數位影像相關法(DIC)為非接觸式量測方法。搭配高放大倍率之取像系統與次像數內插演算法，可以以此精確度測微小變形。數位影像相關法目前已被應用量測牙科複合樹脂的收縮行為。複合樹脂在光激發固化的過程中會有聚合收縮現象，周圍牙齒也會受到收縮的影響而產生所謂的收縮應力，甚至造成填補脫落或填補介面的微滲漏。為了探討不同光照方式對複合樹脂聚合收縮的影響，本研究目的分為光照的強度以及光照方向，以三種方式分析複合樹脂之聚合收縮: DIC量測聚合收縮變形、有限元素法分析收縮應力、micro-CT 檢測牙齒與樹脂介面的損壞。
在光照強度的實驗中，分別以固定的光強度(500 mW/cm2)與步階光強度(100-600 mW/cm2)做比較；光照方向方面則是比較90度與45度。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，藉由DIC的程式去計算出任一時間下收縮所產生的變形，同時定義收縮中心，由收縮中心位置的改變探討光照對整體收縮變形的影響。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 分析結果可知，在受到黏著固定的影響下，收縮方向依然為向心收縮。然而，收縮中心會較無黏著狀態偏向窩洞底部。步階光強度照光方式會使聚合反應集中在表層，導致收縮中心位置比固定光強度照光方式來得高。而45度照光方向會使得收縮中心偏移至靠近光源的位置，並改變全場的變形分布。在模擬的結果中，步階光強度的光照方式明顯降低牙齒在窩洞兩旁的收縮應力。而45度照光導致窩洞兩側受到應力差異變大，顯示收縮方向的變形改變會影響應力分布。與固定的光強度相比，步階光強度的光照所造成的裂縫面積較小，且發生裂縫區域與有限元素所得之高應力區吻合。
臨床上在操作照光固化時，需要考慮照光強度與照光方向的影響。而步階光照強度的方式將能降低牙齒所受到的收縮應力，以及改善微滲漏的現象。數位影像相關法在未來可繼續用來研究不同因素對聚合收縮的影響。

Digital image correlation (DIC) is a non-contact measurement method. With a high magnification of acquisition system and the algorithms of subpixel-interpolation, it measures the minor displacement of an object with great accuracy. This technique has been applied to the shrinkage behavior of dental composite restorations. The polymerization shrinkage of the composite occurs in the process of light-activated curing, and the tooth surrounding the resin would be pulled to cause the contraction stress or microleakage. The purpose of this study was to investigate the effects of light intensity and the light direction on the polymerization shrinkage of composite resin by three approaches: using DIC to measure the shrinkage, finite element analysis (FEA) for contraction stress, and examination of the microleakage by micro-CT.
In the experiment about light intensity, the constant intensity (500mW/cm2) and stepped intensity (100-600mW/cm2) were compared. In the light direction experiment, the 90 degree light direction and 45 degree light direction were compared. Each tooth was separately restored in nonbonding and bonding conditions. During the 40s light-curing, the shrinkage was recorded by continuous images. The DIC program was used to calculate the deformation, and defined the shrinkage center. From the position of shrinkage center changed, the effect of light on the full-field deformation was investigated. The FEA used in the analysis of contraction stress simulated the polymerization shrinkage by a thermal expansion mode. By micro-CT scanning, the 3D tooth model was constructed and used to simulate the different temperatures to obtain the contraction stress by the heat transfer. Finally, the restored and cured teeth were scanned by micro-CT to get the cross-section image, and the interfacial microleakage was determined from the images.
From the result of DIC, the shrinkage started from the surface which was irradiated with light. Under the bonding condition, the shrinkage pattern was still centrifugal. However, the shrinkage center was more closed to the cavity floor compared to nonbonding condition. The stepped light intensity caused polymerization to focus on the top surface and led to higher position of shrinkage center than the constant light intensity. The 45 degree light direction caused the shrinkage center close the light source and the shrinkage deformation was affected.
The simulation result indicated the stepped light intensity reduced the stress in the cavity walls and 45 degree light direction led to the different stresses between two cavity walls. Comparing with the constant intensity, the stepped intensity reduced the microleakage in the cavity wall and the area of debonding is smaller.
In clinical operation of light-curing, the effect of light intensity and direction need to be consider. The stepped intensity technique can be used for reducing the contraction stress in the tooth and the mcrioleakage. In the future, the DIC can continue the investigation of different factors in polymerization shrinkage.

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