Evaluation of micro-tensile bond strengths of composite materials in comparison to their polymerization shrinkage

The present study determined the influence of polymerization shrinkage of eight commercially available hybrid, micro-filled and nano composites (Z100 and Filtek Supreme, 3M-ESPE; Charisma and Durafill, Heraeus Kulzer; Tetric and InTen-S, Ivoclar Vivadent; Enamel plus HFO, GDF; Palfique Estelite Low Flow, Tokuyama) placed in large class 1 cavities on the bond strength to dentin.

Polymerization shrinkage was recorded for 300 s at room temperature with a Stress-Strain-Analyzer (C(FACTOR)=0.3). The maximum contraction stresses after 300 s, the time until gelation (t(0.5 N)) and the coefficient of near linear fit of contraction force/time (gradient) were analysed. For the evaluation of the micro-tensile-bond-strength (micro-TBS), hourglass shape samples obtained from a total of 32 no carious extracted human third and second molars, randomly divided into 8 groups, were used. Micro tensile bond strengths were determined by computing the ratio of maximum load by the adhesion area of the hourglass shape. In order to analyze the quality of the polymerization within fillings, hardness profiles of a surface cut thought the middle of the restored tooth and along the tooth axis were made. Further, the modulus of elasticity, determined in a three-point-bending test, as well as the variation of the modulus of elasticity and of the hardness within a filling were considered. The statistical analyses were conducted by ANOVA (alpha=0.05) and post-hoc Tukey's test.

A significant correlation between polymerization shrinkage and micro-TBS was found (Pearson; -0.44). The correlation between modulus of elasticity in bending test and shrinkage stress (Pearson; 0.77), coefficient of near linear fit m (Pearson; 0.72), time until gelation (Pearson; -0.52) and micro-tensile-bond-strength (Pearson; -0.45) was also significant. A high polymerization tension and modulus of elasticity negatively affected the adhesion of the composite to the tooth hard substance. Further, the number of samples lost during the slice cutting and the hourglass shape preparation was found to correlate highly significantly with the shrinkage stress (Pearson; 0.97), coefficient of near linear fit m (Pearson; 0.94) and modulus of elasticity (Pearson; 0.82). All materials showed sufficient polymerization within the filling in comparison to the filling surface, the hardness at the bottom of the filling was greater than 80% of the maximum hardness value. The modulus of elasticity was not considerably reduced within the filling.

High contraction stress and modulus of elasticity, fast development of contraction force and an early start of stress build-up of materials placed in restrictive cavities cause tension in the material with a possible subsequent distortion of the bond to the tooth structure. A low modulus of elasticity is not necessarily associated with high bond strength. However, it causes a more uniform stress distribution at the restorative composite-tooth interface. This is also evident in a reduced sample loss during the different stages of sample preparation.