Bonding mechanism and ultrastructural interfacial analysis of a single-step adhesive to dentin

Objectives. to characterize the interaction of 4-acryloxyethyltrimellitic acid (4-AET), a key ingredient in a commercial all-in-one adhesive (Reactmer Bond) with dentin apatite, to measure the solubility of 4-AET calcium (4-AETCa), and to examine the ultrastructure of the resin-dentin interface bonded by this adhesive. Methods. solubility of synthesized 4-AETCa in several solutions was measured. Flattened bovine dentin blocks were treated with 0.1wt% NaOCl for 10 min, or 5.0wt% NaOCl for 10 min, or 30 min. Fourier transform infrared analysis using attenuated total reflection (FT-IR/ATR) was performed on these four substrates before and after treatment with a 4-AET mixture (4-AET/HEMA: 40/60wt%). Undemineralized, unstained sections of human sound dentin that were bonded with the same comonomers were examined by transmission electron microscopy (TEM). Results. the respective solubilities of the 4-AETCa in water, 2-hydroxyethyl methacrylate (HEMA; 100wt%) and HEMA/water (70/30wt%) were 0.023, 0.003 and 0.014 mol/l after 2 min. The IR analysis indicated that Ca-carboxylate (new bands: νC=O:1580 and 1413 cm^-1) was formed on all dentin specimens following application of 4-AET, with or without NaOCl pretreatment. TEM showed a 1 μm thick hybrid layer with some remnant apatite crystallites that were encapsulated by a layer of less electron-dense material. Significance. the results suggest that ionized carboxyl groups in 4-AET may interact with Ca²⁺ from apatite crystallites within the partially demineralized hybrid layer to form an insoluble calcium salt (4-AETCa) that may aid in bonding this resin system to dentin.