以光聚合複合樹脂進行牙科窩洞修復，已蔚為復形材料主流。在光照(photo-activation)後，樹脂單體產生聚合反應，產生體積聚合收縮、填補介面間的收縮應力(contraction stress)等現象。這些變化會造成牙體結構的變形可能導致急遽斷裂；殘應力也可能累積而造成黏著界面破壞。一般認為改變窩洞外型、改變聚合時間、應用低彈性模數襯底材(lining material)等方式，可有效減少樹脂聚合收縮及咬合時之種種應力。這些假說有待驗證。
樹脂聚合收縮所造成的種種術後問題，於臨床上始終無法有效改善。因此本研究以複合樹脂進行臼齒二級窩洞復形治療為樣本。研究目的為探討不同幾何型態窩洞中的複合樹脂填補，於樹脂聚合收縮過程所導致的生物力學影響，並探討使用襯底材(lining material)於各種洞中，以降低收縮應力的可能性。研究將以材料性質量測、牙齒樣本實驗、有限元素模擬三方面進行。
研究第一部份探討樹脂於光聚合後的材料性質測定。光聚合樹脂機械性質如彈性模數、不同深度之聚合程度等為測量目標。由於牙科樹脂填補時，光源於上方照射，因此於不同的照射深度也會有不同的聚合程度(degree of conversion)。一般文獻中，決定樹脂於填補內部的機械性質，通常以微小硬度對應聚合程度以計算出彈性模數。本研究以奈米硬度測試儀測量樹脂於不同聚合深度的彈性模數。這兩項測量值比較後，驗證以硬度計算彈性模數的可靠性，並用於後續的有限元素分析材料性質設定上。結果顯示，40秒照射時間所得的微小與奈米硬度、聚合深度都大於20秒照射；而兩段式聚合與標準聚合所得的聚合程度相近。於本實驗中，微小硬度與彈性模數的結果於測量範圍內相近，因此可被應用於後續有限元素分析材料性質的決定。
實驗部分，以真實的臼齒為樣本，探討不同填補技術對於填補邊緣品質與復形牙齒的影響。在不同襯底材料的影響方面，研究結果顯示加上流動性樹脂襯底並無法有效降低齒頸部邊緣微滲漏的改善；使用玻璃離子黏著劑則可改善較深窩洞於牙本質邊緣的密封性。進一步的探討則發現使用較厚的流動性樹脂襯底，於冷熱循環後，邊緣品質受影響更為顯著。本研究並利用數位影像相關變形量測方式測試樹脂填補時之齒質形變，並以此為有限元素模擬觀測之依據。以數位影像相關法所測得的結果，樹脂填補在不同窩洞外型條件中，形變的大小與形式均不同。相較於以往文獻中單純以C-factor定義樹脂所受的限制條件，結果中顯示，樹脂充填量、殘留齒質的韌度均影響牙齒與樹脂的變形量。
而有限元素模擬，是以真實臼齒建立幾何外型，以探討不同窩洞外型中收縮應力的分佈。模擬是利用量測之樹脂收縮性質，建立對等的有限元素模組，並以數位影像相關法實驗所得結果進行比對與修正。研究首先探討如何調整不同變數以符合實驗中所得的牙齒變形量，在包括材料性質與網格化等條件確立後，最終針對窩洞型態、襯底材料等不同影響因子分析，進行在不同二級近遠心窩洞模組的模擬。結果顯示窩洞內部的底部為高主應力承受區。評估以低彈性模數材料襯底之模組，則發現填補與襯底材料的收縮率，對於聚合收縮應力有極大影響。而這些分析結果所得窩洞底部變形率，與數位影像相關法所得有一致性。
本研究透過生物力學的分析研究，探討複合樹脂填補聚合收縮與界面應力的產生之破壞機轉。經研究發現治療後所產生之剩餘齒質變形與微滲漏等問題，被樹脂充填量、樹脂收縮比率、窩洞幾何外型、剩餘齒質強度等多重因子所影響。本研究結合數值分析與實驗驗證，結果可提供臨床上探討複合樹脂或復形技術有利的參考資料。

The light cured resin composites have been widely used in the restorative dentistry. The composite materials polymerize through the photo-activation process, the subsequent polymerization shrinkage is one of the most critical defects that directs to some problems. In a restored cavity, this shrinkage and contraction stress may provoke contraction stress and result in the deflection of bonded tooth structures or the disruption of resin-tooth bonding. Some restorative techniques including modification of cavity configuration, changing curing time, use of materials with low elastic modulus under the composites, were believed to alleviate the contraction stress. These postulation need to be verified.
The clinical consequences of polymerization shrinkage constitute the main reasons for replacement of resin composite restorations. The objective of this study was to investigate the development and distribution of composite contraction stress in a Class II cavity, with specific aim to examine the roles of cavity configuration and lining materials in reducing this stress. By integrating experimental approaches and finite element analysis, the biomechanics of composite polymerization was explored.
The primary part was to collect the material properties including Young’s modulus and degree of conversion of a composite material, which are required in the following finite element (FE) models. The polymerized composite in the cavity is not homogeneous since the halogen lamp only irradiates on the top surface. This study attempted to determine the local Young’s modulus using nanoindentation tester. The obtained local Young’s modulus for different curing depth was compared with microhardness to verify the correlation of hardness and degree of conversion. Results showed that 40 sec curing resins exhibited greater micro- and nanohardness values for all the measured curing depths and also the effective curing depth than 20 sec curing. There was no significant difference between the standard and 2-step curing modes. The obtained microhardness values and Young’s modulus were generally consistent in the measured regions.
With laboratory experimental approaches, composite restorations in human molars were accomplished by different techniques and their polymerization consequences were inspected. With a study comparing the composite restorations with various lining materials, flowable composite lining did not showed reduce the cervical microleakage. Use of glass ionomer lining showed better marginal sealing especially in deep cavities. A subsequent study also showed that a thick flowable composite lining contributed to reduced marginal integrity after the thermocycling test. The cusp deformation generated from polymerization shrinkage was measured by digital-image-correlation (DIC) method. The displacement measurement showed that the composite shrinkage patterns were not solely determined by previously mentioned C-factor, but also the volume of composite materials and the stiffness of the remained cusps.
FE simulation corresponding to the real human molar was performed to analyze the stress distribution in different cavity geometries. After the primary results are verified with data from the experiment approaches, the FE model was adjusted to validate the contribution of multiple variables in a Class II cavity. Using the FE models, the models showed the stress distribution in cavities of different configurations. The stress analysis showed that the high stress was located inside the deep portion of the cavity surfaces. The assessment of models with lining materials showed that the shrinkage rate of restorative and lining materials cast a significant influence on the magnitude of contraction stress. The displacements in the cavity floor from the analytic results were partly in consistence with the DIC measurement.
In conclusion, the current study investigated the polymerization shrinkage and contraction stress provoked by composite restorations through the biomechanical experiments and analysis. The development of remained tooth deformation and microleakage was affected by multiple factors including the volume of resin composite, the polymerization shrinkage rate, the cavity configuration, and the remained cusp stiffness. This“hybrid numerical– experimental” approach provides valuable information to find the most favorable restorative technique in various clinical conditions.

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