Cariology and Operative Dentistry, Department of Restorative Science, Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan.
Dent Mater. 2011 Nov;27(11):1070-8. doi: 10.1016/j.dental.2011.07.008. Epub 2011 Aug 5.
Light-cured composites undergo shrinkage during polymerization. The aim of this study was to evaluate regional shrinkage within a light-cured composite during polymerization by microcomputed tomography and mechanical properties by nanoindentation in bonded or non-bonded class-I cavity.
Zirconium oxide spherical fillers (30 μm diameter) were added as markers to a composite resin, filled into a box-shaped class I cavity with or without a bonding agent. The marker fillers were traced in 3D scans obtained by micro-CT before and after polymerization using a software (TRI/3D-BON). The average hardness of the resin composites determined by nanoindentation at each 250 μm depth was plotted against depth.
In the bonded cavity, the filler particles at the top region moved toward the bottom of cavity, but at deeper depths, the direction of vertical movement changed toward the top of cavity (irradiated surface). A significant linear regression was found between filler displacement and composite depth (R(2)=0.9761). In the unbounded cavity, all the fillers moved toward the light curing source, and a significant power-law regression was found between filler displacement and composite depth (R(2)=0.849). In both groups, the data scattering increased at regions deeper than 3.5 mm, where the hardness, representing degree of conversion of composite, significantly decreased compared to the surface region.
The magnitude and direction of regional polymerization shrinkage depends on boundary conditions, depth and conversion degree. Polymerization shrinkage effect is most significant at the deepest part of the cavity. The application of micro-CT combined with sophisticated image analysis is a novel approach to investigate shrinkage mechanisms of dental composites.
光固化复合材料在聚合过程中会发生收缩。本研究旨在通过微计算机断层扫描评估聚合过程中光固化复合材料内部的区域收缩，并通过纳米压痕测试评估在有/无粘结剂的 I 类腔中粘结的复合材料的机械性能。
在复合树脂中添加氧化锆球形填料（直径 30μm）作为标记物，用粘结剂或不用粘结剂将标记物填入盒状 I 类腔中。使用软件（TRI/3D-BON）在聚合前后通过微 CT 获得 3D 扫描，追踪标记物的位置。通过纳米压痕在每个 250μm 深度处测定树脂复合材料的平均硬度，并绘制深度图。
在有粘结剂的腔中，顶部区域的填料颗粒向腔底部移动，但在更深的深度，垂直移动的方向向腔顶（照射面）改变。填料位移与复合材料深度之间存在显著的线性回归关系（R(2)=0.9761）。在无粘结剂腔中，所有填料都向光固化源移动，填料位移与复合材料深度之间存在显著的幂律回归关系（R(2)=0.849）。在这两组中，在深度超过 3.5mm 的区域，数据分散度增加，与表面区域相比，复合材料的硬度（代表复合材料转化率）显著降低。
区域聚合收缩的大小和方向取决于边界条件、深度和转化率。聚合收缩的影响在腔的最深处最大。将微 CT 与复杂的图像分析相结合是研究牙科复合材料收缩机制的一种新方法。
