Correlation Between Composite %Filler and Strength, Modulus, and Shrinkage Stress

Objectives: Filler content in composite is credited for increasing strength and reducing shrinkage stress of resin composite. This study evaluated the correlation between the filler percentage and strength, modulus, and shrinkage stress of commercially available flowable and packable dental composites.
Methods: Twenty composites were included in the study (see table). Filler percentage by weight (n=5) was determined through the burned ash technique (800°C for 15 minutes). Three-point bend flexural strength and modulus (n=10) were measured according to ISO 4049 with 2x2x25mm bars, with a cross head speed of 0.75mm/min until fracture. Shrinkage stress (n=8) was evaluated using a universal testing machine in which composite was polymerized through two transparent acrylic rods (diameter = 6.45mm) with a controlled distance 2mm. A Teflon cylinder was placed around the rods to contain the flowable resin composites. The maximum force following 500 seconds of measurement was divided by the surface area of the rod to determine shrinkage stress. Linear correlation between mean filler wt% and flexural strength, modulus, and shrinkage stress were measured by Pearson’s correlation coefficient using SPSS.
Results: The mean +/-standard deviation of the filler wt%, flexural strength, modulus and shrinkage stress are presented in the attached table. There is a positive linear correlation between filler wt% and both flexural strength (r=0.70 p<.01) and modulus (r=0.85 p<0.01) but not shrinkage stress (r=-0.14 p=0.54).
Conclusions: Filler wt% is a good indicator of the stiffness (modulus) and flexural strength of a resin composite. Shrinkage stress may not be determined solely based on filler wt%.