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McGinley E L, Dowling A H et al. J Dent Res 2013; 92: 92–97

It is stated that nickel is the most allergenic metal, although many others also induce such a response. There are continuing concerns that that the 'protective' oxide layer formed by chromium with nickel-containing appliances could be disrupted by the low pH encountered in the mouth. In this in vitro study, it was observed that Ni-alloy discs incubated with stationary-phase S. mutans, leached with time increasing levels of Ni and Cr, when compared with untreated discs. A similar pattern was observed with Co-Cr alloy discs. The study also examined the effect of these ions on immortalised human TR146 oral keratinocytes. Viable cell counts, cell morphology, metabolic activity and inflammatory cytokine expression were disturbed. Nickel-based dental casting alloys were more toxic than Co-Cr-based dental casting alloys.