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Chiang, Yu-Chih (2009): Polymerization Shrinkage with Light-Initiated Dental Composites. Dissertation, LMU München: Faculty of Medicine
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Abstract

The present work addressed the determination and visualization of the direction and extent of polymerization shrinkage in the light-initiated composite. Hypotheses about the light-cured composite contraction patterns are controversial. With high resolution µCT images, the displacement vector fields are examined and calculated two-dimensionally via an elastic registration algorithm using vector-spline regularization and three-dimensionally with a local rigid registration (block matching) following images segmentation (corresponding traceable fillers in composite). It appears that the light-initiated resin composites do not always shrink toward the light source. Two major contraction patterns were observed: either shrink toward the top-surface (free surface), or toward one side of the cavity wall, in which the bonding was stronger or remained intact. With the proposed methods, it is possible to describe the contraction patterns in great detail. We could demonstrate that the bonding quality to the tooth affects the material movement more than described so far. In addition, the geometry of the cavity also acts as a factor. The continuation of the studies into the interaction of tooth-adhesive-composite indicated the shortcomings and limitations of the current FEA simulation studies. This meant that the assumption of FEA, especially in adhesive systems (i.e., bonding situations and hybridizations), is too perfect and simplificative to interpret the real condition in clinical. The qualitative and quantitative analysis of the shrinkage vector field along with the µCT datasets supply more insight into the shrinkage behavior in real teeth with all their variations of the boundary conditions than with any currently available method. This new approach has the potential to reevaluate and hopefully unify all the currently available hypotheses concerning the extent and orientation of polymerization shrinkage.