Cause-Effect Relationship of Varying Bonding Thicknesses in Dentin Adhesion of Universal Adhesives

Abstract

Purpose: To evaluate whether varying thicknesses of universal adhesives utilizing the additional coating strategy would affect their microtensile bond strength (µTBS) to dentin, hardness, and elastic modulus (mechanical properties).

Materials and methods: Ninety-nine human maxillary premolars were cut to expose coronal dentin, ground with regular-grit diamond burs, and randomly distributed into 9 groups based on 1. adhesive: Scotchbond Universal Adhesive (SB; universal), G Premio Bond (GP; universal) and Clearfil Megabond 2 (MB; two-step self-etch; control); and 2. application strategy (one, two or three coats; each coat light cured). After adhesive application and resin composite buildup, the bonded teeth were stored in distilled water (37°C; 24 h). Resin-dentin sticks from eight premolars per group (each premolar yielded 3 sticks; n = 24 sticks altogether) were prepared for the µTBS test, followed by measurement of the adhesive thicknesses at their fractured ends using SEM. The mechanical properties of the adhesive layers produced by different coats were evaluated on separate resin-dentin slices (n = 3 teeth per group).

Results: Two coats significantly increased the µTBS (p < 0.001) of all the adhesives. The correlation between adhesive thickness and bond strength was positive for GP but negative for SB. MB did not show any correlation. Additional coating significantly increased the mechanical properties of GP (p < 0.05) but did not affect SB and MB (p < 0.05).

Conclusion: An additional adhesive coating over the manufacturers' recommendations improved the bond strength of all the adhesives tested. However, the increased mechanical properties of the adhesives with additional curing was material dependent.

Keywords: additional coating; adhesive thickness; dentin; mechanical properties.; microtensile bond strength; universal adhesives.

Fig 1

Representative SEM images showing the measuring locations of the adhesive layers from the fractured resin-dentin pairs of (i) G-Premio Bond applied in one coat, (ii) Scotchbond Universal Adhesive applied in two coats, and (iii) Clearfil Megabond 2 applied in three coats. Note that whenever close alignment of the fractured pairs was not achievable (ii and iii), the adhesive thicknesses of two corresponding sides could not be measured under the same focus leading to two separate images. The orange-bordered small-scale images show the complete widths of the fractured pairs. White arrows indicate the adhesive layers; ep: epoxy resin; D: dentin; RC: resin composite.

Fig 2

Schematic of the indentation test for measuring the hardness and elastic modulus of the adhesive layer(s).

Fig 3

Box-and-whisker plot (minimum-(lower quartile-median-upper quartile)-maximum) of microtensile bond strengths obtained by the adhesives applied in different numbers of coats.

Fig 4

Fracture modes of the tested adhesives applied in one, two or three coats. The failures occurring individually or simultaneously at the resin composite-adhesive interface, adhesive-dentin interface, and cohesively within the adhesive were considered adhesive failures. The non-adhesive failures include cohesive failures in dentin or mixed failure types involving dentin. Cohesive failure in resin composite was not observed.

Fig 5

Box-and-whisker plot (minimum-(lower quartile-median-upper quartile)-maximum) of adhesive thicknesses obtained by the adhesives applied in different coats and measured from the fractured resin-dentin sticks following the microtensile bond strength test.

Fig 6

Correlation between adhesive thickness and corresponding microtensile bond strength (µTBS) of the tested adhesives applied in one, two, and three coats.

Fig 7

Correlation between adhesive thickness and corresponding microtensile bond strength (µTBS) of the adhesives applied in one and two coats.

Fig 8

Box-and-whisker plot (minimum-(lower quartile- median-upper quartile)-maximum) of hardness (a) and elastic modulus (b) values obtained by the adhesives applied in different numbers of coats.