The Influence of Testing Protocols on Microhardness Tests of Composite Resin with Different Viscosities

This study tested two hypotheses. The first null hypothesis affirms that there is no statistical difference when composite resin is submitted to knoop or vickers microhardness using different testing protocols. The second hypothesis tested the possible influence of elastic modulus of composite resin in microhardness tests. Two nanohybrid composites were used with different viscosity behavior (Grandio and Grandio Flow). Samples (n=540) were made with 4 mm diameter and 2 mm high using a PTFE mould, stored for 24h at 37°C protected from light and randomly divided into 36 groups according to the resin viscosity (medium and low), microhardness methodology (vickers or knoop), load (50, 100 or 500g) and time (15, 30 or 45s). Two readings were made at the light exposed surface of each sample with Shimadzu HMV tester accomplishing 1.080 indentations. The vickers and knoop results were submitted to a three-way ANOVA with fixed factors the composite viscosity, load and time, and the post-hoc Tukey multiple comparison test at α=0.05. Significant differences were recorded (p<0.001) between the microhradness methodologies with two viscosities rejecting both hypotheses rejecting both null hypotheses. Among the vickers tested samples, the average recorded ranged from 164.94 (50g for 45s) to 210.33 (100g for 45s) for the medium viscosity composite and from 31.2 (100g for 30s) to 88 (500g for 45s) for a low viscosity resin. Knoop microhardness results ranged from 128.92 (500g for 45s) to 184.26 (100g for 15s) for medium viscosity composite and from 23.6 (100g for 15s) to 73.4 (500g for 30s) for flowable compositive. In a controlled in vitro experiment, the microhardness formulas mislead researchers to associate an improvement of diagonal measurements to higher load with a linear and proportional behavior. It is not recommended to clinicians compare both vickers and knoop microhardness results of composite materials without similar protocols applied in the research.