The aim of this study was to determine whether mandibular setback by sagittal split ramus osteotomy (SSRO) influences swallowing function. The subjects were 14 patients with skeletal class III malocclusions who underwent setback surgery by SSRO. Morphological changes were studied on cephalograms, and swallowing function was evaluated by videofluorography before the operation (T0) and at 7–10 days (T1), 3 months (T2), and 6 months (T3) after surgery. The angle between nasion, sella, and hyoid bone (HSN) and the sella–hyoid distance had increased significantly at T1. The hyoid bone returned to the preoperative position at T2. There were no significant changes in the oropharyngeal space at any time. On videofluorographic assessment, lingual movement, soft palate movement, and epiglottic movement had decreased at T1, but all patients recovered at T2. The oral transit time was significantly longer at T1 than at T0. Our results confirm that SSRO influences swallowing function. Swallowing function appears to stabilize by 3 months after surgery.
The sagittal split ramus osteotomy (SSRO) is a common treatment for mandibular prognathism and results in functional and aesthetic improvements. Mandibular setback influences the tongue and pharyngeal airway.
Several studies have shown changes in craniofacial, tongue, hyoid, and pharyngeal morphology after mandibular setback surgery. Such changes include a reduction in pharyngeal airway volume and changes in the tongue and hyoid positions on static imaging techniques, such as lateral and posterior–anterior cephalography, computed tomography (CT), and magnetic resonance imaging (MRI) ; however the functional consequences of these changes remain unclear.
Previous studies have assessed the effects of mandibular setback surgery on masticatory function, stomatognathic function, sleep apnea, psychosocial status, and articulation. However, whether or not mandibular setback affects swallowing movements remains a matter of debate.
Changes in tongue position may influence swallowing function during the oral preparatory phase and oral phase. Altered hyoid position and pharyngeal airway volume may affect swallowing function during the pharyngeal phase. Generally, the effects of various diseases on swallowing function are evaluated by videofluorography, which is used to assess the oral and pharyngeal transit times.
The aim of this study was to investigate the effects of SSRO and mandibular setback on craniofacial and pharyngeal morphology and on swallowing function. Craniofacial and pharyngeal morphology and swallowing function were evaluated by videofluorography before and after SSRO.
Materials and methods
The subjects were 14 patients (two men and 12 women; average age 25.9 ± 10.6 years) with dentofacial deformities, who had skeletal class III malocclusion with or without open bite and asymmetry. The patients underwent SSRO in the department of oral and maxillofacial surgery of the study hospital. Patients who underwent Le Fort I osteotomy were excluded. All subjects received preoperative and postoperative orthodontic treatment. SSRO was performed according to the Obwegeser and Dal Pont method. The fragments were fixed with the use of titanium plates (Würzburg Titanium Plating System; Stryker Leibinger GmbH, Freiburg, Germany) and resorbable fixation devices (Super Fixsorb-MX; Takiron Co., Osaka, Japan). Each subject received a preoperative dose of dexamethasone (mean dose 4.5 mg) immediately prior to surgery, followed by postoperative treatment with dexamethasone (mean dose 2.1 mg/day) for 2 days to control postoperative swelling. The average amount of mandibular setback was 7.65 ± 3.23 mm. Intermaxillary fixation with the use of elastics or steel wires was maintained for 4–6 days postoperatively.
Informed consent was obtained from all subjects after explaining the study procedures in detail. The protocol was approved by the institutional ethics committee.
Morphological changes were evaluated on lateral cephalometric radiographs that were taken with the Frankfort horizontal plane (FH plane) parallel to the floor and with the patient in centric occlusion. Evaluations were performed before surgery (T0) and at 7–10 days (T1), 3 months (T2), and 6 months (T3) after surgery. All cephalograms were traced and analyzed by the same examiner. Fig. 1 shows the landmarks and contours that are commonly used for orthodontic analysis, as well as additional variables that were used to evaluate the pharyngeal airway. Four angles and three linear distances were measured to assess dentoskeletal morphological changes. Five linear distances were calculated to evaluate the pharyngeal airways.
Swallowing was assessed by videofluorography in all patients. Videofluorography was performed before SSRO, at 4–6 days after the release of intermaxillary fixation, and at 3 and 6 months after surgery to qualitatively and quantitatively analyze deglutition. The contrast material used was 10 ml of liquid barium. Images were recorded, converted digitally, and analyzed with InterVideo WinDVD Creator software (InterVideo, Inc., Fremont, CA, USA). Qualitative assessments included lingual movement, barium inflow into the pharynx before swallowing, soft palate movement, epiglottic movement, stasis in the epiglottic valleculae, and stasis in the oral cavity after swallowing.
Swallowing was also assessed quantitatively. The oral transit time, pharyngeal transit time, and total transit time were measured before and after SSRO, and the values were compared. The oral transit time was defined as the time required for the barium to move through the oral cavity, measured from the first backward movement of the barium until the head of the barium passed the lowest point of the mandibular ramus. The pharyngeal transit time was defined as the time required for the barium to move through the pharynx, measured from the time that the head of the barium passed the lowest point of the mandibular ramus until the tail of the barium left the cricopharyngeal region. The oral transit time and pharyngeal transit time were summed to derive the total transit time.
Data were analyzed by conventional statistical methods. The mean, range, and standard deviation were calculated for each variable. Values obtained before and after SSRO were adjusted with the use of the Bonferroni correction. A significance level of 5% was used throughout the study. The statistical analysis was performed using SPSS statistical software (Dr. SPSS II for Windows; SPSS Japan Inc., Tokyo, Japan).