HEMA-rich versus HEMA-poor/free adhesives: effect of HEMA on bonding effectiveness

HEMA presents as a fluid that is well solvable in water, ethanol and/or acetone. This property and its solvent-like nature make HEMA frequently added to dental adhesives. It improves the stability of adhesive solutions containing hydrophobic and hydrophilic components, basically avoiding phase-separations that are typical of HEMA-free/poor adhesives. Another important characteristic of HEMA is its high hydrophilicity, making it an excellent adhesion-promoting agent for dentin.

However, both in uncured and cured state, HEMA will readily absorb water. Uncured, water sorption may dilute the monomers to an extent that polymerization is inhibited. Cured, it still exhibits hydrophilicity, promoting water uptake with consequent nano-leakage and swelling. Like all methacrylates, HEMA is vulnerable to hydrolysis at basic and acidic pH. HEMA also lowers the vapor pressure of water and probably also of ethanol, thereby hindering good solvent evaporation from adhesive solutions. All these unfavorable properties along with the inferior polymerization efficiency of HEMA itself deteriorate the mechanical properties of HEMA-rich adhesives.

There are consequently both PROS and CONS regarding the addition of HEMA to dental adhesives. This presentation will present the effect of HEMA (in varying concentrations) on the eventual bonding effectiveness to dentin and enamel.