Sodium hypochlorite alterations of dentin and dentin collagen

NaOClaq is used as a cleansing and non-specific deproteinizing agent in endodontic treatment, as a component of new chemomechanical caries treatment, and is under study for its alterations of dentin bonding characteristics. We sought to determine the microstructural and nanomechanical changes with such treatments and to test if NaOClaq removed dentin collagen without microstructural or nanomechanical alteration of underlying mineralized dentin. Polished human dentin disks were prepared with a double reference technique that allowed changes to be determined following 10% citric acid etching for 15 s and subsequent treatment of the etched and unetched portions of the sample with 6.5% NaOClaq, using atomic force microscopy (AFM) (Nanoscope III, Digital Instruments, Santa Barbara, CA). Images and measurements were made at intervals up to 1800 s. A Triboscope (Hysitron, Minneapolis, MN) on the AFM was used to measure nanohardness and the reduced elastic modulus. The double reference method allowed measurements immediately following etching and at intervals during deproteinization. Etching caused deep peritubular dentin removal and a small depth change of hydrated intertubular dentin as mineral was removed and left a remnant collagen matrix. NaOClaq removed collagen over time, during which individual fibrils could be resolved; the underlying mineralized dentin was left with a unique porous surface containing numerous channels that are not normally observed in etched or fractured dentin. This could provide an attractive bonding substrate because of the increased surface area and high mineral content, if toughness is not reduced too much. Nanomechanical measurements showed that the reduced elastic modulus and hardness were 75% of original values after removal of the exposed collagen. Current dentin bonding systems rely on hybrid layer formation in which hydrophilic primers/polymers penetrate the opened collagen matrix exposed by etching. However some research suggests that preventing hybrid layer formation by deproteinization, in some cases, can give good bonding, and as shown here, alters the substrate microstructure. There are significant changes as indicated by ANOVA in the reduced elastic modulus that require further study to determine if these changes might affect clinical efficacy.