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This study evaluates the effect of aging in artificial saliva and thermal shocks on the
microhardness of the bulk-fill composite compared to the nanohybrid composite. Two commercial
composites, Filtek Z550 (3M ESPE) (Z550) and Filtek Bulk-Fill (3M ESPE) (B-F), were tested. The
samples were exposed to artificial saliva (AS) for one month (control group). Then, 50% of the
samples from each composite were subjected to thermal cycling (temperature range: 5–55 ◦C, cycle
time: 30 s, number of cycles: 10,000) and another 50% were put back into the laboratory incubator
for another 25 months of aging in artificial saliva. The samples’ microhardness was measured using
the Knoop method after each stage of conditioning (after 1 month, after 10,000 thermocycles, after
another 25 months of aging). The two composites in the control group differed considerably in
hardness (HK = 89 for Z550, HK = 61 for B-F). After thermocycling, the microhardness decrease was
for Z550 approximately 22–24% and for B-F approximately 12–15%. Hardness after 26 months of
aging decreased for Z550 (approximately 3–5%) and B-F (15–17%). B-F had a significantly lower
initial hardness than Z550, but it showed an approximately 10% lower relative reduction in hardness.