Cytotoxicity of dental bonding substances as a function of Degree-of-Conversion

Objectives: The aims of the present study were to develop a preparation method for cytotoxicity-testing of composite specimens in combination with bonding substances modelling conditions in the oral cavity and to evaluate the Degree-of-Conversion of bonding materials cured with and without air-inhibition.

Methods: Composite specimens were prepared in polyethylene blocks containing 5mm diameter cylindrical holes (cylinder-height 2mm), covered with a polyethylene-foil and light-cured from one end for 40s. Combined “bonding/composite”-specimens were produced by applying bonding at first onto the polyethylene-foil and light-curing. Subsequently, polyethylene-moulds were placed on top of the bonding materials and composites prepared as described above. Combined “composite/bonding”-specimens were produced by at first preparing composites in glass-tubes (4mm/8mm) and light-curing from all sides. Therafter bondings were added onto the composite cylinders with a brush and light-cured.

Specimens were added to the cell-cultures immediately after production or after preincubation for 7 days under cell-culture conditions. Specimens were incubated with L-929-fibroblasts for 72h and cell numbers determined by flow-cytometry. In a third series of experiments FTir Spectroscopic measurements were made on thin-films of dentin-bonding agents, cured under both an-aerobic (resembling “bonding/composite”-specimens) and aerobic conditions (resembling “composite/bonding” specimens), to determine Degree-of-Conversion.

Results: Cytotoxicities of all 6 tested composites were significantly different (p<0.0001; ANOVA) and diminished after 7 days of preincubation (p<0.0001; ANOVA). Bonding substances had no statistically significant influence on the cytotoxicity of composites, when applied as “bonding/composite”-specimens but heavily enhanced cytotoxicity when applied as “composite/bonding”-specimens (p<0.0001; ANOVA). A highly significant statistical reduction in the Degree-of-Conversion for each resin cured under air-inhibition conditions was documented (p < 0.01; Student t-test).

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