Biomechanical assessment of unilateral sagittal split ramus osteotomy: a finite element analysis

Introduction

Facial abnormalities can lead to self-deprecation and other psychiatric illnesses in addition to having a negative impact on a patient’s looks, eating, and other mouth functions. Additionally, they may result in temporomandibular joint (TMJ) dysfunction (TMD). The most typical craniofacial abnormality, which makes up about 43% of all maxillofacial abnormalities, is mandibular prognathism (1). Another prevalent mandible malformation, with a frequency varying from 8.7% to 23.3% (2), is facial inequality. These facial abnormalities have frequently been treated with combined orthognathic procedures.

Sagittal split ramus osteotomy (SSRO), a popular procedure for treating the mandible, has raised questions about how it will affect the TMJ (3-5). It is regarded as a routine procedure in mandible orthognathic operations to perform sagittal split osteotomy (SSO) on both symmetrical mandibular rami [bilateral SSO (BSSO)] to lessen unanticipated tension and twisting in the TMJ. It was once thought that only bilateral mandibular approaches could be used to free the tooth-bearing distal mandible segment from both TMJ-attached proximal segments, allowing the proximal segments to freely move into neutral condyle positions and fixing the distal segment with minimal TMJ tension (6,7). However, a recent study used unilateral SSO (USSO) to treat the lateral displacement of the jaw, and the results were encouraging (7).

Operation time and the frequency of postoperative complications, such as inferior alveolar nerve damage, hemorrhage, or a bad split, could be decreased in comparison to those seen in bilateral mandibular surgeries if unilateral surgery could guarantee long-term postoperative stability and favorable results in correcting facial asymmetry (7,8). The opposite mandibular condyle may rotate as a result of USSO, which could have negative consequences like condylar erosion, a restricted range of motion in the jaw, or TMJ disorders (6,9). These factors have led to the highly restricted use of USSO that is currently in place. It may have a positive significance on the relief of temporomandibular disease and postoperative stability.

Through observations on two-dimensional (2D) X-rays or computed tomography (CT), several studies about condylar position alterations reported that the USSRO did not induce undue spin or displacement (10,11). The physiological impact of bilateral SSRO (BSSRO)/unilateral SSRO (USSRO) on the dynamic tension shift of the Temporomandibular structures, however, cannot be fully explained by these steady studies. The USSRO can achieve comparable physical impacts on symmetrical TMJ components immediately following operation as the BSSRO. The present study aimed to assess the biomechanical properties of the mandible following USSO.

Methods

Study design and sample

A CT scan of a fully dentate 20-year-old man was obtained from the database of the Oral and Maxillofacial Radiology Department at Prince Sultan Military Medical City. The criteria for selection of this case were images showing a complete mandible and an absence of metallic restorations in the scanned region to limit radiographic artifacts. We used a scan of a symmetric mandible with almost equal condyle sizes. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was registered in the research center and approved by the Institutional Review Board (IRB) of Prince Sultan Military Medical City (No. HP-01-R079) and informed consent was taken from the individual participant.

Modeling

All CT data were imported in Digital Imaging and Communications in Medicine (DICOM) form to a visualization module using the graphics program Mimics (version 8.11; Materialize HQ, Leuven, Belgium) to filter out the mandible, manually remove any artifacts and outline the structures being examined. This resulted in a highly defined surface converted into a standard triangulation (STL) file to allow further processing (Figure 1). The STL file of the mandible was then imported into the computer-aided design (CAD) software SOLIDWORKS 2014 (SOLIDWORKS Japan, Tokyo, Japan) (12). Two models were generated for biomechanical stability comparison:

• Mandible without any modifications (no osteotomy simulation);
• Mandible treated with USSO using the Obwegeser/Dal Pont technique (13).

Figure 1 Mandible model obtained using cone beam CT. CT, computed tomography.

The study focused on the biomechanical properties of the mandible after USSO. Jaw was not operated to stabilize occlusal forces as we aim to measure intrinsic properties of mandible, internal stress, and strain within the mandible.

The surgical simulation resembled the correction of an asymmetrical mandible by shifting the mandible toward the right side. Eight simulations were performed for the USSO model at 1, 2, 3, 4, 5, 6, 7, and 8 mm deviations of the mandible. The midline of the lower central incisors was considered a reference point for this movement.

The USSO models underwent an additional step before the analysis to determine the stress resulting from the rotation of the mandible. We calculated the amount of rotation needed to displace the mandible to the contralateral side for each model in degrees until the desired amount was reached. This rotation in degrees was then added to the model (Table 1).

Each surgical model was stabilized using mini plates and screws. A four-hole straight titanium plate of 1.0-mm thickness was fabricated, along with 2.0-mm cylinders screws of 7 mm length. The plates and screws were placed at the lateral border of the mandible. The mechanical properties of the bone and materials tested are shown in Table 2. All models were then exported into CAD files.

Meshing

The CAD files were imported to Hypermesh (Altair Hyperworks, 2017) for final meshing and in preparation for the final analysis using the ANSYS software package (ANSYS Inc., Southpointe, PA, USA). For simplicity, the bone was considered a homogenous object. The model consisted of 288,050 elements and 69,802 nodes. The material properties were defined as Young’s modulus of 13,700 MPa and Poisson’s ratio of 0.3 for the bone and 1.2e+005 MPa and Poisson’s ratio of 0.3 for titanium (14).

Boundaries and restraints

Boundaries and restraints were applied to regions in the model where the movement was restricted. The condyles were fully constrained bilaterally by design.

Loading

Each model was examined under the masticatory muscle and occlusal loads. The muscles and their forces are presented in Table 3. Occlusal loads placed on the lower first molars and lower first incisors were 260 and 66.7 N, respectively (14). The previously calculated forces for the USSO models were added to simulate the force required for mandibular rotation in each model.

Pre- and post-procedure condyle stress concerning mandible fossa

The pre-assessment of right and left condyles showed varied stress patterns in masticatory muscle and occlusal loads with a similar resultant force of 98 N in right and left condyles. The resultant forces post-surgery in both TMJs were 79 and 127 N. Although the load was the same pre-procedure, an asymmetrical load was observed through one side of the TMJ. The left side unaffected TMJ through the ramus posterior part, the load was transferred through condyle anterior aspects to the temporal bone anterior aspects. The loading of the right condyle was diffused and the load was transferred through the tympanic bone in a posterior direction. This difference in transfer of load generated high forces of reaction to the temporal bone upper aspect on the side affected. Moreover, identical patterns were observed post-surgery despite different levels of force with lower stress magnitude on the right-side condyle and fossa. We observed reduced stress in the toral force of occlusal (from 320 to 314 N) in comparison to TMJ (Table 4). This led to the generation of more forces of reaction in the molar region.

Data analysis

All models were analyzed using finite element analysis (FEA) for occlusal loads and masticatory forces. The maximum von Mises stresses on the condyle and around the hardware were recorded, following which the displacement of each model was recorded.

Results

The model without any osteotomy procedure was tested first. Masticatory and occlusal loads showed that the maximum stresses were located in the right and left condyle areas, with a maximum von Mises strength of 106 MPa (Figure 2). The maximum stresses on the non-operated condyle and around the fixation area in each USSO model are shown in amount 3. The 7-mm model demonstrated the maximum stress on the condyle at 170.02 MPa (Figures 3,4). No measurements were taken to estimate condylar resorption pre- and post-surgery. This study also found stresses on the condyle contralateral to the sagittal split side in USSO were higher at a more considerable rotational degree.

Figure 2 Maximum von Mises stresses on the non-operated mandible.

Figure 3 Cropped condylar view showing the maximum stresses (non-operative on the left).

Figure 4 Maximum von Mises stresses around the fixation.

Discussion

The USSO technique has not been reported widely in the literature, probably due to limited indications for this procedure and the lack of supporting evidence for its success rates. Twenty years ago, Motamedi (15) reported successful outcomes in the correction of condylar hyperplasia using unilateral ramus osteotomy. And the results of the six cases that underwent unilateral osteotomy were comparable with the other seven cases that underwent bilateral surgeries. However, none of the cases was treated with an SSO, and the follow-up period was up to 2 years for five of the cases.

Another retrospective study by Merkx et al. (16) evaluated the patients who suffered from orthognathic surgery relapse that has resulted due to condylar resorption. Out of a total of 329 patients who underwent surgical correction, only three were treated with USSO. These cases developed TMJ disorders within 3.5 months and reduction of muscle functions and ended up with a relapse. Therefore, this study considered USSO as a contraindication in the management of relapse due to condylar resorption. However, this re-sorption following the second operation could be a continuation of the resorption before the surgery, as no measurements were taken during this study to detect whether the condylar resorption was still active.

Wohlwender et al. (7) was the first to investigate the changes in the TMJ following USSO. They evaluated 23 patients who underwent USSO for the correction of mandibular asymmetry. The patients were examined for the TMJ function clinically, and the radiographs were evaluated for condylar resorption and ramus height, and only one patient showed evidence of resorption in the condyle with bilateral ramus. Nevertheless, similar changes were also reported in cases of BSSO by other studies without observing an increased relapse rate (17). And it was postulated, based on this, that the changes in the non-operative condyle in USSO cases would tolerate these changes without affecting the TMJ and the function.

Furthermore, a CT study by Kim et al. (18) examined the position of the condyle following USSO and found these changes insignificant. The authors evaluated the condyle in a three-dimensional (3D) aspect and noted a change in the condyle position 6 months after the surgery. However, there was no further follow-up. The longest follow-up period found in the literature was in a study of three cases by Lee et al. (10) for USSO setbacks of 6 and 7 mm. The follow-up evaluation was 3 years in one case and 5 and 6 years in the other cases. All patients were free of TMJ symptoms with normal function, and there were no signs of condylar resorption. It is worth noting that condylar resorption was not only documented in USSO cases. Arnett et al. (19) reported condylar resorptions associated with different mandibular osteotomies.

The USSO studies lacked a long-term follow-up and were unpredictable whether the changes in the condyle position and shape would result in any future disturbances. While the choice of USSO as a treatment option would lower the complication rate by half and reduce the surgery time yet, the effect of rotating a non-operative side has not been studied. As one would assume, when one side of the mandible is rotated without osteotomy, it will create an extra load on the non-operated segment. The present study appears to be the first biomechanical assessment study on USSO.

Nevertheless, Ghiasi et al. (13) studied the regulation of micro-movements in micro-environments were advantageous for bone healing and callus formation. Those findings were similar to other FEA studies that compared different fixation techniques of sagittal split osteotomies (20,21). Conversely, the USSO groups showed a reduced displacement at the fixation site with an increased rotation towards that area. This may be explained by the increased bone contact following this movement and increasing the stability of the area.

Relapse following mandibular osteotomies may result from condylar positional changes, condylar resorption, and hardware failure. The condyle position and fixation loss can be detected immediately intraoperatively or with radiographic examination (22). The condylar resorption, however, is slowly progressive with no sudden symptoms, and there were reports of mechanical stress associated with this condition (23). Hence the main objective of the study was to evaluate the stresses around the condyle of the non-osteotomized area. Ureturk and Apaydin (14) studied the stress generated on the condyles following different fixation methods of 5 mm BSSO. Their FEA study concluded that bicortical screws increased the stress on the condyle in contrast to the use of mini plates. Similar results were also reported by Stringhini et al. (24).

The values of stress found in this study could not be compared to others as there is no available literature that compared the effects of osteotomy distance and the amount of correction on the mandibular bone and, specifically, the condyles. We found that the stresses on the condyle contralateral to the sagittal split side in USSO were higher at a more considerable rotational degree. A logical explanation of this is that in the cases of BSSO, the anterior (distal) segment of the mandible would be freely manipulated to the final desired position based on the planned occlusion. Whereas in USSO, the non-osteotomized side would resist these free movements resulting in the seen stresses.

In the present study, it was found that the more mandible deviation is, the more the stresses are on the condyle. This can be explained by the increased forces needed to rotate the mandible as the amount of correction increased. Having the condyles restrained completely may aid in the increase of the stress in this study because this limited the condyle movement that was seen in some studies (7,15,16). There were also reports of USSO cases with no condylar positional changes seen in the condyle following surgery (10).

USSO procedures are not adequately reported in studies and reported in the literature, and most of the published cases lack a long-term follow-up for the patients. Although this procedure will have lesser complication rates and decreased operation time, the indication of USSO is limited to certain cases of asymmetry (15). Moreover, the stresses resulting from unilateral osteotomies must not be ignored, and their effects on condylar position and resorption must be fully understood. The reported cases of USSO that resulted in symptom-free TMJ have also reported changes in the condyle angulation and position. The outcomes of these changes are unpredictable in the future.

This FEA study involved a single mandible scan obtained from a 20-year-old male. Despite the accurate replication that could be achieved in the software, it does not represent any population, especially since the bone quality differs among individuals. It was a stand-alone analysis which should encourage more studies.

To reach the goals of the study, we used a scan of a symmetric mandible with almost equal condyle sizes to analyze the biomechanical behavior of the bone resulted from the simulated surgical movement on a normal-sized condyle to limit the bony defect variables that may be seen in asymmetrical mandibles. This may not represent all clinical scenarios where cases of facial asymmetry may be associated with condylar resorption, hypoplasia, or hyperplasia. The muscle of mastication and the occlusal forces do vary among individuals, and this may affect the outcomes and the stresses following the procedure.

Despite the limitation of the study, it can be concluded that when one side of the mandible is rotated without osteotomy, it will create extra load on the non-operative segment. Therefore, the FEA model helps to understand this biomechanical behavior:

• USSO technique showed increased stress on the condyle of the non-operative side;
• The stresses on the condyle decreased when the mandible deviation increased;
• The USSO models showed high deformation at the fixation site.

One of the major limitations of the study is that the researcher did not use DICOMS from a patient with a USSO indication to better reflect the mandibular changes after the procedure. We recommend for future USSO animal studies study the biomechanical changes in the bone following this procedure to have a better clinical understanding of the stresses generated. We also recommend prolonged clinical studies with a large cohort following USSOs and evaluating the functional and positional condyle changes.

Conclusions

We concluded that USSO procedures carry increased stress on the condyle and fixation. Therefore, the use of this technique should be limited while considering the consequences of the increased stress. The study shows an increase in stress for large mandibular advances (i.e., 7 mm), which does not represent the majority of cases.

Acknowledgments

Funding: None.

Footnote

Data Sharing Statement: Available at https://fomm.amegroups.com/article/view/10.21037/fomm-23-85/dss

Peer Review File: Available at https://fomm.amegroups.com/article/view/10.21037/fomm-23-85/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://fomm.amegroups.com/article/view/10.21037/fomm-23-85/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was registered in the research center and approved by the Institutional Review Board (IRB) of Prince Sultan Military Medical City (No. HP-01-R079) and informed consent was taken from the individual participant.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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