Comparative evaluation of the microtensile bond strength of bulk fill and low shrinkage composite for different depths of Class II cavities with the cervical margin in cementum: An in vitro study.

AIM: The aim of this study was to evaluate the microtensile bond strength (μTBS) of bulk fill and low shrinkage composite for different depths of Class II cavities with the cervical margin in cementum.
MATERIALS AND METHODS: Standardized conservative box-shaped Class II cavities were prepared on sixty sound-impacted human third molars. The samples were randomly divided into two groups: Group I (n = 30) - horizontal incremental technique and Group II (n = 30) - bulk fill technique (SonicFill). They were further subdivided into three subgroups of (n = 10) samples each according to the different occluso-gingival height: subgroup (A - 4 mm, B - 5 mm, and C - 6 mm). The gingival margins for all the samples were located 1 mm below the cementoenamel junction. The restored samples were subjected to thermocycling (500 cycles) followed by μTBS testing. The scores were statistically analyzed using ANOVA and post hoc test using SPSS software version 16.
RESULTS: Subgroups IA and IB showed lower μTBS than subgroups IIA and IIB (P < 0.05) whereas subgroup IC showed higher μTBS than subgroup IIC (P < 0.05). SonicFill showed a significant reduction in μTBS as the depth increased.
CONCLUSION: SonicFill should be used in two increments for cavities with a depth of more than 5 mm.

RCT Entities:   Population Interventions Outcomes

INTRODUCTION

There have been tremendous changes and developments in restorative dentistry over the past few decades and the pace is accelerating. Although various manufacturers have attempted to improve the physical and mechanical properties of composite resins, some drawbacks are still inherent to direct composite restoration, such as polymerization stresses induced during and after their insertion.[12] In addition, optimal occlusal anatomy and approximal contacts are difficult to obtain, especially in large cavities and areas of difficult access. Polymerization stresses generated by polymerization shrinkage may compromise the bond integrity, thereby increasing the potential for mechanical failure by allowing the ingress of bacteria, microleakage, postoperative sensitivity, and ultimately secondary caries, pulpal inflammation, or necrosis.[3] Various clinical methods have been proposed to reduce the shrinkage stresses such as the control of curing light intensity,[4] flowable resin liner application,[5] indirect resin restoration,[6] and incremental layering technique.[7]

Incremental insertion techniques are recommended to reduce the undesirable effects of polymerization shrinkage by decreasing C-factor (ratio of bonded to unbonded surfaces).[8] Despite these benefits, the incremental technique cause incorporation of voids or contaminants between composite layers[9] and increased deformation of restored tooth.[10] Furthermore, increased time is required to place and polymerize each layer. To overcome the shortcomings of incremental filling techniques, bulk fill composites have been introduced.

SonicFill (Kerr Dental, Orange, CA, USA) is a sonic-activated bulk fill composite that has been recently introduced. The composite contains about 83.5% of filler by weight, mainly silica and barium aluminosilicate. The SonicFill system consists of a handpiece, activated sonically and attached to the high-speed multiflex connection. A special composite unidose is screwed on the handpiece. Upon activation, the sonic energy lowers the viscosity and extrudes the composite that has initially a thick consistency. Upon deactivation of sonic energy, viscosity of the composite increases and allows easy adaptation and sculpting morphology of the composites. Herculite Precise (Kerr Dental, Orange, CA, USA) is a low shrinkage, nanohybrid composite which contains prepolymerized filler. The prepolymerized fillers (particle size - 0.4 μm) increase the surface asperity and reduce surface contact with instrument, making the material smooth and nonsticky.

The microtensile bond strength (μTBS) evaluation reduces the nonuniform stress distribution at adhesive interface and has made possible the evaluation of several clinically relevant substrates and conditions.[11] Very few studies have been reported in literature evaluating and comparing the μTBS of bulk fill composite (SonicFill) and low shrinkage composite (Herculite Precise) restored with layering technique for different occluso-gingival depths. Therefore, the aim of this in vitro study was to evaluate the μTBS of bulk fill and low shrinkage composites for different depths of Class II cavities with the cervical margin in cementum.

MATERIALS AND METHODS

Specimen preparation

Sixty sound-impacted human third molars which were freshly extracted and free of any developmental anomalies were selected. Standardized conservative box-shaped Class II cavities were prepared on the proximal surface of each tooth with buccolingual width of 4 mm and axial wall depth of 2 mm in dentin using #245 burs (SS White, Lakewood, NJ, USA) in an air/water-cooled high-speed turbine. A new bur was used after every five cavity preparations.

Grouping of specimens

The samples were randomly divided into two groups of (n = 30) samples each according to the type of restoring technique: Group I - horizontal incremental technique (Herculite Precis) and Group II - bulk fill technique (SonicFill).

Subgrouping of specimens

Occlusal enamel of Class II cavities was abraded with 600 grit SiC paper to obtain different occluso-gingival heights (4 mm - subgroup A, 5 mm - subgroup B, and 6 mm - subgroup C), keeping the gingival margins 1 mm below the cementoenamel junction (CEJ) for all the samples. In each subgroup, there were ten samples. Mylar strip was fixed around each sample. To simulate clinical conditions during restoration placement, the teeth were mounted in models using a silicon impression material. Care was taken that mounting material did not interfere with the cavity finish lines.

Restoration of samples

Group I

In Group I, dentinal surfaces of the specimen's cavity were etched with 37% phosphoric acid for 15 s followed by cleaning with gentle water spray for 10 s. Optibond Solo Plus (Kerr, Orange CA, USA) was applied to etched dentin surface according to manufacturer's instructions. The adhesive was cured for 20 s with a light-guided tip attached to quartz tungsten halogen (QTH) light-curing unit having an in-built radiometer. The cavities were restored with three horizontal increments of Herculite Precis composite using an incremental method with each increment being polymerized for 40 s using QTH. The intensity of curing light was periodically checked.

Group II

In Group II, after completion of bonding protocol as in Group I, the dispensing rate of SonicFill composite was set and the tip was placed at the bottom of cavity floor. The cavity was filled in a steady, continuous stream, withdrawing the tip as the cavity got restored and then cured for 20 s from the occlusal surface. The buccal and the lingual aspects of the tooth were cured for an additional 10 s each.

Thermocycling of specimens

The samples were stored in moist conditions for 24 h at 37°C and then subjected to thermocycling of 500 cycles with the temperature changing from 5°C to 55°C, dwell time of 15 s, and an interval time of 10 s each.

Sectioning of samples

The restored specimens were serially sectioned creating approximately 1 mm thick slabs. Each slab was trimmed from cemental and dentinal sides to obtain dentin restoration interface that had a bonded surface area of approximately 0.9 mm2. Two sticks were obtained from each restoration; therefore, twenty specimens were evaluated in each subgroup. Resin dentin sticks were embedded in cold cure acrylic blocks at both the ends.

Microtensile bond strength testing procedure

The μTBS was evaluated at a cross-head speed of 0.5 mm/min until debonding at the dentin adhesive interface occurred. The maximum force at which debonding occurred was measured. The μTBS was calculated in MPa by dividing the maximum force by the cross-sectional area of the bonding surface for each specimen. The scores were statistically analyzed using ANOVA and post hoc test using SPSS software version 16 (SPSS Inc. Chicago, U.S.A).

RESULTS

Intersubgroup comparison showed significantly higher μTBS (P < 0.05) of subgroups IIA and IIB in comparison to subgroups IA and IB whereas subgroup IIC showed significantly lower μTBS than subgroup IIC (P < 0.05). When intragroup comparison of Group I was done, subgroups IA and IB and IA and IC showed significant difference in μTBS (P > 0.05) whereas subgroups IB and IC showed nonsignificant difference (P > 0.05). When intragroup comparison of Group II was done, subgroups IIA, IIB, and IIC showed significant difference in μTBS (P < 0.05).

DISCUSSION

In our study, μTBS of two currently used protocols in composite insertion was evaluated. The gingival margin of the Class II cavities was kept 1 mm below the CEJ so as to have a standardized substrate for different occluso-gingival depths. The axial depth and buccolingual width of the cavities were also standardized.

QTH curing unit was used as it presents a broad spectrum, which allows efficient activation of different photoinitiators.[12] In addition, QTH curing units have shown better depth of cure, marginal adaptation, and interfacial integrity, when compared to high-intensity light emitting diode.[13]

In Group I, light-guided curing tip was used to ensure a uniform light irradiation of each increment at a constant distance of 1 mm. Prati et al. reported that light guide can be affected by the distance between the light guide tip and the resin composite, and even 1 mm of air can reduce light intensity by approximately 10%.[14] In our study, bonded interface used for checking the μTBS was reduced to 0.9 mm2 from dentinal and cemental sides to have a purely dentin restoration interface.

Pashley et al. have recommended 0.8 to 1 mm2 cross-sectional area of resin–dentin interface to assess the μTBS.[15] Literature has shown that there is an inverse relationship between bond strength and bond area: The smaller the area, the greater is the bond strength. A small surface area of the specimen reduces the stress distribution, thus reducing the number of internal defects which generally results in only adhesive failures.[16]

In subgroup A, bulk fill showed significantly higher μTBS than incremental fill [Table 1]. This might be due to the chemistry of SonicFill and its viscosity. SonicFill incorporates highly filled proprietary resin (83.5% filler by weight) and special modifiers that react to sonic energy. It has increased percentage of photoinitiator which produces adequate degree of conversion at the bottom of cavity. In its initial resting stage, the modifiers form an extended stabilizing network throughout the resin. As sonic waves are applied through the handpiece, the modifiers cause the viscosity of the composite to drop up to 87%. This increases the flowability, quick placement, and precise adaptation of the composite to the cavity walls. Composite returns to a more sticky and nonslumping state as the sonic energy is stopped. Furthermore, the volumetric shrinkage of SonicFill is reported to be 1.6% which reduces the likelihood of composite pulling away from the tooth surface during the polymerization process. Versluis et al. reported that the total amount of composite material to fill a cavity turns out to be low for an incremental filler technique than single bulk filling technique. In incremental technique, polymerization contraction of each individual filling increment causes some deformation of the cavity by forcing the cavity walls to bend in and downward, thereby decreasing the cavity volume. A decreased cavity volume means less composite can be placed for the next filling increment. This results in higher shrinkage residual stresses leading to debonding at tooth restoration interface.[10] Winkler et al. reported that bulk filled technique fills the total volume of the preparation and creates less residual shrinkage stresses than incremental technique.[17]

Table 1

Overall means and standard deviation of microtensile bond strength values (MPa)

Nikolaenko et al. showed contradictory results to our study. In their study, bulk technique (depth: 4 mm) led to low dentin adhesion at the cavity floor.[18] The difference between the results might be due to difference in composite used for bulk filling.

In subgroup B, bulk fill showed significantly higher μTBS than incremental fill [Table 1]. On the contrary, Figueiredo Reis et al. reported that bulk filling technique showed lower μTBS values when compared to incremental techniques. The authors stated that incremental filling technique assures uniform and maximum polymerization at the bottom of the cavity. In addition, by restoring Class II cavities, the C-factor gets reduced to about 1.3.[19]

In subgroup C, bulk fill showed significantly lower bond strength values than incremental fill [Table 1]. This might be due to light-dispersing capability of composite resins. Thus, when light passes through the bulk of the composite, the irradiance is reduced due to light scattering caused by filler particles and resin matrix. Although bulk fill composite (SonicFill) has increased percentage of photoinitiators, they might still be insufficient to facilitate adequate curing at 6 mm depth due to increased distance from the light source. This might have resulted in insufficient polymerization of base of SonicFill composite at 6 mm depth.

Aguiar et al. reported that composite restorations light cured at 2 mm and 4 mm (bottom surface) presented significantly higher microhardness values than samples light cured at 8 mm.[20]

In Group I, intragroup comparison showed nonsignificant reduction in μTBS between 5 mm and 6 mm [Table 1]. This might be due to the use of light-guided tip which produced the uniform and adequate polymerization of each increment at the various depths of the cavity. In Group II, intragroup comparison showed a significant reduction in μTBS as the depth increased [Table 1]. This might be because of insufficient irradiance of the composite at the bottom of the cavity.

CONCLUSION

Both Herculite Precise (incremental technique) and SonicFill (bulk fill composite) showed highest μTBS at 4 mm and least at 6 mm

Bulk fill technique showed superior μTBS at 4 and 5 mm depths and inferior μTBS at 6 mm depth in comparison to incremental technique

SonicFill should be used in two increments for cavities with a depth of more than 5 mm.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.