Confocal Microscopic Determination of Microtensile Failure Patterns of Dental Adhesives

Methods: Class I cavities were prepared in 20 sound molar teeth. The teeth were divided randomly into four groups, comprising those restored with total-etch (Adper Scotchbond, 3M ESPE) and self-etch (Absolute, Dentsply Sankin) dental adhesives, with or without associated flowable composites (Filtek Supreme XT, 3M ESPE and X-flow, Dentsply DeTrey) as liners. Prior to application, adhesives were labelled with Rhodamine B. The teeth were sectioned to obtain 25 specimens per group for testing to failure using a microtensile device. Failure modes were analyzed using a tandem scanning confocal microscope. The two parts of each specimen were reassembled on the stage of the microscope. Samples were examined with a ×20/0.8 NA oil immersion objective lens in conjunction with a ×10 eyepiece, making use of the labelled adhesive to facilitate determination of the location of fracture. All four sides of the reassembled specimen were examined and the failure mode recorded. Failure modes were classified into five categories, with each type scored from 1-3 according to the proportion of each failure type along the examined surface. Specimens were gold coated and the failure modes were further investigated by SEM imaging of the fractured surfaces.

Results: Fractures occurred more toward the dentine-adhesive interface for Scotchbond, whilst Absolute specimens failed more toward the composite-adhesive interface. Failure mode results from confocal and SEM analysis were compatible.

Conclusion: Fracture mode elucidation is facilitated by confocal microscopic analysis of reassembled labelled specimens. This technique both compliments, and provides clearer assessment of interfacial failure locus than SEM alone.