Multiple Short Pulse-Delay Photocuring Technique for Dental Composites

Objectives: To investigate the effect of a multiple short pulse-delay photocuring protocol on the clinically relevant polymerization properties (the degree of conversion (DC), polymerization stress (PS), and temperature change (TC)), and mechanical property (hardness)
Methods: Model dental composites composing of varying weight ratios of the traditional monomers, Bis-GMA, TEGDMA and a constant filler weight were used in the study. The irradiance was varied from 500 mW/cm² to 4000 mW/cm², other protocol parameters, such as, the duration of the pulse, the time interval between consecutive pulses, and the number of pulses were iteratively varied for each composite. A NIST-developed instrument coupled with a NIR spectrometer and a miniature thermocouple was used to measure the development of PS, DC, and TC simultaneously in real time. Hardness tests were performed on the composite samples stored for a week in a post-cure condition. The control curing condition used for comparison was a uniform continuous irradiance of 500 mW/cm² for 20 seconds.
Results:
The protocol conditions which had multiple short duration (≤ 1s) pulses resulted in no statistically significant difference in DC (p > 0.05), reduction of PS by 18% to 33%, TC by 32% to 55%, and had a total radiant exposure generally lower (up to 60%) when compared to the control. Hardness of the composites were at most 12% lower than the control, except for one composite, which was much lower due to the high refractive index mismatch between its resin and fillers.

Conclusions: A multiple short pulse-delay photocuring technique can produce desirable polymerization and mechanical properties comparing to long pulse-delay photocuring. The observation of drastic reduction in PS, without sacrificing DC can be mainly attributed to two factors: 1) the stress relaxation between consecutive pulses, 2) a significant reduction in the contribution to the PS from thermal contraction.