Objectives: The aim of the present study was to correlate differences in mechanical fatigue behavior with surface and in-depth degree of cure of a resin composite (Tetric^® Ceram).

Methods: A variety of light-curing units was used: FL (Freelight std mode, 40s), FE20 (Freelight exp mode 20s), FE40 (Freelight exp mode 40s), TL(Trilight std mode, 40s), AS (Astralis pul mode, 20s), AP (Apollo 95E 3x3s). Specimens with dimensions of 25x2x2 mm were fractured in a universal testing machine (Z2.5, Zwick) after 14d water storage at 37°C. Flexural strength (FS, n = 12) and flexural fatigue limit (FFL, n = 22, 10⁴ cycles, ν = 0.5 Hz, staircase method) were measured. The degree of cure at the uppermost surface (DC0) and at 2 mm depth (DC2) was measured on cylindrical specimens (Ø = 3mm) by micro-multiple internal reflectance FTIR spectroscopy, employing the two frequency technique. Measurements at the curing surface and at 2 mm depth were taken after storage of specimens for 14d in dark and dry storage conditions at the same temperature.

Results: All results are listed below [Mean](S.D.):

	FL	FE20	FE 40	TL	AS	AP
FS [MPa]	75.0 (12.7)	116.0 (9.1)	107.0 (9.8)	68.0 (6.0)	105.7 (5.7)	80.0 (4.3)
FFL [MPa]	23.0 (5.8)	52.2 (7.9)	58.0 (6.5)	42.2 (4.4)	50.0 (6.4)	25.0 (6.3)
DC0 [%]	49.4 (2.4)	49.1 (5.8)	67.1 (1.4)	64.1 (0.4)	56.6 (2.4)	51.2 (2.6)
DC2 [%]	37.4 (2.5)	39.0 (1.5)	55.0 (3.7)	59.0 (2.4)	50.2 (2.8)	36.6 (3.5)

Conclusions: The prominent decrease of FFL compared to FS is supported by a low in-depth DC. However, promising results of initial measurement have to be assessed critically since cyclic FFL and DC2 account for significant decreased results, especially for FL and AP units (ANOVA, p<.05).