Stability of Nanomechanical Properties of Reinforced Enamel by Self-etch Adhesives

Objective: The bonds obtained using phosphoric acid (PA) etching of enamel was the mainstay in restorative dentistry but self-etching adhesive (SEA) systems may be a suitable replacement for etch-and-rinse adhesive (ERA) systems. The aim of the current study is to investigate the nanomechanical characteristics of enamel created by self-etching adhesives, compared with sound tissue and the tissue bonded by etch-and-rinse adhesives. Methods: Human enamel specimens were treated by Clearfil SE Bond (SE, Kuraray Medical, Japan), Clearfil Protect Bond (CP, Kuraray Medical) with or without PA; and Single Bond (SB, 3M ESPE,USA). A universal composite was then built up on the adhesive. After 24h storage in water at 37°C or 6m storage with 10,000 thermocycles the teeth were sectioned at the interface and fine-polished to create a very smooth interface. Nano-indentation (ENT-1100a, Elionix, Japan) with a maximum force of 3 mN at 0.3 mN/s loading rate was employed to perform indentation across resin–enamel interfaces. 120 indentations were made on each specimen at rows of 10 points with a vertical step of 1 micrometer to determine detailed hardness(H) and Young's modulus (Y) profile plot. A total of 10 groups were compared. Results: PA etching of enamel resulted in lower enamel hardness up to 3µm from adhesive interface, compared to enamel treated by SEAs. In the case of PA etching, hardness in the first few micrometers improved when SE or PB were applied compared to SB. After aging, all groups showed somewhat decreased mechanical profile. However, PB showed an advantage in maintaining or improving nanohardness towards enamel subsurface in the long-term perhaps due to fluoride release. Conclusion: Chemical interaction with the tooth-substrate, rather than only a mechanical retention, can provide a reinforced enamel, which may benefit even after PA etching. Fluoride release from PB improved mechanical stability in the long-term.