By ETA do you have a visa exempt passport ? Otherwise you would need an actual visa not an ETA which is only for visa exempt passport holders . An ETA by the way is not a visa it just pre-authorises visa exempts to board a plane.
There is no restriction that I am aware of to visit Canada and to have any type of medical treatment whether you declare that as prime purpose of visit as opposed to tourism in either ETA or visa application or to immigration on arrival is up to you so could go either way for approval.
Be aware that you will of course need to pay for any treatment so may not be cheap plus of course you need to find a dentist in the first place preferably in advance to even treat you.. Do you not have any dentists in your home country would be strange if not.