From: Rabbi Moshe Sofer
To: Rabbi Pinchas Leib Munk
A widow is potentially bound in levirate obligation to her sole surviving brother-in-law, who is an amputee walking on a wooden prosthetic, while the child born of the first brother's marriage died within four days of birth. The Chasam Sofer identifies two combined leniencies: the infant's death within thirty days while ill may be treated as equivalent to stillbirth, potentially exempting the widow from chalitzah altogether, and if some portion of the shin remains below the knee, the amputee may be able to perform valid chalitzah with a leather prosthetic shoe. He declines to rule definitively on his own authority and calls for two leading authorities of the generation to concur before relying on these combined leniencies.
Source: Shu"t Chasam Sofer EH II §69