The purpose of this study was to examine the fracture toughness ( K IC ) of three direct dental composites and one indirect dental composite subject to cyclic loading.
The composites were a micro-filled (Micronew, Bisco INC., Schaumburg, IL, USA), a hybrid (Renew, Bisco INC.), a nano-filled composite (Filtek Supreme Plus, 3M ESPE, St. Paul, MN, USA) and an indirect dental composite (BelleGlass HP, SDS-Kerr, Orange, CA, USA). Rectangular bar specimens (3 mm × 3 mm × 25 mm) were fabricated, notched, aged (5 months) and cyclic loaded in four different environments, air, water, artificial saliva, and a 50/50 by volume mixture of ethanol and water. Specimens were cyclic loaded for 1, 1000, 10,000, and 100,000 cycles.
A 3-way ANOVA (non-aged and aged group, four aging media, four loading cycles) showed a significant difference between non-aged and aged, aging media, and loading cycles. For the control groups as the number of cycles increased, there were no significant differences on the number of cycles completed and fracture toughness, except for Micronew, which showed an increased specimen failure rate and decreased fracture toughness. In the aged groups, cyclic loading in water and artificial saliva did not have a significant effect on BelleGlass HP, Filtek Supreme Plus and Renew for fracture toughness and the number of cycles completed, However for Renew in the 50/50 mixture at 100,000 cycles, Filtek Supreme Plus in air and the 50/50 mixture, and Micronew, there was an increased specimen failure rate and a decreased fracture toughness during cyclic fatigue loading as the number of cycles increased.
BelleGlass HP displayed the best overall resistance to cyclic loading, followed by Renew and Filtek Supreme Plus, and Micronew.
The application of resin based restorative dental composite materials in mastication-load-bearing areas is a concern since failure occurs frequently due to marginal fracture, bulk fracture, and/or wear . These materials are subjected to mastication forces that are low and repetitive (fatigue) with the concern that the reported fatigue behavior of resin composites does not correlate with initial strength values, and materials providing high initial strengths may not reveal the best fatigue resistance .
Restorative dental materials are classified as direct and indirect dental materials depending on how processed. Direct restorative dental materials are utilized directly in the intraoral environment, whereas indirect restorative dental materials are laboratory-processed materials, such as inlays or onlays. Several studies have reported increased mechanical properties of indirect composite compared to direct composite resulting from a post-cure treatment or second polymerization cycle . The increased mechanical properties of indirect dental composites resulted from the increased degree of cure (DC) of monomers which contribute to toughening of the filled resin matrix and possibly an improved filler/matrix adhesion, and the relief of internal stresses by the high temperature treatment. Other studies have indicated that indirect composites are not superior compared to direct composite and that post curing resulted in moderate improvement (∼9%) of selected mechanical properties .
In most instances, stress applied to teeth and dental restorations is generally low in value and repetitive (fatigue) rather than being a single, impact load . The normal occlusal loading of a filling is estimated at between 5 and 20 MPa and occasionally, a higher peak stress may occur during parafunctional movement, such as during maximum clenching increasing to 100 MPa .
The purpose of this study was to examine the fracture toughness ( K IC ) resistance to cyclic loading of three direct dental composites, a micro-filled (Micronew, Bisco INC., Schaumburg, IL, USA), a hybrid (Renew, Bisco INC., Schaumburg, IL, USA), a nano-filled composite (Filtek Supreme Plus, 3M ESPE, St. Paul, MN, USA) and a indirect dental composite (BelleGlass HP, SDS-Kerr, Orange, CA, USA), after aging and cyclic loading fatigue in four different environments, air, water, artificial saliva, and a 50/50 by volume mixture of ethanol and water.
Materials and methods
Four dental composites were included in this study; three direct dental composites micro-filled Micronew, hybrid Renew, nano-filled Filtek Supreme Plus, and the indirect dental composite, BelleGlass HP.
The bar dimensions and testing methodology for fracture toughness were according to ASTM Standard PS070-097 . Rectangular bar specimens (3 mm × 3 mm × 25 mm) were fabricated using a Plexiglas split mold so that no force was required to remove the cured bars. The split mold was placed on a Plexiglas slab lined with a piece of Mylar strip. Uncured composite was inserted into the mold and then a second Mylar strip was used to cover the uncured composite. A second Plexiglas slab was placed on the top, clamps used to stabilize the split mold, and the entire Plexiglas mold assembly cured in an oven (Triad, Dentsply/York Division, Dentsply International, York, PA, USA) for 2 min on each side. Specimens were then hand-polished with 320 grit SiC paper.
The same method was used to fabricate the indirect composite specimens except a metal split mold was used instead of a Plexiglas split mold. The entire metal mold assembly was cured in an oven (Triad, Dentsply/York Division, Dentsply International, York, PA, USA) for 2 min with the open slot side of the mold upward for the first step photo polymerization. Then the assembly was placed into a BelleGlass HP curing unit (SDS-Kerr) for the second step, pressure and heat polymerization under 140 °C and 80 psi nitrogen. Specimens were then hand-polished with 320 grit SiC paper.
A 60° notch was machined 0.75 mm deep at the midspan of each bar except for Micronew, which had 1.00 mm deep notch. A 1.00 mm notch was chosen for Micronew as a continuation of a previous study, that used a fiber reinforced dental composite, which has high flexure strength . Only after the Micronew samples were prepared was it realized that Micronew was very weak, and a 0.75 mm notch later on was decided to be used for the other specimens. Theoretically, the value of fracture toughness is not influenced by notch size since it was calculated by the formula described below, which takes notch size into consideration. Specimens were aged at 37 °C for 5 months in air or 200 ml of artificial saliva or water or a 50/50 by volume mixture of ethanol and water. Composition of the artificial saliva used in this study was in g/L: 0.4 NaCl, 0.795 CaClPO 4 ·2H 2 O, 0.78 NaH 2 PO 4 ·2H 2 O , 0.005 Na 2 S·9H 2 O and 1.0 CO(NH 2 ) 2 .
Both control and aging group 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 on a 858 Mini Bionix II testing device (MTS Systems Corporation, Minneapolis, MN). The load limit was chosen to be approximately 40–50% of the static loaded specimens and a stress level normally encountered during mastication. Following cyclic loading, the intact specimens were tested in three-point loading in their respective media using a Instron 1125 (Instron Corp., Canton MA) material testing system controlled by MTS (MTS Systems Corporation, Minneapolis, MN) digital electronics with a loading speed of 2 mm/min. For every specimen that fractured during cyclic loading, and thus could not be tested in three-point loading, the K IC value was set to zero. The number of cycles for each specimen, fractured and intact, was recorded.
Fracture toughness K IC was computed from a standard fracture mechanics formula :
K IC = P L f 1 ( a ) B H 1.5