Flexural Fatigue Limit of Aged Composite Specimens

Objectives: The strength of filling materials are often characterized after a 1day soak at 36°C in water by load-to-failure tests, where a load is applied until the specimen breaks. Clinically, filling materials exist saturated with water and are not loaded with a single critical force. Under the lower repetitive forces experienced clinically, a crack forms and grows until it becomes fatal. The objective of this study is to characterize the flexural fatigue limit (FFL) of thermocycled filling composites based on different technologies.
Methods: Flexural strength specimens were prepared according ISO4049. Specimens were stored for 2 months in water at 36°C. After 2 weeks they were thermocycled (TC) 10,000 times (5-55°C; 1,000 cycles a day) and placed back into 36C water. The aged specimens were fatigued at 2Hz in water at 36°C for 10,000 cycles using the staircase method (texture analyzer XT plus, stable micro systems). The resulting FFL was compared to the ISO4049 flexural strength (FS) value and FS after the aging protocol above, tested in water at 36C.
Results: ANOVA analysis (Tukey) showed that at p<0.05, initial FS (n=10), FS after aging (n=10) and the FFL (n=21) behavior of composites can be very different. Values with the same letter in the same column are statistically not different. A correlation was found between FFL and ISO4049 FS (linear R²=0,76) and between FFL and FS after aging (linear, R²=0,85).
Conclusions: A high ISO4049 flexural strength is a good indicator for a high FFL. Consequently, a flowable (Filtek^TM Supreme XTE) can perform similar or better than highly filled universal composites. The highest initial FS value does not always mean the best performance under fatigue. Filtek^TM Universal and Filtek^TM One showed the highest FFL after aging. This may be attributed to 3M’s nanofiller and urethane-based resin technology.