ANSWER: 

Tayamum is permissible when one does not have access to water. Also, it is not permissible to make it with a pillow or blanket; it has to be the dust from the earth, or the dust found on walls will suffice.  He has to make normal wudu and wipe over the cast or the covered areas. If this is not possible, he may make tayammum on the parts of his body that cannot be touched by water due to injury. 

Kindly refer to the following link(s):

 

There is person who had motor vehicle accident but can’t do wudu . Could he do tayammum by using his pillow or blanket