Steradent ingredients :
Citric acid, potassium caroate, sodium sulfate, sodium bicarbonate, PEG 150, sodium carbonate, sulfamic acid, malic acid, aroma, PEG 90, sodium dodecylbenzenesulfonate.
So citric acid + fizz action. I have wondeered before if cola works (when it does) because of the dissolved CO2 released. Perhaps bubbles can loosen the deposit. A bit of a long-shot but worth a try. Rinse well after.