The mandibular angle represents an important part of facial aesthetics. Mandibular osteotomy can affect the gonial angle. The aim of this study was to compare the effects of sagittal split osteotomy (SSO) and intraoral vertical ramus osteotomy (IVRO) on the gonial angle. This retrospective cohort study assessed subjects with mandibular prognathism who underwent SSO (group 1) or IVRO (group 2). Lateral cephalograms obtained before and 1 year after the osteotomies were analyzed. In this study, age, sex, the change in occlusal plane (OP) and mandibular plane (MP) angles, and the amount of mandibular setback were considered as variable factors, while the type of surgery (SSO or IVRO) was considered the predictive factor. Fifty-six subjects were studied: 26 in group 1 and 30 in group 2. The changes in MP angle and OP angle were not significantly different between the groups ( P > 0.05). The change in gonial angle was 6.07 ± 4.46° in group 1 and 7.33 ± 5.73° in group 2; assessment of the data did not demonstrate a significant difference between the two groups studied ( P = 0.53). Mandibular osteotomy (SSO or IVRO) may change the gonial angle, but a significant difference between SSO and IVRO was not detected.
The sagittal split osteotomy (SSO) and intraoral vertical ramus osteotomy (IVRO) are the most common techniques used to correct mandibular excess in class III skeletal patients. The gonial region is important in facial harmony and aesthetics. An obtuse gonial angle is a characteristic feature in individuals with mandibular prognathism, and it is desirable that treatment of this deformity also improves the gonial angle.
It has been shown that mandibular osteotomy can increase the gonial angle. The gonial angle change may be considered a predictive factor for relapse after mandibular osteotomy. Resorptive processes in the gonial angle region may cause enlargement of the gonial angle. Few studies have compared the changes in gonial angle after mandibular osteotomy in skeletal class III subjects, and most have not considered all possible variables.
The purpose of this study was to determine whether there are any differences in gonial angle change between the two osteotomy approaches of IVRO and SSO in subjects with a class III skeletal deformity. It was hypothesized that the gonial angle change in IVRO would be greater than that in SSO. This study aimed to have a comprehensive approach and consider all possible variables.
Materials and methods
The sample for this retrospective cohort study was derived from the population of patients attending the oral and maxillofacial department of a medical university in Shiraz, Iran, between 1 September 2008 and 31 October 2013. The research was approved by the medical ethics committee. Subjects eligible for study inclusion had a skeletal class III deformity and underwent either SSO or IVRO. Subjects were excluded from the study if they had asymmetry, previous trauma, fracture of the mandible, previous orthognathic surgery, surgical augmentation, congenital deformities, or if they needed more than 7 mm of setback. All subjects underwent bimaxillary osteotomies (mandibular setback and maxillary advancement). The first author did all surgeries, both the IVROs and SSOs. No genioplasty surgery was performed on any of these subjects.
Subjects were selected randomly and studied in two groups. Group 1 comprised subjects who underwent SSO, and group 2 comprised subjects who underwent surgery by IVRO technique. Using lateral cephalograms taken preoperatively and at 12 months postoperatively as the source of data, several skeletal points were determined and digitized to evaluate two-dimensional skeletal changes. The same X-ray machine and settings were used for all cephalograms. Patients were stabilized in the lateral cephalogram unit (ProMax; Planmeca, Helsinki, Finland) using a cephalostat. The patient was positioned with the head oriented at a 90-degree angle relative to the X-ray beam, at a distance of 1.5 m from the tube. The jaws were in maximum intercuspation, tip of the tongue behind the upper incisor teeth, and lips relaxed. The receptor (CR; Konica Minolta Medical Imaging, Wayne, NJ, USA) was placed 38 cm from the head, and radiographs were obtained with the exposure settings of 60–80 kVp, 10–15 mA, and 16–32 s, and repeated for each case on three occasions. All lateral cephalograms were traced by hand and digitized, superimposed, and evaluated by the same examiner. The tracings were then redone by a second examiner. Thus all cephalograms were traced by two examiners. Matted acetate tracing paper (thickness 0.003 in.; Garware, Mumbai, India) and a 3 H micro lead pencil were used for the tracing.
The mandibular plane (MP) angle (defined as the anterior angle formed by the intersection of the Frankfort horizontal plane and a tangent to the lower border of the mandible and symphysis), the occlusal plane (OP) angle (the angle between the Frankfort horizontal plane and a line touching the cusps of the premolars and second molars), and the gonial angle (the angle between the two lines along the inferior border of the mandible and posterior border of the ramus) were assessed ( Fig. 1 ).
The amount of mandibular setback was assessed on the lateral cephalogram (change in distance between the upper and lower incisors before and after surgery). Age, sex, the change in OP and MP angles, and the amount of mandibular setback (mm) were recorded.
In group 1, the incision was made over the anterior portion of the vertical ramus, extending to the mesial aspect of the first molar. A sub-periosteal dissection was made down to the inferior border of the mandible, where a lateral channel retractor was placed. A long bur was used to make a horizontal bone cut through the medial cortex of the ramus, just above and approximately posterior to the lingula. The vertical cut was made through the buccal cortex, distal to the second molar or further anteriorly, and the vertical osteotomy became bicortical at the inferior border. The two osteotomies were then connected with a 701 fissure bur. A spreader and a narrow osteotome were used to lift the lateral cortex of the sagittal osteotomy gently, and the osteotome was used to step along the connecting cut to ensure that the split stayed close to the lateral cortex. The mandible was positioned in its final position and the condyle was positioned manually by a superior during fixation using a miniplate and monocortical screws or three bicortical screws at each osteotomy site. None of the subjects in this group had intermaxillary fixation ( Fig. 2 ).
In group 2, the mandibular ramus was exposed via a straight-line incision at the level of the mandibular occlusal plane, medial to the external oblique ridge. The incision was carried forward 2–3 mm inferior and parallel to the mucogingival junction so that the buccinator muscle retracted with the lateral flap to improve operative visibility. The periosteum was reflected from the lateral ramus to expose the sigmoid notch and posterior ramus. The inferior border was stripped anterior to the antegonial notch. A Bauer retractor was placed at the sigmoid notch. An oscillating saw (7 mm blade) was used to make a vertical cut from the sigmoid notch to the inferior border (anterior to antilingula if it was identified, or 10 mm anterior to the posterior border of the ramus). Inferiorly the vertical cut was ended 10 mm anterior to the posterior border of the mandible. All subjects had intermaxillary fixation for 3 weeks ( Fig. 3 ).
Statistical analyses were performed using IBM SPSS Statistics for Windows, version 19.0 (IBM Corp., Armonk, NY, USA). An independent t -test was used to compare age and the amount of setback between the two groups. The χ 2 test was applied to compare sex between groups 1 and 2. The Mann–Whitney U -test was used to assess variables and outcomes between the two groups. An inter-examiner reliability analysis was performed using the kappa statistic to determine consistency between the examiners.
Fifty-six subjects were studied in the two groups. Group 1 comprised eight males and 18 females who underwent SSO. Group 2 comprised 16 males and 14 females who underwent IVRO. There was no difference between the two groups with regard to sex ( P = 0.46). The mean age of the subjects in group 1 was 22.30± 2.42 years and in group 2 was 23.66 ± 3.59 years. Comparison of the data did not demonstrate a statistically significant difference in subjects’ age between the two groups ( P = 0.09). The mean amount of mandibular setback was 4.85 ± 0.99 mm in group 1 and 4.06 ± 0.79 mm in group 2. The results did not show a significant difference for the amount of setback between groups 1 and 2 ( P = 0.23).
Preoperative cephalometric parameters (MP angle, OP angle, and gonial angle) were compared between the two groups. The results did not reveal a significant difference between the two groups for any of these angles ( P > 0.05, Table 1 ).
|Variables||Group 1, SSO||Group 2, IVRO||P -value|
|Sex, M/F||8/18||16/14||0.46 *|
|Age, years||22.3 ± 2.42||23.66 ± 3.59||0.09 †|
|Setback, mm||4.85 ± 0.99||4.06 ± 0.79||0.23 †|
|MP angle,°||26.54 ± 7.20||22.61± 6.94||0.75 ‡|
|Gonial angle,°||132.38 ± 8.60||130 ± 7.80||0.88 ‡|
|OP angle,°||6.70 ± 4.60||5.10± 2.60||0.50 ‡|