This observational longitudinal study was approved by the Research Ethics Committee of the Federal University of São Paulo, Brazil (number 0301/10) and all patients were provided with a written Informed Consent Form (IFC), which they were required to sign in order to participate in the study.Patients who had previously been diagnosed with OSA and were referred for MAD therapy were selected. Initially, 63 patients were recruited, but 21 patients did not meet the study's eligibility criteria, resulting in the selection of 42 patients. However, 20 patients did not undergo all the necessary exams, and 2 patients dropped off from the study, resulting in a final sample of 20 patients (9 males and 11 females) (Figure 1). The patients underwent clinical, polysomnographic, and cone-beam computed tomography (CBCT) examinations before starting MAD treatment (T0) and after achieving therapeutic protrusion (TP) (T1), with the exams being performed with the appliance placed in the oral cavity. At T0, the sample included 15 patients with mild OSA, 3 with moderate OSA, and 2 with severe OSA (Table 1).
The study analyzed demographic, tomographic, and polysomnographic variables. The demographic variables included information such as sex, age, weight, height, and body mass index (BMI) of the individuals. The three-dimensional image variables encompassed linear and angular measurements of the cranial base, TMJ, and mandible, as well as linear and volumetric measurements of the airways. Finally, the polysomnographic variables consisted of the AHI and minimum and average oxyhemoglobin saturation (SpO2).
Inclusion criteria
Eligible for the research were adult patients of both genders, aged between 18 and 65 years, with a body mass index (BMI) ≤35 kg/m2, clinically and polysomnographically diagnosed with OSA (AHI ≥ 5/h) according to the International Classification of Sleep Disorders. They also had a negative diagnosis for temporomandibular disorders, as determined by the Research Diagnostic Criteria for Temporomandibular Disorders – RDC/TMD questionnaire (adapted to the Portuguese language) [10], and a minimum mandibular protrusion of 7 mm (measured from maximal retrusion to maximal protrusion), clinically assessed with the George Gauge® device (Great Lakes Orthodontics).
Exclusion criteria
Patients using psychoactive medication or medications that induce or reduce sleep or could modify the electroencephalographic pattern were excluded from the study. Those who had undergone any previous surgical treatment for OSA were also excluded. Additionally, patients with unsatisfactory dental conditions (such as active periodontal disease, cavities, or an insufficient number of teeth for MAD retention) and/or a crown-to-root ratio of 1 to 1 or inverted (2 to 1) were excluded.
Sample size calculation and patient selection
The sample size calculation was based on the study by Consellu et al. [11], where a significant increase in the mean volume of the airways (+1261.6 ± 1476.2 mm³) was observed in 16 patients with OSA after treatment with MAD. Therefore, it was estimated that a minimum of 16 patients evaluated at two time points would be needed to obtain a sample with a 95% confidence interval and 90% power (paired t-tests). Thus, the total sample of the study consisted of 20 patients, with a mean age of 48.35 ± 10.42 and a mean BMI of 27.10 ± 4.29.
Polysomnographic examination
All-night PSG was performed at the Instituto do Sono de São Paulo using digital-based polysomnography equipment (Embla® N7000, Embla Systems, Inc., Broomfield, CO, USA). Surface electrodes were used to record electroencephalography, submental and tibial electromyography, bilateral electrooculogram, and electrocardiography. Breathing was monitored with a nasal cannula, and nasal flow was measured using a pressure transducer and oronasal thermistor. Respiratory effort was assessed using chest and abdomen inductance plethysmography. Pulse oximetry was used to measure oxyhemoglobin saturation. The analysis of the polysomnographic examination data and the diagnosis of OSA were performed by specialized physicians. Apnea was defined as a complete cessation of airflow for at least 10 seconds, while hypopnea was defined as a decrease in airflow by 30% or more for at least 10 seconds, followed by a reduction of 4% or more in oxyhemoglobin saturation (SpO2). The Apnea-Hypopnea Index (AHI) was defined as the number of obstructive events (apnea and hypopnea) occurring per hour of sleep. SpO2 was defined as the percent of hemoglobin in the blood that is carrying oxygen. Minimum SpO2 was defined as the lowest observed SpO2 value during the examination, while mean SpO2 was defined as the average value of SpO2 recorded throughout the examination [1]. In this study, treatment success was defined as an AHI reduction below 5 obstructive events per hour. This criterion indicates a successful reduction in the severity of OSA, as treatment success is usually expressed as a ≥50% reduction in AHI from baseline or achieving an AHI of <10 events per hour [12].
Mandibular advancement device and measurement of protrusion amount
The mandibular advancement device used in this study was the Brazilian Dental Appliance (BRD) [4], which is an individualized intraoral appliance designed to gradually advance the mandible. It consists of two acrylic bases, one in each dental arch, that cover all the teeth. The appliance incorporates anteroposterior expansion screws on both sides. Additionally, the design of the appliance allows for small lateral movements by the patient [4]. The MAD was installed with an initial advancement of 50% of the maximum mandibular protrusion capacity, until reaching the therapeutic protrusion, which ranged from 85 to 100% of the maximum mandibular protrusion amount. The average therapeutic protrusion was 94.4 ± 4.6%. The therapeutic protrusion amount was determined based on the improvement in signs and symptoms of OSA documented in the medical records, and the treatment time to achieve it varied between 4 and 6 months.
Acquisition of tomographic images
The CBCT scans were performed at a private dental radiology clinic using an i-CAT® scanner (Imaging Sciences International, Hatfield, PA). The scanner was configured with 120 kVp, 3-8 mA, a voxel size of 0.4 mm, and a field of view (FOV) of 23 cm x 17 cm, allowing for full vertical framing of the head [13, 14]. The images were acquired in the Digital Imaging and Communications in Medicine (DICOM) format. The scans were performed before the initiation of OSA treatment with MAD (T0) and after achieving TP (T1). At T0, the scan was performed with the mandible occluded in maximum intercuspidation, and at T1, with the intraoral appliance in place [14,15,16]. During the scans, the patients were required to be awake, with the head in a natural position (Frankfurt horizontal plane parallel to the ground), and with their gaze fixed on the horizon line. Additionally, they were instructed not to move, swallow, or take deep breaths during the scan to avoid changes in the airway space [17,18].
Image processing
The T0 and T1 data were processed using open-source imaging platforms. Two open-source software programs were used for processing the T0 and T1 data as described below:
1. ITK-SNAP 3.8 software (https://www.itksnap.org) was employed for segmentations of the cranial base, TMJ, mandible, and airways, necessary for image processing. It was also used for the conversion of DICOM files to NifTI files [19].
2. Slicer 4.11 software (www.slicer.org) was utilized to orient and register the tomographic images and patient segmentations. For the T0 images, orientation was achieved by aligning the models with the Frankfurt horizontal, sagittal, and transporionic planes, which corresponded to the axial, sagittal, and coronal planes, respectively, in a standard coordinate system within the Slicer software. The alignment of the T1 images with the previously oriented T0 images was accomplished through voxel-based registration [16,20].
3. Sixteen 3D points were directly defined on the tomographic images using a 3D marker in the ITK-SNAP software. These points comprised 3 on the cranial base, 3 on the maxilla, 4 on the mandible, 4 on the TMJ, and 2 on the cervical spine (Supplementary Table 1). These landmarks served as reference points for measuring and quantifying the condyle, TMJ, mandible, and airway volume using the 3D Slicer software [21].
4. After defining the reference points, linear, angular, area, and volumetric measurements of the cranial base, TMJ, mandible, and airway were obtained. These measurements were expressed in millimeters (mm) for linear dimensions, degrees (°) for angular measurements, square millimeters (mm²) for area measurements, and cubic millimeters (mm³) for volumetric measurements [16,20].
Quantification of measurements in the cranial base, TMJ, and mandible
For bilateral measurements, the arithmetic mean of both sides was calculated. The following parameters were analyzed in the skull base, TMJ, and mandible: vertical dimension, height of the articular eminence, condylar rotation and translation, mandibular advancement, rotation, and translation (see Table 2). Linear measurements were assigned positive and negative signs. Positive values indicated anterior movements in anteroposterior displacement, while negative values indicated posterior movements. For superoinferior displacement, positive values indicated superior movements, and negative values indicated inferior movements (Figures 2, 3, and 4).
Quantification of measurements in the upper airway
The volume and area of the UA, superior oropharynx, and inferior oropharynx were calculated. The upper boundary of the UA was defined superiorly by the basion (Ba) and posterior nasal spine (PNS) points, while the lower boundary was defined by a tangent line to the most inferior and anterior point of the fourth cervical vertebra (C4), parallel to the Frankfurt plane. The UA was divided into two regions, namely the superior oropharynx and the inferior oropharynx, separated by a tangent line to the most inferior and anterior point of the second cervical vertebra (C2), also parallel to the Frankfurt plane (Figure 5).
Calibration and intra-operator agreement analysis
The tomographic measurements were conducted by a single examiner with a 15-day interval between each assessment, followed by an analysis of intra-operator agreement. The data were exported to Microsoft Excel spreadsheets (Microsoft Corporation, Redmond, WA) and analyzed using the Statistical Package for the Social Sciences (SPSS®) version 20.0 for Windows (IBM Corporation, Sommers, NY). The intra-class correlation coefficient (ICC) with a 95% confidence interval was employed to evaluate systematic errors concerning the numerical data.
Statistical analysis
The data were stored in Microsoft Excel and exported to the SPSS® version 20.0 software for Windows. Analysis was conducted using a 95% confidence interval. The variables were expressed as mean and standard deviation and assessed for normality using the Kolmogorov-Smirnov normality test. Paired t-tests were used to compare measurements at T0 and T1 (for parametric data), while Pearson's correlation was employed to evaluate correlations.