Roughness Correlated to CoJet^® Conditions of Use and its Efficacy

Objectives: CoJet^® is a chairside surface treatment system used to improve the bond of resin cements to various substrates. The aim of this study was to check whether surface roughening plays a significant part in the mechanism by which this system works. Methods: 6mm diameter specimens were made from a Nickel Chrome alloy (Nicor), a gold alloy (Argenco60S), CADCAM ceramic (Vita-CEREC), Techceram^® and Z250 composite. They were mounted in resin blocks, lapped and finished to a high gloss. CoJet^® was used to sandblast the specimens' surfaces (50μ particle size) with the distance of the nozzle from the surface and the application time as variables (n=15). A profilometer (Surftest) was used to record Ra and Rt readings for each specimen. ESEM micrographs were taken of specimens of each group. Columns of a resin cement (Nexus) were bonded to the sandblasted surfaces following the manufacturer's instructions. Shear bond strength of 24 hour water stored specimens was tested using an ISO standard test rig in an Instron Universal Testing Machine at a cross head speed of 1.0 mm min^-1. These data were analysed using ANOVA and linear regression. Results: Sandblasting distance significantly affected bond strengths only for Techceram and composite (p<0.05) while time had a significant effect on bond strength for all materials (p<0.05) except NiCr. There was no significant correlation between distance and both Ra and Rt (p>0.05) except for composite. Roughness was highly significantly influenced by time with composite, Au and CADCAM (p<0.001), with high R² values for composite (45-61%) and moderate for both CADCAM and Au (between 17-32%). The topographic changes seen with ESEM supported these results. Conclusions: Time of CoJet^® sandblasting influenced bond strength with resin cements more than distance. Roughness was found more related to sandblasting time specifically with softer materials.