Measuring Full-field Polymerization Shrinkage of Dental Composites using Image Correlation

Objectives: The aim of this study was to measure the full-field polymerization shrinkage of dental composites using the optical image correlation method.

Methods: Digital Image Correlation (DIC) was employed to measure the polymerization shrinkage of a commercial dental composite along the curing depth. A stainless steel block with a rectangular slot (4mm ×2mm) was used as a mould to prepare the specimens. The bar-shaped specimens, which stayed in the mould during testing, were light-cured from one end. Two different light intensities, 450mw/cm2 and 180mw/cm2, were used to examine their influence on material shrinkage. The deformations of the specimens were recorded by a CCD camera for 30 minutes after curing at a frequency of 0.1Hz. The images were then analyzed with specialist software and the volumetric shrinkage determined along the curing depth.

Results: The shrinkage strain field was highly non-uniform even at constant depths, indicating possible heterogeneity in material composition and shrinkage behavior in the composite. The maximum volumetric shrinkage strain of ~1.5% occurred at a subsurface distance of ~1mm, instead of at the irradiation surface. The depth of cure was about 5mm where a turning point in the shrinkage curve was clearly observed. Using an irradiation intensity of 180mW/cm2 resulted in only slightly less polymerization shrinkage than using an irradiation intensity of 450mW/cm2; the difference being statistically insignificant (p > 0.05).

Conclusions: DIC is a non-contact optical method that can be used to measure the full-field shrinkage of dental composites. The shrinkage strain field was highly non-uniform, with the maximum volumetric shrinkage occurring subsurface instead of at the irradiation surface. Using higher light intensity did not cause much more increase in the overall shrinkage and depth of penetration.