Wash everything you are able to wash, and over the cast or bandage that cannot be removed or wetted without harm, pass a wet hand (mash) in place of washing that limb. The narrations specifically about wiping over a splint are weak, so scholars derive this ruling from the general principles of the Sharia: "So fear Allah as much as you are able" (Quran 64:16) and "He has not placed upon you in the religion any hardship" (Quran 22:78). If even wiping would harm the injury, perform tayammum for that area after washing the rest of the limbs, based on the concession for the sick in Quran 4:43. Once the cast is removed you return to normal washing, and according to the majority of scholars you do not repeat the prayers you prayed in that state.
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.