| Dental crowns fabricated from modern ceramics inevitably retain tooling marks and minor crevices, especially at their cut and polished edges. Cutting fluids and other contaminants are also generally present, which can interfere with the bonding potential of the ceramic and can become biologically disadvantageous, as well as non-sterile. Radio Frequency Glow Discharge Treatment (RFGDT) is a recently introduced, novel cleaning and sterilization method for finishing ceramic crowns. The procedure involves a quick (3 minute), ambient temperature gas plasma-based process that removes all organic material and modifies the surface oxide layer. The goal of this study was to determine whether the use of RFGDT significantly reduces the flexural strength of the ceramic. Nearly 80 rectangular slabs of "blue state" (partially crystallized) and crystallized lithium disilicate (LS2) specimens were cut from ingots and polished to standard clinical finishes, followed by washing with detergent. Physical features of LS2 slabs before and after 3-minute exposures to RFGDT conditions in air were compared. Groups of six slabs each were examined by ellipsometry, energy-dispersive X-ray spectroscopy (EDS) analysis, and Multiple Attenuated Internal Reflection Infrared (MAIR-IR) spectroscopy to assess changes in surface chemistry and thickness of the oxide layer. Contact angle measurement was used to analyze changes in critical surface tension, and scanning electron microscopy (SEM) was used to identify fracture initiation sites created during three-point bending trials. Our results demonstrate that after RFGDT, the crystallized LS2 surface zones were substantially cleared of low-energy residues. The cleared surfaces were more hygroscopic as a result of contaminant removal, as well as due to production of a silicon oxide-enriched surface layer approximately 85 Angstroms (A) thick. There was a slight, but insignificant, decrease in flexural strength after RFGDT (p > 0.05) as measured by a three-point bending test. Subsequent SEM imaging of fractured samples revealed fracture initiation sites at the slab surface in 8 of 10 cases and at internal flaw sites in 2 of 10 cases. Together, these results support the conclusion that RFGDT treatment provides benefit in the absence of negative effects on the physical strength of fabricated LS2 appliances. Based on these results, we recommend RFGDT as a surface cleaning technique for LS2 constructs. |
