Etch-and-rinse versus Self-etch Pretreatment: the Ultra-structural Investigation on Intact Enamel

Abstract

Objective: To investigate the ultra-structural changes produced by phosphoric acid (PA) and self-etching primer (SEP) on intact enamel surface. Methods: Sound human teeth were collected with ethical approval (IRB No.: UW13-262) and randomly assigned to four groups: Group A (40% PA liquid), Group B (40% PA gel), Group C (Clearfil SE Bond primer, Kuraray Co.), and Group D (Transbond Plus SEP, 3M Unitek). Four coronal central buccal enamel specimens with surface dimension of 1x1.5 mm2 were sectioned and gently cleansed. In groups A and B, specimens were etched for 0, 15, 30, and 60 seconds, respectively, rinsed thoroughly with de-ionized water and dried afterwards. In groups C and D, SEPs were applied following manufacturer’s instructions. Specimens without treatment were set as control. Surface chemical composition, crystalline structure, and morphology were determined by energy dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD) and scanning electron microscopy (SEM).

Results: Calcium to phosphate (Ca/P) ratio was calculated and analyzed by two-way analysis of variance (α=0.05). After PA-etch, Ca/P ratio was initially increased and then decreased, without significant differences among various etching durations (p>0.05). Statistical difference was demonstrated between liquid and gel (p=0.016). Application of SEP dramatically decreased the Ca/P ratio (p<0.001), which was also significantly influenced by the type of SEP (p=0.003). XRD spectra revealed existence of new phosphate compounds in all groups, while intensity changes of hydroxyapatite characteristic peaks were only found in groups A and B. SEM exhibited typical etch patterns generated by PA-etch. Whereas, very conservative etch pattern was produced by SEP, which masked enamel surface with a hybrid layer.

Conclusion: PA-etch dissolved the superficial layer of intact enamel, changed the crystalline structure of enamel hydroxyapatite and efficiently roughed the surface. SEP had limited dissolution ability and less actively reacted with enamel substrate. However, new phosphate compounds were formed by either technique.