Effect of Resin Composite Filling Thickness in Zirconia Abutment Screw-Access on the Fatigue Behavior of a Cement-Retained Lithium Disilicate Material

This study evaluates the effect of varying resin composite thicknesses for sealing the screw-access hole of zirconia abutments on the fatigue mechanical behavior of a lithium disilicate cement-retained material. One hundred lithium disilicate discs (Ø = 10 mm, 1 mm thickness; IPS e.max CAD, Ivoclar AG) were prepared, alongside zirconia abutments (Ø = 10 mm, 3 mm thickness, 2.5 mm of screw-access hole diameter; IPS e.max ZirCAD MO, Ivoclar AG). The specimens were randomly assigned to five groups based on the thickness of sealing resin composite (Tetric N-Ceram Bulk fill, Ivoclar AG): Ctrl (only PTFE tape); PTFE tape +0.5 mm composite; PTFE tape +1.0 mm composite; PTFE tape +1.5 mm composite; and PTFE tape +2.0 mm composite. Surface treatments were conducted on ceramics before luting with dual-cure resin cement (Multilink N, Ivoclar AG). Monotonic testing was conducted at a loading rate of 1.0 mm/min until crack detection (n = 5). Cyclic fatigue testing was performed (n = 15; 100 N for 5000 cycles, followed by increments of 100 N every 10,000 cycles at 20 Hz) until failure. Finite element and Scanning Electron Microscopy analyses were also performed. One-way ANOVA and Tukey post hoc tests were used for monotonic data, while Kaplan–Meier and Mantel-Cox tests assessed survival rates (α = 0.05) based on fatigue test. No significant differences in monotonic tests were found. However, the 1.5 mm and 2.0 mm groups exhibited significantly higher fatigue failure loads compared to the Ctrl, 0.5 mm, and 1.0 mm groups (0.5 mm: 1093 N = Ctrl: 1120 N = 1.0 mm: 1127 N < 1.5 mm: 1426 N = 2.0 mm: 1307 N, p ≤ 0.05). To improve the fatigue behavior of lithium disilicate restorations bonded to zirconia abutments, more than half of the screw-access hole (greater than 1.5 mm) should be filled with resin composite.