Microleakage and bond strength of 'open-sandwich' restorations in primary molars

The term ‘sandwich technique' describes a laminated restoration where glass-ionomer cement (GIC) replaces dentine and resin composite (RC) replaces enamel; it is applied as ‘open' or ‘closed' to Class II restorations. In cavities lacking cervical margin enamel, an incremental layer (eg. GIC) restores the proximal box at the cervical instead of RC. Objectives: This in-vitro study compared two material combinations: RC (TPH3,Dentsply) + resin-modified GIC (Photac Fil,3M ESPE), termed RC/RMGIC; and RC (TPH3,Dentsply) + polyacid-modified RC (Dyract Extra, Dentsply), termed RC/PMRC, for restoring Class II preparations in primary molars using the open-sandwich technique. Methods: Microleakage (ML) was studied in restored cavity preparations; microshear bond strength (MSBS) was measured with material combinations bonded to substrates of dentine, RMGIC or PMRC. Fifty Class II preparations were restored as open-sandwich restorations; ML was assessed progressively during lapping. The MSBS of RC bonded to RMGIC or PMRC substrates, and of RMGIC and PMRC bonded to dentine substrates, was measured. Debonded surfaces were studied under scanning electron microscopy. Results: More extensive ML occurred in RC/PMRC than RC/RMGIC at RC/enamel cavo-surfaces and gingival floor of proximal boxes. Bonded to RC, RMGIC showed lower mean (±SD) MSBS (MPa) than PMRC (16.04±1.96 vs 19.85±3.76, p<0.001) and failed cohesively within both materials; RC/PMRC failed cohesively within PMRC and adhesively between RC and PMRC. Bonded to dentin, RMGIC showed higher mean MSBS than PMRC (11.81±3.24 vs 8.50±3.63, p<0.001) and failed in cohesion or mixed (cohesion/adhesion); PMRC/dentin bonds showed mixed failure. After debonding, RMGIC substrates showed irregular topography, voids, and large adherent RC fragments; PMRC substrates showed smooth topography and small adherent RC fragments. Conclusion: Acknowledging superior MSBS of RC bonded to PMRC, open-sandwich restorations of RC/RMGIC appear preferable to RC/PMRC based on higher MSBS to dentine, less ML, and less likelihood for adhesive failure to dentine.