牙科複合樹脂修補失敗的其中一個原因為光聚合所造成的收縮。複合樹脂在光激發固化的過程中會有聚合收縮現象，而造成周圍牙齒受到收縮的影響產生收縮應力，甚至造成樹脂脫落或填補介面的微滲漏。對於牙科第五級窩洞修復，其窩洞的幾何形狀和對樹脂非保留的窩洞空間以及位於牙釉質或牙本質邊緣間，複雜的幾何及位置不利於樹脂黏著。本研究目的是探討當牙科第五級窩洞填補在光照過程中其樹脂聚合收縮的機制，以及利用實驗及數值分析樹脂在光照結束後可能造成的收縮釋放。此外，亦探討窩洞形狀的影響以及不同樹脂對於光照收縮的影響。
在本研究中，使用六十顆完整的小臼齒並且將窩洞形狀分為: box-cavity 和V-cavity，接著進一步地將樹脂分三大類填補: Z350、LS、Z350 flowable以單獨的形式或混和的形式填補在牙齒上。Z350組為填補材料為Z350； Z350/F組為填補Z350 flowable墊在Z350的下面；LS組為填補材料為LS。在照光40秒的過程中，以連續取像的方式記錄下複合樹脂聚合收縮的影像，及記錄光聚合收縮一個小時內的影像，藉由DIC的程式去計算出收縮過程中和光照後一個小時內所產生的變形。在收縮應力分析方面，採用有限元素法，以熱膨脹的方式模擬樹脂聚合的體積收縮。以micro-CT掃描的方式建構出3D的牙齒模型，再依照光照不同的方式時所測得收縮條件，以熱傳方式下不同溫度的改變進行模擬可得收縮應力。最後再將填補且聚合完的牙齒經過 micro-CT的掃瞄得到牙齒的剖面圖，由影像去判斷是否有發生微滲漏的情形。
由DIC量測發現，其位移及應變隨時間增加。Box- cavity 的位移量大於V-cavity的位移量，而且其樹脂下端邊緣的位移量大於上端邊緣。在樹脂內，Z350表現較大的應變量及位移量. Z350/F 在下端有較小的位移量。LS 在應變量以及位移量皆是最小的除了V-cavity 在下端邊緣的位移。Z350 和 Z350/F 在內部邊緣皆有拉伸應變，但是LS 在內部有大的壓應變。在模擬結果中發現，box-cavity 的收縮應力大於 V-cavity 的收縮應力，樹脂靠近牙釉質有最大的應力。對於LS的應力表現，其下端邊緣應力大於上端，容易在下端邊緣造成樹脂黏著失效。在為滲漏的觀察下，發現到各組在上端邊緣的維滲漏是有差異的，box-cavity 的維滲漏現象相較於V-cavity 來的嚴重，特別是在Z350/F。LS 並沒有改善為邊緣的完整性。
從結果得知，使用不同複合樹脂以及窩洞的形狀在光聚合收縮行為是必須同時考量。Box-cavity 有著較好的窩洞依附空間但有較大的收縮。LS 雖然有低的收縮，但其較低的黏著強度並不一定能改善的收縮應力的表現。

One cause of dental composite restoration failure is the polymerization shrinkage. The contraction stress counteracts with the tooth-composite bonding, and causes marginal microleakage and debonding. In dental class V restorations, the non-retentive cavity geometry and complicated margin locations on enamel or dentin are especially unfavorable for bonding. This study is aimed to measure the shrinkage behaviors of class V composite restorations during polymerization, and its possible relaxation after polymerization using an experimental-numerical analysis. In addition, the effects of cavity form and composite materials on the shrinkage behaviors were investigated.
Sixty premolars were prepared with two cavity designs: a box-cavity and a V- cavity; and were further divided into three restorations: a nanocomposite Z350 filling (Z350); Z350 filling with a flowable composite lining (Z350/F); and a low-shrinkage composite filling (LS). After restorations, the specimens were irradiated for 40 seconds and their images were captured during- and post-irradiation for 1 hour. A digital-image-correlation (DIC) program Vic-2009 was used to calculate the full-field deformation. A corresponding finite element analysis (FEA) simulated the polymerization shrinkage by a thermal expansion module, and accordingly to analyze the stress on tooth and restorations. Finally, the microleakage of these restorations was examined.
From the shrinkage measurements, the displacements and strain increased with time. The displacements were greater in Box-cavities than in V-cavities, and greater on the bottom than on the top margins. For all locations, Z350 presented the greatest displacements and strains. Z350/F reduced the displacements on the bottoms. LS presented the least displacement and strain except on the bottom of V-cavities. Z350 and Z350/F showed tensile strain on the lateral margin, while LS showed a compressive strain. FEA showed that the stresses of box-cavities were greater than those in V-cavities. The maximum stress existed on the top enamel margin. The stress of LS in V-cavity was greater on the bottom than that on the top margin, which may increase the debonding. For the microleakage observation, there was difference among three restorations on the top margins, and box-restorations revealed more microleakage than those in V-restorations especially for Z350/F. LS did not improve marginal adaptations.
With these results, the effect of composite material and cavity form should be considered. A box-cavity is more retentive but shows a great shrinkage. LS reduced the shrinkage and stress but did not present an improved, which may be due to the low bond strength.

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