In-vitro evaluation of stressing patterns in loaded glass-ionomer-cement sandwich restorations

Objectives: The study aimed to evaluate the impact of a conventional glass-ionomer-cement (GIC) and a novel resin-modified glass-ionomer-cement (RMGIC) on the stressing patterns arising in loaded resin-based-composite glass-ionomer-cement sandwich restorations (RBC-(RM)GIC).

Methods: The flexural moduli of a universal RBC, Tetric-Ceram (TC), a conventional GIC, Ketac-Molar (KM) and a ‘nano' RMGIC, Ketac-N100 (N1) were characterized. Materials were activated, mechanically mixed and/or photo-initiated according to the manufacturer's recommendations. Disc-shaped specimens of TC, KM and N1 and were fabricated (n=20) using a split mould (12mm diameter, 1mm thickness). Additional groups of KM and N1 specimens were coated with a 1mm increment of TC and further KM and N1 specimens were alumina air abraded prior to coating with TC. Following 24h wet storage, mono- and bi-layered specimens were loaded in bi-axial flexure and failure stresses calculated at axial vectors throughout the specimen thickness. Group means were compared using analyses of variance at a 95% significance level.

Results: The flexural moduli of TC, KM and N1 were 9.9±0.9, 5.7±0.7 and 3.5±0.2GPa respectively, and differed significantly (P<0.05). Significant differences were also observed between the mean bi-axial flexure strengths of TC, KM and N1 mono-layers (103.6±19.7, 21.4±6.0 and 51.2±9.0 MPa, respectively) (P<0.05). The maximum tensile stresses observed in the RBC-(RM)GIC bilayer were generated at the exposed (lower) surface of the (RM)GIC. The maximum tensile stress generated within the (RM)GIC layer were significantly increased (P<0.05) from 20.4±6.0 for the TC-KM bilayer to 42.5±10.8 for the TC-N1 bilayer. Roughening of the RBC-(RM)GIC interface resulted in significantly increased failure stresses for the TC-N1 bilayer only.

Conclusion: The stressing patterns arising in RBC-(RM)GIC layered restorations were sensitive to the mechanical response of the GIC substrate and the nature of the RBC-(RM)GIC interface. The combination of dissimilar materials results in patterns of functional variability not fully represented by testing individual structural components.