Role of the smear layer in adhesive dentistry and the clinical applications to improve bonding performance

Abstract

Currently, dental adhesives can be divided into two systems; a smear layer-removal approach with etch-and-rinse adhesives or a smear layer-modified approach with self-etching adhesives. After phosphoric acid etching, the smear layer is completely removed. More attention is, however, required when using self-etching adhesives. The smear layer is partially demineralized by the weak acidic monomer and subsequently incorporated into the hybrid layer. Therefore, the characteristics of the smear layer play an important role on the bonding performance of self-etching adhesives. Such characteristics, for instance, smear layer thickness and smear layer density, are influenced by many factors, e.g., instruments used for dentin surface preparation, cutting speed, and the abrasive particle size of the cutting instruments. This review discusses the contributing factors that affect the smear layer characteristics, and the influence of the smear layer on the bonding performance of dental adhesives. Also, the application techniques regarding how to improve the bonding performance of self-etching adhesives - the smear layer removal by using chemical agents, or the modification of the adhesive application procedures - are provided.

Keywords: Dentin; Self-etching adhesive; Smear layer; Surface preparation; TEM.

Fig. 1

Schematic diagram of smear layer treated with self-etching adhesive. (a) Smear layer covering dentin surface after tooth preparation. (b) After being treated with self-etching adhesive, the smear layer was partially demineralized and superficial collagen was exposed. (c) After resin polymerization, the residual smear layer was incorporated into hybridized smear layer between the adhesive layer and the hybrid layer.

Fig. 2

Representative TEM images of dentin surfaces with different surface preparations. (a) No smear layer was detected in the fractured dentin specimen. (b) The smear layer prepared by a carbide bur was loosely bound to the dentin surface with approximately 300 nm-in-thickness. (c) The smear layer prepared by SiC paper was non-homogenous. (d) The smear layer prepared by a regular diamond bur was the most compact.

Fig. 3

TEM observation of dentin surface prepared by superfine diamond bur in longitudinal view (a). The dentin surface was etched with 35% phosphoric acid solution (b) and treated with Clearfil SE2 primer (Kuraray Noritake; Tokyo, Japan) (c).

Fig. 4

Representative TEM images of superfine diamond bur-prepared smear layers treated with Clearfil SE2 primer (Kuraray Noritake; Tokyo, Japan) with the inactive application (a) or active application (b).

Fig. 5

Representative TEM images of superfine diamond bur-prepared dentin treated with Scotchbond Universal adhesive (3 M Oral Care; St. Paul, MN, USA) with single application (a) or double application (b).