Cytotoxic effects of dental bonding substances as a function of degree of conversion

Recently, we found that dental bonding substances tested alone or in combination with composites are far more cytotoxic than composite materials alone. These data are in line with several cytotoxicity reports but contradict in vivo studies showing the beneficial effects of bonding materials. The aims of the present study were to develop a preparation method for composite specimens modelling conditions in the oral cavity and to analyse the influence of bonding substances on the cytotoxicity of six different composite materials.

Cylindrical composite specimens were prepared in polyethylene blocks containing 5 mm diameter cylindrical holes (cylinder height 2 mm), covered with a polyethylene foil and light cured from one end for 40 s. In a second series of experiments, composite specimens were combined with bonding materials. Bonding was applied onto the polyethylene foil in one or two layers and light cured according to the manufacturers' instructions. Subsequently, polyethylene moulds were placed on top of the bonding materials and composites prepared as described above. After unilateral light curing from the top of the cylindrical holes, visual confirmation of adherence at the base was obtained. Specimens were added to the cultures immediately after production or after preincubation for 7 days under cell culture conditions. Specimens were incubated with L-929 fibroblasts for 72 h and cell numbers determined by flow cytometry. To evaluate the degree of conversion (DC) of bonding materials cured with and without air inhibition a third series of experiments was performed. FTIR spectroscopic measurements were made on thin-films of dentin-bonding agents, cured under both an-aerobic and aerobic conditions, to determine degree of conversion.

Cytotoxicities of all six tested composites were significantly different (p<0.0001) and diminished after 7 days of preincubation (p<0.0001). Bonding substances had no statistically significant influence on the cytotoxicity of composite materials (p=0.159). A highly significant statistical reduction in the degree of conversion for each resin cured under air inhibition conditions was documented (p<0.01).

Our study demonstrates that cell culture toxicity data are highly model dependent and that internationally standardized test protocols for toxicity screening of dental materials in line with the existing standards are clearly needed to obtain comparable results.