Determining the temporal development of dentin-composite bond strength during curing

Graphical abstract

Highlights

  • A new method is presented for measuring the rate of tooth-composite bond formation.

  • The peak rate of bond formation is ∼0.6 s −1 .

  • Rule of reciprocity does not hold; >2 J/cm 2 is required to form adequate bonding.

  • The adhesive-dentin interface is the weakest in the fully formed bond.

  • The current results will help determine the likelihood of interfacial debonding.

Abstract

Objectives

As composite restorations cure a competition develops between bond formation and shrinkage stress at the composite-dentin interface. Thus, understanding the temporal development of tooth-composite bond strength should enable better assessment of tooth-composite debonding.

Methods

In this study, bond strengths of composite-dentin specimens obtained from tensile test at different curing times were used to determine the bond formation rate. By varying the composite thickness and output from the curing light, their effects on the rate of bond formation for two different materials (a conventional and a bulk-fill composite) were also investigated. The proportions of cohesive and adhesive failure were determined by analysis of electron micrographs of the fractured surfaces.

Results

The development of dentin-composite bond strength ( S ) with time ( t ) can be described by the equation: S = S max (1 − exp(− αt )), where S max is the final bond strength (∼12 MPa for both composites) and α the rate of bond formation. Using bulk-fill and thinner specimens gave faster bond formation. In fact, the higher the irradiance at the interface, the higher the rate of bond formation. However, α had a maximum value of ∼0.6 s −1 and the rule of reciprocity did not hold. A minimum dose of ∼2 J/cm 2 was required to achieve adequate bond strength. The predominant failure mode changed from cohesive in the composite and adhesive to interfacial at the adhesive-dentin interface, indicating the latter to be the weakest link in the cured dentin-composite assemblies considered.

Significance

When combined with the temporal development of shrinkage stress, the current results will help determine the likelihood of tooth-composite debonding.

Introduction

Since their first introduction to restorative dentistry in the late 1950s, light-cured resin-based composites (RBCs) have undergone many improvements to enhance their mechanical properties and esthetics . These developments have made RBCs the most widely used dental materials for the repair of dental caries, crown fractures, and tooth wear. However, polymerization shrinkage, which can lead to marginal gaps and, ultimately, failure of the restoration through the development of secondary caries, remains a major drawback of RBCs . During polymerization, the methacrylate double bonds of the resin re-hybridized and link together to form polymeric networks resulting in volumetric shrinkage as the molecular distances and free volume are reduced .

The shrinkage kinetics of dental composites during light curing has been studied extensively using dilatometry , the bonded-disc method and digital image correlation (DIC) . The results show that the majority of shrinkage in light-cured composite resins occurs during the first few seconds of the polymerization process . Clinically, composite shrinkage is restricted by adhesion to the cavity walls, which results in the development of internal stresses . The latter are a product of the polymerization shrinkage, the constraints imposed by the surrounding tooth and the rigid nature of the reinforced cross-linked polymer network formed during curing . The shrinkage stress produced in thin disk specimens also shows rapid increase at the very beginning .

In conjunction with the development of RBCs, adhesive technology has evolved considerably during the last 50 years, leading to much improved tooth-composite bond strength , bonding procedures and antibacterial capabilities of adhesive compositions . For self-etch systems, the micro-tensile bond strength can reach 30 MPa ; whereas 40 MPa has been reported for etch-and-rinse systems . However, debonding of composite restorations still occurs clinically . Some studies further show that debonding can happen during the curing process . It should be recognized that, like shrinkage strain and stress, bonding between the tooth and composite also takes time to form, and the rate of formation among these entities could be different. There is, thus, a competition at the composite-tooth interface between bond formation and development of shrinkage stress. Even though the final bond strength may be higher than the final shrinkage stress, it is entirely possible that debonding can still happen when, for example, shrinkage stress is developing much faster than the tooth-composite bond. To avoid debonding, therefore, the developing bond strength must be higher than the developing shrinkage stress at all times. Studies on dental adhesion almost exclusively focus on measuring the final value of bond strength. To date, only Davidson et al. have investigated the bond strength development of a chemically cured micro-filled composite, which showed a gradual increase over a period of 30 min. They also investigated the small increase in bond strength of a light-activated composite over a period of 30 min post-curing. For a light-cured resin composite during curing, the increase will be much faster and its measurement more challenging. This probably explains the lack of studies on the temporal bond strength development of composite during its light curing.

The objective of this study is to determine the time-dependent formation of the dentin-composite bond by measuring the bond strength at different time points during light curing. The effects of the composite’s thickness and the intensity of the curing light on the rate of bond strength development will also be investigated for a conventional and a bulk-fill composite, in response to the latter’s introduction for deep cavities in recent years.

Materials and methods

Preparation of the specimens

Bovine incisors that had been cleaned and stored in distilled water at 4 °C for 2 weeks were used for preparing the specimens. Five tooth slices of about 3.5-mm thick were obtained from each bovine incisor ( Fig. 1 a and b ) by cutting the teeth perpendicular to the long axis using an Isomet™ low-speed diamond saw (Buehler, Lake Bluff, IL, USA). After removing the residual pulp tissue, a high-speed handpiece with a diamond bur was used to cut the tooth slices into two halves ( Fig. 1 c). These were then trimmed into dentin slabs ( Fig. 1 d) along the dashed lines shown in Fig. 1 c using the high-speed handpiece. The final dimensions of the dentin slabs were: width 3 mm × length 4 mm × height 2 mm ( Fig. 1 e). A total of 540 dentin slabs were prepared as described. Two commercial materials, a conventional composite (Filtek™ Z250, 3M ESPE, St. Paul, USA) and a bulk-fill composite (Filtek™ Bulk Fill, 3M ESPE, St. Paul, USA), both shade A2, were used in this study. The composition and other product information of these composites are listed in Table 1 . In order to investigate the effect of composite thickness on the rate of bond formation, blocks of cured composite (length 11 mm × width 8 mm) with three different thicknesses (1, 3 and 4 mm) were fabricated using the Teflon molds shown in Fig. 1 f, g and h. When filled with one of the composite materials, the molds were pressed with a glass slide, and the composite cured for 20 s using a LED curing unit (Elipar™ S10, 3M ESPE, St. Paul, USA) with an irradiance of 1200 mW/cm 2 . The three subgroups generated for each material were: Z1, Z3 and Z4 for Z250, and B1, B3 and B4 for Bulk Fill, representing 1-mm, 3-mm and 4-mm thick specimens, respectively. There were 45 blocks in each subgroup, giving a total of 270 blocks.

Fig. 1
Preparation of dentin and composite samples: (a) bovine incisor; (b) root dentin ring; (c) splitting of dentin ring; (d) dentin slabs; (e) dimensions of the dentin slab; (f) 4.0-mm thick composite block and its mold; (g) 3.0-mm thick composite block and its mold; (h) 1.0-mm thick composite block and its mold.

Table 1
Product information of composites used in the study.
Materials Code Shade Resin system Fillers Filler content Lot # Expiration date
Filtek™ Z250 Z A2 • UDMA
• Bis-EMA(6)
• Bis-GMA
Zirconia/silica 82 wt%
60 vol%
N670334 2018-02
Filtek™ bulk fill posterior restorative B A2 • UDMA
• AUDMA
• 1,12-dodecane-DMA
Ytterbium trifluoride
Zirconia/silica
76.5 wt%
58.4 vol%
N665130 2018-02

Bonding procedure

Before assembling a dentin-composite specimen, a layer of adhesive resin was pre-cured on both the inner (next to the root canal) surface of the dentin slab and the upper surface of composite block. To standardize the procedure, dentin surfaces were polished with a 600-grid SiC paper (Buehler, Lake Bluff, IL, USA) under water irrigation for 30 s. The exposed dentin surfaces were etched with 35 percent phosphoric acid (Scotchbond Etchant, 3M ESPE) for 15 s, rinsed for 15 s using distilled water and blot-dried for 2 s. Immediately after blot drying, two consecutive coats of adhesive (Adper™ Single Bond Plus, 3M ESPE, St. Paul, USA) were applied and agitated gently for 15 s in total using a fully saturated applicator. The adhesive was then gently air dried for 5 s, followed by light curing for 10 s using the same LED curing unit used for curing the composite blocks. A similar placement and curing procedure was used for the adhesive layer on the cured composite block.

Tracking the development of bond formation

Bond strength measurements to capture the time-dependent dentin-composite bond kinetics required bond strength measurements at precise time points during cure because the photo-curing reaction is very fast. Thus, the universal testing system (MTS 858 Mini Bionix II, Eden Prairie, MN, USA) used in this study for bond strength testing was augmented with a curing light (S-10™, 3M ESPE, St. Paul, USA) equipped with an adjustable power supply (3540A, Agilent, Santa Clara, CA, USA) and a USB Multifunction DAQ control system (UV-H3, LabJack, Lakewood, CO, USA) that could handle simple analog and digital signals. A schematic diagram of the testing system is shown in Fig. 2 a .

Fig. 2
Schematic diagram of the test system: (1) upper holder, (2) dentin slab, (3) lower holder, (4) cured composite block, (5) curing unit, (6) uncured composite, (7) uncured composite pressed into a thin layer.

During each test, the composite block was secured in the lower holder with the pre-cured adhesive facing upward; while the dentin slab was held by the upper holder (labeled 4, 3, 2, 1 in Fig. 2 a, respectively). First, the dentin slab was lowered onto the composite block with minimal force to establish a reference position ( Fig. 2 b). Next, the dentin slab was returned to the starting position ( Fig. 2 c) and on top of the composite block was placed 2.5 μg of the same but uncured composite (6 in Fig. 2 c). The dentin slab was then lowered back down to a position 0.2 mm from the reference position ( Fig. 2 d), pressing the uncured composite into a thin layer of 0.2-mm thick (7 in Fig. 2 d) in the process. The dentin slab was kept at this position for 10 s allowing the soft uncured composite to fully wet the dentin slab and the cured composite block. Then, a signal was sent from the control system to trigger the curing light (5 in Fig. 2 a) placed underneath the cured composite block. The intensity of the curing light was set at 1200 mW/cm 2 for the first set of tests; other light intensities were used for subsequent tests (see later). After a pre-defined length of time, another signal given by the control system turned off the curing light and triggered the universal testing machine to pull the dentin slab away from the composite block. The dentin-composite bond strength at that particular time point was measured by the load cell to which the lower holder was fixed. Measurements at 8 time points (0.5, 1.0, 2.5, 5.0, 7.0, 10, 15 and 20 s) were made to determine the development of bond strength during curing. Another measurement was made 580 s after the 20-s cure to establish the final bond strength. 45 specimens were prepared for each thickness, thus providing 5 measurements per time point.

Quantitative analysis of failure surfaces using SEM

Scanning electron microscopy (SEM) was used to examine the morphology of the dentin surface of all the specimens after bond strength testing. The fractured dentin slabs were air-dried overnight before being mounted on aluminum stubs with carbon tapes. They were examined by a semi-environmental tabletop SEM (TM-3000, Hitachi, Japan) operated at a 15-kV accelerating voltage. No conductive coating was applied to the samples.

SEM pictures taken at ×50 magnification were processed to analyze the facture surfaces of the dentin slabs quantitatively. Using an image processing software (Adobe Photoshop CS6, California, USA), an area covering most of the fracture surface (780 × 960 pixels) was clipped from each SEM image. The format of the clipped images was then changed to 8-bit, and the brightness and contrast of each component (composite, adhesive and dentin) were adjusted to maximize their differences with the other components. Using Image-J, available as freeware from the National Institutes of Health ( ), the gray value threshold of the images was adjusted to highlight the composite, adhesive or exposed dentin regions, separately. Area of the highlighted region was calculated and divided by the entire area of the clipped fracture surface to obtain the fractional area of each component.

Measurement of irradiance reaching the composite-adhesive interface

The actual irradiance reaching the composite-adhesive interface would be different from that output from the curing light, due to specular reflection, scattering and absorption by the composite block. Therefore, to better quantify the effect of composite thickness on the formation of bond strength, the irradiance emitted from the top surface of the composite block was measured with a radiometer (P-9710, Gigahertz-Optik, Munich, Germany). To ensure accurate measurement, the optical detector of the radiometer was placed as close as possible to the upper surface of composite block. Five tests were performed for each thickness. The irradiance measured was then multiplied by the time of curing to estimate the dose supplied to the composite-adhesive interface.

Evaluating effect of interfacial irradiance on the rate of bond formation

To more fully evaluate the effect of irradiance on the rate of bond formation, further blocks of Bulk Fill with different thicknesses were tested using different outputs from the curing light. Table 2 summarizes the thickness and light intensity values used in this second set of tests for these additional specimens ( n = 45 per group), which were prepared and tested using the same procedures as described above for the first set of tests.

Table 2
Thickness of composite blocks used and interfacial irradiance estimated.
First set of tests (Z 250 and Bulk Fill) Second set of tests (Bulk Fill only)
Thickness (mm) Output from curing light (mW/cm 2 ) Interfacial irradiance (mW/cm 2 ) Thickness (mm) Output from curing light (mW/cm 2 ) Interfacial irradiance (mW/cm 2 )
1.0 (Z) 1200 246.7 2.0 200 28.1
3.0 (Z) 1200 79.2 2.0 407 77.4
4.0 (Z) 1200 28.7 2.0 1475 240.9
1.0 (B) 1200 328.9 2.0 1910 328.9
3.0 (B) 1200 104.3 1.5 1980 406.3
4.0 (B) 1200 65.8

Statistical analysis

The software SPSS (v.15.0, IBM Corp, Armonk, NY) was employed for the statistical analysis. The bond strength measured at 600 s of the different groups ( n = 5) were analyzed using the Kruskal–Wallis test, with the p -value for statistical significance set at 0.05. The least-square method was used to fit a theoretical model to the bond strength vs. time data.

Materials and methods

Preparation of the specimens

Bovine incisors that had been cleaned and stored in distilled water at 4 °C for 2 weeks were used for preparing the specimens. Five tooth slices of about 3.5-mm thick were obtained from each bovine incisor ( Fig. 1 a and b ) by cutting the teeth perpendicular to the long axis using an Isomet™ low-speed diamond saw (Buehler, Lake Bluff, IL, USA). After removing the residual pulp tissue, a high-speed handpiece with a diamond bur was used to cut the tooth slices into two halves ( Fig. 1 c). These were then trimmed into dentin slabs ( Fig. 1 d) along the dashed lines shown in Fig. 1 c using the high-speed handpiece. The final dimensions of the dentin slabs were: width 3 mm × length 4 mm × height 2 mm ( Fig. 1 e). A total of 540 dentin slabs were prepared as described. Two commercial materials, a conventional composite (Filtek™ Z250, 3M ESPE, St. Paul, USA) and a bulk-fill composite (Filtek™ Bulk Fill, 3M ESPE, St. Paul, USA), both shade A2, were used in this study. The composition and other product information of these composites are listed in Table 1 . In order to investigate the effect of composite thickness on the rate of bond formation, blocks of cured composite (length 11 mm × width 8 mm) with three different thicknesses (1, 3 and 4 mm) were fabricated using the Teflon molds shown in Fig. 1 f, g and h. When filled with one of the composite materials, the molds were pressed with a glass slide, and the composite cured for 20 s using a LED curing unit (Elipar™ S10, 3M ESPE, St. Paul, USA) with an irradiance of 1200 mW/cm 2 . The three subgroups generated for each material were: Z1, Z3 and Z4 for Z250, and B1, B3 and B4 for Bulk Fill, representing 1-mm, 3-mm and 4-mm thick specimens, respectively. There were 45 blocks in each subgroup, giving a total of 270 blocks.

Fig. 1
Preparation of dentin and composite samples: (a) bovine incisor; (b) root dentin ring; (c) splitting of dentin ring; (d) dentin slabs; (e) dimensions of the dentin slab; (f) 4.0-mm thick composite block and its mold; (g) 3.0-mm thick composite block and its mold; (h) 1.0-mm thick composite block and its mold.

Table 1
Product information of composites used in the study.
Materials Code Shade Resin system Fillers Filler content Lot # Expiration date
Filtek™ Z250 Z A2 • UDMA
• Bis-EMA(6)
• Bis-GMA
Zirconia/silica 82 wt%
60 vol%
N670334 2018-02
Filtek™ bulk fill posterior restorative B A2 • UDMA
• AUDMA
• 1,12-dodecane-DMA
Ytterbium trifluoride
Zirconia/silica
76.5 wt%
58.4 vol%
N665130 2018-02
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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Determining the temporal development of dentin-composite bond strength during curing
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