Can Initial Low-intensity Curing Reduce Cuspal Flexure from Composite Shrinkage?

Objectives: Reducing polymerization shrinkage stress without compromising depth-of-cure is desirable. For the same total light energy, reducing light-intensity has been shown to reduce post-gel shrinkage. This study investigated cuspal flexure and depth-of-cure in composite-restored teeth when polymerization was initiated with a lower light-intensity.

Methods: Large mesio-occlusal-distal slots (ca 3 mm wide, 3.5 mm deep) were prepared in 10 matched-pairs extracted molars, mounted in stainless steel rings featuring reference spheres.  The preparations were etched, rinsed, bonded, and restored with Filtek Supreme composite (3M ESPE) in two horizontal increments.  Each increment was cured with a standard regimen (SR) using LED light (600 mW/cm²) from the occlusal, mesial, and distal directions (32 seconds each).  Within each pair, the SR-cure of one of the teeth was preceded by a 16-seconds initial occlusal light-activation with low light-intensity (IL, 75 mW/cm²) by retracting the curing-tip 20 mm.  The teeth were digitized after preparation and restoration (Comet xS).  The scans were fit (Cumulus ©Regents University of Minnesota) using the reference spheres.  Cuspal flexure was calculated from lingual and buccal volume change/area (CuspFlex software).  Twenty-four hours depth-of-cure was determined on the bucco-lingual restoration cross-sections using Vickers Hardness at 0.5 mm increments from the occlusal surface.

Results: One pair was discarded because of cusp fracture in the SR group.  Cuspal flexure with the IL+SR was lower than the SR, but the reduction was not significantly different (t-test, p=0.08).  Hardness values of the two groups were not significantly different at any depth.

 

	Cuspal flexure (µm)	Vickers Hardness Number
		0.5mm	1.0mm	1.5mm	2.0mm	2.5mm	3.0mm	3.5mm
SR	–12.80±2.46	94.7±2.8	94.1±2.2	94.3±3.1	94.9±3.1	94.3±1.5	94.8±3.0	93.1±3.0
IL+SR	–10.59±2.61	94.4±2.4	94.3±2.7	94.3±2.8	94.4±2.5	94.1±2.1	94.2±2.2	93.7±2.0

Conclusions: Curing composite with low initial light-intensity did not compromise depth-of-cure and may reduce cuspal flexure.

Acknowledgment: Supported in part by a Non-Tenured Faculty Grant (3M Foundation).