Water uptake of bonding systems applied on root dentin surfaces: a SEM and confocal microscopic study

Dentin adhesives have been proposed as desensitizing agents to seal exposed root dentin surfaces. Simplified 'one-step' dentin adhesives are highly permeable to water. The authors hypothesized that a lactic acid challenge may increase permeability of simplified adhesives and may induce fast degradation of bonding. This phenomenon adversely affects their durability as long term desensitizing agents. The aim of this in vitro study was to evaluate the ability of four dentin adhesives to seal root dentin surfaces that were exposed to water and lactic acid challenges.

Four commercially-available dentin adhesives were applied with a small sponge to the root dentin of extracted human molars as de-sensitizing agents. Impression replicas of the adhesive-covered root dentin were fabricated after water immersion, as a control, and after lactic acid challenge. The replicas were examined with SEM for quantitative comparison of fluid droplet formation on the surfaces. The bonded specimens were also examined using reflected light confocal microscopy.

Replicas of water droplets were observed on the adhesive surfaces, by SEM which corresponded with direct confocal observation of blisters and voids from the surface of the bonded specimens. There were significantly more water droplets from samples that were subjected to lactic acid challenge than water only immersion.

Although the dentin adhesives examined were able to cover exposed root dentin, they all exhibited fluid transudation through the polymerized adhesives. Dentin adhesives were also susceptible to surface degradation after lactic acid challenge. As simplified self-etch adhesives were highly water permeable and exhibited the most extensive surface damage, they may not be the best adhesives to be used for long-term dentin desensitization. These preliminary in vitro findings warrant validation in vivo.