Dentin surface conditioning : the role of collagen in dentin bonding

Abstract

Preparation of dental cavities inevitably produces a thin layer of smear, which covers dentin and enamel surfaces. This smear layer, which is between 1 and 10 gm thick, consists of hydroxyapatite, denatured collagen and remnants of cariogenic bacteria. If left untouched, it will weaken the bond between filling material and cavity wall. Thus, etching of cavity walls has been recommended since the mid-1950's. In addition to removing smear, adds demineralize peri- and intertubular dentin and expose collagen fibers. The concept of a hybrid layer as part of adhesive restorative dentistry introduced by Nakabayashi and co-workers in 1982 relies on infiltration of the exposed collagen fibres by a primer.

However, recent results have indicated that acids operating at low pH such as phosphoric acid may dissolve not only the mineral component of dentin but also the collagenous matrix. The overall aim of the studies on which the present thesis is based was to investigate how cavity preparation may modify the quality of the dentin surface and its bonding properties. The shear strength of bonding to dentin etched with EDTA was tested after the efficacy of EDTA in removing smear and exposing collagen had been confirmed. To test the significance of collagen for dentin bonding, surfaces with an ascending amount of collagen were prepared. The following conclusions represent the most significant findings:

· Acid etching altered the collagen structure and showed a less homogenous appearance compared to the collagen etched with EDTA, which showed distinct cross-striated pattern with no apparent difference to the non-etched control.

· By combining of two different etchants, EDTA on dentin and phosphoric acid on enamel, the structures on each tissue surface could be optimized for retention compared to any single one of the two etchants applied to both tissues of the cavity walls.

· Caries excavation using CarisolvTM always resulted in a smear layer, covering surfaces of the cavity.. Residual caries, predominantly in the enamel dentin interface, were found in six of ten cavities excavated with CarisolvTm.

· Shear strength for All Bond 2 bonded to dentin was significantly increased then the dentin was treated with EDTA-gel compared to phosphoric acid etching.

· A smear-free dentin surface showing abundant fiber structures and patent dentin tubuli compared to a smear-free dentin surface showing traces of fiber structures and clearly outlined dentin tubuli provided a significantly better surface for dentin bonding. These surfaces were consistently the result of EDTA treatment regardless of exposure time, while, phosphoric acid treatment, the current clinically accepted standard dentin etching treatment resulted in fewer and also compromised collagen fibers in the dentin surface.

· Similarly, a smear-covered dentin surface provided a dentin surface with less optimal bonding characteristics comparable to a phosphoric acid etched and Na0Cl-treated surface.

· The importance of an intact collagenous fiber network is further emphasized by the results in which Na0Cl was used to remove all traces of denuded collagen fibers resulting in no significant difference between phosphoric add or EDTA treatment. These results also lend support to the theory that an intact denuded collagen network provides a significantly better environment for resin penetration and entanglement, thus increasing the thickness of the hybrid layer.

Thus, the present thesis has shown an essential role of an intact collagen network in dentin cavity surfaces for optimizing bond strength of adhesive restorations. In this context, EDTA treatment has been shown to meet all clinical requirements of a dentin conditioner.