Morphological Analysis and Microtensile Bond Strength Evaluation of a Self-Etch Adhesive to Caries Excavated Human Dentin Prepared by Mechanical, Chemomechanical and Er, Cr: YSGG Laser Systems: An In Vitro Study

The advent of ‘Adhesive Dentistry’ has simplified the guidelines for cavity preparation enormously. In light of the minimally invasive dentistry concept, the macroretentive cavities have been replaced by cavities limited to removal of carious dentin that are at most extended with some additional rounding off sharp margin edges and/or beveling. Caries dentin removal by mechanical means is non-specific nature of excavation that may result in excessive loss of tissue thus affecting the prognosis of the treated tooth. This in vitro study was aimed to evaluate the surface characteristics of caries-excavated dentin and to compare the microtensile bond strength values of an adhesive bonded to the residual dentin remained after three different contemporary caries-excavation methods namely Mechanical, Chemomechanical (CarisolvTM) and Er, Cr: YSGG laser (BiolaseTM). Forty five extracted cavitated human permanent molars with the lesion limited to the central portion of the occlusal surface were selected. The teeth were randomly divided into three groups (n=15), according to the caries removal methods, as follows: Group I: Mechanical rotary preparation by diamond bur/air turbine. Group II: Chemo-mechanical preparation with carisolvTM gel. Group III: Laser preparation by Er. Cr: YSGG Laser (BiolaseTM). Morphological changes in the residual dentin surfaces were evaluated using scanning electron microscopy (SEM). These specimens were immersed in 4% glutaraldehyde solution for 1hr at room temperature. Then they were rinsed in distilled water. The samples were then placed in cold buffer solution of sodium cacodylate for 90 minutes to fix the organic matter. Specimens were then dehydrated in ascending grades of ethanol and then dried in a Critical Point Drier based desiccator. The dried specimens were mounted on a metal stand and gold sputter coated by cathode atomization under vacuum. SEM images of the caries excavated surfaces were obtained. After caries excavation the samples were bonded using a “Mild” two-step self-etch adhesive, Clearfil SE Bond and then restored using a nanohybrid resin composite. The specimens were serially sectioned into multiple slices with a diamond disc under water cooling to create a 1mm² stick shaped samples for the Micro-tensile bond strength test (μTBS). The prepared rods comprised of residual dentine to which composite resin was bonded using suitable dentin adhesive. Silicon sleeves were attached to both the end of the samples. Specimens were mounted on a pneumatic jig. The test was taken out at a tension of 0.5mm/min until it failed. Bond strengths were calculated by dividing the load at failure by the cross-sectional bonding area. In this study, the morphological analysis of the residual dentin excavated with CarisolvTM produced a reduced and more inconsistent smear layer with large areas of open tubular orifices. The dentine surfaces that received Er, Cr: YSGG laser irradiation showed a scaly, irregular and rugged appearance whereas the teeth prepared by mechanical method appeared rougher with irregular particles scattered throughout the surface area. A well defined smear layer was detected. The results were subjected to statistical analysis by means of parametric Kruskal-Wallis one-way ANOVA and Mann-Whitney ‘U’ test. The Microtensile bond strength evaluation showed the highest bond strength values for the CarisolvTM group (6.37MPa) and the mechanical group (6.25MPa), and the lowest value was found with the laser group (4.47MPa). CONCLUSION: Within the limitations of this in vitro study it can be concluded that the caries-removal effectiveness and minimal-invasiveness potential varied amongst the three caries-excavation techniques tested. It was found that the surface remaining after CarisolvTM excavation and bonding with a mild self-etch adhesive seem to be very compatible, allowing deeper penetration of the adhesive and resulted in a thick and homogenous hybrid layer. The fracture mode analysis of the resin-dentin interface revealed that the three caries excavation techniques have no effect on the failure mode and loci of failure. A substantial increase in cohesive dentin failure was observed in the Carisolv group, whereas mechanical and laser groups showed a mixed fracture pattern