Is it permissible for a female dentist to treat non-mahram males while observing the Islamic hijab?

If a doctor treats a man or a woman who is unlawful for him (to be in seclusion with), or if a woman's treatment of a man leads to touching or exposure of awrah (private parts), then it is not permissible for her to directly treat him. However, if there is no man or a mahram (unmarriageable kin) available, and the patient is in a state of necessity or need that is equivalent to necessity, then there is no harm in a woman treating a man.