Validity of OCT in Detecting Shrinkage Behavior of Composite Restorations

Objectives: The polymerization shrinkage of composite resin has been investigated by microscopic and finite element analysis, but intrinsic shrinkage behaviors were not revealed yet. In previous studies, the optical coherence tomography (OCT) images facilitated the recognition of structural detail in dental tissues. This investigation was aimed to examine the practicability of OCT in examine the shrinkage behaviors of resin composites.

Methods: In this study, the OCT images of composite restorations were analyzed in combination of a digital image correlation method to examine the polymerization shrinkage. Four composites were prepared with Filtek Supreme-Ultra flowable composites (3M ESPE) as the base materials, or mixing with either medium-size (75 µm) glass beads, large-size (150~212 µm) glass beads, or zirconia particles (75-150 µm) at 20 wt%. Each composite was filled in a tapered cylindrical cavity on a labial surface of incisors, and OCT scanned the restoration to obtain the image. These images were moved digitally to different directions and distances, and the displacements were analyzed by a DIC software to assess the effects of different speckle patterns. These materials were also filled into the cavities and polymerized at unbounded states. Series OCT images of these composite were also recorded during light curing for the analysis of shrinkage behaviors.
Results: The deformed images of four composites showed high correlations with the original images. The calculated displacements by DIC accurately matched with the moving distances and no difference was found between composites with/without different particles. The setting of subset size affects the accuracy of measured displacement, and should exceed the inter-particle distance to get the DIC results. The large glass beads caused high noises to interfere the image recognition.
Conclusions: A new technique set up by combining OCT and DIC was proposed to investigate composite shrinkage non-destructively. Further validation is required to improve the accuracy.