Evaluation of shear bond strength of various adhesives under Simulated intrapulpal pressure: An in vitro study.

INTRODUCTION: The presence of pulpal fluid can influence dentin bonding. This study aimed to evaluate the shear bond strength of universal dental adhesives under simulated intrapulpal pressure.
MATERIALS AND METHODS: Forty intact maxillary premolars were distributed into four groups (Group 1 - 3M ESPE Adper Single Bond 2 total etch adhesive, Group 2 - 3M ESPE Single Bond Universal, Group 3 - Prime and Bond universal adhesive, and Group 4 - Ivoclar Tetric N Bond Universal). Adhesive restorative procedures were carried under intrapulpal pressure simulation following which it was subjected to a shear load of 0.5 mm/min. Fracture mode analysis was performed using a stereomicroscope. The data obtained were subjected to ANOVA and Tukey post hoc test with a P < 0.05.
RESULTS: Highest bond strength was exhibited by Group 3 (7.01 ± 2.02 MPa) and least by Group 1 (5.36 ± 3.03 MPa). However, there were no statistical differences among the groups. Group 1 and 2 showed mostly cohesive failure, whereas Group 3 and 4 showed mixed failure.
CONCLUSION: The results demonstrated that the experimental universal adhesive agents exhibited comparative shear bond strength when subjected to pulpal pressure. Pulpal pressure has a significant effect on bond strength. Copyright:

INTRODUCTION

Retention while bonding is considered a significant aspect of composite resin restoration. The outcome of a composite restoration relies on its ability to bond to the tooth structure. Achieving a good bond between the resin and tooth substrate reduces microleakage and improves restoration.[1] Universal or multimode adhesives were brought into the market in 2011. Clinical procedures were more comfortable with these adhesives. They can be used in etch-and-rinse or self-etch mode on dentin and etch-and-rinse mode on enamel.[2] Functional monomers in these adhesives help in chemical adhesion to dentin.[3]

Recent concepts of self-etching primers and adhesives showed promising results, both scientifically and clinically.[34] Reduced clinical time with good bonding to dentin and postoperative comfort popularized these adhesives among clinicians.[5] Dentin is inherently a hydrated tissue. Due to complex hydrated structure, bonding to dentin is considered more difficult than bonding to enamel.[67] There is a small but constant pressure exerted by the pulp through the dentinal tubules, which results in an outward flow of the dentinal fluid at the dentinoenamel junction. This small flow opposes the monomer resin's diffusion toward the pulp, affecting the bonding of the composite resin restoration.[8910] Currently, several universal adhesives in the market are said to have achieved acceptable levels of bond strength.

AdperTM Single Bond (3M ESPE, St. Paul, MN) is a total-etch, visible light-activated dental bonding agent comprising 10% by weight of 5 nm diameter silica filler. It is used in etch-and-rinse mode. Single bond Universal Adhesive (3M ESPE, St. Paul, MN, USA) consists of MDP, which helps in chemical bonding to hydroxyapatite through “nano-layering.” When used in self-etch mode on dentin, the lowest immediate nanoleakage can be achieved after 1 year of water storage.[1112] Prime and bond adhesive (Dentsply Caulk, Milford, MA) is a universal acetone- and water-based HEMA-free adhesive system. It interacts chemically with dentin.[13] Tetric N-Bond Universal (Vivadent-Ivoclar, Schaan, Liechtenstein) is a universal adhesive used for direct and indirect bonding procedures.

Literature is replete with several studies conducted to evaluate the bond strength of various universal adhesives. However, the effect of pulp pressure on the universal adhesives bonding mechanism has not been explored.[14] Hence, the study aimed to collate and compare the shear bond strength of universal bonding agents and evaluate the fracture mode of this composite resin restoration under simulated intrapulpal pressure.

MATERIALS AND METHODS

The study was conducted after obtaining approval from the institutional ethical committee (Protocol ref number 19007). The sample size for the study was calculated using the group comparison formula.

DF = N−k = kn−k = k (n−1) N = n × k

DF = Degrees of freedom,

N = Total number of subjects,

n = Number of subjects in the group,

k = Number of groups.

Forty extracted maxillary first premolars with straight, noncalcified, and nontortuous canals were chosen and placed in a 0.1% thymol solution. Forty coronal segments were obtained by slicing the roots at 2 mm below the cement-enamel junction (CEJ) with a water-cooled diamond disc (Isomet 1000, Buehler, Lake Bluff, IL, USA). A parallel cut 3 mm above the CEJ was made on each coronal segment's occlusal enamel exposing the dentin. The surface was polished with silicone carbide papers (Volo Waterproof Paper, Grit: 100, India). This helps to create a smooth substrate for bonding on dentin. The pulpal remnants were removed without altering the predentin surface. A caliper (Pincer Dental Lab Crown Tools, India) was used for measuring the dentin thickness of 0.7–0.8 mm. Each specimen was then attached to a Plexiglas (Universal Plexiglas, India) using cyanoacrylate adhesive. A perforation was created on the Plexiglas, and a rubber catheter (Bhairavi surgical, Maharashtra) was connected to it. It was glued on to the Plexiglas using cyanoacrylate adhesive. Before the application of the bonding agents, the other end of the rubber catheter of each specimen was connected to a hydraulic pressure device designed that creates water pressure during the restorative procedure based on the model designed by Sauro et al. (2007).[15]

A wide flat tub with water was placed at a height. An elevated setup was prepared to pressurize the water into the pulp chamber. The elevated setup helped to pressurize the water that moved into the dentinal tubules. A wide tub was used as a reservoir to decrease the uncertainty of pressure variation. The uncertainty could be caused due to the drop in the level of water. Bernoulli's equation was applied to maintain the height of the water.

ρ (Hg) × g × h (Hg) = ρ (w) × g × h (w)

h (w) = ρ (Hg) × g × h (Hg)/ρ (w) × g

h (w) is the height of the water column required to produce 8–10 mmHg pressure (normal pulpal pressure). ρ is the density and g is the acceleration due to gravity. The water column's height from the dentinal surface was kept at 13 mm in accordance with the formula. 10% of the original height was introduced to overcome the fluid pressure losses due to the flow-through narrow pipes and other minor factors. The final height was maintained at 14.3 mm. Figure 1 depicts the schematic representation of the apparatus setup to simulate intrapulpal pressure.

Figure 1

Hydraulic device to simulate intrapulpal pressure

Bonding procedures:

Tooth sections attached to the Plexiglas was connected to the hydraulic unit that simulated the pulpal pressure during the bonding procedure.

Group 1: Adper single bond 2 (n = 10)

Etchant (37% phosphoric acid gel, Joinville, SC, Brazil) was applied over the dentin using microtip brushes for 15 s and then washed for 10 s. The cotton pellet was used to blot excess water, following which 2–3 subsequent adhesive coats were applied to the etched dentin. It was then air thinned gently for 5 s for the solvent to evaporate. It was then light cured for 10 s using (high mode/1200 mW/cm2, Blue phase Style 20i, Ivoclar Vivadent, Schaan, Liechtenstein) LED curing light.

Group 2: Single bond universal adhesive (n = 10)

The adhesive was applied over the tooth surface by scrubbing action for 20 s. It was air thinned gently for 5 s, followed by light curing for 10 s.

Group 3: Prime and bond universal adhesive (n = 10)

The adhesive was taken on the applicator tip and agitated on the dentinal surface for 20 s. It was air thinned gently for 5 s. It was then cured for 10 s with a light cure unit.

Group 4: Ivoclar Tetric N bond universal adhesive (n = 10)

The desired amount of Tetric N-Bond Universal was dispensed into a mixing well and applied over the dentinal surface. The adhesive was scrubbed into the tooth surface for at least 20 s. It was air thinned gently for 5 s for the solvent to evaporate. It is light cured for 10 s.

The following bonding procedures were carried out as per the protocols. LED light-curing unit (high mode/1200 mW/cm2, Blue phase Style 20i, Ivoclar Vivadent, Schaan, Liechtenstein) was aimed at 5 mm from the tooth surface. Following bonding, composite (Filtek Z350; 3M EPSE, St Paul, MN) was built up incrementally up to 5 mm thickness.

Shear bond strength evaluation

Following the restoration, the tooth section was removed from the Plexiglas. It was then embedded onto an acrylic block that fits onto the jig of a universal testing machine. A universal testing machine (Instron 3342, Canton, USA) was used to subject the specimens with an occlusal-gingival load applied at a speed of 1.0 mm/min. A ball bearing, 4.76 mm in diameter, was mounted on a specially constructed testing head that comes into contact with the restoration. Specimens were examined under the stereomicroscope (Leica MZ6, Mannheim, Germany) for fracture mode.

The modes of fracture are categorized as:

Adhesive failure – dentinal surface does not show traces of composite resin

Mixed failure – <50% of the dentinal surface is partially covered with composite resin

Cohesive failure – <70% of the dentinal surface is covered by composite resin.

Statistical analysis

Descriptive statistics were calculated, and one-way ANOVA was used to analyze intergroup comparison, whereas the post hoc Tukey test was applied for intragroup analysis to check for statistical significance (P > 0.05) using software SPSS version 17.0 (IBM SPSS Inc, Chicago, IL, USA) [Table I]. The P value was kept <0.05.

RESULTS

Among the four adhesive systems, prime and bond universal exhibited higher bond strength values (7.01 ± 2.02 MPa) when compared to single bond universal adhesives (6.74 ± 1.59 MPa) and Tetric N Bond (6.02 ± 1.68MPa). Adper Single Bond (5.36 ± 3.03MPa) showed the least shear bond strength. However, the groups did not show any statistically significant differences as shown in Table 1.

Table 1

The mean and standard deviation of bond strength values and compressive load applied of the dentin adhesives

	Mean±SD
	Compressive stress (MPa)	Compressive load (n)
Group 1 - Adper single bond	5.36±0.03	−269.89±1.52
Group 2 - single bond universal adhesive	6.74±1.59	−359.6±75.68
Group 3 - Prime and Bond Universal	7.01±2.02	−355.72±95.79
Group 4 - Tetric N-bond	6.02±1.68	−302.85±84.61

SD: Standard deviation

A stereomicroscope was used to detect the type of fracture after shear bond strength testing was represented in Table 2. Fracture analysis predominantly showed two types of failures, that is, a cohesive and mixed failure. Adper Single Bond 2 showed 62% of cohesive failure (between the bonding agent and composite resin) and 37% mixed failure. Single Bond Universal adhesive showed 12.5% adhesive failure (between the bonding agent and dentin), 25% mixed failure, and 62.5% cohesive failure. Prime and bond universal showed 62.5% of mixed failure and 37.5% cohesive failure. Tetric N Bond showed 75% mixed failure and 25% cohesive failure.

Table 2

Frequency of mode of fracture

Group	Mode of fracture	Frequency of fracture (%)
Group 1	1	0
	2	37.5
	3	62.5
Group 2	1	12.5
	2	25
	3	62.5
Group 3	1	0
	2	62.5
	3	37.5
Group 4	1	0
	2	75
	3	25

1:Adhesive failure, 2: Mixed failure, 3: Cohesive failure

DISCUSSION

The success of composite resin restoration vastly depends on its ability to bond to the enamel and dentin, facilitated by the dental adhesives. This study aimed to evaluate the shear bond strength of four different dental adhesives applied under simulated intrapulpal pressure.[16] Most of the studies done in vitro fail to replicate pulpal conditions. This study aimed to simulate the normal intrapulpal pressure using a hydraulic unit.

Fluid movement in the dentinal tubules and the intrapulpal pressure created in a vital tooth are essential factors to be considered during a bonding procedure as it can influence the outcome of the procedure. Water diffuses through the underlying hydrated dentin structure to the polymerized hydrophilic adhesive layer by an osmotic gradient during the slow setting process of composite resin.[17] According to Yiu et al., this water uptake through the adhesive layer can lead to a porous region.[18] This leads to the deterioration of the interface between the adhesive and resin. As a result, a weak link is created at the bonding interface when pulpal pressure is simulated. Extensive research has confirmed that minimum bond strength of 17–20 MPa is required to withstand the contraction forces of composite materials and help retain resin restoration.[19] Various in vitro studies have shown higher bond values when conducted without the application of simulated intrapulpal pressure, contrary to in vitro studies performed under simulated conditions.[2021]

Since a simulated setup should mimic the actual scenario, it is prudent to conduct in vitro studies in such an environment. Hence, a hydraulic pressure device was used to create water pressure during the restorative procedure. Different authors have used various models for simulating intrapulpal pressure.[222324] The model used in the present study to simulate intrapulpal pressure was based on the model created by Sauro et al.[15]

This study reveals that pulpal pressure has an impact on the shear bond strength of the composite resin restoration and is lesser than minimally recommended.

Among all the four adhesive systems, Prime and Bond universal adhesive (Group 3) showed the highest bond strength values. Higher bond strength is attained with single-step universal adhesive systems when compared to two step total-etch technique.[252627] Self-etching adhesives consist of both hydrophilic and hydrophobic monomers with greater solvent content.[928] The presence of water helps in self-etching as it can act as an ionization medium. Outward fluid flow through the dentinal tubules occurs when positive hydrostatic pressure is applied to the dentin, resulting in “wet bonding.” This water that percolates thought the dentin affects the bond strength. The low pH of Prime and Bond universal adhesive (Group 3) (2.1) may dissolve the smear layer and the smear plug. This allowed clearing of the dentinal tubules, increasing dentin permeability, hence exhibiting higher bond strength values.

It is reported that HEMA-containing adhesive systems, Adper single Bond (Group 1), enhance water absorption, resulting in hydrogel formation. This can lead to hydrolytic degradation, resulting in the deterioration of their mechanical properties.[729] This could have been the reason for reduced bond strength in Adper single bond group.

Overall, all the experimental groups’ bond strengths ranged between 5.36 Mpa and 7.01Mpa, which is low to resist contraction force and retain the restoration under mastication forces. The reason could be the hydrostatic pressure, which leads to fluid seepage through the tubules that contaminated the bonded surface affecting resin infiltration.[3031] This pathway permits water percolation along with the dentin-resin interface, deteriorating the longevity of the bond.[253233]

The present study's major limitation was the specimens were not subjected to thermal cycling or artificial aging, which would have better replicated clinical conditions. Future studies utilizing microshear bond tests of universal adhesive would be more advantageous since it is claimed to be more reliable, whereas macroshear bond strength test results in an overestimation of bond strengths.[34]

Minimal adhesive failure was exhibited by most groups, suggestive of good bond strength shown by the bonding agents. Adhesive failure of bonding agent shows no remnants of the resin composite on the dentin substrate, whereas cohesive failure is the failure within the composite resin. Mixed failure is a combination of the two.[35] Cohesive failures and mixed failure indicate that bond strength of the adhesive system, and the resin composite exceeds that of the cohesive strength of the substrate. Most of the specimens showed cohesive failure implying a good bond strength exhibited by universal adhesives.

CONCLUSION

All the experimental universal adhesive agents have shown recommended bond strengths required for successful bonding of composite restoration to dentin. The highest bond strength was recorded by one step, HEMA-free universal adhesive system. However, this group also exhibited adhesive failure under fracture loading. All the groups exhibited cohesive and mixed fracture under fracture loading.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.