下顎骨大範圍邊緣切除術後之骨折預防效能--有限元素分析

Abstract

臨床上，下顎骨邊緣切除術與部分切除術一直是以10mm的殘餘骨高為分界，但此分界點僅立基於一個體外的乾下顎骨實驗。施行邊緣切除術後的下顎骨若以金屬板加強，通常使用由切除區前到後連續的鈦金屬板，但鈦金屬板的彎折費力且費時、客製化又昂貴。本研究之目的在以三維有限元素模型分析下顎骨大範圍邊緣切除術後殘餘骨高度對術後下顎骨應變分佈的影響，並且探討以連續或不連續金屬板對術後下顎骨加強以防止骨折的效果。本實驗將電腦斷層掃描影像輸入ABAQUS 6.13-2建立下顎骨模型，選擇術後所受應變最大的大臼齒區大範圍（48mm）切除區作為實驗對象，在模型上切割出八種殘餘下顎骨高（22.5mm、20.0mm、17.5mm、15.0mm、12.5mm、10.0mm、7.5mm、5.0mm），並且裝設不同厚度（1.5mm、2.0mm、2.5mm、3.0mm、3.5mm、4.0mm、4.5mm、5.0mm）的連續或不連續金屬板與螺絲。將下顎骨模型的海綿骨與螺絲以十節點之四面體元素、皮質骨與金屬板以三節點之三角形殼元素網格化之後，以ABAQUS 6.13-2求解。研究分為三個部分：第一部分將最大開口時第一小臼齒位置的下顎骨彎曲量與文獻做比較，以探討此下顎骨模型及材料特性、邊界條件、荷載等參數設定之合理性 ；第二部分為分析在右大臼齒咬合時，左側大臼齒大範圍切除區所受之最大拉應變與最大壓應變的位置及數值大小，並分別以3000 microstrain與4000 microstrain的應變門檻探討其骨折風險；第三部分承接第二部分，在同樣的切除區及咬合型態，分析裝設連續或不連續金屬板與螺絲的術後下顎骨最大拉應變與最大壓應變的位置及數值大小，並分別以3000 microstrain與4000 microstrain的應變門檻探討其預防下顎骨骨折的效果。 結果：(1)在最大開口時，兩側下顎骨體上緣互相靠近，越往髁頭處靠近的量越多；下緣則是在前面先擴張，往後再逐漸靠近。第一小臼齒位置的下顎骨彎曲量為2.2μm，與文獻數值比較結果在合理的範圍，確認此有限元素模型之設定合理。(2)右大臼齒咬合時，左側大臼齒大範圍切除區若殘餘骨高越少則應變值越大，所有組別皆無法避免骨微損傷出現而開始有骨折風險。(3)裝設金屬板可以將切除區的最大拉應變及最大壓應變值均降低，金屬板越厚，應變值降低的量越多。裝設任一種形式的金屬板後，最大壓應變值均降低到4000 microstrain以下。在骨脊越低的情況下，裝設不連續金屬板比起連續金屬板可使最大拉應變值降低更多，但即使將不連續金屬板的厚度增加到5.0mm，仍無法使最大拉應變值降低到3000 microstrain以下。 本研究顯示：下顎骨大範圍邊緣切除術後，無論殘餘下顎骨脊高度為何，皆會有骨微損傷產生。裝設金屬板與螺絲可強化並使邊緣切除術後下顎骨的應變值降低，但仍無法預防骨折。

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 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. The aim of this study was using three-dimensional finite element analysis to investigate the stain distribution on mandible after extensive marginal mandibulectomy 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 experimental models were set as left extensive resected area (48mm) under the occlusal scheme of right molar biting. Eight groups of residual bone height (5.0, 7.5, 10.0, 12.5, 15.0,17.5,20.0 and 15.0 mm) were investigated. A continuous reconstruction plate or two separate mini-plates were fixed to one of the resected mandibles 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 while maximum mouth opening. Part II. The maximum tensile strain and compressive strain were evaluated on extensive defect during molar biting on the contralateral side. 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 defect site which was reinforced with a continuous reconstruction plate or two separate mini-plates were 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) When the mandible was under the conditions of maximum mouth opening, 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 2.2 μm. The data coincided to the literature. (2) While the residual ridge height was lesser, the stain became larger on the left extensive defect site during right molar biting. The risk of bone fracture couldn’t be avoided in all groups. (3) The maximum tensile and compressive strain decreased after plate reinforcement. The thicker the plate was, the better the effectiveness. On both continuous and separated plate design, maximum compressive strain decreased beyond 4000 microstrain. If the ridge height was limited (5.0mm and 7.5mm ), separated plate performed better than continuous plate on maximum tensile strain. However, the maximum tensile strain was higher than 3000 microstrain persistently. The study suggested that after extensive marginal mandibulectomy, the bone fracture risk existed no matter what the residual ridge height was. Plate reinforcement decreased the strain, but fracture cannot be effectively prevented.