Guided Tissue Remineralization of Resin-dentin Interfaces. I. Moist Bonding

Objectives: Guided Tissue Remineralization (Tay and Pashley, Biomaterials, 2008) refers to the use of nanotechnology concepts for regenerating mineralized dentin from phosphoric acid-demineralized dentin under the guidance of biomimetic organic molecules. This technique is based on the non-classical crystallization pathway (Cölfen and coworkers; 2006-2007). In the context of dentin bonding, although collagen degradation within incompletely-infiltrated hybrid layers may be postponed/arrested using matrix metalloproteinase inhibitors, the soft collagen matrices that remain are susceptible to creep during function and may eventually fail by cyclic fatigue. This work explored the possibility of remineralizing incompletely-infiltrated zones in resin-dentin interfaces created by etch-and-rinse adhesives. Methods: Flat dentin surfaces were prepared from twenty extracted human molars and bonded with One-Step (Bisco) or Single Bond Plus (3M ESPE) using a moist bonding technique followed by composite placement. One-mm thick slabs of resin-dentin interfaces were prepared. Baseline interfacial ultrastructure and silver nanoleakage were characterized using TEM. Additional slabs were immersed in a Portland cement/simulated body fluid system that generates carbonated apatites from amorphous calcium phosphate precursors. The latter also contained polycarboxylic acid and polyphosphonic acid biomimetic analogs of dentin noncollagenous proteins for precursor dimension regulation and collagen targeting. Specimens were examined by FE-SEM, TEM (unstained, undemineralized sections) and selected-area electron diffraction at 1-3 months. Results: Electron-dense irregular amorphous phases first appeared around interfibrillar spaces in isolated regions of the hybrid layers that were transformed into segregated and eventually apatite nanoclusters. These initially subtle changes became readily discernible at 2-3 months, with interfibrillar and intrafibrillar apatite depositions within the top and middle portion of the hybrid layers. Apatite nanocrystals were also identified heterogeneously within the polymer matrix of the overlying adhesive layers. Conclusion: Amorphous calcium phosphate nanoprecursors created by Guided Tissue Remineralization can penetrate hybrid/adhesive layers of resin-dentin interfaces created using a moist bonding technique to produce apatite nanocomposites.