Effect of spherical silica additions on marginal gaps and compressive strength of experimental glass-ionomer cements

Purpose: To clarify the effects of the addition of silanized (S) and unsilanized (U) spherical silica filler to resin-modified glass-ionomer cement and of powder-liquid ratio on (1) the early marginal gap-width of restorations in both tooth cavities and Teflon molds, (2) the gap-formation of restorations in Class V cavities, and (3) the compressive strength of the cement. Methods: Resin-modified glass-ionomer powder (Fuji II LC EM, GC) was modified by adding 5 and 10 wt% of powder respectively, of S and U, and then the powder-liquid ratio was increased up to 4.8. Human premolars, extracted for orthodontic reasons, were used for this study. Cylindrical cavities (1.5 mm deep, 3.5 mm in diameter; one cavity was prepared in each tooth in the coronal region and medial surface) were prepared in extracted human premolar teeth and restored with resin-modified glass-ionomer cements. Each restoration margin was inspected immediately after curing and polishing (as the immediate condition was the most severe), the maximum gap-width and the opposing width (if any) were determined microscopically (n= 10). An additional test was conducted in model Class V cavities. After finishing of restorations in model Class V cavities, each tooth was sectioned in a bucco-lingual direction through the center of the restoration, and the presence or absence of gaps along the cavity interface was evaluated (n=10). Additionally, the maximum marginal gap-width and the opposing-width along margins of restorations in cylindrical Teflon molds were measured (n= 10). The compressive strengths of the restorative materials were determined immediately after light-activation (n=10). Results: Marginal gap (tooth cavity: 0.32 to 0.25-0.20%, P< 0.05; Teflon cavity: 0.94 to 0.6-0.8%, P< 0.05) and cavity adaptation (no gap in the Class V: 22 to 40-50%, P< 0.05) of the restorations improved with increasing powder-liquid ratio (3.0 to 4.4-4.8) and compressive strength increased (111 to 150-170 MPa, P< 0.05). Highly significant correlation coefficients were found for the relationships between powderliquid ratio and (1) percentage of marginal gap width in the tooth cavity (r= -0.96, P= 0.002, n= 6), (2) gap-free tooth/cement interfaces (r= 0.90, P= 0.015, n= 6), (3) percentage of marginal gap widths in the Teflon mold (r= 0.98, P= 0.0004, n= 6) and (4) compressive strengths of the cements (r= 0.95, P= 0.004, n= 6).