Investigation into the nature of dentin resin tags: A scanning electron microscopic morphological analysis of demineralized bonded dentin

Statement of problem. While the formation of the dentin/adhesive hybrid layer has been generally established, the infiltration and flow of the adhesive resin inside the acid treated dentinal tubules remains controversial. Purpose. The aim of the present study was to investigate and review the current interpretation of resin tags by means of scanning electron microscopic (SEM) observation. Material and methods. Eight noncarious, human third molars were cut transversally and then longitudinally to obtain 8 middle-to-deep dentinal surfaces. The dentin was etched with 37% phosphoric acid (H3PO4) gel for 10 seconds and then rinsed with water for 20 seconds. The dentin was kept moist by removing the excess water with a damp cotton pellet. The conditioned dentin was treated with a dentin bonding agent (Single Bond) and was light-polymerized for 20 seconds. A 0.2- to 0.5-mm layer of flowable composite (Tetric Flow) was then applied to the bonded dentin followed by 2 layers (2 mm each) of composite (Z 250). Each composite was light-polymerized for 40 seconds. Subsequently, the specimens were cut lengthwise into 2 halves and randomly divided into 4 groups (n=4), according to the surface preparation modality of the sectioned surface: Group EA: ethylenediamine tetraacetic acid, Group PA3: H3PO4, Group PA120: H3PO4 + NaOCl, and Group CA: HCl + NaOCl Two additional teeth (Group N) were cut lengthwise into 2 halves and served as the control. The sectioned surfaces were treated with HCl and NaOCl All specimens were processed for SEM observation. Results. Specimens from Groups EA, PA3, PA120, CA, and N showed filamentous structures that were tens of microns long. Some filaments presented split-ends with hollow structures and very thin walls. Others made sharp hairpin turns indicating they were soft and compliant. Conclusions. Conventional SEM techniques, which arc currently used to detect resin tags, actually identified filamentous organic structures, supposedly glycosaminoglycans, which were resistant to conventional specimen preparation techniques. The organic component showed a strong resemblance to the lamina limitans contained within the dentinal tubules. Over-reliance on SEM morphology has led to much confusion about the depth of penetration of resin tags.