咀嚼肌肌力對下顎骨切除術後之殘餘下顎骨應力分佈之影響

Abstract

臨床上，部分下顎骨切除術(partial mandibulectomy)中的邊緣下顎骨切除術(marginal mandibulectomy)與整段式下顎骨切除術(segmental mandibulectomy)一直是以10 mm的剩餘骨脊高度為選擇術式的判斷標準，但以10 mm為判斷標準的法則是建基於一個體外的乾下顎骨實驗[6]。在下顎骨進行邊緣切除手術後，會造成應力集中在缺損處造成發生骨折風險提高，然而在手術區附近的咀嚼肌可能也會受到手術的影響，導致施加在下顎骨的力量可能會減少，進而使應變分布產生改變，影響我們對於下顎骨切除手術後風險的評估。 因此本研究分為兩個部分，第一個部分希望利用三維有限元素模型分析下顎骨大範圍邊緣切除術後，模擬咀嚼肌被部分移除之後咀嚼肌力量減少的情況，觀察下顎骨缺損處應變分佈的變化，以評估術後的骨折風險。第一部分實驗將電腦斷層掃描影像輸入ABAQUS/CAE 6.13-1建立下顎骨模型，選擇術後所受應變量最大的左側大臼齒區大範圍(48 mm)切除區作為實驗對象，在模型上切出三種殘餘下顎骨脊高度(12.5mm、10.0mm、7.5mm)。缺損處附近的咀嚼肌共有四條，分別是：淺咬肌(superficial masseter muscle)、深咬肌(deep masseter muscle)、內翼肌(medial pterygoid muscle)與前顳肌(anterior temporalis muscle)。這四條肌肉分別設定施力荷載量為100%、90%、80%、70%、60%、50%、40%、30%、20%、10%，以表示不同程度的肌肉殘餘量。將下顎骨模型的海綿骨以十節點之四面體元素(C3D10)、皮質骨以三節點之三角形殼元素(S3R)網格化之後，探討在右側大臼齒咬合時，左側大臼齒大範圍邊緣切除區在不同程度肌肉力量施力時，所受之最大拉應變(MTS)與最大壓應變(MCS)的位置及數值大小，並分別以3000 microstrain(MTS)與4000 microstrain(MCS)的應變門檻探討其骨折風險。 第二個部分為臨床觀察手術術後對於咀嚼肌肌力的影響。由於文獻上對於接受過局部下顎骨切除術患者術後的咬合力變化的探討，大部分使用較傳統的應變計作為測試儀器，精準性與參考性不如近日新一代的測試系統，而且並沒有同時配合測試咬合力、肌電圖與咀嚼效率作為實際上咬合力與肌肉效能變化相互參考的研究。近年來數位咬合計的發展(如T-Scan、Prescale系統)可以讓患者在較為接近正常咬合狀態下測量咬合力量。因此第二部分實驗將採用無線傳輸肌電生理回饋系統監測7位受試者術後咬力，讓受試者以最大咬合力量(maximum bite force)進行咬合的動作，使用壓力感測片紀錄壓力系統(GC Dental Prescale II System)記錄咬點分布與咬點面積來量測咬合力大小，並同時搭配肌電圖(EMG)記錄肌肉活性(muscle activity)，最後使用義美寶吉果汁QQ軟糖測試咀嚼效率。藉由臨床觀察量測來比較術後有無假牙重建以及切除側和正常側的肌肉活性、咬力與咬點分布、咬點面積間以及咀嚼效率的關係。第二部分的實驗結果將有助於了解接受過局部下顎骨切除手術的患者，在術後的咀嚼肌咬合功能和肌電圖的變化，並依據此臨床測試結果驗證在有限元素分析模擬模型的受力設定，讓我們在探討相對應應力以及應變的影響時，更加符合臨床實際情況。 實驗二的實驗結果顯示在接受過下顎骨單側局部切除術且術後使用植體固定假牙重建之患者的最大咬合力量(maximum bite force)上，切除側相較於正常側(非切除側)具有顯著的下降，而是否使用植體重建假牙在切除側的最大咬合力量上無顯著差異，但咀嚼效率(chewing efficiency)在有植牙假牙重建時，與無植體假牙的情況相比有顯著的增進，在進行最大咬合力量時的咀嚼肌效能(muscle activity)方面，整體而言則沒有顯著的差異。在實驗一有限元素分析的模擬模型中得知在肌肉荷載下降的情況下，剩餘骨脊高度不足時依然有骨折的風險，因此即使在此實驗中實際觀察在切除區的最大咬合力量顯著低於正常側(非切除側)，但我們仍然不能低估在剩餘骨脊高度不足下的骨折風險的可能性。

Clinically, many surgeons follow the “ 10 mm rule” to decide to perform marginal mandibulectomy or segmental mandibulectomy. This rule was based on the result of Barttlebort’s in vitro study performed on a dry mandible with two condyle heads fixed in the cement. Strain concentration at the corner of the defect would lead to higher risk of mandibular bone fracture after marginal mandibulectomy. However, the surgery would also affect the activity of surrounding masticatory muscles, which may alter the strain distribution and affect the evaluation on the risk of post-surgical complication. This study contained two parts. The first part was aimed at using numerical analysis to investigate the effect of the reduced force of surrounding masticatory muscles on the strain distribution of the mandible after marginal mandibulectomy. A Digital Imaging and Communications in Medicine (DICOM) data file was created using cone beam computed tomography (CBCT) image data of the mandible of the patient with left side marginal mandibulectomy. A CAE software (ABAQUS/CAE 6.13-1) was used to reconstruct and analyze 3D mandibular models built up from the DICOM data file. Models were designed to test the effect of different remaining bone height on strain distribution under different functional loadings. Antero-posterior defect width was 48 mm to simulate the resected defect from the left first premolar to ascending ramus. Three different remaining bone heights of 7.5 mm, 10 mm and 12.5 mm were designed. Depending on the position closed to the defect area, four muscle groups could be affected during the operation: superficial masseter muscle, deep masseter muscle, medial pterygoid muscle and anterior temporalis muscle. We set the different proportion (100%, 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20% and 10% ) of muscle force of each muscle group to simulate the different degree of the amount of the remaining muscle. The models consisted 20930 ten-node tetrahedral elements (C3D10) in solid part and 81721 three-node triangular general-purpose shell (S3R) elements after meshing. Linear 3-dimensional FE analyses were performed. To evaluate mechanical strain on the mandible after marginal resection under right molar clenching motion, maximum principle tensile and compressive strains were calculated. Thresholds of 3000 microstrain and 4000 microstrain for tension and compression sites respectively were used to evaluate the fracture risk of the resected mandibles. The second part was aimed at evaluating the effect on muscle activity, biting force and chewing efficiency after the partial mandibulectomy. In previously literatures, traditional instruments which were less precise and poorly consistent compared to modern instruments were used to investigate the effect on masticatory system after the surgery. Moreover, there was no research on evaluating the combination of muscle activity, biting force and masticatory performance. Recently, modern digital occlusion analysis systems (T-Scan, Dental Prescale II) can evaluate occlusal force on almost normal intermaxillary relation. In this study, Teethan (BTS Bioengineering Corp., Italy) was applied to investigate the muscle activity .The subjects were tested with GC Dental Prescale II system for the maximum bite force and EMG for muscle activity at the same time. The subjects were also tested with gummy jelly to examine their chewing efficiency. Therefore, the results were investigated the relation in muscle activity, biting force and masticatory performance between normal side and defected side, as well as with or without the implant prosthesis reconstruction. The results were helpful for setting up the protocol and strategy of fine element analysis model for stress and strain analysis in partial mandibulectomy patients more similar to clinical condition. The results of the first part of the study showed the risk of the bone fracture exist even though the degree of the masticatory force decreased, if the remaining bone height was insufficient. The second part of the study presented the reduced amount of the maximum bite force(N) compared defect side to normal side under the implant-assisted prosthesis applicated condition was statistically different. The chewing efficiency obviously increased under the implant-assisted prosthesis applicated condition. But the maximum bite force(N) showed no statistical difference with or without the implant-assisted prosthesis over the defect side. About the muscle activity(EMG), neither the normal side compared to the defect side nor the condition with prosthesis compared to without prosthesis, no statistical difference was presented. According to our the study, the risk of the bone fracture after the marginal mandibulectomy cannot be underestimated even the force of the masticatory muscle was reduced. The remaining bone height of the mandible still played an important role on the evaluation of the risk of bone fracture even the load of the masticatory muscle was reduced.