Effect of intratooth location and thermomechanical cycling on microtensile bond strength of bulk-fill composite resin.

CONTEXT: The initial bond strength and potential durability of an adhesive restoration are significantly affected by regional variations in dentin composition. AIM: The aim of this study is to evaluate the influence of dentin location and thermomechanical cyclic loading on microtensile bond strength of bulk-fill composite resin to coronal dentin.
MATERIALS AND METHODS: Disto-occlusal cavity preparations were done on 60 extracted human mandibular molars with pulpal floor depth of 1.5 and 3.5 mm for superficial and deep dentin and 1.5 mm axial wall depth and are restored with bulk-fill restorative composite. Bond strength evaluation was done using universal testing machine, and mode of bond failure was observed under scanning electron microscope. STATISTICAL ANALYSIS: Statistical analyses were done using ANOVA and Tukey's multiple post hoc test. For comparison of failure mode, Mann-Whitney U-test was used.
RESULTS: Superficial dentin has shown higher bond strength compared to deep dentin and gingival wall dentin (P = 0.001). The bond strength values decreased with the thermomechanical cycling (P = 0.001). Deep dentin and gingival dentin have exhibited more of adhesive failures.
CONCLUSION: Bond strength of bulk-fill composite was negatively influenced by the depth of dentin and thermomechanical loading.

INTRODUCTION

Dental composite that can mimic the natural teeth has become the restorative material of choice for replacing the lost or damaged tooth structure. However, durability of restoration should be the most important criterion in material selection, as longevity of posterior composite restoration is considered less than optimal. An annual failure rate reported for posterior composite restorations has ranged from 0% to 9%.[1] Secondary caries and marginal breakdown are the most frequent causes cited for the replacement of composite restorations.[2]

Due to its complexity and dynamism, dentinal tissue continues to represent a challenge as regards to bonding with resin-based adhesive restorations. Adhesive bonding to dentin may be influenced by substrate-related factors such as location of the preparation wall, diameter and density of the dentinal tubules, their course and orientation, pulpal fluid flow, and sclerotic or caries-affected substrate.[34] Complex three-dimensional cavities such as Class II preparations present different dentinal sites for adhesion. The density of dentinal tubules varies with dentinal depth, occupying approximately 1% of the total surface area at dentino-enamel junction and 22% near the pulp chamber.[5] The relative contribution of resin tags and hybrid layer formation to the total bond strength are dependent on the orientation of the dentinal tubules and dentin depth which exhibit a nonuniform bond surface inside a prepared cavity.[46]

The difficulty in achieving a tooth – restorative seal relates to the shrinkage stresses developed during polymerization of the resin composite. Another important factor is depth of cure of the composite, especially at the gingival margin of Class II restorations, which is most difficult to access with the light-curing unit. Insufficient curing will allow the “washout” of the material during aging, leaving an open space for plaque retention.[7] It has been reported that 80%–90% of secondary caries will be found at the gingival margin for Class II restorations.[8] Recently, bulk-fill composite resins have been introduced that are suitable for insertion in a 4–5 mm thick bulk placement due to the presence of high reactive photoinitiators. The changes in monomer and organic matrix of these composites reduce polymerization shrinkage stresses upto 70%.[910] Thus, problems related to polymerization shrinkage such as gap formation causing secondary caries, pulp irritation, and cuspal deflection due to high “C” factor can be minimized.[1112]

Interface between the restoration and the tooth is exposed to diverse forces during function. It was reported that degradation occurs rapidly within 6 months throughout the dentin-bond interface leading to bond failure due to bacterial biochemical activities and thermal or mechanical load stresses.[1314] Host-derived matrix metalloproteinases (MMPs, mainly MMP-2 and MMP-9) have been reported to play a major role in the collagenolytic activity within the hybrid layer during aging.[15] The distribution of these matrix metalloproteinases was found to be different with different depths of coronal dentin indicating their different proteolytic potentials.[16] The application of in vitro methodologies using thermal and mechanical stresses to simulate the conditions in the oral environment could enable better evaluation of adhesive materials exposed to stresses. The rationale behind bond strength testing is that the higher the actual bonding capacity of an adhesive restoration, the better it will withstand functional stresses and the longer the restoration will survive in vivo.[17] Hence, the present study was designed to evaluate the influence of dentin depth, location, and tubule orientation on the bond stability of bulk-fill composite resin under thermomechanical challenge.

MATERIALS AND METHODS

Sixty noncarious human mandibular molars of approximately similar dimensions were collected and were used within 2 months after the extraction. Preoperative radiographs were taken to ensure that the collected teeth did not have root caries, pulpal calcifications/resorptions, or fractures/craze lines. After extraction, teeth were disinfected using 0.5% chloramine-T solution and their use in research was approved by the local biomedical research ethics committee (D158601032).

Standardized disto-occlusal cavities were prepared using # Ex 41 diamond abrasives (Mani Utsunomiya, Tochigi, Japan) and # 245 tungsten carbide burs (SS White, New Jersey, USA). Each bur was changed after every five cavity preparations. Superficial dentin was prepared with a depth of 1.5 mm and deep dentin with 3.5-mm depth from the occlusal central fossa. Gingival floor was placed 1 mm above the cemento-enamel junction, and the depth of the axial wall was maintained at 1.5 mm from the external surface. The cavosurface margins were prepared at 90°, and all the internal line angles were rounded. Application of etchant (Eco Etch – 37% phosphoric acid, Ivoclar, Schaan, Europe) was done first on the enamel walls and then to dentin so that 20-s etching time for enamel and 10 s for dentin were maintained. Then, single bond universal adhesive (3M ESPE, St. Paul, MN, USA) was applied and light cured using Bluephase C8 LED-curing unit (Ivoclar Vivadent, Schaan, USA). Filtek bulk-fill posterior restorative composite (3M ESPE, St. Paul, MN, USA) was placed as a single increment of 4-mm thickness into the prepared cavities and light cured for 20 s. Restorations were finished with Sof-Lex discs (3M ESPE, St. Paul, MN, USA), and polishing was done using rubber cups at slow speed.

Thermomechanical load cycling

After the restorative procedure, teeth were randomly allocated into two groups (n = 30 each). Group 1 teeth samples were stored in distilled water at 37°C for 24 h and 100% humidity after which they were assigned for immediate bond strength testing. Group 2 teeth samples were subjected to thermomechanical cyclic loading after embedding their roots in cold-cure polystyrene resin in order to obtain a flat occlusal surface for mechanical load application. The teeth were thermally stressed in a Wileytec thermocycler machine (Haakeek 30, Thermo electron Corporation, Germany) at 5°C in cold cycle followed by hot cycle at 55°C for 10,000 cycles with a dwell time of 30 s and the transfer time of 5 s. The specimens were submitted to one lakh mechanical cycles with an intermittent vertical occlusal loading of 50 N at 20 cycles/min. With a chewing simulator CS-4.8 (SD Mechatronik, Germany), the axial force was applied with a round end piston of 5-mm diameter that touched the occlusal internal cuspal inclines at 1 HZ frequency.

Microtensile bond testing

The restorations were sectioned along the long axis of the teeth into three slabs with a slow-speed diamond saw (Leica SP1600, Germany) under copious water coolant. These specimens were then trimmed and shaped to produce three beams of 0.9 mm ± 0.1 mm bonded surface area, so that one section each from superficial dentin, deep dentin, and gingival floor were obtained, with a total of 90 sections from each group. The specimens were then mounted with a cyanoacrylate adhesive in a universal testing apparatus (Autograph, AG-15, Shimadzu Inc., USA) and debonded at a crosshead speed of 1 mm/min until failure. Mean bond strength values and standard deviations were calculated and expressed in MPa. Statistical analysis was performed with SPSS software version 22.0 (IBM Corp, Armonk, NY, USA) using ANOVA and Tukey's post hoc test at a significant level of 5%.

Fracture mode analysis

All fractured specimens were dry mounted on aluminum stubs, gold sputter coated (ion sputter coater, Hitachi E-1010), and observed with a scanning electron microscope (LJSM 5600, Joel Inc., MA, USA) to evaluate the fracture pattern. The predominate fracture modes observed were classified into three types: Adhesive/cohesive/mixed failures [Figure 1]. The percentage of failure modes was calculated; Fischer's exact Chi-square and Mann–Whitney U-tests were used with α = 5%.

Figure 1

Scanning electron microscope photographs of specimens showing different failure modes. (1) Cohesive failure within bulk fill composite. (2) Adhesive failure at dentin and composite interface with broken resin tags. (3) Adhesive failure at the dentin and adhesive interface. (4) Mixed failure – cohesive failure within the bulk fill composite and adhesive failure at the dentin and adhesive interface. (5) Mixed failure – cohesive failure within the composite and adhesive failure with broken resin tags. (6) Adhesive failure between the dentin and bonding agent with broken resin tags

RESULTS

For all the dentin regions, bond strength values were significantly lower when the specimens were submitted to thermal and mechanical cyclic loading (P = 0.001) [Table 1]. Higher mean bond strength was recorded for superficial dentin and has shown statistically significant difference compared to deep and gingival dentin (P = 0.001) [Table 2]. Gingival floor presented lower bond strength values but did not show significant difference compared to deep dentin (P = 0.807).

Table 1

Mean microtensile bond strength (MPa) comparison between two groups and subgroups by independent sample t-test

Table 2

Comparison of microtensile bond strength (MPa) among subgroups with and without thermomechanical cyclic loading using Tukey’s honestly significant difference post hoc test

Without thermomechanical load cycling, the mode of failure was not statistically different in different dentinal regions (P = 0.079) [Table 3]. Superficial dentin has shown more number of cohesive failures without and after thermomechanical loading. Deep dentin and gingival wall dentin have exhibited more of adhesive failures. In all the tooth regions, the percentage of mixed failures were increased after thermomechanical cyclic loading, which has shown statistically significant difference in the fracture mode within the dentin regions (P = 0.001).

Table 3

Comparison of failure modes (adhesive, cohesive, and mixed) by Mann-Whitney U-test

DISCUSSION

While placing a composite restoration, the incompatibility between the strong demineralizing ability of acid etchant (37% phosphoric acid) and the insufficient infiltration of the hydrophilic monomers may develop a gap below the hybrid layer causing the exposure of certain amount of collagen fibrils and matrix-bound MMPs. These MMPs activated by the acidic monomers may induce the degradation of the denuded collagen on thermal changes during aging. Niu et al. reported that the distribution of MMP-2 and MMP-9 was reduced from the deep dentin to superficial dentin, but the distribution of their specific tissue inhibitors of metalloproteinases were not commensurate with that.[16] Hence, to stimulate intraoral functional environment on the restorations, thermomechanical cyclic loading procedures as per the guidelines given by the Academy of Dental Materials for in vitro testing[18] were performed in the study.

The results of the study confirmed the research hypothesis, as the dentin region and the depth of dentin proved to be influencing factors on bond strength values. Superficial dentin demonstrated higher bond strength values than deep and gingival dentin, which is in accordance with previous studies.[319] Superficial dentin contains fewer and tapering dentinal tubules, whereas deep dentin is composed mainly of larger funnel-shaped dentinal tubules with minimum intertubular dentin. To establish good bond, permeation of the resin into intertubular dentin by hybrid layer formation is more important than resin tag formation in dentinal tubules.[19] In clinical scenario, the presence of greater amount of dentinal fluid in deep dentin may further decrease the bond strength values. Contrary to these findings, some studies have shown that the deeper dentin is capable of producing higher bond strength due to an increase in the total surface area available for forming hybridized tubule walls and intertubular dentin.[2021] In another recent study, no significant difference in bond strengths was reported between superficial and deep dentin.[22]

Gingival wall has shown lowest bond strength values among the tested tooth regions. Gingival wall presents the dentinal tubules that run obliquely and parallel to the preparation; as a result, greater amount of peritubular dentin and only a smaller area of intertubular dentin are available to form the hybrid layer.[4] Apart from that, due to less mineralized dentin at the gingival margin, acids are expected to etch dentin at the gingival wall faster. In clinical conditions, Perdigão stated that patent tubules with dentinal fluid may contaminate the prepared surface leading to reduced adhesive infiltration and lower monomer/polymer conversion of the adhesive at the gingival margin as compared to the proximal wall.[5]

In agreement with other studies,[1723] the bond strength values were decreased in all dentin regions after thermomechanical cyclic loading indicating bond degradation with aging. Although hydrophilicity of the adhesive and water sorption of adhesive interfaces are considered the principal mechanism of resin bond degradation, enzymatic degradation of hybrid layer by MMPs contribute to the degradation process and loss of bond strength with time.[222425] It was reported that released carboxy telopeptides from type 1 collagen values in deep dentin were higher than those in superficial dentin and exhibited higher MMP activity and collagen degradation during thermocycling process.[22]

Failure mode analysis revealed that independent of depth of dentin, thermomechanical cyclic loading increased the number of mixed failures due to the partial degradation of resin-dentin interface with aging. These findings are in accordance to the study results of Mitsui et al.,[17] where the number of mixed failures were increased with increasing number of thermomechanical cycles. Due to the partial degradation of resin-dentin interface with aging, mixed failure mode exhibited some cohesive and some adhesive fractures.

CONCLUSION

The dentin region proved to be an important influential factor on bond strength as there was a significant fall in the bond strength of bulk-fill composite, in the deeper levels of dentin

After thermomechanical cyclic loading, the gingival wall exhibited significant resin-dentin bond degradation as compared to superficial dentin.

The findings of the study indicate that adhesive restorations might experience early leakage or bond failure at some preparation walls.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.