Microtensile Bond Strength of Lithium Disilicate Ceramics to Resin Adhesives

Purpose: To evaluate the influence of the internal structure of lithium disilicate glass ceramics (LDC) on the microtensile bond strength to a resin adhesive using two surface treatments. Materials and Methods: Milling blocks of three types of LDC were sectioned (4 mm thick) using a precision cutting machine: IPS Empress 2 (conventional LDC), IPSe.max CAD (a refined crystal high strength LDC), and Celtra (zirconia reinforced LDC). Cut specimens received crystallization heat treatment as suggested by the manufacturers. Two surface treatments were performed on each group: hydrofluoric acid etching (HF) and airborne particle abrasion using 50-μm glass beads, while the as-cut surface served as control. Treated surfaces were examined using scanning electron microscopy (SEM). The disks were coated with a silane primer and bonded to pre-aged resin composite disks (Tetric EvoCeram) using a resin adhesive (Variolink II) and then stored in water for 3 months. Bonded specimens were sectioned into micro-bars (1 x 1 x 6 mm) and microtensile bond strength test (MTBS) was performed. Data were analyzed using two-way ANOVA and Tukey's post-hoc test (α = 0.05). Results: Statistical analysis revealed significant differences in microtensile bond strength values between different LDCs (F = 67, p 0.001), different surface treatments (F=232, p 0.001), and interaction between LDC and surface treatments (F = 10.6, p 0.001). Microtensile bond strength of Celtra ceramic (30.4 ± 4.6 MPa) was significantly higher than both IPS Empress 2 (21.5 ± 5.9 MPa) and IPSe.max ceramics (25.8 ± 4.8 MPa), which had almost comparable MTBS values. SEM images demonstrated homogenous glassy matrix and reinforcing zirconia fillers characteristic of Celtra ceramic. Heat treatment resulted in growth and maturation of lithium disilicate crystals. Particle abrasion resulted in abrasion of the glass matrix and exposure of lithium disilicate crystals, while HF etching produced a microrough surface, which resulted in higher MTBS values and reduction in the percentage of adhesive failure for all groups. Conclusions: Within the limitations of this study, bond strength to lithium disilicate ceramics depends on proper surface treatment and on the chemical composition of the glass ceramic. Keywords: MTBS, bond, lithium disilicate, SEM