The use of oxalate to reduce dentin permeability under adhesive restorations

Purpose: To test a novel approach to reducing dentin permeability that loca lizes occlusion of dentin tubules by calcium oxalate crystals to the subsur face without lowering resin bond strengths to oxalate-treated dentin surfac es. Materials and Methods: Flat dentin surfaces of extracted human third mo lars were etched with a mass fraction of 32% phosphoric acid (Bisco) for 15 s and rinsed. Half of the surface was treated with a potassium oxalate gel (mass fraction of 3% monopotassium monohydrogen oxalate) for 2 mins. The e ntire surface was then moist bonded with either One Step (OS) or Scotchbond Multi-Purpose (SB) adhesive systems. A resin-based composite (Z-100) build up was made over the entire surface. After storage for 24 hrs in distilled water, the teeth were longitudinally sectioned to separate the oxalate-trea ted half from the untreated (control) half. Each half was serially sectione d into several 0.7 mm thick slabs which were then trimmed to an hour-glass shape to reduce the bonded area to approximately 0.8 mm(2) and tested in te nsion. Pre- and post-treatment hydraulic conductance (Lp) of dentin was det ermined using dentin discs with a standard protocol. SEM images were obtain ed to examine the effects of treatment both on the dentin surface and along the tubules beneath the surface. The bonded interface was also examined by SEM. Results: Treating etched dentin discs with the oxalate gel caused sig nificant reduction of the Lp of dentin (80% reduction, P < 0.05). There wer e no statistically significant differences between bond strengths of oxalat e-treated and untreated surfaces for either adhesive system. OS resulted in a bond strength of 25.8 +/-9.2 MPa to untreated and 27.8 +/-8.9 MPa to oxa late treated surfaces (P> 0.05). SE showed bond strengths of 22.9 +/-7.9 MP a and 22.9 +/-9.6 MPa to untreated and treated surfaces, respectively (P> 0 .05). SEM images showed that the application of potassium oxalate gel on et ched dentin resulted in the formation of crystals inside the tubules rather than on the surface. Examination of the bonded interfaces demonstrated tha t the crystal formation inside the tubules did not compromise the formation of a typical hybrid layer on the top of dentin surfaces. Resin monomers pe netrated into the tubules filling the spaces around the crystals forming re sin tags with a jagged-like feature.