Effectiveness of self-adhesive luting cements in bonding to chlorhexidine-treated dentin

Abstract

Objectives

The aim was to investigate the microtensile bond strength (μ-TBS) and failure pattern of self-adhesive luting cements (SLC) to dentin pre-treated with different concentrations of chlorhexidine (CHX) solutions.

Methods

The occlusal enamel was removed from 30 extracted sound human molars in order to expose a flat dentin. Resin-composite (Filtek Z250, 3 M ESPE) discs (12 mm in diameter, 6.0 mm thickness) were cemented to the smear layer-covered dentin using the SLC [RelyX U100, 3 M ESPE (U100); Multilink Sprint, Ivoclar Vivadent (MS)] with 0.2% or 2.0% CHX solutions. Results were compared with the control, untreated dentin. Six groups were then created based on the SLC and dentin pre-treatment ( n = 5). After 24 h of water storage, restored teeth were serially sectioned into beams with a cross-sectional area of 0.8 mm 2 at the bonded interface. Subsequently, specimens were tested in tension with a crosshead speed of 0.5 mm/min in a universal testing machine, and the failure patterns were classified. Two-way ANOVA and Tukey’s tests were performed ( α = 0.05). Additionally, 18 teeth were subjected to energy-dispersive X-ray spectroscopy (EDS) analysis and micromorphology characterization of the smear layer-covered dentin and 0.2% or 2.0% CHX-treated dentin surfaces.

Results

The μ-TBS obtained for both control groups were significantly higher, regardless of the CHX concentration and/or the cement used. Bond strengths were significantly higher for U100 than for MS, except when 2.0% CHX was applied. Fractographic analysis indicated that most failures in the control groups occurred cohesively in the SLC. Pre-treated dentin with 0.2% and 2.0% CHX solutions presented higher incidences of adhesive failures. EDS/SEM analysis exhibited varied concentrations of chlorine ions and crystal-shaped precipitates, depending upon the CHX concentration.

Significance

Pre-treatment of dentin with 0.2% or 2.0% CHX adversely affects the bonding efficacy when associated with the SLCs tested.

Introduction

Active bacteria can remain in enamel and dentin after preparation procedures due to the incomplete removal of caries-infected tissue . In this condition, bacteria may continue to multiply within the preparation, producing toxins that may diffuse into the pulp, causing irritation and inflammation . Such harmful effects, induced either by residual bacteria or bacterial microleakage, can be eliminated by means of antibacterial agents . Chlorhexidine (CHX), for example, has a broad spectrum of action and is effective against both Gram-positive and Gram-negative microbiota, although it is less effective against certain Gram-negative types . Due to its antibacterial action, CHX has been used as a cavity disinfectant prior to the application of restorative materials and, more recently, as a root canal irrigant . Moreover, CHX is claimed to serve as a potent inhibitor of endogenous collagenolytic/gelatinolytic activity of the matrix metalloproteinases (MMPs), preserving the bonding integrity .

The presence of deposited debris on the surface was previously reported when 2.0% CHX was applied to the dentin . Although CHX diminishes the loss of bond strength over time , little is known about the influence of the application of CHX solution prior to the cementation of indirect restorations, particularly when a new category of resin cements, known as self-adhesive luting cements (SLC), is used. In a recent study , it was speculated that the deterioration of the bonding efficacy of a commercial SLC might be related to the presence of moisture contamination on the dentin surface.

The central idea behind introducing SLCs is to overcome to a certain degree the drawbacks associated with using other types of cements to bond indirect restorations to the tooth tissues , as they require no technique-sensitive steps, such as acid-etching, priming, and bonding . This adhesion approach also induces secondary reactions between self-adhesive resin monomers incorporated in the SLC formulation with the hydroxyapatite, characterized as chemical bonding . This innovative bonding mechanism represents an important characteristic of this technique when compared with both etch-and-rinse and self-etching adhesive approaches, which are essentially micromechanical in nature . Since intimate contact of restorative material with dental tissues is crucial for bonding to take place , modification of the dentin surface was naturally thought to interfere with the adhesion process.

Hence, the present study focused on investigating the effect of dentin pre-treatment with CHX, which is recommended as a cavity disinfectant in a variety of clinical procedures. The aim of the study was to compare the bond strength of SLCs to pre-treated dentin, using different concentrations of CHX solutions. These solutions were applied to the dentin surface as a therapeutic primer prior to the placement of the indirect restoration. The research hypothesis stated that bonding of SLCs is negatively affected when the dentin is pre-treated, irrespective of the concentration of CHX solution used.

Material and methods

Forty-eight non-carious human third molars were collected in accordance with the local Institutional Review Board (# 1105) and with the informed consent of the donors. The teeth were stored in a 1.0% chloramine T solution at 4 °C and used within a month following extraction.

Composite overlay preparation

Resin composite (3 M ESPE Filtek Z250, St. Paul, MN, USA) discs were constructed using Teflon molds (12 mm in diameter, 6 mm in thickness). One side, designated as the bonding surface of each composite disc, was sandblasted with 50 μm aluminum oxide glass spheres (Sandblaster Micro Etcher, Buffalo Dental, San Ramon, CA, USA) for 10 s. The discs were then ultrasonic cleaned in distilled water for 3 min Silane primer (3 M ESPE RelyX Ceramic Primer, St. Paul, MN, USA) was applied by means of a mini-sponge (Microbrush International, Grafton, WI, USA) to the sandblasted surfaces for 1 min and air-dried.

Microtensile bond strength evaluation and SEM fractographic analysis

The occlusal enamel of thirty teeth was sectioned to expose flat dentin using a diamond-impregnated disc (Extec, Enfield, CT, USA) under water lubrication in a specific cutter machine (Isomet 1000, Buehler, Lake Bluff, IL, USA). In the event of pulp exposure, the tooth was discharged. The middle-crown dentin surfaces were further polished with a #600-grit silicon carbide (SiC) paper for 60 s in order to standardize the smear layer . The teeth were divided into six groups of five teeth each. The surfaces either received no further treatment (control) or were applied with a solution of 0.2% or 2.0% CHX in 0.05 M phosphate buffer (pH 7.4). Both CHX aqueous solutions were prepared using a chlorhexidine digluconate solution (C9394, Sigma–Aldrich, St. Louis, MO, USA). An aliquot of 50 μl of CHX solutions was applied onto the smear layer-covered dentin. The solution was gently rubbed by means of a sterilized microbrush and left on the dentin surface for 60 s. Excess solution was removed with a new brush, and the dentin surfaces were gently blow-dried in order to cause the water to evaporate until signs of visible moisture could no longer be observed. In the control group, the dentin surface was only gently blow-dried.

Two commercial SLC systems were selected for the present study. The composition and manufacturers of the self-adhesive resin luting cements are listed in Table 1 . The SLCs were manipulated and applied according to the manufacturer’s directions. Thereafter, cements were applied to the bonding surface of the composite discs and gently seated on the dentin surface using finger pressure. Thereafter, the teeth were placed under a constant seating pressure of 3.0 kg for 3 min . Cement excesses were removed and then polymerized for 60 s on all tooth surfaces: buccal, lingual, mesial, distal, and occlusal, using an LED light unit (Bluephase, Ivoclar Vivadent, Schaan, Liechtenstein) operating at 1000 mW/cm 2 .

Table 1
Self-adhesive resin luting cements used in this study.
Material Composition Manufacturer Batch#
RelyX U100 Glass powder, silica, calcium hydroxide, pigment, substituted pyrimidine, peroxy compound, initiator. Methacrylated phosphoric ester, dimethacrylate, acetate, stabilizer, initiator 3 M ESPE, Seefeld, Germany 287269
Multilink Sprint Dimethacrylates, adhesive monomers, fillers, initiators/stabilizers Ivoclar-Vivadent, Schaan, Liechtenstein K25709

Specimens were then stored in distilled water at 37 °C for 24 h. After storage, the teeth were longitudinally sectioned in order to obtain bonded stick-shaped specimens with a cross-sectional area of 0.8 mm 2 (±0.1) using the “non-trimming” technique for microtensile bond strength (μTBS) testing . Individual bonded sticks were positioned in a Universal Testing Machine (Instron model 4411, Canton, MA, USA) by means of cyanoacrylate-based glue (Zapit, Dental Ventures of America, CA, USA) and were then subjected to tensile forces at a cross-head speed of 0.5 mm/min until failure. The exact dimensions of each stick were measured with a digital caliper (Absolute Digimatic, Mitutoyo, Tokyo, Japan), and the ultimate tensile stress values were calculated and converted into MPa. Statistical differences among the mean bond strengths of the six groups were investigated by two-way ANOVA (factors-‘resin cements’ and ‘dentin pre-treatment’) and Tukey’s test at a pre-set alpha of 0.05.

The fractured samples were stored in plastic containers (Eppendorf Multi-vials, Electron Microscopy Sciences, WA, USA) containing a saline solution (0.9% sodium chloride) for 24 h. In order to chemically dry the specimens, a dehydration process was performed in ascending concentrations of ethanol (25%, 50%, 75%, 95%, and 100%) for at least 20 min in each concentration, and the specimens were immersed in hexamethyldisilazane (HMDS) for 10 min . The specimens were then sputter-coated (40 mA for 120 s) with gold/palladium (SCD 050; Balzers, Schaan, Liechtenstein) for scanning electron microscopy (SEM) analysis (JSM 5600LV, JEOL, Tokyo, Japan) operating at 15 kV. Failure modes were classified into the following categories : type I – Cohesive failure in the resin cement; type II – Cohesive failure in the dentin; type III – Adhesive failure; type IV – Mixed failure: cohesive in resin cement and dentin; and type V – Mixed failure: cohesive in resin cement and bonding region.

Energy-dispersive X-ray spectroscopy (EDS) analysis and micromorphology characterization of the smear layer

Eighteen teeth were used. Flat dentin surfaces were prepared as described above. The dentin surfaces were ground with a #600-grit SiC paper under running water for 60 s in order to create a standardized smear layer. The teeth were then divided into three groups according to dentin surface treatment: 0.2% CHX, 2.0% CHX, or no CHX treatment (control). The CHX solutions were applied using the same protocol as that described above. The teeth were then dehydrated in silica gel for 2 h. Afterwards, the specimens were submitted to carbon evaporation (SCD 050, Balzer Union AG, Balzers, Lichtenstein) for EDS analysis using an SEM (JSM 5600LV; JEOL, Tokyo, Japan) equipped with an X-radiation detector EDS (Voyager, Noran Instruments, Middleton, WI, USA). The EDS was equipped with an ultra-thin Norvar window and operated with a Windows NT-based (Vantage) digital microanalysis system. An elemental analysis of the dentin surface was then conducted in backscattered electron mode operating at 15 kV, at a working distance (WD) of 20 mm.

Material and methods

Forty-eight non-carious human third molars were collected in accordance with the local Institutional Review Board (# 1105) and with the informed consent of the donors. The teeth were stored in a 1.0% chloramine T solution at 4 °C and used within a month following extraction.

Composite overlay preparation

Resin composite (3 M ESPE Filtek Z250, St. Paul, MN, USA) discs were constructed using Teflon molds (12 mm in diameter, 6 mm in thickness). One side, designated as the bonding surface of each composite disc, was sandblasted with 50 μm aluminum oxide glass spheres (Sandblaster Micro Etcher, Buffalo Dental, San Ramon, CA, USA) for 10 s. The discs were then ultrasonic cleaned in distilled water for 3 min Silane primer (3 M ESPE RelyX Ceramic Primer, St. Paul, MN, USA) was applied by means of a mini-sponge (Microbrush International, Grafton, WI, USA) to the sandblasted surfaces for 1 min and air-dried.

Microtensile bond strength evaluation and SEM fractographic analysis

The occlusal enamel of thirty teeth was sectioned to expose flat dentin using a diamond-impregnated disc (Extec, Enfield, CT, USA) under water lubrication in a specific cutter machine (Isomet 1000, Buehler, Lake Bluff, IL, USA). In the event of pulp exposure, the tooth was discharged. The middle-crown dentin surfaces were further polished with a #600-grit silicon carbide (SiC) paper for 60 s in order to standardize the smear layer . The teeth were divided into six groups of five teeth each. The surfaces either received no further treatment (control) or were applied with a solution of 0.2% or 2.0% CHX in 0.05 M phosphate buffer (pH 7.4). Both CHX aqueous solutions were prepared using a chlorhexidine digluconate solution (C9394, Sigma–Aldrich, St. Louis, MO, USA). An aliquot of 50 μl of CHX solutions was applied onto the smear layer-covered dentin. The solution was gently rubbed by means of a sterilized microbrush and left on the dentin surface for 60 s. Excess solution was removed with a new brush, and the dentin surfaces were gently blow-dried in order to cause the water to evaporate until signs of visible moisture could no longer be observed. In the control group, the dentin surface was only gently blow-dried.

Two commercial SLC systems were selected for the present study. The composition and manufacturers of the self-adhesive resin luting cements are listed in Table 1 . The SLCs were manipulated and applied according to the manufacturer’s directions. Thereafter, cements were applied to the bonding surface of the composite discs and gently seated on the dentin surface using finger pressure. Thereafter, the teeth were placed under a constant seating pressure of 3.0 kg for 3 min . Cement excesses were removed and then polymerized for 60 s on all tooth surfaces: buccal, lingual, mesial, distal, and occlusal, using an LED light unit (Bluephase, Ivoclar Vivadent, Schaan, Liechtenstein) operating at 1000 mW/cm 2 .

Table 1
Self-adhesive resin luting cements used in this study.
Material Composition Manufacturer Batch#
RelyX U100 Glass powder, silica, calcium hydroxide, pigment, substituted pyrimidine, peroxy compound, initiator. Methacrylated phosphoric ester, dimethacrylate, acetate, stabilizer, initiator 3 M ESPE, Seefeld, Germany 287269
Multilink Sprint Dimethacrylates, adhesive monomers, fillers, initiators/stabilizers Ivoclar-Vivadent, Schaan, Liechtenstein K25709
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Effectiveness of self-adhesive luting cements in bonding to chlorhexidine-treated dentin
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