Origin of Interfacial Droplets with One-step Self-etch Adhesives

Objectives: Contemporary one-step self-etch adhesives (1-SEAs) have often been documented with interfacial droplets. The objective of this study was to investigate the origin of these droplets.

Methods: The adhesives included in the study are shown in the following table.

The adhesives were applied to enamel and dentin according to the manufacturers' instructions, with the lining composite either cured immediately (non-delay group) or only after 20 min (delay-group). The µTBS was determined according to a standardized protocol, and the interface was investigated using TEM and SEM.

Results:The µTBS to enamel was stable when the composite was delay-cured, but the bond-strength to dentin dropped significantly for the HEMA-rich adhesives in the delay group. All one-step adhesives exhibited droplets at the interface; however, the droplets had two different origins. With the HEMA-free adhesives, droplets of varying size (0.5–10 µm) were located throughout the adhesive layer and their number was stable over time. They could be observed both on enamel and dentin. With the HEMA-rich adhesives, smaller droplets (0.3-1.5 µm) were observed exclusively at the adhesive resin/composite interface and their number increased significantly when the composite was delay-cured. They could only be found in dentin samples. These droplets in HEMA-rich adhesives caused a significant drop in bond strength after delay-curing.

Conclusion: This study showed that there are two distinct kinds of droplets with 1-SEAs, different in appearance, location and behavior. While the droplets in the HEMA-free one-step adhesives should be ascribed to phase-separation, those observed with HEMA-rich adhesives result from water absorption from dentin through osmosis.

KL Van Landuyt is appointed as aspirant FWO.