Methods: Filtek Z100 and Filtek Z250 (3M-ESPE), examples of direct resin composite restorative materials, and Sinfony (3M-ESPE) and an experimental composite (Heraeus Kulzer), examples of indirect composites, were used in this study. In accordance with ISO 4049, 3-point bending flexural strength tests (FS) were carried out on bar shaped specimens (2x2x25mm; n=10). Equally sized specimens (n=20) were modified to hour-glass shaped specimens to determine the rotary fatigue resistance (RFR) in a rotating-bending cantilever test. In this test the maximum stress was located at the constriction of the hour glass. RFR tests were conducted according to the staircase approach (10,000 cycles, 1.0Hz). The mean RFR was calculated using logistic regression.
Results: FS ranged between 99 and 152 MPa, while the RFR ranged between 52 and 90 MPa. After 10,000 cycles the strength of the materials decreased (23% to 48%). RFR of Filtek Z100, Filtek Z250, and the experimental resin composite (77-90 MPa) were significantly higher than Sinfony (52 MPa).
Conclusion: Exposure of the resin composite specimens to rotational fatigue results in a significant decrease of the flexural strength (23-48%). From a clinical perspective the resistance to rotational fatigue test might be of more value than the flexural strength test.