Bond Strength of Thiol-ene/Methacrylate Composite Restorative Materials

A novel photopoplymerizable polymer is being developed as the matrix for composite restoratives. The polymerization involves a hybrid radical chain/step-growth reaction between multifunctional thiol and multifunctional vinyl monomers and produces polymers with high conversion and strength. Bonding to tooth structure requires the continuous polymerization of the resin in the bonding agent with the resin matrix of the composite restorative. A different polymerization mechanism could affect the bonding with current commercial agents.

Objective: Determine the dentin bond strength of methacrylate/thiol-ene-based composite systems with either a total etch or self-etching bonding agent.

Methods: Extracted human teeth were sectioned, dentinal surfaces polished to 600 grit, and tested by the Watanabe technique. Either a total etch technique (37% phosphoric acid, followed by Optibond Solo cured for 40 sec), or a self-etching technique (Clearfil S3, cured for 40 sec) was used. There were 4 formulations: 1) conventional BisGMA/HDDMA, and methacrylate:thiol-ene ratios of 2) 80:20, 3)70:30, and 4) 50:50. Each resin system was filled to 75wt% with barium glass/microfill silica. Each layer of composite was cured for 40 seconds at 500 mW/cm2. After 2 days storage in a humidor at 37oC the specimens were allowed to equilibrate to ambient conditions before testing at a rate of 1 mm/min.

Results:

(mean±sd) MPa)    Conventional        80:20           70:30          50:50

Total etch                       38.2±7.7     41.4±8.5     32.1±9.9     15.0±10.8

Self etch                         28.6±4.7     26.3±4.7     23.5±4.5     20.8±6.2

The 50:50 mixture had statistically significantly lower bond strengths. Trends in the data suggest that the greater the thiol-ene content, the lower the bond strength for the total etch technique. The self-etching system did not show such compromise possibly due to the potential for acid catalysis of the thiol-ene system.

Conclusion: Methacrylate/Thiol-ene matrix composite restoratives will bond to dentin using either conventional bonding system.

Supported by NIH/NIDCR 2R01DE14227 and a gift from Septodont