Rheological properties of veneer trial pastes relevant to clinical success

The porcelain veneer is a relatively conservative means of improving the appearance of teeth. As these restorations are usually highly visible it is essential to gain the approval of the patient before final cementation takes place. As a result a variety of trial pastes are available that match the shades of the resins that are used ultimately to finally cement the veneer in place. Such restorations, by virtue of their small size and fragility, are difficult to handle and position. It is therefore important that the application of trial paste does not impede placement and retention upon the tooth surface at try in whilst the patient assesses its appearance.

To determine the rheological properties of three makes of trial pastes (Calibra - Dentsply Caulk, USA; Nexus - Kerr, USA; Rely X - 3M Espe, USA) and assess if temperature affected these. Design In vitro rheological measurement.

For each product the rheological properties of three shades were determined at both 25 degrees C and 35 degrees C three times. Measurements of both the Initial Yield Stress and Shear Rate Index were undertaken using a controlled stress rheometer (Carri-med, Dorking, UK) in flow mode with a cone-and-plate test configuration.

One way analyses of variance of the initial yield and shear rate index for each shade of material at 25 and 35 degrees C demonstrated no significant effects of temperature with the exception of the initial yield of Nexus (P = 0.0024) where the yield stress of the white shade was significantly (P <0.01) higher at 35 degrees C than at 25 degrees C. In all cases the yield stress for Nexus was of the order of ten times higher than the other materials. All materials demonstrated pseudoplastic behaviour.

The variation in yield stress displayed by the materials will affect the handling of veneers and offers a potentially useful choice parameter to the clinician as to which paste bests suits their requirements.