Orthognathic surgery: bimaxillary approach reduces stress on the TMJ

Biomechanical impact of maxillary repositioning on the TMJ: a finite element analysis

Orthognathic surgery, while corrective in terms of occlusion and aesthetics, radically alters the mechanical balance of the temporomandibular joint (TMJ). The practitioner is faced with the challenge of preventing postoperative joint overload, particularly during complex multidirectional movements. This study specifically addresses this issue by evaluating the biomechanical effects of maxillary repositioning combined with mandibular advancement and rotation on TMJ components.

The central objective was to quantify the stress distribution on the condyle, the articular disc, and the glenoid fossa via a personalized finite element analysis (FEA). The authors used three-dimensional models reconstructed from preoperative CT scan data of six patients presenting with various dentofacial deformities. These models allowed for the simulation of six virtual surgical configurations, including isolated mandibular advancement, bimaxillary repositioning, vertical maxillary movements, and rotational asymmetries.

The study is based on the hypothesis that the surgical configuration directly influences the homogeneity and intensity of joint loads. The researchers sought to verify whether bimaxillary approaches offer a more favourable biomechanical environment than isolated mandibular advancements, while evaluating the impact of asymmetric and vertical components on unilateral joint loading.

Finite element analysis methodology

This biomechanical study is based on a patient-specific finite element analysis (FEA), aiming to model the impacts of surgical movements on the temporomandibular joint (TMJ). The protocol was based on the reconstruction of three-dimensional (3D) models from preoperative CT scan data of 6 distinct patients presenting with various dentofacial deformities.

Based on these anatomical models, six virtual orthognathic surgery configurations were generated to simulate different repositioning vectors:

• Isolated mandibular advancement;
• Combined bimaxillary repositioning;
• Vertical maxillary movements;
• Asymmetric rotational components.

To ensure the comparability of the results, the experiment applied strictly identical load and boundary conditions for each configuration. The analysis focused specifically on evaluating the stress distribution within three structural components of the TMJ: the condyle, the articular disc, and the glenoid fossa.

Impact of surgical configurations on TMJ structures

The finite element analysis (FEA), performed on 3D models specific to six distinct patients, quantified the stress distribution within the condyle, the articular disc, and the glenoid fossa. The results highlight significant load variations depending on the surgical repositioning vector used.

Qualitative and biomechanical observations

The study highlights notable differences in the behavior of soft and hard tissues of the temporomandibular joint depending on maxillary and mandibular movements:

• Homogeneity: Bimaxillary configurations promote a more balanced distribution of forces across all TMJ components (condyle, disc, and fossa), unlike monomaxillary procedures.
• Rotational asymmetry: The introduction of asymmetric rotational components during maxillary repositioning correlates directly with an increase in unilateral stress peaks.
• Joint overload: Large-amplitude mandibular advancements, when not compensated by adequate maxillary repositioning, generate stress concentrations potentially deleterious to the integrity of the articular disc.

Although data confirm a clear statistical trend towards the biomechanical superiority of the bimaxillary approach, the authors specify that the planning of complex rotations must be approached with caution to limit excessive postoperative joint loads.

Stress analysis: the mechanical benefit of the bimaxillary approach

This finite element analysis, based on 6 patient-specific models, provides quantitative insight into the biomechanical response of the temporomandibular joint (TMJ) to surgical movements. The major finding lies in the mechanical superiority of bimaxillary configurations: they generate a more homogeneous and reduced stress distribution at the level of the condyle, the articular disc, and the glenoid fossa compared to isolated mandibular advancement.

The study demonstrates that mandibular advancement alone induces more marked asymmetric stress peaks. Conversely, the integration of maxillary repositioning appears to balance the joint load. However, vigilance is required during vertical repositioning or asymmetric rotations, as these vectors significantly increase unilateral load and unbalance the joint. These data suggest that the choice of a bimaxillary approach is not solely aesthetic or occlusal, but addresses a need for joint preservation.

The limitations of this work lie in the small sample size (n=6) and the virtual nature of the modeling, which does not take into account long-term biological remodeling or neuromuscular adaptation. Nevertheless, these results confirm current literature on the importance of limiting postoperative mechanical overloads to prevent TMJ disorders, particularly in cases of significant mandibular corrections.

Study summary

This finite element analysis, conducted on 6 patient-specific models, demonstrates that isolated mandibular advancement generates higher and more asymmetric stress concentrations on the condyle, the disc, and the glenoid fossa. Conversely, bimaxillary configurations ensure a more homogeneous stress distribution, although the addition of vertical movements or asymmetric rotations systematically increases the unilateral joint load.

In practical terms, for the practitioner:

• Prioritize the bimaxillary approach: For large-scale corrections, the bimaxillary approach offers a more protective biomechanical environment for the TMJ than isolated mandibular advancement, which overloads the joint structures.
• Moderate rotations: Be particularly vigilant when planning asymmetric rotational movements or significant vertical repositioning, as they induce deleterious postoperative load imbalances.
• Caution with large advancements: High-amplitude mandibular advancements without maxillary compensation must be planned with caution to limit the risk of excessive mechanical stress on the articular disc.

Technical glossary of the study

Finite Element Analysis (FEA): Numerical simulation method used in this study to calculate mechanical stress distributions on TMJ components from 3D models reconstructed via CT scan.

Articular disc: Fibrocartilaginous component of the TMJ whose stress levels were evaluated to compare the impact of monomaxillary versus bimaxillary surgeries.

Glenoid fossa: Temporal anatomical structure whose biomechanical load was analyzed to identify areas of increased pressure induced by rotational or mandibular advancement movements.

Bimaxillary repositioning: Surgical configuration combining maxillary and mandibular movements, associated in this study with a more homogeneous and reduced stress distribution at the TMJ level.

Rotational asymmetry: Surgical movement component included in the virtual models, identified as a factor significantly increasing unilateral and unbalanced joint load.

Isolated mandibular advancement: Surgical repositioning procedure involving only the mandible which, according to the analysis results, generates higher and more asymmetric stress concentrations than bimaxillary approaches.

Stress distribution: Central biomechanical parameter measured on the condyle, the disc, and the fossa to evaluate the balance of postoperative loads.

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