Behavior of resin-based endodontic sealer cements in thin and thick films

For root canal fillings, a thin layer of sealer cement is generally recommended. However, with resin-based sealers, lower bond strength to dentin has been shown in thin layers compared to thick, contrary to typical behavior of adhesive layers between two adherents. The aim of this study was to evaluate tensile and shear bond strength of thin and thick films of three resin-based sealers (one epoxy-based and two methacrylate-based) materials and to investigate corner effects of one methacrylate-based resin sealer.

Freshly mixed sealer cements were placed between metal-to-metal surfaces of plano-parallel stainless steel aligned rods with diameter 4.7 mm. Ten samples were prepared for each type, thickness (0.1 and 1.0 mm) of sealer and test. Tensile and shear strengths were measured after 48 h for the methacrylate-based materials and after 7 days for the epoxy-based material using a universal testing machine at a crosshead speed of 1mm/min. Corner effects were investigated using one methacrylate-based resin material.

Film thickness had a highly significant influence on both tensile and shear strengths. For methacrylate resin-based sealers, thin films had higher bond strength than thick (p<0.001 for both tensile and shear bond strength). With the epoxy-based sealer either no difference (shear) or lower bond strength in thin films (tensile; p<0.05) was found, and appeared to result from numerous voids created during mixing. The methacrylate based sealer demonstrated typical engineering behavior for an adhesive material, with corner effects shown as a material property and in good agreement with the tensile bond strength results.

The higher tensile and shear bond strength of resin-based sealer in thin films is the opposite of that previously reported for bonding to dentin. The substrate clearly has an important role in failure behavior.