[Stress distribution of mandible under different loading and biting condition].

OBJECTIVE: To study the stress distribution of intact mandible, especially that of the mandibular angle under different muscle loading and biting condition. METHODS: Develop a more accurate, more objective mandibular model, measure the different stress patterns on the outer surface of the mandible by strain gauges due to different biting sites (INC and ICP) and different loading methods (masseter, temporalis and four pairs of muscles). RESULTS: It was found that the strain in the zone of mandibular angle is more markedly under masseter loading; that the strain in the zone of anterior mandibular ramus is more markedly under temporalis loading; and that the stress in the zone of mentum becomes a tension because of the medial pterygoid under the load of four pairs of muscles. During anterior teeth biting, the stress of mandible angle is larger than that of the bilateral molar biting. When an occlusal load is on the ipsilateral molars, there is a reversal result of the stress direction in the upper line of the mandibular angle. CONCLUSION: Different muscular loading and biting condition can change the stress distribution of the mandible. It is important to develop a functional model of human mandible to study its complex biomechanics behavior.

Biomechanical evaluation of Le Fort I maxillary fracture plating techniques.

PURPOSE: This study used a biomechanical model to examine fundamental questions about rigid plate fixation treatment for maxillary Le Fort I fractures. Specifically, we sought to elucidate the principal strain patterns generated in miniplates and bite force transducers secondary to all masticatory forces, as well as the amount of permanent deformations incurred due to these loading forces. MATERIALS AND METHODS: Forty polyurethane synthetic maxillary and mandibular replicas were used to simulate the mandible and maxilla. Ten replicas were controls (group A). The other 30 were divided into 3 groups (10 each), according to the fixation techniques of 3, 2, and 1 miniplates each side (groups B-D), that were osteotomized in the Le Fort I fracture line on the maxilla. Different forces of masseter medial pterygoid, temporalis, and lateral pterygoid muscles were loaded onto the replicas to simulate different functional conditions (anterior incisor, premolar, and molar clenching). Rosette strain gauges were attached at predefined points on the plates and the bite force transducer to compare the stability and bite force of the different fixation methods for maxillary Le Fort I fractures. RESULTS: Statistically significant differences were found for the deformation of the plates among fixation techniques. The order of stability for each technique was: group B greater than group C greater than group D. In regard to bite force, no difference was found between those found with group A and group B (P > .05), whereas the bite forces of groups C and D were less than those of group A (P < .05). CONCLUSIONS: The fixation of 3 miniplates on each side provides sufficient stability and restores the bite force to the level of the intact maxilla. "The ideal fixation" with 2 miniplates on each side restores 90% of the bite force, and there were more deformations of the miniplates with the "ideal fixation" compared to those found with group B. Group D fixation produced the worst effects for the treatment of maxillary Le Fort I fractures with a weak bite force and insufficient stability.

[Biomechanical study of rigid internal fixation for mandibular angle fracture].

OBJECTIVE: To develop a functional biomechanical mandibular model, and to observe the stress distribution of angle-fractured mandible under different rigid internal fixation (RIF) methods. METHODS: A biomechanical model of mandible was built which include the simulative temporal-mandibular joint (TMJ) and was under mechanical loads of masseter, temporalis, medial pterygoid and lateral pterygoid. The different stress pattern was measured by strain gauges. Under the standard mandibular angle fracture, bilateral molar biting and four pairs of muscles loading, the strains were compared to evaluate the stability of one-or two-miniplate fixation. RESULTS: The fixation of miniplate in the lateral oblique line can recover the main stress on the non-fracture side, but it was broken in the fracture side. The tension increased in the lower border of mandibular angle. CONCLUSION: Through the biomechanical study based on the functional mandibular model, only one miniplate fixation in lateral oblique line for mandible angle fracture was insufficient buttressing of the segments, while the additional miniplate in the lower margin can recover the stress pattern and provide more stability.

[The effect of cryotreat on tensile properties of medium melting-point and high melting-point castable alloy].

OBJECTIVE: To introduce cryotreat technique into prosthetic dentistry by testing the tensile properties of CW-H Co-Cr-Mo cast alloy before and after cryotreat and to observe the image changes by SEM to study the mechanism that changes the tensile properties of the alloy. METHODS: 15 CW-H cast alloy were divided into 3 groups, i.e. control group (G1), cryotreated group (G2) and cryotreated plus post-cryogenic treated group (G3). The gauge test technique was employed to test the modulus of elasticity. Then the strength and percentage elongation (PE) were tested. SEM images were used to analyze the mechanism that improved the tension properties of the alloy. RESULTS: For CW-H alloy the strength and the modulus of elasticity of both G2 and G3 were effectively increased but PE effectively decreased than G1. There was no effectively difference between G2 and G3, but G3 was larger than G2. SEM images of G2 and G3 showed that secondary-carbonide separated out all over the alloy. CONCLUSION: The results obtained above suggest that cryotreat is an effective method in enhancing tensile properties of CW-H Co-Cr-Mo cast alloy.