Behavior of failed bonded interfaces under in vitrocariogenic challenge

Highlights

  • This is the first study to evaluate the caries susceptibility of failed bonded interfaces.

  • The failed bonded samples showed an intermediate caries development between the non-bonded groups.

  • Lesion development was lowest for the non-bonded samples with bonding applied on the dentin and highest for the non-bonded samples with no bonding on dentin.

  • For the non-bonded groups, the self-etch adhesive showed deeper lesion and more mineral loss than the etch-and-rinse bonding material.

  • Mechanical loading aging resulted in deeper caries lesions than the water storage aging.

Abstract

Objective

This in vitro study aimed to compare dentin wall caries development at different composite–dentin interfaces.

Methods

Dentin samples (10.4 mm 2 ) were restored with composite resin using two adhesive systems (etch-and-rinse and self-etch techniques). Different composite–dentin interfaces with gaps were produced: (a) failed bonded, which were fractured at interface after being submitted to aging protocols (no aging, mechanical loading or water storage); (b) non-bonded interfaces, both without any adhesive material or with adhesive material applied only on the dentin. Adhesively fractured and non-bonded samples were subjected to a lactic acid gel (pH = 5) caries model with a continuous opening/closing movement of the interfacial gap for 10 days. Transverse wavelength-independent microradiographs were taken, and lesion depth and mineral loss were measured. Data were analyzed with linear mixed-effects regression models.

Results

Caries development differed among the composite–dentin interfaces ( p < 0.001). The non-bonded interface with adhesive material on the dentin showed less lesion depth than the failed bonded groups, while the non-bonded interface without adhesive on dentin showed the deepest wall lesions. Difference between the adhesive systems was observed only in the non-bonded groups ( p = 0.003), with the self-etch adhesive applied on the dentin showing more severe lesions. Samples broken after mechanical loading aging showed deeper lesions than those broken after water storage ( p < 0.001).

Significance

Composite–dentin interfaces failed after aging presented different demineralization from interfaces that were never bonded, indicating that the restorative treatment changes the tissue in a way relevant to secondary caries development.

Introduction

In dental practice a large proportion of time is devoted to replacing restorations. The most common reasons for restoration failure are caries and fracture, with caries at the restoration margin as the main cause of composite restoration failures in high caries-risk patients . It has been proposed that secondary caries is actually primary caries at a restoration margin ; however, interfacial gaps have been reported to result in the unique feature of secondary wall lesions .

The adhesive interface is reported as an instable factor in composite restorations . Water sorption has been shown to contribute to hydrolysis, plasticization of the polymer, promoting deterioration of the mechanical properties of the materials . Moreover, cariogenic bacteria may show esterase activity at a sufficient level to induce hydrolysis-mediated degradation of the composite and adhesive, leaving the restorative materials open to biological breakdown .

Several studies have evaluated caries wall lesion development in restoration gaps, but most did not include the adhesive bonding step. The presence of a bonding material has already been shown to influence caries development . However, in some studies bonding was only applied either on the restorative material, or on the dentin, but did not include the complete adhesive procedure, as this would not leave any gaps to investigate. Clinically, it is much more likely to encounter restoration gaps where the adhesive bond was present at baseline but has failed over time. Failure could start already during the adhesive procedure due to polymerization shrinkage, incomplete polymerization, or technical errors, or could be the result of aging processes. A failure of the adhesive interface between a composite restoration and enamel or dentin may thus result in different types of ‘failed bonded’ interfaces, which may react differently to cariogenic challenges. For clinical relevance of in vitro secondary caries models, it would be interesting to observe whether distinct failed bonded interfaces react differently to a caries challenge.

The objective of this study was to compare in vitro dentin caries wall lesion development of ‘failed bonded’ composite–dentin interfaces with non-bonded situations where no adhesive was used or adhesive was applied only on the dentin, for two different adhesive systems. We hypothesized that lesion depth and mineral loss would be deeper for the ‘failed bonded’ interfaces than for interfaces with adhesive applied on dentin, but less deep than a situation with no adhesive.

Materials and methods

Seventy-eight freshly extracted sound human molars were selected, cleaned and stored in water. Flat midcoronal dentin surfaces were exposed using #150 grit SiC paper (Siawat Abrasives, Frauenfeld, Switzerland) under running water. Complete removal of enamel was confirmed by stereomicroscopic examination. Subsequently, the dentin surfaces were polished using #600 grit SiC paper (Siawat Abrasives, Frauenfeld, Switzerland).

Sample preparation

One trained operator performed all adhesive and restorative procedures. The samples were prepared in different ways in order to produce different composite–dentin interfaces, all with an interfacial gap. Fig. 1 illustrates schematically the steps of the study. All groups with adhesive material were made with one of two adhesive systems: a 2-step self-etch adhesive (Clearfil™ SE Bond – CSE, Kuraray Noritake, Tokyo, Japan) or a universal adhesive system applied with a 2-step etch-and-rinse technique (Scotchbond™ Universal – SU, 3M ESPE, St. Paul, MN, USA). This resulted in the following 9 experimental groups:

  • Failed bonded, bond broken after no aging (CSE/SU).

  • Failed bonded, bond broken after mechanical aging (CSE/SU).

  • Failed bonded, bond broken after water storage aging (CSE/SU).

  • Non-bonded, with adhesive material on the dentin (CSE/SU).

  • Non-bonded, with no adhesive material.

Fig. 1
Schematic drawing of the study design: (a) bonded groups; (b) creating the failed bonded interfaces; (c) non-bonded groups; (d) cariogenic challenge; (e) lesion measurement at three locations. Abbreviations : Comp (Composite), Dent (Dentin), CSE (Clearfil SE Bond), SU (Scotchbond Universal).

Failed bonded samples ( Fig. 1 a)

Sixty teeth were used to produce failed bonded samples. These samples were optimally bonded with one of the two adhesive systems ( n = 30 teeth). The prepared teeth were embedded in acrylic resin samples (16 mm high), leaving the prepared occlusal dentin surface free. The adhesive systems were applied following the manufacturers’ instructions and light-cured for 10 s, using a LED curing device with an intensity of ≈900 mW/cm 2 (Fusion™ S7 Curing Light, DentLight Inc., Richardon, USA). Restorations of ±4 mm in height were built up incrementally with a composite resin (Clearfil™ AP-X, Kuraray Noritake, Tokyo, Japan). Each composite resin increment was light-cured for 20 s. After the restorative procedures, the teeth remained stored in distilled water for 24 h, at 37 °C. Subsequently, the restored teeth were randomly allocated ( n = 10 per group) to one of three aging conditions: no aging, water-storage aging and mechanical loading.

All restored teeth were sectioned into rectangular composite–dentin samples with an approximate cross-sectional adhesive area of 10.4 mm 2 (3.2 mm × 3.2 mm × 8 mm), using a low speed diamond saw under continuous water-cooling. Every restored tooth yielded two composite–dentin samples ( n = 20 samples per group).

Mechanical loading aging was performed before sectioning on whole restored teeth, using a Rub&Roll mechanical device, at 30 N of force, 0.4 Hz, during 3-week, resulting in 750,000 mechanical cycles. The device used for applying mechanical loading (Rub&Roll) is described in detail elsewhere ( Fig. 2 ). Water storage aging was performed after sectioning, and consisted of storage in distilled water (37 °C, changed weekly) for 5 months. The non-aged samples were stored in distilled water, at 37 °C, for 24 h after sectioning.

Fig. 2
General schematic overview of the Rub&Roll device used for mechanical load aging. The Rub&Roll consists of a container in which a cylinder is placed that is driven by a stirring motor (A). The samples are placed in the cylinder (B). Between the cylinder and the inner wall of the container there is a space where a rod (black arrow) is placed (C). When the stirring motor (cylinder) starts rotating, the rod rotates in an opposite direction, rolling over the specimens mounted in the cylinder, applying a standardized loading force.

Creating the failed bond ( Fig. 1 b)

For the non-aged group after 24 h, and for the aged groups after mechanical and water storage aging conditions, the composite–dentin blocks were broken to create failed bonded interfaces. The samples were fixed onto polystyrene bars (Stripstyrene, Item 32, No. 176, 100 × 125″, Evergreen scale models, Kirkland, WA 98034) of 3.2 mm × 3.2 mm × 25 mm, with the adhesive interface placed in the middle of the bar. Subsequently, the samples were subjected to 3-point flexural loading to promote the fracture of the composite–dentin interface, using Universal Testing Machine (Materials Testing Machine LS1, Lloyd Materials Testing, Hampshire, UK) at 1 kN, and 1 mm/min cross speed. The stylus was positioned on the bar exactly above the interface, thus promoting interface fracture. The polystyrene bar remained intact during this procedure, and the dentin and composite blocks, now with an interfacial gap, remained attached to the bar. The load at fracture for each sample was recorded and the bond strength ( σ ) in MPa was obtained with the formula σ = F / A , where F = load for specimen rupture (in Newton) and A = bonded area (mm 2 ). To determine the area, the formula to calculate A (10.4 mm 2 ) = width (3.2 mm) × height (3.2 mm) was employed.

The broken samples were observed in stereomicroscope (40× magnification) and baseline microradiographs (see later) were assessed to categorize the type of fracture. Only samples with adhesive fractures were included in the cariogenic challenge test for further analysis. Samples showing mixed or cohesive failures were discarded, as the focus of this study was to evaluate the behavior of the adhesively failed restoration interface with respect to the caries development. Microradiographs were also used to measure gap size.

Non-bonded samples ( Fig. 1 c)

Eighteen teeth were used to produce non-bonded samples with interfacial gaps. Adhesive material was either not used at all (no adhesive), or placed on the dentin wall using one of the two adhesive systems (adhesive on dentin) ( n = 6 teeth). Dentin blocks of 3.2 mm × 3.2 mm × 4 mm dimensions (two dentin blocks per teeth, n = 12 samples per group) were prepared using a low speed diamond saw under continuous water-cooling and were fixed onto polystyrene bars (3.2 mm × 3.2 mm × 25 mm). For the groups with the adhesive on dentin, the adhesive materials were applied on the dentin surface according the manufacturers’ instructions. For the other samples no adhesive was applied.

The polystyrene bars with mounted dentin samples were secured in a vice, and matrices were placed to create composite–dentin gaps after composite application. The gap sizes as measured in the adhesively failed bonded samples ranged between 50 and 300 μm. In the non-bonded samples matrices were used to create gaps of similar size distribution. Three matrix types were used: plastic matrices of 50 and 200 μm thickness and metallic matrices of 300 μm thickness. The dentin blocks mounted on the polystyrene bars with the matrix in position were restored with resin composite material (Clearfil™ AP-X, Kuraray) parallel to the dentin wall, creating composite resin blocks (3.2 mm × 3.2 mm × 4 mm) using a mold. In this way, final sample configuration was similar for failed bonded and non-bonded samples.

Cariogenic challenge ( Fig. 1 d)

All samples, fixed on the polystyrene bars, were suspended in a cariogenic medium and submitted to cariogenic challenge using a hydrodynamic flow model, to enhance caries development in the gaps . The polystyrene bars rested on the edges of the reservoir, and horizontal movement was limited by a putty mold. Two layers of an acid-resistant varnish had been painted onto the dentin surfaces of the samples before the cariogenic challenge, except the composite–dentin interface. Demineralization was produced with lactic acid gel (10 g of 0.1 M lactic acid + 980 ml distilled water + 9.5 ml of 10 M KOH) for 10 days (pH = 5). The solution was renewed every 5 days. A modified brushing machine (instead of toothbrushes, acrylic points were mounted) was used to load the samples in such a way that the gaps intermittently opened up and returned to their resting state (16×/min with 300 g of load), creating a cariogenic fluid movement in and out of the gap, enhancing the demineralizing challenge at the interface .

Transversal wavelength independent microradiography ( Fig. 1 e)

Caries wall lesion development in dentin of the interfaces was evaluated using Transversal Wavelength Independent Microradiography (T-WIM). Microradiographs were made at baseline ( T 0 ) and after 10 days ( T 10 ) of cariogenic challenge. The microradiography settings were 60 kV, 30 mA and an exposure time of 8 s. A step wedge with the same absorption coefficient as the tooth material (94% Al/6% Zn alloy) was used for proper quantitative measurement of lesion depth (LD, μm) and mineral loss (ML, μm.vol%). After exposure, the films were developed (10 min), fixed (7 min), rinsed, and dried. A digital image of each sample was recorded with a light microscope (Leica Microsystems, Wetzlar, Germany) with a magnification of 10× and a CMOS camera (Canon EOS 50D, Tokyo, Japan).

Lesion depth and mineral loss for T-WIM were measured using thresholds of 8% (tissue edge) and 43.2% (sound) mineral. Each sample was measured with a software program (T-WIM calculation program, version 5.25, J.de Vries, Groningen, NL) at three locations ( Fig. 1 e): location 1 (near to gap entrance, 200 μm distance from the sample surface); location 2 (in the middle of the sample) and location 3 (at 200 μm distance from the base). Baseline measurements ( T 0 ) were subtracted from measurements after 10 days ( T 10 ) for estimation of true lesion depth and mineral loss. The subtracted values were used in the statistical analysis.

Statistical analysis

The bond strength values in MPa were subjected to 2-way ANOVA (aging conditions × adhesive materials) and post hoc Tukey test. The correlation between lesion depth and mineral loss was evaluated using Pearson’s correlation coefficient. The effect of interface conditions on both lesion severity outcomes was analyzed using linear mixed-effects regression models, with gap size and lesion location as added factors. All tests were conducted using the statistical software package R (version 3.0.1, R Foundation for Statistical Computing, Vienna, Austria), with the significance level set at 5%.

Materials and methods

Seventy-eight freshly extracted sound human molars were selected, cleaned and stored in water. Flat midcoronal dentin surfaces were exposed using #150 grit SiC paper (Siawat Abrasives, Frauenfeld, Switzerland) under running water. Complete removal of enamel was confirmed by stereomicroscopic examination. Subsequently, the dentin surfaces were polished using #600 grit SiC paper (Siawat Abrasives, Frauenfeld, Switzerland).

Sample preparation

One trained operator performed all adhesive and restorative procedures. The samples were prepared in different ways in order to produce different composite–dentin interfaces, all with an interfacial gap. Fig. 1 illustrates schematically the steps of the study. All groups with adhesive material were made with one of two adhesive systems: a 2-step self-etch adhesive (Clearfil™ SE Bond – CSE, Kuraray Noritake, Tokyo, Japan) or a universal adhesive system applied with a 2-step etch-and-rinse technique (Scotchbond™ Universal – SU, 3M ESPE, St. Paul, MN, USA). This resulted in the following 9 experimental groups:

  • Failed bonded, bond broken after no aging (CSE/SU).

  • Failed bonded, bond broken after mechanical aging (CSE/SU).

  • Failed bonded, bond broken after water storage aging (CSE/SU).

  • Non-bonded, with adhesive material on the dentin (CSE/SU).

  • Non-bonded, with no adhesive material.

Fig. 1
Schematic drawing of the study design: (a) bonded groups; (b) creating the failed bonded interfaces; (c) non-bonded groups; (d) cariogenic challenge; (e) lesion measurement at three locations. Abbreviations : Comp (Composite), Dent (Dentin), CSE (Clearfil SE Bond), SU (Scotchbond Universal).

Failed bonded samples ( Fig. 1 a)

Sixty teeth were used to produce failed bonded samples. These samples were optimally bonded with one of the two adhesive systems ( n = 30 teeth). The prepared teeth were embedded in acrylic resin samples (16 mm high), leaving the prepared occlusal dentin surface free. The adhesive systems were applied following the manufacturers’ instructions and light-cured for 10 s, using a LED curing device with an intensity of ≈900 mW/cm 2 (Fusion™ S7 Curing Light, DentLight Inc., Richardon, USA). Restorations of ±4 mm in height were built up incrementally with a composite resin (Clearfil™ AP-X, Kuraray Noritake, Tokyo, Japan). Each composite resin increment was light-cured for 20 s. After the restorative procedures, the teeth remained stored in distilled water for 24 h, at 37 °C. Subsequently, the restored teeth were randomly allocated ( n = 10 per group) to one of three aging conditions: no aging, water-storage aging and mechanical loading.

All restored teeth were sectioned into rectangular composite–dentin samples with an approximate cross-sectional adhesive area of 10.4 mm 2 (3.2 mm × 3.2 mm × 8 mm), using a low speed diamond saw under continuous water-cooling. Every restored tooth yielded two composite–dentin samples ( n = 20 samples per group).

Mechanical loading aging was performed before sectioning on whole restored teeth, using a Rub&Roll mechanical device, at 30 N of force, 0.4 Hz, during 3-week, resulting in 750,000 mechanical cycles. The device used for applying mechanical loading (Rub&Roll) is described in detail elsewhere ( Fig. 2 ). Water storage aging was performed after sectioning, and consisted of storage in distilled water (37 °C, changed weekly) for 5 months. The non-aged samples were stored in distilled water, at 37 °C, for 24 h after sectioning.

Fig. 2
General schematic overview of the Rub&Roll device used for mechanical load aging. The Rub&Roll consists of a container in which a cylinder is placed that is driven by a stirring motor (A). The samples are placed in the cylinder (B). Between the cylinder and the inner wall of the container there is a space where a rod (black arrow) is placed (C). When the stirring motor (cylinder) starts rotating, the rod rotates in an opposite direction, rolling over the specimens mounted in the cylinder, applying a standardized loading force.
Only gold members can continue reading. Log In or Register to continue

Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Behavior of failed bonded interfaces under in vitrocariogenic challenge
Premium Wordpress Themes by UFO Themes