In-situ Analysis of Degree of Conversion of 2-step etch-and-rinse Adhesives

Objectives:

To investigate the degree of conversion of the hybrid layer (HL) created by three commercial 2-step etch-and-rinse adhesives. The tested hypothesis was that no difference could be observed within the tested adhesives when applied under simulated pulpal pressure.

Methods: The tested adhesives were SB (Scotchbond 1XT, 3M ESPE), PB (Prime&Bond NT, Dentsply), AM (Ambar, FGM). Adhesives were applied on human dentin disks (N=3) following manufacturers’ instructions under simulated pulpal pressure (20mm H₂O). A 2mm-thick composite layer was applied on the adhesive and light cured for 20 s with a quartz-halogen lamp at 600mW/mm² (Elipar 2500; 3M ESPE). The adhesive interface was exposed for the spectroscopic micro-Raman analysis (Renishaw InVia; laser wl 785 nm). One Raman spectrum was collected every 1µm along the dentin/adhesive interface in the intertubular dentin region. The relative intensities of bands associated with mineral dentin components (PO functional group at 960 cm^-1) and adhesive (C=C at 1640 cm^-1 as reaction peak, phenyl C=C at 1610 cm^-1 as reference peak) across the hybrid layer were used to identify the adhesive layer and calculate the degree of conversion (DC) of the tested adhesives. Data were analyzed by one-way ANOVA (statistical significance was set at p=0.05) and Tukey’s post-hoc.

Results: The DC of the adhesives ranked as follows: SB(79±7%)=AM(78±7%)>PB(65±10%) (p<0.05).

Conclusions: The tested hypothesis was rejected since PB showed lower DC values than SB and AM. An optimal DC is needed to obtain a stable polymeric network and to guarantee the longevity of the adhesive interface. Further studies are needed to investigate the mechanical properties of the hybrid layer created by the tested 2-step etch-and-rinse bonding agents. 

Supported, in part, by grants: FIRB RBAP1095CR and PRIN 2009SAN9K5 and 2009FXT3WL from MIUR, Italy.