A Novel Intraoral Mandibular Osteotomy For Set-Back Surgery In Complex Mandibular Anatomy

Mandibular setback surgery is a complex process when challenged by anatomical variations such as rolled-out inferior mandibular border, thin rami with cortically adherent inferior alveolar nerve (IAN) and inferiorly seated IAN canal [1]. Traditional jaw osteotomies, including Bilateral Sagittal Split Osteotomy (BSSO), Vertical Ramus Osteotomy (VRO) and the traditional Inverted-L Ramus Osteotomy (ILRO), have been widely used to correct mandibular deformities [2‐4]. However, these techniques are limited in cases with complex anatomical variations, as they pose risks of unfavorable splits, increased nerve injury, and extensive distal interferences during setback movements [5‐7].

BSSO is the workhorse for correction of different mandibular deformities owing to its versatility [2]. It also allows for rigid internal fixation with maximum bony contact. Nevertheless, Mandibular anatomical variations such as thin rami, low marrow to cortex ratio with cortically adherent IAN and rolled out inferior mandibular border are considered as major limitations for this osteotomy. These variations complicate the splitting procedure and increase the risk for bad split and IAN injury [1, 6]. VRO is an established method in the management of mandibular prognathism with minimal risk of bad splits and IAN injury. However, it doesn’t allow for rigid fixation that necessitates prolonged maxilla-mandibular fixation. Moreover, the massive distal interferences in setback cases hinder its applicability [3].

The traditional ILRO is a competent alternative to the BSSO in terms of versatility and amenability to rigid fixation with a lower risk of bad splits and neurosensory disturbance. Despite its advantages, ILRO is performed via an extraoral approach, and it faces limitations in mandibular setback cases due to distal interferences and the need for IAN protection [4, 8, 9]. Subsequently, the C-osteotomy introduced by Caldwell et al. 1968, modified the traditional ILRO to be performed via an intraoral approach by extending the long descending arm anteriorly below the IAN canal, this anterior arm is usually thin and prone to unfavorable splits with neurosensory disturbances, especially in cases with inferiorly positioned IAN canals [8‐10].

Recently, Senior et al. modified the C-osteotomy’s anterior extension to be performed at the level of the inferior border of the IAN canal while protecting the IAN upward to run as a sagittal split osteotomy instead of bicortical traditional anterior extension. This modification possesses the same limitations of the traditional BSSO [10]. These limitations highlight the need for an innovative technique that combines safety, versatility, and effectiveness in managing complex mandibular anatomical variations. Specially that the advances in digital planning and customized surgical approaches have offered solutions to some of these limitations. This study aims to introduce a modified intraoral ILRO involving IAN lateralization and intra-canal osteotomy for management of complex mandibular anatomical variations with IAN protection and preservation of split segments integrity during mandibular setback surgery.

The study was registered on ClinicalTrials.gov under the registry number NCT05397002. It adhered to the Declaration of Helsinki on medical research ethics and received approval from the Institutional Research Ethics Committee of the Faculty of Dentistry, Cairo University (IRB number: 151020). This prospective, single-center case series included 20 patients consecutively recruited from the outpatient clinic of the Department of Oral and Maxillofacial Surgery, Faculty of Dentistry, Cairo University. Patient recruitment began in October 2020 and continued until April 2022. The intervention phase was conducted from December 2020 to April 2022, with follow-up extending from December 2020 to April 2023. Data collection was carried out throughout the study duration, concluding in May 2023. Patients were selected according to the following inclusion criteria: (1) Adult patients complaining of skeletal class III malocclusion requiring mono-maxillary or bi-maxillary surgical intervention; (2) CBCT examination revealed anatomical hindrances that interfere with the application of BSSO indicating the application of the currently modified intraoral ILRO osteotomy (thin mandibular rami with minimal or no medullary bone, inferior alveolar nerve proximity to the buccal cortex along the length of the BSSO cuts, lateral bending of the inferior mandibular border at molar angle region, high mandibular foramen, inferiorly seated IAN canal and extensive distal interferences) (Fig. 1) and (Fig. 2). Patients with previous extensive jaw surgery, mandibular pathological lesions, or temporomandibular joint dysfunction were excluded from the study.

This case series included 20 patients (mean age: 21.6 ± 3.3 years; 13 females, 7 males) with skeletal class III malocclusion requiring mandibular setback surgery. 14 patients (70%) underwent bimaxillary surgery, while 6 patients (30%) underwent mono-maxillary surgery. The mean magnitude of the mandibular setback was 6.05 ± 1.1 mm. The average duration of the mandibular procedure, from the initial incision to suturing, was 2.26 ± 0.21 h. Inferior Alveolar Nerve (IAN) exposure was successfully achieved bilaterally within 11 ± 3.2 min per side (Table 1).

Mandibular anatomical variations as rolled-out inferior mandibular border, thin rami with cortically adherent IAN and inferiorly seated IAN canal are considered a critical diagnostic finding which obligates the orthognathic surgeon to select a mandibular osteotomy over another [1]. Conventional mandibular osteotomies introduced in the literature, such as BSSO, ILRO, and C-osteotomy, are utilized for mandibular setback surgery but have notable limitations which direct the authors to find an alternative solution for management of such limitations aiming to reduce the incidence of neurosensory disorders and bad splits along with adequate mandibular setback without distal interferences [2‐10].

Based on the literature, BSSO is associated with a variable incidence of bad splits, ranging from 0.2 to 14.6%, influenced by anatomical factors such as thin cortices and impacted molars [12, 13]. Moreover, it induces long-term neurosensory disturbances (NSD) which persist in 10–15% of patients at one year [14]. ILRO reduces the risk of nerve injury compared to BSSO but distal interferences limit its application in setback procedures [4]. To address these challenges, Senior et al. introduced a C-osteotomy modification with an intraoral sagittal split below the IAN canal which enhances bone contact, but it remains prone to bad splits and nerve injury in patients with thin buccal cortices (< 7–10 mm), inferiorly positioned IAN or rolled out mandibular border [10]. This directed the authors to designate this new modification for the intraoral ILRO to incorporate nerve lateralization with intra-canal osteotomy.

The incorporation of nerve lateralization ensured safe application of the intra-canal osteotomy and removal of distal interferences with non-blind nerve protection, while the intra-canal osteotomy enhanced the structural integrity of the anterior arm of the proximal segment with 0% incidence of bad splits and adequate rigid intraoral fixation through placement of 4-holes titanium mini-plate.

The fixation method used in this study differs from conventional approaches owing to the novel osteotomy design. Typically, two miniplates with monocortical screws are placed in mandibular body fractures or osteotomies, one in the traction zone and another in the compression zone to enhance stability [15]. However, in the current modified intraoral inverted-L ramus osteotomy, the anterior vertical osteotomy extends from the inferior border and terminates at the level of the inferior alveolar canal, meaning no osteotomy is present at the usual location for the superior plate typically applied in mandibular body osteotomies or fractures. Additionally, the current osteotomy follows a bicortical design, distinct from the traditional BSSO, which may result in different load transmission characteristics [16]. Given these factors, the use of a single miniplate for fixation was deemed appropriate within the scope of this study.

The nerve lateralization of the current osteotomy prompted an evaluation of its NSD risks that relied on subjective assessments such as Shintani Y. et al. who emphasized that subjective evaluations reflect the patient’s experience and provide a more representative outcome compared to objective assessments of neural damage [17]. Abayev et al. reported transient NSD in 99.47% of cases within 1 to 6 months, with permanent NSD in only 0.53% of procedures. Similarly, this study observed transient NSD in 90% of cases, with progressive recovery to 5% at one year [18]. These findings surpass those reported for both BSSO and ILRO, where NSD recovery is slower and incomplete in a significant number of cases [19]. Roychoudhury et al. reported 13.3% NSD for BSSO using subjective assessments after one year follow-up [14]. Moreover, Kobayashi et al. found that NSD rates over one year after ILRO were consistently lower than those for BSSO, with 9.5% vs. 15.2% of cases respectively [20]. In comparison, the current study’s osteotomy achieved only 5% NSD at one year.

Despite the added steps of nerve lateralization and intra-canal osteotomy, the mean surgical duration (2.26 ± 0.21 h) was comparable to traditional methods. The quick nerve exposure time (11 ± 3.2 min per side) and absence of intraoperative complications, such as nerve transection or major bleeding, highlight the technique’s feasibility [21]. Additionally, the use of patient-specific guides through utilizing CBCT imaging and digital planning ensured precise segment separation without unfavorable splits. This is consistent with several studies, that reported improved outcomes with digital integration in orthognathic surgery [11, 22].

The combination of preoperative planning and intraoperative guidance minimizes human error and ensures better preservation of anatomical structures [23]. While the study demonstrates promising outcomes, its limitations include a single-center design. Future multicenter studies are necessary to validate these findings. Additionally, comparing this technique directly with other modifications, such as the C-osteotomy or variations of BSSO, would provide more robust evidence of its advantages. Future studies also should compare different fixation techniques to validate optimal fixation methods in relation to this novel osteotomy design and assess its long-term stability.

The introduced osteotomy overcomes the limitations of the traditional mandibular osteotomies in dealing with mandibular complex anatomy with adequate IAN protection and split segments integrity during mandibular setback surgery.