The effects of cyclic fatigue and aging on the fracture toughness of dental composites

Objective. The objective of this study was to examine the fracture toughness (KIC) of three direct dental composites, including one micro-filled (MicronewRTM, Bisco INC., Schaumburg, IL, USA), one hybrid (RenewRTM, Bisco INC., Schaumburg, IL, USA), and one nano-filled composite (Filtek(TM) Supreme Plus, 3M ESPE, St. Paul, MN, USA) and one indirect dental composite (BellGlass HP, SDS-Kerr, Orange, CA, USA), after aging and cyclic loading fatigue in four different environments, namely, air, water, a 50/50 by volume mixture of ethanol and water, and artificial saliva. Methods. The rectangular bar specimens were fabricated and a 60 degree notch was machined at the midspan of each bar. Aging group specimens were aged at 37°C for five months in air, artificial saliva, water, and a 50/50 by volume mixture of ethanol and water. The specimens were loaded in each corresponding media at a frequency of 5 Hz with sinusoidal loads cycling between 5 N and 10 N for 1, 1000, 10,000 and 100,000 cycles. Following cyclic loading, the specimens were tested in three-point loading in their respective media. Results . BellGlass HP displayed overall the best mechanical properties, followed by RenewRTM and Filtek(TM) Supreme Plus. Micronew RTM had the worst performance of the four composite materials investigated in this study. The non-aged group of Filtek(TM) Supreme Plus showed a strikingly similar performance to RenewRTM, a hybrid composite, both in the number of cycles completed and in the fracture toughness in all testing conditions. Aged Filtek Supreme Plus however, showed increased failure during cyclic fatigue loading, and also showed significantly decreased fracture toughness when compared to RenewRTM. Media did not have a significant effect on the number of cycles completed and the fracture toughness in the control group of BellGlass HP, Filtek(TM) Supreme Plus, and Renew RTM specimens. For MicronewRTM, air was the media which had the most detrimental effect on fracture toughness and the number of cycles completed in all four media investigated. For the aged groups, none of BellGlass HP specimens fractured during cyclic loading in all four media; for the remaining three materials, the 50/50 mixture was the most detrimental one on fracture toughness and the number of cycles completed in all four media investigated. Aging had the most detrimental effect on Filtek(TM) Supreme Plus and MicronewRTM. In terms of the effect of cyclic fatigue, there were no significant differences on the number of cycles completed and fracture toughness as the predetermined set of cycle numbers increased in the control group of BellGlass HP, Filtek(TM) Supreme Plus, and Renew RTM. For MicronewRTM, there was an increased specimen failure rate and decreased fracture toughness as the predetermined set of cycle numbers increased. In the aged groups, cyclic fatigue did not have a significant effect on BelleGlass HP and Filtek(TM) Supreme Plus in water and artificial saliva, and RenewRTM in terms of both fracture toughness and the number of cycles completed, except for RenewRTM in the 50/50 mixture at 100,000 cycles. For Filtek(TM) Supreme Plus in air and the 50/50 mixture and MicronewRTM, there was an increased specimen failure rate and a decreased fracture toughness during cyclic fatigue loading as the predetermined set of cycles increased. BellGlass HP displayed overall the best mechanical properties, followed by RenewRTM and Filtek(TM) Supreme Plus. Micronew RTM had the worst performance of the four composite materials investigated in this study.