Impact of Contrast Ratio on Photocure Depth of Low-Stress Composites

Objectives: Successful photo-curing of dental composites depends on many factors including the composition and light source. Clinicians desire deeper curing preparations in order to reduce steps and save time.  Photo-curing of large composites, however, is typically limited by insufficient light transmission in the composite resulting in limited depth of cure (DOC) coupled with undesirable polymerization shrinkage-stress. Several recent commercial composites claim low stress polymerization with 4+mm DOC. The aim of this study was to investigate the impact of contrast ratio (CR) on the DOC of five experimental low-shrinkage-stress composites.

Methods:  Five experimental dental composite compositions with CR's between 40 and 70 were prepared by adding an opacifying pigment to a base photo-polymerizable composition (35% resin and 65% filler, by weight).  Specimens (1mm thick x 30mm diameter, n=3) of each composition were cured for 20 seconds at an irradiance 750mW/cm² and the CR's were determined using a Hunter UltraScan SE Spectrophotometer with a 1” aperture. DOC specimens (n=3) for each composition were evaluated per ISO4049 using a 20 seconds cure with an LED (550 mW/cm²@455nm peak output).

Results: A plot of CR versus DOC fit an exponential model: y=9.78e^-0.018x, where y=DOC(mm), x=CR, and B= fit constant (R²=0.98).  Table 1 summarizes the materials, their respective CR, DOC and standard deviations.

Conclusion : The data demonstrates that DOC decreases exponentially as the CR increases for this model low-stress composite system. This relationship between composite depth of cure and light transmission indicates the importance of having adequate light availability at the 4+mm depths to ensure sufficient deep curing. Several examples of commercialized materials will also be presented.