Table of Contents:
Aim of this study:
The aim of this in vitro study was to evaluate the relevance of the incremental technique, which was developed in the 1980’s and to give a statement of its influence on the problematic polymerisation shrinkage of resin composites and therewith the effect on the dentin bond strength.
Material and methods:
40 human, caries free third molars were prepared with class I cavities (3 mm x 4 mm x 4 mm). After that they were randomly divided into eigth groups each with five teeth. All teeth were etched with email preparator and bonded with a fifth generation dentin adhesive (iBOND® Total Etch). Four groups received as filling material the nano hybrid composite Tetric EvoCeram®, the other four groups the nano hyrid composite Grandio®SO. Each of the both resin composite groups was divided into four subgroups (n=5) and each of them filled with one, two, three or four increments.
After 180 days of aqua dest storage the decapitated molars were sawed into discs of 0,9 mm and afterwards sawed into sticks with measures of 0,9 mm x 0,9 mm by a diamond saw with a speed of 150 Rpm.
For the evaluation of the dentin bond strength all samples were subjected to the microtensile testing device. For this purpose all composite-dentin sticks were horizontal fixed at each end with wax at the extractor device and drawed-out with a movement speed of 1 mm/min. During fracture of the samples the values of the tensile strength in N were measured and calculated with the stick face in mm² to determine the dentin bond strength in MPa.
To analyse the mode of failure some samples of each group were subjected to the scanning electron microscope.
The Tetric EvoCeram® group as well as the Grandio®SO group showed significant higher dentin bond strength values by using the incremental technique compared to the bulk method (p<0,05).
While Tetric EvoCeram® exhibited a significant increase of dentin bond strength from the group using one increment to the group with two increments and to the group with three increments (p<0,05), Grandio®SO only showed a significant increase of bond strength from the group with one increment to the group with two increments (p<0,05), but no significant difference from the two incremental group to the group with three increments (p>0,05). However unlike Tetric EvoCeram®, Grandio®SO exhibited significant higher values in the group with four increments compared with the group with three increments (p<0,05).
By comparing the both nanohybrid composites among each other concerning their dentin bond strength it was noticeable that there was no significant difference in the bulk group as well as in the incremental group with four layers (p>0,05). Using two increments Grandio®SO showed higher bond strength values while using three increments Tetric EvoCeram® exhibited a higher bond strength.
The results of this in vitro study show that using the incremental technique leads to higher bond strength values at both composite groups (Tetric EvoCeram® and Grandio®SO) so that this filling method should be prefered to the bulk method.
However the compiled values only refer to the used cavity size (3 mm x 4 mm x 4 mm) of this study. It has to be proved in further researches if smaller cavity sizes will show simillar results.