Comparison of condylar morphology changes and position stability following unilateral and bilateral sagittal split mandibular ramus osteotomy in patients with mandibular prognathism

Deviated mandibular prognathism is one of the common craniofacial deformities with a lateral shift of the mandible midline [1]. It is a three-dimensional dentofacial deformity that involves asymmetry of the mandible, maxilla and chin [2]. The geometric complexity of the facial asymmetry makes it an extremely challenging prospect in orthognathic surgery.

Bilateral sagittal split mandibular ramus osteotomy (BSSRO) is considered a standard procedure in the correction of facial asymmetry [3]. By completely separating the distal mandible segment from bilateral proximal segments, it allows the tooth-bearing segment to move freely to achieve midline alignment and also ensures that the condyles in the proximal segments is seated in the neutral position of glenoid fossa with minimal temporomandibular joint (TMJ) tension [4]. However, some disadvantages of BSSRO include neurovascular injury, relapse, unintentional fractures, and malocclusion.

In recent years, unilateral sagittal split ramus osteotomy (USSRO) has been applied for correction of asymmetric mandibular prognathism and yielded favorable outcomes [5]. USSRO has the advantages of reducing operation time and the incidence of postoperative complications. In contrast to BSSRO, USSRO operates on only one side of the mandible and requires a rotation of the tooth-bearing segment to align the lower and upper dental midline, which would inevitably rotate the mandibular segment [5]. The key point to assess a successful operation is whether the movement of the mandibular segment during USSRO would affect the postoperative stability. Only if long-term postoperative stability is guaranteed, unilateral surgery could be accepted as a standard procedure for correcting facial asymmetry. Although many studies have proved the validity and postoperative stability of BSSRO, limited data about USSRO are available.

With regard to the relation of the condylar rotation and the postoperative stability, different scholars have different views. Some believe that the displacement of condyles during SSRO has a significant influence on early postoperative relapse and TMJ dysfunction, such as TMJ pain, clicking sounds, disc displacement and condylar resorption [6]. In contrast, some researchers considered that although three-dimensional changes in condylar head position are observed in patients post SSRO, there are no significant changes that would clinically affect the patients [7]. Therefore, larger sample size evaluation of patients following USSRO surgery and longer postoperative follow-up study are needed.

Whether the mandible movement during surgery would cause abnormal morphology changes in TMJ condyles is another key to determine whether USSRO can be regarded as a standard surgical procedure for the correction of deviated mandibular prognathism. Orthognathic surgery-induced changes of condylar location will impart physical stress on the condylar surface and subsequently cause condylar remodeling of the TMJ structure. If the stress exceeds the adaptation ability of TMJ, joint dysfunction will occur and ultimately lead to postoperative relapse.

With the development of imaging technology, numerous CBCT-based analysis methods have been introduced to assess condylar remodeling. For comprehensive evaluation of condylar surface changes, three-dimensional (3D) reconstructed models and a method of superimposition over the ramus region are necessary [8]. da Motta et al. used a 3D CBCT overlapping technique to evaluate mandibular anatomy and condylar location, proving its effectiveness [9]. An, S., et al. evaluated TMJ remodeling by the superimposition of the 3D reconstructed condylar models in patients with BSSRO or two jaw surgery and found that condylar surface changes before and after the surgery were significantly different [10]. In addition, numerous studies revealed postoperative condylar morphological changes following orthognathic surgery [11]. However, to the best of our knowledge, no study has assessed the effect of unilateral osteotomy on the condylar surface morphology and mandibular positioning changes in patients with lateral mandibular prognathism using 3D-reconstructed models and condylar superimposition.

The objective of this study was to quantitatively evaluate TMJ condylar surface morphology changes and postoperative stability following USSRO in patients with mandibular lateral prognathism compared with the classic BSSRO technique. In addition, the purpose of the study was to determine the relationship between the amount of midline deviation and condylar surface morphology changes after SSRO surgery.

Of the 134 patients we reviewed, 56 patients (23 males/33 females, mean age 26.4 years) underwent USSRO surgery with or without Lefort I osteotomy, and 78 patients (37 males /41 females, mean age 23.1 years) underwent BSSRO surgery. In the USSRO group, there was an average mandibular dental deviation of 4.3 mm (range from 2.7 mm to 8.3 mm) before surgery. In the BSSRO group, there was an average mandibular dental deviation of 5.1 mm (range from 2.8 mm to 13.5 mm) before surgery. All of the patients had a negative ANB angle and underwent mandibular segment setback osteotomy (Table 2).

In the USSRO group, the average condylar volume changes of the deviated and nondeviated side of TMJ (from before surgery (T0) to a year after surgery (T2)) were 98.3 ± 29.3 mm³ and 58.7 ± 9.7 mm³, respectively. In the BSSRO group, the average volume changes of the deviated and nondeviated side of TMJ (from T0 to T2) were 71.6 ± 11.9 mm³ and 59.3 ± 8.4 mm³, respectively. A significant difference was observed between the USSRO and BSSRO groups on the deviated side of the TMJ (P < 0.01).

Regarding the comparison of surface size between the USSRO and BSSRO groups, a significant difference was found on the deviated side of TMJ between T0 and T2 (P < 0.05). For the evaluation of the average surface deviation, a significant difference was also noted between the USSRO and BSSRO groups on the deviated side of TMJ (P < 0.01). No significant differences in the morphology parameters were noted on the nondeviated side (Table 3).

In the evaluation of dental midline correction across the different time intervals, the BSSRO group exhibited a significant difference between T0 and T1 (P < 0.01), but no significant difference was noted between T1 and T2 (P = 0.235). The same evaluation in the USSRO group revealed a significant difference between T0 and T1 (P < 0.01) but no significant difference between T1 and T2 (P = 0.163). Regarding the SNB angle and SN-MP angle, also significant differences were noted between T0 and T1 within each group, whereas no significant difference was found between T1 and T2. These results indicated that patients in both groups exhibited satisfactory asymmetry correction and postsurgical stability without obvious mandibular segment rotation (Table 4).

To uncover the relationship between the amount of dental midline deviation and the condylar surface morphology changes, a Pearson correlation analysis was performed. After surgery, significant coefficients were observed for the amount of dental midline deviation and the morphology changes of the deviated side following USSRO surgery (volume change, r = 0.784, P < 0.01; surface size change, r = 0.717, P < 0.01; average surface deviation, r = 0.782, P < 0.01). However, no significant coefficients were noted for the other groups (Table 5).

The evaluation results of TMJ function stability were presented in Table 6. For USSRO group, the average values of maximum mouth opening were 48.1 mm, 24.8 mm, and 45.4 mm for T0, T1 and T2, respectively. There were significant differences between T0-T1 and T1-T2 (P < 0.05, P < 0.01). Similar results were observed in the BSSRO group, the average values of maximum mouth opening were 49.2 mm (T0), 21.6 mm (T1), and 48.4 mm (T2), respectively.

As for the evaluation of the incidence rate of TMJ pain in USSRO group, 19.6% (11/56) of the patients reported joint pain at T0, while at T1 and T2, the proportion were 48.2% (27/56) and 14.3% (8/56). In the BSSRO group, the incidence rates of TMJ pain were 24.4% (19/78, T0), 41.0% (32/78, T1) and 14.1% (11/78, T2), respectively. The incidence rates of TMJ clicking of patients in USSRO and BSSRO were 26.8% (15/56, T0), 32.1% (18/56, T1), 35.7% (20/56, T2) and 29.5% (23/78, T0), 21.8% (17/78, T1), 26.9% (21/78, T2), respectively.

Currently, the research hotspots in orthognathic surgery include operation safety, in particular the protection of alveolar nerve; postoperative complications, such as the bony healing; long-term postoperative stability of skeletal movements and the stability of TMJ structure and function. In recent years, USSRO has gradually been used to correct lateral deviation of the mandible [15]. Increasing research interests have been focused on whether USSRO can be applied as an effective operation for the correction of mandibular deviation.

The postoperative stability of USSRO is the first concern, as it is closely related to the rotation of the articular condyle during USSRO. Some studies have been conducted to analyze the movement pattern of joints following SSRO surgery. Hu et al. reported that the condyles were displaced posteriorly and rotated anteriorly in 22 patients who underwent mandibular advancement surgery using SSRO [16]. Some papers indicated that the location changes of the condylar head during orthognathic surgery caused condylar resorption and led to recurrence [17, 18]. However, Kim, M.I., et al. noted that although positional changes in the condylar head are observed post USSRO in 3 cases during 6-month follow-up, there are no significant changes that would clinically affect patients [7]. The postoperative stability of patients with USSRO requires studies with larger sample size and longer follow-up.

TMJ constantly undergoes a remodeling process as responses to pressure stimuli during mandibular movement [19]. Joint function can be maintained as long as the changes of condylar position are within its physiological adaptive capacity. Given that changes in the axial direction of the TMJ condyles do not indicate postoperative stability, additional indicators are needed to assess the true postoperative outcome. In the present study, we selected three cephalometric parameters for post USSRO mandibular positioning evaluation from horizontal, sagittal and vertical dimensions. No significant differences between T1 and T2 were noted in the measurement of deviated dental midline, SNB and SN-MP in both of the BSSRO and USSRO groups. These results indicate good postoperative stability with at least 1 year of follow-up in both groups. Other studies also demonstrate that changes in the joint can maintain postoperative stability as long as it is within the joint adaptive capacity. Farina, R., et al. observed 14 patients with USSRO and reported that no patient showed signs or symptoms of temporomandibular joint dysfunction during the 1-year postoperative follow-up [15]. Wohlwender, I., et al. conducted a study on 23 patients to evaluate the long-term clinical and radiological effects on the TMJ following USSRO. Two years postsurgery, no significant differences were noted compared with the literature on BSSRO, and no patient displayed functional alterations on the side that was not subject to the operation [5].

Another important point regarding the maintenance of the joint function is that the morphology changes of joint surface morphology are within the capacity of joint self-regulation [20, 21]. Condylar surface morphology changes, such as joint resorption after orthodontic surgery, are common in clinical practice. With advances in anthropometry techniques, various studies attempt to objectively quantify TMJ with CBCT. For assessment of condylar remodeling, several CBCT-based analysis methods have already been introduced [22, 23]. However, most of the reports are limited to the linear measurement of the condyle, such as lengths, angles and vectors. Saccucci et al. demonstrated that the optimum size or volume of the mandibular condyle are indicative and predictive of a precise clinical situation [24]. In the study, we measured the volume and surface size changes of condyles in the USSRO and BSSRO groups and observed significant differences at the deviated side of TMJ before and 1 year after the surgery between the two groups.

For comprehensive evaluation of condylar surface changes, 3D reconstructed images and a method of superimposition are necessary. Ok, S.M., et al. and Park, S.B., et al. introduced CBCT superimposition methods for evaluating condylar surface morphology changes after orthognathic surgery in TMJ OA patients [14, 25]. However, there have been few 3D CBCT studies that quantitatively analyzed the condylar head surface changes in patients with USSRO. For model overlapping, the key is to identify the correct regions that are not subject to volumetric changes after orthognathic surgery for registration. Theo J.M. Hoppenreijs, et al. noted that regional superimposition of condyles with voxel-based registration on the coronoid process may particularly be interesting [26]. In the study, we superimposed reconstructed models with three registration areas (condylar neck, mandibular notch, and posterior border of ramus) according to the method of An, S.B. and Park, S.B. [10, 14]. It is found that condylar surface morphology changes at the deviated side of TMJ before and 1 year after the surgery were significantly different between the USSRO and BSSRO groups. Considering that the positional parameters of the mandible are not significantly changed, the result indicated that the condylar remodeling on the deviated side of TMJ in USSRO group is within the normal range of physiological resorption. Other authors used a margin of 6% as the threshold between physiological remodeling and resorption [14].

In the study, we demonstrate that the dental midline deviation was related to changes in condylar volume, surface size and surface deviation (r = 0.784, 0.717, and 0.782, respectively; all of the P-values were < 0.01). Our research is consistent with the study by An, S.B. [10], which found that an increase in the change in the condylar axis increased condylar resorption. Therefore, the inevitable rotation of the condyle of the major segment during correction of mandibular asymmetry by a USSRO cannot be neglected. It is necessary to clarify to what extent mandibular deviation can be corrected by USSRO with stable postoperative outcomes. In the study by Lee, S., et al., 7-mm unilateral mandibular movement after USSO would rotate the mandibular condyle 3° to 4° in nonosteotomy side without influencing TMJ function [27]. However, Beukes, J., et al. suggested that USSRO should only be performed for small mandibular asymmetries of less than 5 mm [2]. In the study, the average deviation of the lower arch midline is 4.3 mm, ranging from 2.7 mm to 8.3 mm, and all of the patients reported satisfying postoperative outcomes.

Some limitations do exist in this study. The work presented is still limited to a relative short-term follow-up study. One year follow up is not sufficient to show postoperative stability, also not with regard to TMJ-disorders. Future investigations will focus on long-term postoperative stability evaluation. Moreover, a prospective study with a larger sample size assessing the application of USSRO in the correction of more severe mandibular deviation is needed in the future.

Both BSSRO and USSRO have demonstrated favorable postoperative stability in the correction of mandibular lateral prognathism. After USSRO surgery, condylar surface changes occurred at the deviated side of the TMJ, and the dental midline deviation was closely related to changes of condylar surface morphology. USSRO is a great alternative for minor asymmetric mandibular prognathism correction with the advantages of reduced operating time and surgical trauma with a stable outcome. Long-term postoperative stability evaluation of USSRO needs follow-up study.