Influence of cleaning solutions and hydrothermal aging on the flexural strength and microhardness of resins for additively manufactured definitive fixed restorations

Statement of problem: Studies on the effects of cleaning solutions and hydrothermal aging on the flexural strength and microhardness of additively manufactured (AM) resins are lacking. Purpose: The purpose of this in vitro study was to evaluate the effect of cleaning solution on the flexural strength and microhardness of resins marketed for AM definitive restorations. In addition, the effects of hydrothermal aging and material type on these properties were examined. Material and methods: Rectangular or disk specimens (n=19) were additively manufactured for flexural strength and microhardness tests from 2 commercially available resins for definitive restorations: a glass-filled composite resin (AM-CR) and a urethane acrylate-based resin (AM-UA). The specimens were divided into 4 groups based on the cleaning solution: 96% ethanol, 98% isopropanol (IPA), water-based solvent, and methyl ether solvent. The specimens were divided into 2 subgroups, nonaged and hydrothermally aged (10 000 thermal cycles), and flexural strength and microhardness tests were performed before and after aging. Results: AM-CR had higher flexural strength with ethanol, IPA, and water-based solvent than AM-UA (P<.004), whereas it had lower flexural strength than AM-UA with methyl ether solvent (P<.001), regardless of aging. AM-CR and AM-UA had higher microhardness with ethanol and IPA compared with the methyl ether solvent (P<.031). AM-CR had higher microhardness with all cleaning solutions than AM-UA, regardless of aging condition (P<.001). Conclusions: Resin type, cleaning solution, and aging condition affected the flexural strength and microhardness of AM resins intended for definitive restorations. Regardless of hydrothermal aging, AM-CR had the lowest flexural strength when the methyl ether solvent was used, while cleaning solution type did not affect the strength of AM-UA; AM-CR's strength was lower than AM-UA's with methyl ether. Regardless of hydrothermal aging, AM-CR and AM-UA had higher microhardness with ethanol and IPA than with methyl ether solvent. AM-CR had higher microhardness than AM-UA, regardless of cleaning solution. After hydrothermal aging, the microhardness of AM-CR decreased with the water-based solvent.