One of the cases involved a young widow whose yavam was missing his right foot, preventing her from performing chalitza.
Pischei Teshuvah (165 – 3) brings several sources which deal with such a case.
Most hold to do yibum. Beis Meir ads that cherem d’Rabeinu Gershom does not apply in such a case. The cherem not to give a get without her consent also does not apply, says Bais Meir, because he only married her for the mitzvah. It is implied that they may remain married if they wish.