Objectives . The ability of chlorhexidine (CHX) to slow the resin-dentin bonds degradation is likely related with its capacity to inhibit the collagenolytic/gelatinolytic activity of dentin. The aim of this study was to investigate the substantivity of CHX to human dentin.

Materials and methods . Thirty extracted non-carious human molars were used according to protocol reviewed and approved by the Human Assurance Committee of the University of São Paulo, Brazil. Dentin disks were obtained from the mid-coronal portion of each tooth using a slow speed diamond saw under water cooling. The inner dentin surfaces were covered with a varnish. One-third of dentin disks were kept mineralized (MD), while the other two-thirds had their outer uncovered surface partially demineralized with 37% phosphoric acid for 15 s (PDD) or they were totally demineralized with 10% phosphoric acid for 12 h (TDD). Disks of sintered hydroxyapatite (HA) were also prepared, having one of their surfaces also covered with varnish. The uncovered surface of all specimens was treated with one of the following solutions: (1) 10 μL of distilled water (controls) or (2) 10 μL of 0.2% chlorhexidine diacetate. Solutions were applied using a micro-syringe and they were left undisturbed for 30 s. Specimens were then transferred to plastic tubes containing 1 mL of sterile phosphate buffer (PBS, pH 7.4, 37 °C). Substantivity was evaluated as a function of CHX desorption after the following periods of specimens’ incubation: 30 min, 1 h, 3, 6, 24, 168, 672 and 1344 h. Five disk-like specimens of neat varnish were prepared and individually incubated in PBS. CHX concentration in eluates was spectrophotometrically analyzed. UV absorbances were measured at 260 nm. Data were analyzed by two-way repeated measures ANOVA and Holm-Sidak tests ( α = 0.05)

Results . Substantivity of CHX was expressed in percentage retention of the applied dose of CHX (32 nmoles). The highest CHX desorption was observed for specimens of HA, while the specimens of PDD exhibited the lowest CHX desorption ( p < 0.05). Statistically significant differences were also detected between the CHX-treated specimens of PDD and TDD ( p < 0.05). After 1344 hours of incubation, the percentages of CHX desorption from PPD, MD, TDD and HA were 3, 5, 26 and 96% respectively. This means that 97, 95, 74 and 4% of the applied CHX to, respectively, PPD, MD, TDD and HA remained bound to the structure of these substrates.

Conclusions . A significant amount of CHX remained absorbed in dentin specimens that were freely exposed to an aqueous solution. It is expected that even higher concentrations of CHX would remain bound to dentin matrix if CHX-treated dentin was sealed with adhesive/resin composites. The substantivity of CHX to dentin and its effect on the inhibition of dentinal proteases may explain why CHX can slow the degradation of resin–dentin bonds.