Effects of conditioners on microshear bond strength to enamel after carbamide peroxide bleaching and/or casein phosphopeptide–amorphous calcium phosphate (CPP–ACP) treatment

Abstract

Objectives

To evaluate (a) the enamel microshear bond strength (MSBS) of a universal adhesive and (b) the effects of conditioning with a self-etching primer adhesive with/without prior bleaching and/or casein phosphopeptide–amorphous calcium phosphate (CPP–ACP) application.

Methods

Thirty-five molars were cut into four sections, assigned randomly to four groups (no treatment; 16% carbamide peroxide bleaching; CPP–ACP-containing paste (Tooth Mousse, TM); bleaching and TM) and treated accordingly. Specimens were divided into two for bonding with either a self-etching primer (Clearfil SE Bond, CSE) or a total-etch adhesive (Single Bond, SB). Specimens for CSE bonding were subdivided for one of four preconditioning treatments (no conditioning; 30–40% phosphoric acid (PA); 15% EDTA; 20% polyacrylic acid conditioner (Cavity conditioner, CC) and treated. The adhesives were applied and resin composite bonded to the enamel using microtubes (internal diameter 0.75 mm). Bonds were stressed in shear until failure, mean MSBS calculated and data analysed using ANOVA with Tukey's HSD test (α = 0.05). The modes of bond failure were assessed and classified.

Results

Two-way ANOVA revealed significant differences between treatments (P < 0.0001), conditioners (P < 0.0001) and a significant interaction between treatments and conditioners (P = 0.001). One-way ANOVA revealed no significant differences in MSBS following any of the treatments for SB; following TM application for CSE without preconditioning; and significant differences in MSBS following bleaching with and without TM application for CSE. With preconditioning, applying PA before CSE post-bleaching and either PA or CC before CSE post-TM application, resulted in significant differences in MSBS (P < 0.05).

Conclusions

The use of conditioners prior to bonding with the self-etching primer adhesive system on treated enamel may significantly improve bond strengths.

Introduction

Nightguard vital bleaching is becoming increasingly popular due to its relative ease of use, time-saving and cost effectiveness. First described by Haywood and Heymann,¹ this technique allows bleaching to be carried out by the patient at home by wearing an intraoral tray containing the bleaching agent, usually overnight. Teeth are bleached for a number of reasons, including the elimination of intrinsic and extrinsic stains, pre-restorative lightening of tooth colour or simply for better aesthetics.² Nightguard vital bleaching using low concentrations of carbamide peroxide, carried out outside the dental office has been reported to have a high-success rate.³ Tooth bleaching has, however, been reported to result in alterations to surface morphology related to demineralization of enamel,4, 5 quantitative mineral loss6, 7, 8 and often, a reduction in bond strengths of restorative materials to enamel9, 10, 11, 12 which may persist for 1 day to 3 weeks after bleaching.11, 12 Some reasons put forward to explain the reduction in bond strengths are poorly defined etch patterns and the presence of polymerization-inhibiting residual oxygen in the tissues.13, 14, 15 One study, however, reported no mineral loss in enamel following vital bleaching.¹⁶ Bleaching can also be associated with side effects including tooth hypersensitivity, which has been reported in 15–78% of patients undergoing vital tooth bleaching.17, 18

A number of treatments has been applied to manage tooth hypersensitivity, which include the use of oxalates and nitrates of potassium, or a combination of potassium nitrate and fluoride in gels, toothpastes³ as well as the use of amorphous calcium phosphates (ACP).19, 20 The topical application of ACP has been shown to reduce root surface hypersensitivity.²¹

Casein phosphopeptide–amorphous calcium phosphate (CPP–ACP) is ACP that is complexed with the milk protein, casein phosphopeptide.²² Casein phosphopeptides stabilize ACP,²² localize ACP in dental plaque, thereby concentrating calcium and phosphate at the tooth surface.23, 24, 25 By maintaining a high-concentration gradient of calcium and phosphate ions, CPP–ACP helps to suppress demineralization and promote remineralization of enamel by the deposition of apatite.22, 26 CPP–ACP has been shown in laboratory and in situ studies to remineralize human enamel subsurface lesions.23, 26, 27 The remineralized enamel has also been shown to be more resistant to subsequent acid attack.²⁷ CPP–ACP has thus been incorporated into oral health products such as chewing gum and mouthrinse. CPP–ACP in the form of a paste is used in conjunction with, or following treatments such as bleaching, in order to reduce tooth sensitivity. The effect of applying CPP–ACP to an enamel surface and following enamel bleaching on bond strength, is however, unknown.

Self-etching primer adhesive systems present the dentist with the option of fewer application steps, and therefore less technique sensitivity, reduced operative time, and have been shown to form reliable bonds to enamel and dentine.28, 29, 30, 31 They incorporate an acidic monomer which infiltrates as it etches the dentine and thereby allows for more complete penetration of the resin into the exposed collagen. However, the acidity of the resin is weak and does not consistently etch enamel when compared with phosphoric acid etch used in other resin-based adhesive systems.29, 30, 31, 32 The use of phosphoric acid etching with self-etching primer adhesives while increasing bond strengths to enamel, has however, been reported to significantly reduce bond strengths to dentine.³²

Tooth conditioners are mild etching agents more commonly used on dentine but their efficacy on enamel is questionable. They have been applied as cavity cleansers before placement of restorations with the aim of removing the smear layer and a small amount of dentine mineral.33, 34, 35, 36

The application of CPP–ACP may result in a more highly mineralized enamel surface and hence the etching effect of self-etching primers may be less pronounced than on untreated enamel, which may therefore compromise resin infiltration and the bonds formed. The application of a conditioner with a mild etching action, prior to the use of a self-etching primer adhesive system on enamel, may help to enhance the etch and improve enamel bond strengths while maintaining good bond strengths to dentine. A recent study suggested that microshear bond strength of a self-etching primer adhesive may be reduced following application of a CPP–ACP-containing paste.³⁷

With the aid of a CPP–ACP-containing paste (Tooth Mousse), this study aims to evaluate the effects on enamel microshear bond strength (MSBS) of: (1) CPP–ACP application to untreated and carbamide peroxide bleached enamel; (2) the use of conditioners prior to application of a self-etching primer adhesive; (3) the use of conditioners prior to application of a self-etching primer adhesive following bleaching and/or CPP–ACP application.

Three hypotheses are to be tested: (1) there is no difference in MSBS of a universal adhesive to enamel following application of CPP–ACP, with or without prior carbamide peroxide bleaching; (2) there is no difference in MSBS to untreated enamel when conditioners are applied prior to the use of a self-etching primer adhesive; (3) there is no difference in MSBS to enamel when conditioners are applied prior to the use of a self-etching primer adhesive following bleaching and/or CPP–ACP application.