Shrinkage stress kinetics of Bulk Fill resin-based composites at tooth temperature and long time

To determine the shrinkage stress kinetics at up to 12h after light exposure and at tooth temperature during placement of selected Bulk Fill resin-based composites (RBCs).

Five representative Bulk Fill RBCs from four companies were chosen with a wide range of viscosity and filler volume content. The shrinkage stress kinetics at T=33°C was measured continuously over a period of 12h using a modified tensometer with the ability to measure the cantilever beam deflection to better than 40nm accuracy at a sampling rate of up to 200 samples/s, and thermally stable resulting in a measurement accuracy better than 0.05MPa at 12h. The tensometer compliance was 0.105μm/N. A custom made heater was used to control the RBC sample temperature at T=33°C with a temperature gradient across the sample of less than 1°C. The samples were irradiated for 20s with irradiance of 1.1W/cm² and total energy density of 22J/cm². Three samples (n=3) were used for each RBCs.

The shrinkage stress at 12h for the five Bulk Fill RBCs ranged from 2.21 to 3.05MPa, maximum stress rate ((dS/dt)_M) varied from 0.18 to 0.41MPa/s, time at which the maximum stress rate occurred (t_Max) were between 1.42 to 3.24s and effective gel time (t_gel) varied from 50 to 770ms. Correlations were observed between (dS/dt)_M and t_Max (r=-0.946), t_Max and filler volume fraction (r=-0.999), and between the shrinkage stress at 12h and t_gel (r=0.994). However, no correlation was observed between the stress at 12h and filler volume fraction.

The shrinkage stress for four of the five Bulk Fill RBCs were not significantly different (p<0.05) at 6h and beyond after photo-curing and that fully developed stress induced by photo-cured RBCs may only be reached at times longer than 12h.