An Assessment of Residual Monomer in Endodontic Retrofill Material

BACKGROUND: Polymer-Clay Nanocomposites(PNC) are a new class of polymeric materials composed of nano-sized clay particles that have high aspect ratios(20-1000) and high surface areas. When dispersed in a polymer matrix, the nanoparticles tremendously improve overall strength, dimensional stability, and drug elution control of the matrix. A previous in vitro study reported that an endodontic retrofill using a Polymer/Monomer Matrix(PMM) with nanocomposite significantly reduced apical bacterial leakage(p<0.01) than the commercially available Geristore®. In the same study, however, Geristore® showed lower leakage than PMM without nanocomposite. HYPOTHESIS: This finding is due to a lower degree of polymerization in the PMM material. OBJECTIVES: The objective of this study was to identify and evaluate the release of unpolymerized/residual monomer and to compare two photoinitiator formulae in reducing residual monomer release. METHODS: Two monomer matrices(MM) were compared—MM1: 2,2-bis[p-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane(Bis-GMA), triethylene-glycol-dimethacrylate(TEGDMA), and hydroxyl-ethyl-methacrylate(HEMA); and MM2: HEMA and urethane-dimethacrylate(UDMA). Each monomer matrix was photopolymerized with each of the two formulae of photoinitiators, I₁(Camphorquinone and ethyl-4-N,N-dimethylaminobenzoate) and I₂(Camphorquinone, ethyl-4-N,N-dimethylaminobenzoate, 2-dimethyl-amino-ethylmethacrylate, and Diphenyliodonium-chloride) at 20, 40, and 60seconds. Kinetic UV absorbance was utilized to identify the release of residual monomer in sterile water over a 5-day period. Photoinitiator formulae I₁ and I₂ were compared using ¹H NMR analysis to determine the presence of unpolymerized monomer. RESULTS: The UV absorbance results showed a release of unpolymerized HEMA from both monomer groups coupled with I₁. The ¹H NMR analysis revealed alkene proton peaks for monomer groups with the I₁ formula confirming the existence of unpolymerized HEMA. These were not observed in the polymerization utilizing the I₂ formula indicating a great reduction of unpolymerized HEMA. There was no significant difference among 20, 40, and 60seconds of photopolymerization(p<0.01). CONCLUSION: Based on this pilot study, future experiments will compare PMMs with I₂ formula.