Analysis of acid-treated dentin smear debris and smear layers using confocal Raman microspectroscopy

Smear layers are generally present on any dentin surface prepared with cutting instruments and are often the only available substrate for bonding. It is commonly reported that acid removes these layers, but to date there has been no chemical evidence to support this observation. Confocal Raman microspectroscopy was used to investigate changes in the composition and molecular structure of acid-treated smear debris and in situ dentin smear layers. The exposed dentin in human molars was abraded with 600-grit silicon carbide sandpaper. Raman spectra were acquired on the smear debris and collected from the sandpaper before and after treatment with 10% citric acid, 35% H3PO4, or 0.5M ethylenediaminetetraacetic acid (EDTA). The resultant smeared dentin samples were treated with one of the aforementioned reagents, and spectra were acquired at 1.0-microm intervals across the interfaces of the smear layers/demineralized dentin/mineralized dentin. Corresponding specimens were morphologically analyzed with scanning electron microscopy (SEM). The results showed that the composition of the smear debris and the in situ smear layers was a mixture of disorganized collagen and mineral. Spectral changes in the smear debris suggested that the disorganized collagen was denatured by acid treatment. The denatured collagen formed a gelatinous matrix around the mineral in the smear layer, thereby shielding it from the acid. The smear layers were not apparent in the SEM micrographs of acid-etched dentin prepared and processed with conventional techniques. The micro-Raman spectroscopic results presented in this study provide the first direct evidence that partially denatured collagen within smear layers is not removed and that the mineral is only partially removed with acids that represent conventional dentin adhesive etchants.