The influence of light-curing time on the bacterial colonization of resin composite surfaces

Bacterial colonization of composite surfaces represents the main factor in the etiology of secondary caries around adhesive restorations. The authors' aim was to evaluate the influence of light-curing time on mutans streptococci colonization (MS) of a resin composite material.

Specimens obtained from a dental resin composite were divided into 12 groups and light-cured with the same light source respectively for 10, 20, 30, 40, 60 or 80 s using two different curing-power levels: 400 and 800 mW/cm2. A wild strain of MS was isolated and a 24-h-monospecific biofilm, adherent to the surfaces of the samples, was obtained. A colorimetric technique (MTT assay), based on the reduction of a yellow tetrazolium salt to a purple formazan, was used to evaluate the biomass adherent to the specimen surfaces. ANOVA and Scheffé's tests were used to statistically analyze the results.

Two-way ANOVA demonstrated there was no interaction between curing-time factor and curing-power factor (p=0.970); one-way ANOVA was used to analyze separately the data obtained from each curing-power level. Both levels showed highly significant differences (p<0.0001) among the different curing time groups. The non-parametric test for trend showed in both levels the existence of a highly significant trend (p<0.0001) for bacterial colonization reduction as curing time increases.

A reduced curing time seems to be responsible for increased in vitro colonization of composite surfaces by MS; this phenomenon is likely to be related to the presence of unpolymerized monomers on the material surface.