1. All of the rules that we have studied over what constitutes pikuach nefesh on Shabbat apply to cases where a definite danger to life exists. However, Rishonim disagree as to whether a potential loss of a limb is also considered a danger to life that allows violating Shabbat in any manner needed. The halacha is that it is not considered a case of pikuach nefesh, and one may only violate a rabbinic prohibition to prevent loss of a limb, if one’s life is not in danger (Shulchan Aruch, O.C. 328:1; Mishnah Berurah 328:49; Orchot Shabbat 20:119) (though see #2 below).

2. However, based on what we know today using modern medicine, in almost all cases, potential loss of a limb also comes with a potential risk to life due to infection and the like. Therefore, in practice, any potential loss of limb will permit violating even a Torah prohibition on Shabbat (Nishmat Avraham, O.C. 328:49).

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Dedicated by Fran Broder as a zechus for everlasting peace in Eretz Yisrael in the merit of learning Hilchos Shabbos.