During root canal treatment, steel or nickel-titanium files scrape and widen the canal walls, leaving behind a sticky, amorphous organic-inorganic layer of microbacterial debris, fragmented pulp tissue, and mineralized tooth dust; this is called the “smear layer” (cutting residue). This layer seals the openings of the dentin tubules like a plug. If the canal is filled without cleaning this smear layer, the microbes inside become trapped within the tubules, preventing irrigation solutions from reaching them. Furthermore, the root canal filling paste cannot adhere microscopically to the tooth. Immediately before filling, a 17% EDTA solution, a chelating agent, is injected into the canal to chemically dissolve this smear layer and fully open the tubules. Failure to use EDTA allows latent bacteria to continue multiplying and, in the long term, leads to root apex abscesses.
