Palomo (2014), which was related to the primary aim of this study (ie, evaluating the volumetric change in the oropharynx). Considering an α of 5%, and a power of 80%, and with the intent to detect volumetric changes >2000 mm 3 (effect size = 0.80), the recommended sample size is at least 18 patients in each group using a paired-sample t test. Thus, a convenient retrospective search on the database of the Graduate Program in Orthodontics of the Pontifical Catholic University of Minas Gerais allowed the identification of 42 patients with Class II malocclusion(mean age at initial records: 13.8 ± 1.2 years with a range of 12.0-16.9 years; 25 male and 17 female).
All patients presented at baseline (T0) with Class II malocclusion with an ANB angle >4°, and a facial appearance of mandibular retrusion, based on the Fränkel maneuver (ie, it is observed that facial convexity improves when patients have a mandibular advancement with the bite into appropriate overbite and overjet). The stage of skeletal maturation at T0, based on the cervical vertebral maturation method, ranged from cervical stage CS2 to CS5. The majority of the patients (85%) began treatment during the circumpubertal period (stages CS3-CS4). No syndromes, clefts, dentofacial deformities, or temporomandibular joint dysfunction were present. Patients who had undergone previous orthodontic treatment using high-pull headgear and mandibular advancement devices, fixed orthodontic appliances with Class II elastics mechanics, or corrective jaw surgery were excluded from the parent sample. The same orthodontic team performed all treatments.
Forty-two patients were assigned to 2 groups: HA group (HAG; n = 24; 15 male and 9 female) that comprised patients with Class II malocclusion treated with HA for 8 months, and the comparison group (CG; n = 18; 10 male and 8 female), that consisted of patients with Class II malocclusion needing prior dental treatments before Class II correction, such as marsupialization of cysts, traction of impacted permanent canines or restorative dentistry.
The present study was submitted and approved by the Ethics and Human Research Committee of Pontifical Catholic University of Minas Gerais (Ethics Committee approval number: 21534013.8.0000.5137).
Cone-beam computed tomography (CBCT) was obtained for all patients at 2 time-points (T0 and T1). For the HAG, the T0 scans were acquired before the placement of HA, and T1 was obtained after appliance removal. In the CG, the T0 scans corresponded to the beginning of dental treatment, and the T1 scans were acquired before orthodontic treatment was initiated.
The interval between T0 and T1 was 8.9 ± 0.8 months (range, 8-12 months) for both the HAG and CG groups, depending on the individual needs of each patient. All images were obtained using the i-CAT tomograph (Imaging Sciences International, Hatfield, Pa), with a field of view of 23 cm × 17 cm, voxel 0.3 × 0.3 × 0.3 mm, 36.90 mA, 120 kV, and exposure time of 40 seconds. The generated images were saved in digital imaging and communications in medicine format for further analysis.
3D virtual models were constructed from CBCT slices to measure the changes between T0 and T1. The CBCT scans and virtual models were processed and analyzed using Dolphin 3D Imaging 11.5 software (Dolphin Imaging and Management Solutions, Chatsworth, Calif) by 1 researcher (P.M.O.).
To standardize measurements and minimize errors, the head of each patient was orientated in 3 planes of space. In the coronal perspective (frontal view), the head was positioned relative to a line passing through the left and right frontozygomatic sutures parallel to the floor. In sagittal perspective (lateral view), the Frankfort horizontal plane was orientated parallel to the floor. Finally, in axial perspective (superior view), a line connecting crista galli and basion was orientated coincidently to the medial sagittal plane and perpendicular to the floor.
The upper airway was divided into 3 regions: nasal cavity, nasopharynx, and oropharynx ( Fig 1 ). For volume calculation of each of the 3 regions, the sinus/airway tool available in the Dolphin software was used. The limits were determined in the sagittal, coronal, and axial planes ( Fig 1 ). Next, the hypodense regions were added manually using the seed points tool for reconstruction of the airways. The update airway tool was used to calculate the volume ( Fig 2 ).
Statistical analysis
SPSS (version 21.0; SPSS, Chicago, IL) was used to analyze the data. The images of 25 randomly selected patients were remeasured 30 days following T0 scans, and the intraclass correlation coefficients (ICC) were calculated. The random error was tested with Springate method, and systematic error was tested with a paired-samples t test. The Kolmogorov-Smirnov test was used to check the normal distribution of the data.
A paired-samples t test was used to evaluate the differences between the volumes of the airways in both groups at T0 and T1 (intragroup analysis). Finally, to compare the volumetric data between the treated group and the comparison group at T0 and T1 (intergroup analysis), the independent-samples t test was used. The level of significance was set at 5%.
Results
A high ICC score was found (ICC >0.9), indicating excellent agreement between first and second readings of the same scan. The random error was smaller than 6% for volumetric changes and 8% for minimum cross-section. Systematic error assessment showed no significant differences between 2 readings of the same scan.
We observed a small and not statistically significant ( P >0.05; Table ) decrease of the nasopharynx and oropharynx volumes in CG during the observation period (9% and 3%, respectively), whereas the nasal cavity volume increased significantly (12%; P = 0.046; Table ) in this group. In HAG, there was an increase in the volume of the 3 compartments (nasal cavity, 5.5%; nasopharynx, 11.7%; oropharynx, 29.7%; Table ). However, only the oropharynx showed a statistically significant increase after the advancement of the mandible with the HA ( P = 0.003; Table ). The volumetric increase of the oropharynx of a patient with HA following treatment is illustrated in Figure 3 .
| T0 | T1 | T1 – T0 | P † | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Mean | SD | Mean | SD | Mean | SD | T1 – T0 (%) | T1 – T0 | ||
| Nasal cavity | HAG | 24863.95 | 5272.85 | 26239.04 | 5212.83 | 1375.08 | 4449.16 | 5.50 | 0.144 |
| CG | 23811.94 | 6200.13 | 26700.78 | 6046.44 | 2888.83 | 3861.91 | 12.13 | 0.046 | |
| P ∗ HAG vs CG |
0.496 | 0.908 | 0.282 | ||||||
| Nasopharynx | HAG | 8341.08 | 3801.58 | 9320.75 | 3246.13 | 979.66 | 2330.22 | 11.75 | 0.071 |
| CG | 8954.78 | 4805.40 | 8296.33 | 3694.91 | –658.44 | 2150.36 | –8.96 | 0.211 | |
| P ∗ HAG vs CG |
0.645 | 0.338 | 0.072 | ||||||
| Oropharynx | HAG | 14751.12 | 5833.61 | 19130.92 | 7544.56 | 4379.79 | 6345.66 | 29.69 | 0.003 |
| CG | 12764.11 | 6515.53 | 12345.56 | 5705.77 | –418.56 | 4038.64 | –3.20 | 0.666 | |
| P ∗ HAG vs CG |
0.763 | 0.006 | 0.007 | ||||||
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