Influence of HEMA on Bond Strength and Phase Separation

In spite of its high allergenic potential, HEMA, a low molecular-weight monomer, has previously been shown to positively influence bond strength. In addition, the presence of HEMA in one-component one-step adhesives can prevent phase separation. Objectives: In search for improved bonding effectiveness, the 24-hour bond strength of four experimental one-step self-etch adhesives (4-MET and phoshoric acid monomer as functional monomer) with different concentrations of HEMA to bur-cut enamel and dentin was tested using a micro-tensile bond strength protocol. Methods: The tested experimental adhesives (exp-0, exp-10, exp-19 and exp-36) only differed in their concentration of HEMA, which was 0%, 10%, 19% and 36% respectively. With increasing concentration of HEMA, the concentration of acetone was decreased. Besides bond strength, the adhesives were also examined by light microscopy for homogeneity and by SEM. Results:

µTBS in MPa (SD); n = 11, statistics by Kruskal Wallis; p<0.05 ; statistical significance is indicated by letters in superscript.

	Exp-0	Exp-10	Exp-19	Exp-36
enamel	17.2 (4.3) a	28.3 (4.0) b	25.6 (5.4) b	22.0 (4.9) b
dentin	25.3 (6.6) A	37.4 (4.5) B	26.0 (5.6) A	18.3 (4.7) A

Only exp-36 did not exhibit phase separation on a glass plate. Nevertheless, this adhesive yielded the lowest bond strength. In addition, droplets could be observed by SEM in the adhesive layers of all adhesives, except for exp-36 on enamel. Conclusion: 10% of HEMA improved the bond strength of a one-step self-etch adhesive, however only significantly for dentin. This favourable result must be attributed to better wetting and infiltration properties of the adhesive. When added in higher concentrations, this beneficial effect of HEMA on the bond strength may be lost due to increased hydrophilicity, decreased removal of water and reduced co-polymerization.

This study was partly supported by Toshio Nakao Chair and by FWO.