Methods: Forty bovine lower central incisors were used, twenty teeth for the fatigue test and twenty for the tensile test. Dentin slabs 1mm in thickness were cut from the teeth using a low-speed cutting machine. The slabs were sectioned from the radicular dentin of the tooth. A dentin slab was harvested from each tooth. After preparation, a rectangular block (3.0 x 3.0 mm) was cut from each dentin slab, and dumbbell-shaped specimens with a narrow central portion (1.0 x 1.0 x 1.5 mm) were obtained using an air turbine hand piece attached to a profiling machine. For the fatigue test, specimens were mounted on a servo-hydraulic test machine in 37°C Hank's balanced saline solution with a haversines cyclic load of 1 Hz for frequency, maximum load from 35 to 50 MPa and minimum load 5 MPa. The staircase method was employed to determine the fatigue strength and the standard deviation. A texture analyzer was used for the tensile test. Mean fatigue and tensile strengths were compared statistically by one-way ANOVA and Fisher's PLSD test (p<0.05). Fracture surfaces were observed by scanning electron microscopy.
Results: The mean fatigue strength and tensile strength were 44.3 and 87.4 MPa, respectively. There was a difference of fracture surfaces between the fatigue and tensile test groups; the fracture surfaces of the fatigue test were smooth while those of the tensile test were rough.
Conclusion: The fatigue strength of radicular dentin is significantly lower than the tensile strength. In this study the ratio of fatigue strength to tensile strength was 0.5.