Influence of S.mutans on Biocompatibility of Non-Precious Dental Casting Alloys

Objectives: To determine the influence of S. mutans on the biocompatibility of non-precious dental casting alloys using two-dimensional (2D) TR146 oral keratinocyte cell monolayers. Methods: Disc-shaped specimens of nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr) dental casting alloys were cast and polished to a clinically relevant polished surface finishing condition. Alloy-discs were exposed to growing cultures of S. mutans. Biocompatibility was determined following direct (S. mutans-treated alloy-discs) and indirect (S. mutans-treated alloy-immersion solutions) exposure to the TR146 cells for up to 72h. Cellular morphology, cell density measurements using trypan blue dye exclusion assay, cellular metabolic activity using a XTT reduction assay and cellular toxicity using the lactate dehydrogenase (LDH) release assay were investigated. Inflammatory cytokine expression was quantified using enzyme-linked immunosorbent assays (ELISAs) specific for interleukin-1α (IL-1α), interleukin-8 (IL-8), prostaglandin E₂ (PGE₂) and tumour necrosis factor-α (TNF-α). Elemental ion release was determined by inductively coupled plasma mass spectrometry (ICP-MS) within the detection limits of the apparatus (ng/L). Results: Cellular morphology was altered following direct and indirect exposure to the S. mutans-treated dental casting alloys. Significant decreases in cell density measurements and cellular metabolic activity were observed. Significant increases in LDH levels and the expression of IL-1α, IL-8, PGE₂ and TNF-α inflammatory cytokines were observed following exposure to S. mutans-treated Ni-Cr dental casting alloys. The model could detect significant decreases in cellular density measurements and cellular metabolic activity between S. mutans-treated Ni-Cr dental casting alloys and cellular toxicity was attributed to nickel ion release levels in solution, as detected by ICP-MS analysis. Conclusion: Given the occurrence of oral carriage in the human population for S. mutans, and the enhanced loss of biocompatibility of the TR146 cells observed following exposure to the S. mutans-treated dental casting alloys, the clinical implications for the stability of such restorations in the oral environment long-term are of concern.