Can Esterase Degradation of Dental Resins be Detected Using Attenuated Total Reflection IR Spectroscopy?

Objectives: Esterases associated with host saliva or S. mutans have been shown to degrade the ester linkage responsible for dimethacrylate resin polymerization. HPLC has been used in many studies to measure bulk degradation, but a less explored method is infrared [IR] spectroscopy. This study used attenuated total reflection [ATR] spectroscopy to specifically analyze the interface between sample and internal reflection element (“prism”).
Methods: BisGMA and TEGDMA mixtures were cast and cured directly on germanium prisms (Group 1), and cast onto glass slides, pressed under polyester strips, and removed after curing to create freestanding thin films (Group 2). Both groups were treated with porcine liver esterase as well as negative controls (carbonic anhydrase, distilled water). Group 1 samples were analyzed directly on the germanium prisms with ATR-IR, while Group 2 samples were analyzed by both (a) traditional transmission IR, and (b) ATR-IR by clamping the thin films against zinc selenide prisms.
Results: Group 1 samples exhibited permeation of water at the resin-to-Ge interface as early as 15 minutes, while no polymeric degradation was detected within the first 90m of exposure to esterase. Group 2a samples (transmission IR) did not demonstrate significant degradation, but Group 2b samples (ZnSe ATR-IR) did demonstrate degradation within two days of exposure to esterase at 37°C. Results from Group 1 suggest that, within 90m of exposure, water molecules permeate the polymer film while larger enzyme molecules remain trapped at the far surface. Group 2a transmission IR results suggest that degradation may be localized to the extremely thin "skin" of the film, but such changes are beyond the sensitivity of the IR instrument in transmission mode. Group 2b samples showed significant differences between esterase-treated samples and negative controls.
Conclusions: Study data suggest that ATR-IR spectroscopy allows for useful analysis of resin degradation within the outermost 0.1 μm of the polymer surface.