Micromorphological spectrum of acid-conditioned dentin following the application of a water-based adhesive

Objectives. The goal of this study was to illustrate the micromorphological spectrum along the resin-dentin interface when a water-based, dentin adhesive (Scotchbond Multi-purpose) was applied to acid-conditioned dentin under different dry and wet bonding conditions. Methods. Twenty-eight 1 mm dentin discs were each conditioned with 10% maleic acid for 15 s. Twenty-four of these discs were randomly divided into four groups, based upon the status of the remaining surface moisture: Group I (30 s dry); Group II (3 s dry); Group III (visibly moist) and Group IV (overwet). They were bonded using Scotchbond Multi-purpose. The remaining four discs were bonded using an experimental water-based primer containing 35 vol.% HEMA (Group V). Laminated dentin disc pairs were prepared for transmission electron microscopic examination. Results. In all groups, diffusion of the polyalkenoic acid copolymer component of the primer into acid-conditioned dentin was localized to the surface region of the hybrid layer. The remaining part of the hybrid layer (the subsurface region) appeared variable. In Group I (30 s dry), collagen fibrils were collapsed. In Group II (3 s dry) and Group III (visibly moist), stained collagen fibrils were surrounded by wide electron-lucent interfibrillar spaces. In Group IV (overwet), a marked diffusion gradient was observed representing dilution of part of the primer components. In addition, an electron-dense primer phase, containing electron-lucent globular domains was invariably observed, irrespective of the hydration status of the demineralized collagen matrix. This electron dense phase was absent when HEMA alone was used as the primer (Group V). Significance. With the use of a water-based adhesive, one could briefly air-dry the acid-conditioned dentin and allow the water in the primer to rehydrate the collapsed collagen matrix, without the risk of incomplete hybridization or tubular seal along the resin-dentin interface. However, care should be exercised to minimize dilution of the water-soluble primer component.