High irradiance curing and anomalies of exposure reciprocity law in resin-based materials

The aim of this work was to investigate the effect of high irradiance curing on resultant degree of conversion of 'flowable' resin composites and their counterpart higher viscosity paste materials.

Five commercial flowable materials (Venus; Heraeus Kulzer, Synergy D6; Coltene, Premise; Kerr, Grandio; Voco and Gradia; GC Corp) and their counterpart higher viscosity restorative versions were tested. Specimens were cured with a halogen Swiss Master Light (EMS, Switzerland) using five different curing protocols with similar radiant exposure (18J/cm(2)): 400mW/cm(2) for 45s, 900mW/cm(2) for 20s, 1500mW/cm(2) for 12s, 2000mW/cm(2) for 9s and 3000mW/cm(2) for 6s. Degree of conversion (DC) was measured in real time by Fourier transform near infrared spectroscopy (FT-NIRS).

Three- and subsequent two way ANOVA testing revealed significant differences (p≤0.02) with respect to "composite type" and "cure protocol" for DC for all 5 product comparisons. Supplementary one-way ANOVA also revealed significant differences between curing protocols (p<0.05). The majority of higher viscosity resin composite paste materials exhibited similar DC regardless of curing protocol. However, a significant decrease in DC for specimens cured at 3000mW/cm(2) for 6s compared with 400mW/cm(2) for 45s was observed for the flowable materials, Grandio (41±0.36 and 62±1.15%, respectively) and Venus (44±0.44 and 67±0.44%, respectively). Conversely, other flowable materials exhibited little or no significant differences between curing modes. Generally, a higher degree of conversion was observed for flowables compared with their more viscous counterpart, except at high irradiance for those materials where a reciprocal relationship with exposure time was not observed.

The validity of exposure reciprocity law and final degree of conversion depends on several factors, amongst which resin viscosity and filler content were important. Practitioners should be aware of the importance of resin composite constituents and irradiation protocols. Information on material composition and appropriate radiation sources by manufacturers may assist practitioners with the selection of appropriate curing protocols for specific material/light curing unit combinations with the aim of reducing the incidence of under-cured restorations and the clinical impact thereof.