Optimizing Bond Strength of RMGI and Conventional Composite

Objectives: The “sandwich technique” of placing resin modified glass ionomer (RMGI) as the initial restorative material increment, followed by overlaying with light-cured composite, is a common clinical procedure. This project evaluated the effects of RMGI surface abrasion as well as dentin bonding agent components on the shear bond strength (SBS) of composite to RMGI.
Methods: Cylinders (10 mm dia, 3 mm thick) of a commercially available, light-cured resin modified glass ionomer (RMGI) (Fuji II LC, GC) were photocured (Dem iUltra, Kerr, 20s) in the wells of 3D printed holder, without any Mylar covering of the light-exposed surface, to simulate the clinical condition. The as-cured surface was then abraded (240 grit SiC, water irrigation) or not. Each surface was etched with phosphoric acid (Uni-Etch, BISCO) (20s), rinsed and dried, and then the components of a 4^th generation dentin bonding agent (DBA) (Optibond FL, Kerr) were placed (either only adhesive, or with primer and then adhesive). The DBA was light-cured (Demi Ultra) for 10s. The prepared specimen was then placed in an Ultradent specimen holder, and a small cylinder (2mm x 2mm) of unpolymerized composite (Premise Body, A2, Kerr) was placed and light-cured (20s, Demi Ultra). Specimens were stored in a humidor (37°C, 24h) after which they were thermocycled 500x (5-55°C). SBS was determined in an Ultradent mini-tester. (n=10 per condition).
Results: 2-way ANOVA (pre-set alpha 0.05) indicated neither factor (abrasion p=0.057; DBA components p=0.791) nor the interaction term (p=0.427) significantly affected SBS. Most all failures occurred cohesively within the RMGI.
Conclusions: Generally, clinicians would be advised not to surface abrade freshly placed RMGI prior to bonding resin composite, to enhance bond strength between the substrates.