Sheikh `Umar al-Muqbil, professor at al-Imâm University
If a person for some legitimate reason cannot wash a part of his body, he may wipe over it with water.
In the event that even wiping it with water might also harm him, he may perform tayammum in lieu of the body parts he is unable to wash.
He should first perform ablutions (wudû’) as best he can. Then he should perform tayammum with the intention that it is for the parts of his body that were not washed or wiped by water.
And Allah knows best.
Source: Islam Today