Fatigue Behaviour of Direct and CAD/CAM Indirect Restorative Materials Bonded to a Dentine Analogue Substrate

Abstract

<p>Purpose / Aim: This study aimed to investigate the fatigue behaviour of direct and CAD/CAM indirect restorative materials cemented to a dentine analogue substrate. Materials & Methods: Two resin-based direct restorative materials (G-ænial Universal Injectable, GU; Filtek Supreme Flowable Restorative, FS), and two CAD/CAM indirect restorative materials (IPS e.max CAD, EM; Tetric CAD, TE), were tested. Flexural properties (flexural strength and flexural modulus) and energy dissipation parameters (modulus of toughness, elastic recovery, and their difference) were determined via the three-point bending (3PB) test (n=10). Disc-shaped specimens were fabricated by adhesively bonding the restorative materials to the dentine analogue substrates. After the monotonic pilot trial (n=5), the wet cyclic fatigue test was performed on bonded tri-layer assemblies following the staircase approach (initial load: approx. 40% of the monotonic load; step size: approx. 5% of the monotonic load; 500,000 cycles per specimen; 20 Hz; n=15). Mean fatigue failure load (FFL), standard deviation (SD), and 95% confidence interval (CI) were calculated using the Dixon and Mood method. FFLs were also evaluated with Weibull statistics. Specimen survival or failure was examined visually and under a stereomicroscope, then fractographic analyses were conducted on failed specimens. Finite Element Models of the fatigue test were established to analyse the stress distributions. SPSS 26.0 was used for statistical analysis and the significant level was pre-set as α=0.05. Results: EM showed significantly higher flexural strength and elastic modulus than the three RBCs (p < 0.001). Nevertheless, three energy dissipation parameters of RBCs were significantly higher than those of lithium disilicate, ranking TE > FS = GU > EM (p < 0.05). The mean FFL (Fig 1) showed a similar trend, ranking TE (353.93 ± 43.01 N) > GU (276.79 ± 34.24 N) > FS (255.33 ± 47.30 N) > EM (184.00 ± 76.92 N). EM presented the lowest Weibull parameters and the highest RSD values, while TE presented superior fatigue resistance and structural reliability. Distinguishable failure characteristics and stress distribution patterns between EM and three resin-matrix materials were found. Conclusions: Distinct fatigue behaviours were discovered between lithium disilicate and resin-based materials, including the fatigue failure load, failure mode, and stress distribution pattern. RBC materials outperformed lithium disilicate in the cyclic fatigue test, among which the CAD/CAM indirect RBC material presented the best fatigue resistance. Energy dissipation parameters may serve as predictors to the fatigue properties.<br></p>