Effect of functional monomers on bond durability of self-etch adhesives

Objectives: The aim of this study was to determine the durability of bonding to dentin of three self-etch adhesives that contain three different functional monomers, and to relate it to the underlying mechanism of bonding as investigated ultra-morphologically as well as chemically. Methods: The three adhesives tested were Clearfil SE Bond (Kuraray) that contains 10-methacryloxydecyl dihydrogen phosphate (10-MDP), Unifil Bond (GC) that contains 4-methacryloxyethyl trimellitic acid (4-MET), and Clearfil Liner Bond 2 that contains 2-methacryloxyethyl phenyl hydrogen phosphate (phenyl-P). The bond durability was determined using a micro-tensile bond strength testing protocol after 0 (control), 10000, 20000, 30000, 50000 and 100000 thermo-cycles (60 sec immersion alternatively in a 5 and 55oC water bath, three teeth per adhesive and per thermo-cycling session). The interfacial interaction of the self-etch adhesives with dentin was characterized using Transmission Electron Microscopy, and the chemical interaction of the three functional monomers with synthetic HAp was analyzed using X-ray Photoelectron Spectroscopy and Atomic Absorption Spectrophotometry. Results: The micro-tensile bond strength of the 10-MDP-based adhesive did not decrease significantly after 100000 cycles, but did after 50,000 and 30,000 cycles, respectively, for the 4-MET-based and the phenyl-P-based adhesive. The interfacial ultra-structure remained unchanged after 100000 thermo-cycles for the 10-MDP-based adhesive, while that of both the 4-MET- and phenyl-P-based adhesives showed voids and less defined collagen. Chemical analysis revealed that the monomer 10-MDP readily and stably adhered to hydroxyapatite, as confirmed by the low dissolution rate of its calcium salt in water. The chemical interaction potential of 4-MET was substantially lower, while the monomer phenyl-P and its bond to hydroxyapatite appeared not very hydrolytically stable. Conclusion: The findings of this study support the concept that the chemical bonding potential of the functional monomer contributes to the long-term durability of adhesive-dentin bonds.