Over Yom Tov, I read a sefer on the halachic positions of the Rav. One of the cases involved a young widow whose yavam was missing his right foot, preventing her from performing chalitza.
The Rav investigated the matter and ruled that due to the circumstances (i.e. the alternative being that she remain an agunah), yibum could be performed with the proviso that the couple divorce immediately afterward.
The Wolf