The impact of long-term aging in artificial saliva media on resin-based dental composite strength

Although salivary liquid can degrade constituents in resin-based dental composites in short-term incubations, there is a knowledge gap on how longer-term aging impacts their bulk strength. We address this through extended aging studies with resin-based dental composites in different environments. Two commercial composites (FIL and AEL) were aged aseptically at 37°C in air (A, control), artificial saliva (AS), and esterase enzyme amended AS (EAS). Diametral and pushout strength were measured after periods of 120-180 days. At 120 days, the diametral strength of composites aged in air was 69.9 ± 11.0 and 57.7 ± 3.31 MPa in FIL and AEL, respectively. These were significantly greater compared to composites aged in AS (32.1 ± 7.01 and 46.2 ± 9.38 MPa in FIL and AEL, respectively) or EAS (36.7 ± 8.49 and 43.5 ± 5.51 MPa in FIL and AEL, respectively). In contrast, pushout strength for both composites were smaller in A compared to those aged in AS and EAS, results attributed to AS absorption and polymer expansion. No significant change in either diametral or pushout strength occurred after 120 days. There was no significant difference between aging in AS and EAS, suggesting that esterase did not significantly decrease the bulk material strength to a greater extent than AS under the test conditions. Aqueous diffusivities for the composites ranged from 8.4 to 11 × 10^-13  m² /s, with associated porosities ranging from 0.06% to 0.10%. These results indicate that saturation of a typical dental composite occurs over a time frame of 4-5 months, longer than typical aging studies. Together, the results demonstrate the importance of aging time on composite strength.