It is stated in Hashiya Tahtāwī wa Maraqīlfalah, Pg 143,

If one has a bandage wrapped over a wound and opening it is difficult or will prove harmful to the wound then wiping over the bandage will be sufficient, likewise, if washing a limb causes harm, due to an illness or pain, then one can wipe it with his hands. The bandage must not cover any area of the skin unnecessarily, or else wiping it will not be sufficient. If it is not possible to wrap the bandage without covering unwounded areas of the skin (e.g. if the wound is on the upper surface of the arm but the bandage has to be wrapped around the back of the arm as well, resulting in an area of unaffected skin being covered up) then one must try to see if he can open up the bandage and wash that unwounded area, if not then wiping over the whole bandage is acceptable. Washing the unwounded areas will no longer be necessary in such cases.