From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger rules that if the entire shin is amputated, valid chalitzah is impossible since the shoe would rest on the thigh, constituting me'al d'me'al. If the ankle bone remains, a sfek sfeika may permit chalitzah in pressing circumstances. On the viability question, he distinguishes between piheik u'meis and chalah u'meis, permitting chalitzah in the latter case where some shin remains, while deferring to greater authorities in more stringent scenarios.
Source: Shu"t Rabbi Akiva Eiger §98