下顎第一大臼齒根叉侵犯前後之應力分布變化—有限元素法

大臼齒由於其所處牙弓之位置與解剖型態之關係，是口腔中最易發生牙周病之牙齒。由於是多根牙根，一旦牙周病侵犯，在牙根分叉處，很難長期維持此處之清潔，故有極大比例之臼齒在早期常因牙周病而喪失。而牙根切除之手術治療，長期追蹤之結果，成功率並不高。經治療後失敗的原因不外乎蛀牙、牙周破壞、根管治療失敗及牙根斷裂，當中除了根管治療失敗的原因之外，其餘失敗的原因均與應力的發生有關。本研究利用電腦輔助分析配合電阻應變實驗驗證的方式，在不同齒槽骨高度，以及不同方向受力負荷狀況下，對根叉受侵犯齒與齒槽骨應力分布之影響，進行探討，以供臨床診斷與治療參考之用。實驗模擬下顎第一大臼齒及第二小臼齒在牙釉牙骨質交界下 2 mm之齒槽骨高度為樣本，樣本經由鑽針修磨、牙髓組織樹脂複製及掃描後，經影像處理製成三維有限元素模型，並由電阻應變實驗分析印證有效性之後進行有限元素分析，施力設於下顎第二小臼齒之頰側咬頭與第一大臼齒之近心頰側咬頭與頰側咬頭，方向計有垂直咬合平面(P1)、頰側45°(P2)、舌側45°(P3)三個不同施力方向。本研究針對六種不同齒槽骨高度設計形式在三種方向咬合力下對根叉受侵犯齒與齒槽骨之應力分布情形來加以探討。結果顯示，受咬合力時，齒頸部之齒槽嵴與牙齒均有高應力集中現象，隨著齒槽骨高度降低，應力亦往根尖方向移動，正常齒槽骨高度之壓應力值、張應力值及von Mises值，均比齒槽骨下降後之應力值低。其中，施力方向為P2及P3時，在牙齒與齒槽骨上所得到的主應力及von Mises應力明顯比方向為P1時高，故根叉受侵犯齒應盡量避免側向力。本實驗結論是根叉受侵犯之牙齒在模型CEJ(5)、模型CEJ(6)處，即齒槽骨破壞達26 ~ 35 % (根叉下1~2 mm)時，為應力與形變量大輻增加之起始點，亦為臨床治療關鍵處。就齒槽骨觀點而言，張應力多出現在第一大臼齒牙根周圍處，一旦根叉受侵犯後，則分叉處之張應力更加集中於第一大臼齒近心根周圍齒槽骨處，代表一旦根叉受侵犯後，牙周破壞可能會從近心根周圍齒槽骨先破壞，若再加上清潔不佳，則破壞會更形加速。就牙齒而言，必須要盡量減少不必要的側方力量，以得到較佳的臨床結果。關鍵詞：根叉受侵犯，齒槽骨，有限元素法，應力分析，電阻應變實驗

關鍵字

電阻應變實驗；應力分析；有限元素法；根叉受侵犯；齒槽骨

並列摘要

Abstract Molars are the most frequently periodontal invasion in the oral cavity due to arch position and anatomical factors. Because molar has multiple roots,it is difficult to maintain cleansing at furcation area long-term when periodontal invasion. Molars had missing loss early due to periodontal disease. The success rate of root resection is not high after the results of long-term follow-up.The reasons of failure are caries, periodontal destruction, endodontic failure and root fracture. Exception endodontic failure, they are associated with stress. The purpose of this study was analysis the stress of furcation involved molars at different alveolar bone height during various load (P1, P2, P3) by CAD/CAE and experimental strain measurem- -ent for validity test. Furthermore, attempt to provide the clinical diagnosis and reference to treatment. The sample simulated the mandibular first molar and second premolar when the alveolar bone height below 2 mm of CEJ. The three-dimensional finite element models were constructed through imaging processing after tooth preparation, resin casting of pulp tissue and scaning. After confirmation of effectiveness through the analysis of strain gauge technique, finite element analysis has been used. Force was applied to the mesiobuccal cusp and buccal cusp of the lower first molar and buccal cusp of second premolar, with the force being applied in three different directions: directly perpendicular to the occlusal plane (P1), buccal 45°(P2), and lingual 45°(P3).This study analysis the stress distribution of tooth and alveolar bone when furcation involvement according to six alveolar bone height designs under three direction of occlusal loads. The results show that high stress is concentrated around the cervical region of alveolar ridge and tooth structure during mastication. As alveolar bone height reduced, the stress moved toward apex and the principal stress and von Mises of tooth and alveolar bone has been raised. In particular, when the direction of pressure was P2 and P3, the principal stress and the von Mises stress which resulted at the alveolar bone and the tooth was significantly higher than P1; this means that it is avoided lateral force when furcation involvement. The conclusion of this study is the great increasing point of stress and deformation at model CEJ(5) and model CEJ(6) (ie, the destruction of the alveolar bone is reached to 26 ~ 35 % and the position is 1 ~ 2 mm below furcation area. It is the key point of clinical treatment. For the view of alveolar bone, the tensile stress was observed around the root of the first molar. Once furcation involvement , the compressive stress was more concerated around the alveolar bone of mesial root of first molar. This means the periodontal destruction begins from the alveolar bone of mesial root of first molar when furcation involvement. The destruction is more accelerated when poor oral hygiene. For tooth structure, it should be decrease the unnecessarily lateral force as possible. Keywords: furcation involvement, alveolar bone, finite element method, stress analysis, strain gauge technique.