Influence Of Tooth Thickness On Depth Of Cure And Degree Of Conversion Of A Photo-activated Resin Composite Irradiated Through The Tooth

Objective: To investigate the influence of trans-dental photo-curing on the depth ofcure (DoC) and degree of conversion (DC) of a resin composite irradiated by a light curingunit operating in different curing modes. Cavity preparation principle based on toothstructure conservation provided a theoretical basis for the research. Methods: A curing lamphaving six different curing modes was used for indirect photo-activation of a nano-compositeFiltekTM3M Z350through different thicknesses of tooth tissues. DoC was evaluated bymeans of an ISO4049standard scraping technique. Six main groups were formed inaccordance with tooth thicknesses (0.5,1,1.5,2,2.5and3mm) which were further dividedinto six subgroups (n=5) according to light modes. The uncured soft material was scrapedaway with a spatula and the height of remaining material was measured by a digital caliperin three spots to get a mean value,50%of the mean value was taken as DoC. Fouriertransformation infrared (FTIR) spectroscopy was used to determine DC. Three mainexperimental groups were formed in accordance with the tooth thicknesses0.5,1and3mm， which were further subdivided into six minor groups （n=3） formed according toirradiation modes. Control groups employed direct irradiation. Results: For each of the sixlight curing modes, respectively, the average DoC and DC through different thickness oftooth structure show that: DoC and DC decreases with increasing intervening tooth thickness.At3mm tooth thickness, except for Turbo mode, there is no obvious curing performance.Turbo and High light modes could achieve the resin composite2mm DoC even when toothstructure was1.0mm thick. However for the rest of the light modes, the2mm compositelayer could adequately be cured when the tooth thickness was0.5mm only. FTIR analysismean DC values for the through-tooth-structure indirect curing were lower than the45%-75%direct curing values. Among the groups,0.5mm group had the highest DoC and DC mean values,2.5mm and3mm groups had the lowest mean values. Using StatisticalPackage for Social Sciences (SPSS) version18.0, one-way ANOVA with multiplecomparison tests and paired sample t-tests were done at P=0.05. The composite resinshowed lower DoC and DC mean values when irradiated indirectly compared to controldirect curing (P<0.005).0.5mm and Turbo light groups recorded the highest DoC and DCvalues than other groups, differences between tooth thickness groups were statisticallysignificant with P<0.005. DoC and DC mean values for Turbo and High light modes weregenerally statistically different compared to Normal, Soft Start, Pulse and Pulse Soft lightmodes. Although the performance of Turbo light was slightly better than High light on bothDoC and DC mean values, the difference between them was not statistically significant.Conclusions:(1) A nano-composite resin cured through different tooth thickness will havelower DoC and DC compared to direct curing.(2) In clinical operation, composite should bedirectly cured; indirect curing must be avoided at all possibilities.