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I. Introduction
Sagittal split ramus osteotomy (SSO) is one of the most common procedures to correct mandibular deformities including prognathism, retrognathism, and asymmetry since its introduction by Trauner and Obwegeser1. Performing SSO on bilateral mandibular rami (bilateral SSO, BSSO) to reduce unexpected stress or torsion in the temporomandibular joint (TMJ) is considered a standard technique in mandibular orthognathic surgeries. Previously, it was believed that only bilateral mandibular approaches could be used to separate the tooth-bearing distal mandible segment from both TMJ-attached proximal segments, allowing free repositioning of the proximal segments into neutral condyle positions and fixing the distal segment with minimal TMJ tension23.
However, in a recent report, unilateral SSO (USSO) was used to correct lateral deviation of the mandible and yielded favorable outcomes3. If unilateral surgery could guarantee long-term postoperative stability, as well as favorable results in correcting facial asymmetry, operation time and the incidence of postoperative complications, including inferior alveolar nerve damage, hemorrhage, or a bad split, could be reduced compared to those observed in bilateral mandibular surgeries34. However, USSO may cause rotation of the opposing mandibular condyle, likely leading to adverse effects such as condylar resorption, limited mouth opening, or TMJ disorders25. Due to these considerations, current use of USSO has been extremely limited. Thus, the use and acceptance of USSO as an orthognathic surgical method could be improved if thoroughly documented clinical guidelines or criteria are provided for unilateral approaches.
To further validate the feasibility of USSO, we report three consecutive cases with long-term follow-up in which USSO was used to correct asymmetric mandibular prognathism.(Tables 1, 2) In this case series, changes in the vertical condylar height (CH), ramal height (RH), and mandibular width (MW) were analyzed at both operative and non-operative sites using preoperative and 2-year postoperative panoramic and cephalometric radiographs (Fig. 1, Table 2), similar to previous reports678. In addition, for 3 years following the USSO, any evidence of mandibular condylar resorption at the non-osteotomized side was evaluated by computed tomography (CT). Informed consents were obtained from the patients for publication of the present report and accompanying images.
II. Cases Report
1. Case 1
A 39-year-old woman visited the clinic complaining of facial asymmetry. Initial examinations revealed mandibular prognathism with right-sided deviation, right posterior crossbite, absence of the lower first molars bilaterally, extrusion of the left upper first molar, and a narrow maxillary arch with crowded incisors.(Fig. 2. A-C) Radiographs showed that both mandibular condyles were symmetrical and equal in size, with elongation of the left mandibular ramus and rightward deviation of the chin point.(Fig. 2. B) The patient was diagnosed with asymmetric mandibular prognathism (laterognathism), and Le Fort I osteotomy and BSSO with preoperative orthodontic treatment were recommended to correct the jaw deformities. However, the patient was reluctant to undergo conventional bimaxillary surgery and requested a less invasive option. Therefore, the protocol was revised to include a minimally invasive approach (MIA), consisting of miniscrew-aided orthodontic intrusion of the extruded upper left molar, left unilateral surgically-assisted rapid palatal expansion (USARPE) to expand the constricted maxilla, and USSO on the mandible for its rotated setback on the left side. The patient underwent 3 months of preoperative orthodontic treatment initially to align the dentition. Intraoperatively, the mandible was moved to a 6-mm setback on the left side only, the right side of the mandibular condyle was smoothly rotated without any resistance.(Fig. 2. D) After 12 months of postoperative orthodontic treatment and dental implantation for the missing teeth, the patient had normal occlusion and symmetric facial morphology.(Fig. 2. E-G) Follow-up examinations over 5 years did not show any postoperative complications, including TMJ disorders. The preoperative and 2-year postoperative RH, CH, and MW were measured using panoramic and cephalometric radiography, and were not significantly different after USSO.(Fig. 3, Table 2) Moreover, the condylar shape in the non-osteotomized site was preserved without any long-term postoperative osteolysis on follow-up panoramic radiographs.(Fig. 3)
2. Case 2
A 28-year-old woman was referred to the clinic for treatment of facial asymmetry. Clinical and radiological examinations revealed left posterior crossbite, right mandibular ramus elongation, leftward chin deviation, absence of both mandibular first molars, extrusion of the upper right second premolar, and symmetrical mandibular condyles.(Fig. 4. A-C) The patient was diagnosed with asymmetric mandibular prognathism with missing teeth (bilateral mandibular first molars) and consented to correction of her facial asymmetry using MIA rather than bimaxillary surgeries. After 3 months of preoperative orthodontic dental alignment, the patient underwent unitooth osteotomy for intrusion of the upper right second premolar and USSO with a 7-mm setback on the right mandible. She showed symmetrical facial morphology and stable occlusion after 18 months of postoperative orthodontic treatment.(Fig. 4. D-F) Follow-up examinations over 6 years showed no evidence of TMJ dysfunction or condylar resorption on routine panoramic radiography.(Fig. 3) Moreover, the RH and CH remained unchanged postoperatively and the condylar shape appeared intact on CT at approximately 6 years after USSO in both the operative and non-operative sites.(Fig. 5, Table 2)
3. Case 3
A 43-year-old woman visited the clinic complaining of an asymmetric face and multiple teeth losses. Clinical and radiological examination revealed the absence of multiple mandibular posterior teeth bilaterally (#35, #36, #45, #46, and #47), extrusion of the right upper posterior teeth (#14, #15, and #16), left unilateral cross bite, leftward chin point deviation, and normal and symmetrical mandibular condyles.(Fig. 6. A-C) Based on these findings, the patient was diagnosed with asymmetric mandibular prognathism with multiple missing teeth. Right unilateral posterior maxillary segmental osteotomy (PMSO), USSO for intrusion of the extruded posterior maxillary segment and unilateral 6-mm setback of the right mandible were performed after 3 months of preoperative orthodontic treatment. After 16 months of postoperative orthodontic treatment and restoration of the missing teeth with dental implants, the patient achieved balanced occlusion and symmetrical facial morphology.(Fig. 6. D-F) On preoperative and postoperative routine radiography and CT at 3 years postoperatively, there was no significant change in the RH or CH, and no evidence of cortical bone resorption in the non-osteotomized mandibular condyle.(Table 2, Fig. 3, 5) Clinically, she had normal TMJ function and mouth opening range for more than 3 years postoperatively.
III. Discussion
Facial asymmetry can be either developmental or functional, occurring along the horizontal, vertical, and transverse planes according to predisposing or contributing factors related to the onset period. The developmental type of facial asymmetry, which includes unilateral condylar hyperplasia, unilateral mandibular hypertrophy, and hemifacial hypertrophy, presents as skeletal asymmetry with complete dentition, elongated condylar neck on the non-deviated side, and resorptive condyle contour on the deviated side with bilateral posterior crossbite9. The functional type of facial asymmetry, such as asymmetrical mandibular prognathism (laterognathism), is characterized by positional facial asymmetry with dento-alveolar compensation. This condition presents as unilateral posterior crossbite with symmetrical mandibular condylar shape and size, which are usually accompanied by unilateral absence of the lower posterior teeth8910.
Irrespective of the type of facial asymmetry, bimaxillary surgeries, including Le Fort I osteotomy and bilateral mandibular surgeries, are the traditional treatment modalities. However, several recent studies have described the correction of facial asymmetry with minimally invasive surgical techniques. Motamedi11 reported favorable outcomes in 6 cases of unilateral mandibular osteotomy to correct unilateral condylar hyperplasia. Either unilateral extraoral subcondylar osteotomy or extraoral vertical ramus osteotomy with or without Le Fort I osteotomy was used to treat skeletal-based facial asymmetry caused by unilateral condylar hyperplasia with mandibular movement in the osteotomy side ranging from 5 to 10 mm11. In addition, cohort studies have reported the use of USSO in orthognathic surgeries, but have not detailed the magnitude or direction of mandibular movement12131415. The USSO technique has also been used to correct post-traumatic malocclusion caused by unilateral condyle fracture or to remove intrabony tumors such as odontoma or myxoma in the mandible16171819. Thus, USSO has not been readily accepted as a common orthognathic surgical technique to correct jaw deformity.
Recently, researchers completed the first evaluation of long-term outcomes and TMJ function after USSO as an orthognathic surgical technique3. In their study, 26 patients underwent USSO to correct laterognathism; three underwent isolated USSO, and the remainder underwent USSO combined with Le Fort I osteotomy. In these cases, the mandibular movement was between 2 to 7 mm at the affected site, and two cases of condylar resorptions were noted within 1 year postoperatively. However, there was no significant impairment in condylar movement on the non-operated side. They reported that the 2-year postoperative condylar resorption rate of USSO was not higher than that of BSSO (approximately 4%)2. Prior studies that utilize USSO in mandibular orthognathic surgery are summarized in Table 331112131415.
In the present study, three patients are described who underwent USSO as a unilateral mandibular setback to correct functional facial asymmetry caused by laterognathism. TMJ function was evaluated both preoperatively and postoperatively, looking specifically at noise, pain and mouth opening. The unilateral mandibular setback ranged between 6 and 7 mm, and patients did not show any TMJ dysfunction with normal mouth opening (≥40 mm) after at least 3 years of follow-up.(Table 1) In both the operative and non-operative sides of the mandibles, the RH and CH were not significantly changed on panoramic or cephalometric radiography over more than 3 years of follow-up. Moreover, the condylar shapes at the non-osteotomy sites remained intact without any evidence of resorption on CT.(Table 2, Fig. 3, 5) Along with USSO, USARPE, uni-tooth osteotomy, or unilateral PMSO was also used in each case to correct maxillary dentoalveolar compensation. These minimal and selective surgical approaches in the maxillary dento-alveolus region could reduce the need for Le Fort I osteotomy, which would minimize the risk of potential surgical complications associated with more aggressive surgical approaches.
Similar to previous cases, the maximum unilateral mandibular movement of USSO was 7 mm in the present study, which rotated the mandibular condyle within 3° to 4° on the non-operated contralateral site3. In a previous report, the mean proximal segment's rotation was 5.6° after mandibular movement with BSSO, and the maximum rotational tolerance of the mandibular condyle was approximately 10° to 15° after orthognathic mandibular surgery1120. Thus, unilateral mandibular movement of 7 mm after USSO, which would rotate the mandibular condyle 3° to 4° in non-osteotomy side, should be tolerable and would rarely affect TMJ function.
In conclusion, this case series shows that in cases of functional facial asymmetry with unilateral posterior crossbite, dento-alveolar compensation for partial missing teeth, and predicted unilateral mandibular movement of less than 7 mm, USSO could be considered a useful mandibular orthognathic surgical technique for reducing operation time and the incidence of postoperative complications.
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