Factors affecting in vitro bond strength of no-mix orthodontic cements

The effects of cement thickness and time of exposure of primer to a stimulated oral environment on tensile bond strength were determined in vitro for three no-mix cements and a two-paste cement. Five shims produced increasing cement thicknesses, and a humidor at 37 degrees C simulated an oral environment. SY had the greatest tensile bond strength (0.92 kg/mm2), whereas UN and ML had similar strengths (0.60 kg/mm2 and 0.66 kg/mm2, respectively), when no shims were used. Generally, there was a decrease in tensile bond strength as thickness increased for all no-mix cements. The failure site was essentially at the cement-base interface. Failures within the cement were characterized by incomplete polymerization of the resin. There was a decrease in tensile bond strength which ranged from 11% to 24% after primed teeth were stored in a simulated oral environment for 1.0 minute for ML and 2.5 minutes for UN and SY. This decrease in strength was accompanied by an increase in percentage failure at the paste-primer interface.