| Summary: | 碩士 === 國立臺灣大學 === 臨床牙醫學研究所 === 104 === Clinically, surgeons follow a rule of 10 mm to decide whether to perform marginal mandibulectomy or segmental mandibulectomy. However, this rule was based on an experiment performed on a dry mandible with two condyle heads fixed in the cement. Traditionally, reconstruction plates of mandible were bridging two ends of the defect area to reinforce the resected mandible. To manually bend a ready-made reconstruction plate to make it fit the contour of the resected mandible takes time and efforts, whereas to order a custom-made reconstruction plate is expensive. The aim of this study was using three-dimensional finite element analysis to investigate the effect of defect location, defect extent and residual bone height on the stress distribution in resected mandible stress distribution and to investigate the effect of fracture prevention of the continuous reconstruction plate and the separate mini-plates. A basic solid model of mandible was built from CT image and imported into ABAQUS 6.13-2 software. The basic model was transformed into different test models which were designed according to (1) defect location (anterior, left premolar region and left molar region), (2) defect extent (34 mm and 48 mm), and (3) residual bone height (5.0, 7.5, 10.0, 12.5, and 15.0 mm). A continuous reconstruction plate or two separate mini-plates were fixed to one of the resected mandibles (molar defect with 48 mm extent and 5 mm residual bone height) with eight screws. In the mandible model, cancellous bone part and screw parts were meshed with ten-node tetrahedral elements, and cortical bone part and plate parts were meshed with three-node triangular shell elements. The solutions were performed by ABAQUS 6.13-2 software. The study includes three parts. Part I. The finite element model was verified by comparing the volume of mandibular flexure between bilateral first premolars with data in the literature when the mandible was under the conditions of maximum mouth opening and protrusion. Part II. The maximum tensile strain and compressive strain were evaluated in different defect patterns when the mandible was under the conditions of incisor biting and right molar biting. Thresholds of 3000 με and 4000 με for tension and compression sites respectively were used to evaluate the fracture risk of the resected mandibles. Part III. The maximum tensile strain and compressive strain of the molar defect with extent of 48 mm and residual bone height of 5 mm reinforced with a continuous reconstruction plate or two separate mini-plates was evaluated. Thresholds of 3000 με and 4000 με for tension and compression sites respectively were used to evaluate the effect of fracture prevention.
Results: (1) When the mandible was under the conditions of maximum mouth opening and protrusion, the upper border of the bilateral mandibular bodies came close to each other. The amount of closure was the largest between two condyle heads. While observing the lower border, the bilateral mandibular bodies became far from each other at the anterior region and close to each other at the posterior region. The amount of closure due to mandibular flexure over bilateral first premolars was 6.3 μm. The data coincided to the literature. (2) The maximum tensile strain was higher but the maximum compressive strain was lower during right molar biting than incisal biting. The wider the defect extent or the less the remained bone height, the higher the strain. To prevent microdamages and reduce the risk of fracture, the maximum tensile strain and the maximum compressive strain of both biting conditions should be considered. The suggested residual bone height was 12.5 mm for anterior region and 15.0 mm for premolar and molar regions at least to prevent microfracture, regardless of the defect extent. (3) Resected mandibles reinforced with plates showed lower value of maximum tensile strain and maximum compressive strain. The thicker the plates were designed, the more the strain value decreased. The maximum compressive strain was decreased to less than 4000 με with either type of the plate reinforcement. The maximum tensile strain was lower when the mandible was reinforced with two separate mini-plates than with a continuous reconstruction plate. However, even if the resected mandible was reinforced with 3 mm-thick separate mini-plates, the maximum tensile strain was higher than 3000 με persistently.
The study suggested that the fracture risk of mandible with marginal mandibulectomy was related to the defect location, defect extent and residual bone height. And even if reinforced with plates lower the strain of mandible with marginal mandibulectomy, fracture cannot be effectively prevented.
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