Effect of Curing Light Distance/Curing Time on Composite Microhardness

Objective: The complete cure of composite resin may be difficult when the ability to bring the polymerization light in direct contact with the surface is impaired.  The purpose of this study is to determine the effect on composite polymerization of distance from the light source during polymerization.

Method: Teflon blocks of three thicknesses (1.7mm, 3mm, and 4mm) were prepared with a 3mm hole in the center were prepared. Composite resin specimens were prepared (3M Filtrek-Supreme Plus, St Louis Mo) utilizing shades A2 and A3.5. Teflon blocks were placed over a black base to prevent light reflection and glass microscopic slides was utilized above to ensure a flat surface. A VALO broadband LED curing light (Ultradent, South Jordan UT) was utilized to cure the composite material. Ten specimens for each thickness and shade specimen and were light cured for 20 or 40 seconds with and without a 4mm spacer (total 120 specimens). Vickers microhardness values for each test specimen for the top and bottom of each specimen were obtained following dry storage for at least 24 hours in the dark.  Differences in microhardness were evaluated utilizing ANOVA statistics.

Result: The outer surface hardness of both composite shades at 0 and 4mm spacing from the light was the same.  The bottom microhardness of A2 shade at 0 and 4mm spacing from the light was the same for the 1.7mm specimens was the same and the A3.5 specimens were 15% weaker with 4 mm spacing.  In all other specimens 3 and 4mm, the bottom specimens were less polymerized between 15-100% (p<0.001) no matter what the increased light exposure time.

Conclusion: At ideal composite resin thickness (<2.0mm) composites polymerize completely through even with increasing distance to light.  With resin thicknesses greater than 2mm, increasing light-curing time does not result in complete polymerization.