Characterization of an Experimental Two-Step Self-Etch Adhesive's Bonding Performance and Resin-Dentin Interfacial Properties

Abstract

This study evaluated an experimental two-step self-etch adhesive (BZF-29, BZF) by comparing it with a reference two-step self-etch adhesive (Clearfil Megabond 2, MB) and a universal adhesive (G-Premio Bond, GP) for microtensile bond strength (μTBS) and resin-dentin interfacial characteristics. Twenty-four human third molars were used for the μTBS test. Bonded peripheral dentin slices were separated to observe the resin-dentin interface and measure the adhesive layer thickness with SEM. μTBS data of the central beams were obtained after 24 h and 6 months of water storage. Fracture modes were determined using a stereomicroscope and SEM. Nine additional third molars were used to determine the elastic modulus (E) employing an ultra microhardness tester. Water storage did not affect μTBS of the tested adhesives (p > 0.05). μTBS of BZF and MB were similar but significantly higher than GP (p < 0.05). BZF achieved the highest adhesive layer thickness, while GP the lowest. E of BZF and MB were comparable but significantly lower than GP (p < 0.05). Except for GP, the predominant fracture mode was nonadhesive. The superior bonding performance of BZF and MB could be attributed to their better mechanical property and increased adhesive thickness imparting better stress relief at the interface.

Keywords: dentin bonding; elastic modulus; microtensile bond strength; resin-dentin interface; scanning electron microscopy; two-step self-etch adhesive; ultra microhardness tester; universal adhesive.

Figure 1

Schematic of specimen preparation and test set-ups for determining µTBS, fracture mode, and interface characterization. * GP, G-Premio Bond; MB, Clearfil Megabond 2; BZF, BZF-29.

Figure 2

Schematic explaining the specimen preparation methodology and test set-up for determining elastic modulus. The resin-dentin interface was microscopically divided into three target regions—the adhesive layer (R1), adhesive-dentin interface (R2), and sound dentin (R3).

Figure 3

Representative SEM images of the adhesives’ (GP—G-Premio Bond; MB—Clearfil Megabond 2; BZF—BZF 29) fracture modes. The images under 3000× columns demonstrate the magnified specific features of the preceding 80× images. Row A is adhesive failure, row CD is cohesive failure in dentin, and row M is the mixed type of failure involving adhesive and dentin. The white arrows indicate a cohesive failure in the adhesive (ii,iv,vi); the white rectangle includes bubbles (ii); white-bordered black arrows show darker adhesive areas indicating failure at the composite resin-adhesive interface (ii,iv,xiv,xviii); transparent white arrows indicate the dentinal tubules’ openings (viii,x,xii,xiv,xvi,xviii). The mixed failure patterns (xiv,xvi,xviii) also show complete or partially occluded dentinal tubules (white hands).

Figure 4

Representative SEM images of interfacial structures (1000×) of the tested adhesives—GP (G-Premio Bond), MB (Clearfil Megabond 2), and BZF (BZF-29). Double-ended white arrows represent the extension of the adhesive layers measured at left lateral (i,iv,vii), central (ii,v,viii), and right lateral (iii,vi,ix) spots of each bonded dentin slice. The electron-lucent area between the twin yellow arrows demarcates the hybrid layer. Blue arrows indicate resin tags. The striped arrows show gap formation at the resin-dentin interface (iii). CR—composite resin; Ad—adhesive layer; D—dentin.

Figure 5

The graph shows a gradually increasing elastic modulus (E) across the adhesive-dentin interface. G-Premio Bond (GP), Clearfil Megabond 2 (MB), and BZF-29 (BZF).