In vitro cytotoxicity of orthodontic bonding resins on human oral fibroblasts

Polymerization of bonding resins is compromised by atmospheric oxygen, givi ng rise to a layer of low molecular weight chemical species commonly known as the oxygen inhibited layer. The aim of this study was to evaluate the cy totoxic effect of this layer on primary cultures of human oral fibroblast. The cytotoxic effect related to the modes of polymerization of seven commer cially available orthodontic bonding resins was also evaluated statisticall y. Each material was polymerized into 12 resin disks of standardized dimens ions. Half of them were washed with 99% acetone to remove the oxygen inhibi ted layer. In duplicates, human oral fibroblasts were exposed to the intact and washed resin disks in tissue culture inserts. Cell viability was asses sed by tetrazolium bromide reduction assay (MTT) 1, 3, and 6 days after exp osure. Glass disks served as controls. ANOVA was used to test for statistic al significance. Overall, the presence of an oxygen inhibited layer renders bonding resins 33% more cytotoxic (P < .01, F = 11.83, DF = 1). Light-cure d and chemically cured 2-pastes materials had their mean cytotoxicities app roximating their inert controls over 6 days. In chemically cured liquid-pas te materials, the viability of human oral fibroblasts was only 37% (P < .00 1, F = 26.4, DF = 2) comparing to the control, 64% on day 1, 30% on day 3 a nd 14% on day 6. This suggested that the oxygen inhibited layer formed on t he surface of bonding resins is an important cytotoxic source in vitro. Che mically cured liquid-paste materials were more cytotoxic than light-cured a nd chemically cured 2-paste materials. Further investigation into the influ ence of the modes of polymerization on materials' toxicodynamic effect is w arranted to verify its clinical implication.