Depends on the country. I read about someone who was denied in Singapore because the treatment 'was not for a long-term disease.' He tried with the same documents in Vietnam and succeeded. The treatment was Invisalign.
Another person—I don’t remember which country he applied in—tried with a letter for two dental appointments (a root canal or something similar) and succeeded as well. However, that was early on. I guess they could get stricter over time.