| Summary: | 碩士 === 國立臺灣大學 === 臨床牙醫學研究所 === 107 === Compared to segmental mandibulectomy, marginal mandibulectomy keeps the continuity of mandible after tumor resection, and then gets better esthetic and functional outcomes. Clinically, many surgeons follow the “ 10 mm rule” to decide to perform marginal mandibulectomy or segmental mandibulectomy. However, this rule was based on the result of Barttlebort’s in vitro study, performing on a dry mandible with two condyle heads fixed in the cement. Traditionally, reconstruction plates of mandible are bridging two ends of the defect area to reinforce the resected mandible. It takes time and efforts to manually bend a ready-made reconstruction plate to make it fit the contour of the resected mandible, whereas it’s expensive to order a custom-made reconstruction plate. Although the mandibles were bridged with reconstruction plates, there were still some fractures occur after surgery. Therefore, the aim of this study was using numerical analysis to investigate the strain distribution on mandible after extensive marginal mandibulectomy and to investigate the effect of fracture prevention of different cutting angle and different reconstruction plates. And the risk of fracture in segmental mandibulectomy patients repaired with fibular free flap was also investigated.
A basic solid model of mandible was built from CT image and imported into ABAQUS 6.14-1 software. The experimental models were set as left extensive resected area (48mm) under the occlusal scheme of right molar biting. Four groups of residual bone height (7.5, 10.0, 12.5, and 15.0 mm) were investigated. 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 6.14-1 software. The study includes three parts.
Part I. Left extensive marginal resected defect under right molar biting. The cutting line angle with higher tensile strain was designed as right angle or curved. The differences of maximum tensile strain and compressive strain in these two designs were evaluated. Thresholds of 3000 microstrain and 4000 microstrain for tension and compression sites respectively were used to evaluate the fracture risk of the resected mandibles.
Part II. In the same situation as part I, we investigate the reinforcement effects of three types of reconstruction plates: I shape, L shape, and T shape. Thresholds of 3000 microstrain and 4000 microstrain for tension and compression sites respectively were used to evaluate the fracture risk of the resected mandibles.
Part III. The maximum tensile and compressive strain on extensive segmental defect site, which was restored with fibular free flap, was evaluated. Thresholds of 3000 microstrain and 4000 microstrain for tension and compression sites respectively were used to evaluate the effect of fracture prevention.
Results: (1) Comparing to right cutting angle, curved cutting angle can reduce MCS and MTS. While the residual bone height was lesser, the strain became larger on the left extensive defect site during right molar biting. The risk of bone fracture couldn’t be avoided in all groups. (2) Curved cutting angle combined with reconstruction plate reinforcement can decrease MCS and MTS effectively, especially in the thinner residual bone height group. (3) If the residual bone height was limited (less than 15 mm), each plate design performed better than no reinforcement group on MTS and MCS. However, the maximum tensile strain was higher than 3000 microstrain persistently. (4) There is no difference in using two screw fixation or three screw fixation. (5) Segmental mandibulectomy patients restored with fibular free flap still have risk of bone fracture.
The study suggested that after extensive marginal mandibulectomy, the bone fracture risk existed in the residual ridge height less than 15 mm. Curved cutting angle and plate reinforcement is necessary to decrease the strain, but fracture cannot be effectively prevented. The bone fracture risk still existed in segmental mandibulectomy patient repair with fibular free flap.
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