Determining the initial viscosity of 4 dentinal adhesives. Relationship with their penetration into tubuli

The main aim of this study was to determine the viscosity of four amelo-dentinal adhesives based on the same adhesion principles: the complete elimination of the dentinal smear layer using a 37% orthophosphoric acid and the building of a hybrid layer in a single stage. The secondary aim was to study the filling time for a tubulus using 2 approaches: analytically and using finite elements.The viscosity of four amelo-dentinal adhesive systems (Vivadent’s Syntac Sprint®, De Trey Dentsply’s Prime & Bond 2.1®, Prime & Bond NT® and Kuraray’s Panavia F®) was determined using a viscometer (a StressTech® rheometer) with a Couette rheometer-type tool mounted on it.A tubulus model was fixed: the assumption was of a cylindrical–conical cavity with its lower part closed, a depth of 11 μm, and a radius of more than 1.5 μm and less than 0.5 μm. This allowed us to perform an analytical study on the time it took these tubuli to be filled by the adhesives according to their viscosity and the various pressures involved when they were introduced into the dentinal tissue. The Forge 2 software application allowed us to reduce the geometry of the tubuli referring to a 3D problem to an axisymmetric 2D configuration. Thus modelling using finite elements was also performed and the results obtained by the two approaches were compared.The results from the analytical calculation and from the modelling showed different patterns of behaviour by the four adhesives with different viscosities at 25 °C: Syntac Sprint®: 50×10–3 Pa s, Prime & Bond 2.1®: 1.1×10–3 Pa s, Prime & Bond NT®: 1.4×10–3 Pa s, Panavia F®: 10 Pa s.The four adhesives would fill up the dentinal tubuli very quickly indeed. At identical surface tension values, the viscosity would have an influence on the filling time for the dentinal tubuli. Clinically, given the 20 s recommended by manufacturers in order for these products to impregnate the dentinal tissue, the viscosity of these adhesives did not appear to be a decisive factor in the formation of the hybrid layer.