A deep dive into the static force transmission of the human masticatory system and its biomechanical effects on the temporomandibular joint.

OBJECTIVE: This study aims to investigate the biomechanical behavior and reveal the force transmission patterns of the human masticatory system through advanced three-dimensional finite element (FE) models. METHODS: The FE model was constructed according to the medical images of a healthy male adult. It contains full skull structures, detailed temporomandibular joints (TMJs) with discs, complete dentitions, masticatory muscles, and related ligaments. Several static bite scenarios were simulated to demonstrate the effects of bite positions and muscle force recruitments on the force transmission patterns. RESULTS: Molar occlusal surfaces are the primary force transmission region for clenching. Sensitivity analysis demonstrated that the stiffness of the bite substance would not alter the force transmission patterns but could affect the maximum contact stresses on the discs and the occlusal surfaces. During the unilateral clenching tasks, the high-stress region on the discal surfaces shifted ipsilaterally. The presence or absence of the molar cushions would significantly affect the biomechanical response of the masticatory system. SIGNIFICANCE: FE analysis is an effective way of investigating biomechanical responses involving complicated interactions. Enriching the static analysis of the masticatory system with a detailed model can help understand better how the forces were transmitted and the significance of TMJs during the clenching process.

Effects of the lining material, thickness and coverage on residual stress of class II molar restorations by multilayer technique.

OBJECTIVE: This study aims to analyse the influence of different lining material, thickness and coverage on residual stress of class II molar restoration by the multilayer technique through the three-dimensional (3D) finite element (FE) method. The objective is to reveal the correlations between the base layer configurations and the residual stress distributions of the tooth. METHODS: A 3D reconstructed model of an extracted first molar was built through micro-CT images and a class II mesio-occlusal-distal cavity was prepared using computer-aided engineering (CAE) software. A bilayer technique was then applied, and 3D FE analyses were performed under polymerization contraction loading. Glass ionomer composites (Vitrebond and Ionosit) and flowable composite resin (Luxa Flow and Z350 Flowable Restorative) were used as lining materials. Moreover, several sensitivity analyses with dozens of hypothetical lining materials were conducted to provide definitive results. The thickness of the base layers was set to be distinct (0.5 mm or 1 mm) so as to reveal its effect on the stress alleviation. Various lining strategies with the liner covering different areas of the cavity walls (Closed Sandwich, Open Sandwich and Special Open Sandwich) were adopted to determine the effects of the base layers. RESULTS: Adoption of an appropriate liner could effectively reduce the polymerization shrinkage stress. The mechanical properties of the base layer can affect the residual stresses, basically a lower Young's modulus and lower Poisson's ratio (of lining material) result in better stress mitigation, therefore reducing the stress transmitted to the dentin. Increase of the lining thickness within a certain range could lead to a decrease in the probability of stress concentration formation. Lining strategies had the strongest influence on the stress distributions. Different lining coverage could cause various stress responses, and covering all cavity walls with the lining material had the optimal performance among all the simulations in this study. SIGNIFICANCE: The multilayer technique is an effective way to prolong the service life of resin composites restorations. A thorough evaluation of the sandwich technique through the FE method can provide a better understanding of the stress distributions of the restoration, and reveal its internal mechanisms.

[Three-dimensional morphological changes in the temporomandibular joints of patients with anterior disc displacement with reduction].

OBJECTIVES: To investigate the differences in the temporomandibular joints (TMJs) between patients with anterior disc displacement with reduction (ADDwR) and asymptomatic subjects by using 3D morphometric measurements. METHODS: A total of 15 patients with ADDwR and 10 asymptomatic subjects were enrolled. Then, 3D models of the maxilla and mandible were reconstructed using MIMICS 20.0. Nine morphologic parameters of TMJs on both sides were measured on the 3D solid model. The differences in the parameters were analyzed between the patients and the asymptomatic subjects and between the left and right sides of each group. RESULTS: The horizontal and coronal condylar angles on the ipsilateral side of the patients were significantly greater than those of the asymptomatic subjects (P<0.01). Meanwhile, the sagittal ramus angle (SRA), medial joint space, lateral joint space, superior joint space, anterior joint space, and posterior joint space in the patients were significantly lower than those in the asymptomatic subjects (P<0.01). CONCLUSIONS: ADDwR will increase the condylar angles to be significantly greater than the normal level and decrease SRA and articular spaces to be significantly smaller than the normal level. The condyles will be displaced upward, closer to the fossa.

Biomechanical influence of anchorages on orthodontic space closing mechanics by sliding method.

This study aims to analyse the stress distributions and initial displacements of teeth during the space closing stage through a three-dimensional finite element method. Computed tomography images of a patient were used to reconstruct the detailed teeth and alveolar bone, and brackets with stainless steel archwire were modelled according to the orthodontic prescriptions. The second premolars and first molars were chosen as the anchorages in the model 6-force, with buccal tubes attached to the second molars in the model 6-force-7, and the second molars as additional anchorages in the model 7-force. The results indicated that a movement of lingual lateral inclination occurred on the incisors during the retraction, and the frictional force between the teeth and the archwire significantly reduced the stress on the teeth and periodontal structures. Graphical abstract Malocclusion is one of the most common issue in dentistry with high prevalence and orthodontic treatment need. The extraction of first premolar teeth was normally needed at the beginning of the treatment. And the straight wire appliance together with the sliding mechanics was used for space closure at the end of the treatment. However, side effects like root resorption also found after the surgery. Biomechanically, the stress distributions and initial displacements of teeth during space closing stage might be a crucial factor contributed to those undesirable side effects. And different selections of anchorages might alter the biomechanical environment during the treatment. Thus, the purpose of the current study was to analyse the stress distributions and initial displacements, with the different anchorage selections, of teeth during space closing stage through 3D finite element method.