Responsa su Salmi 35:30

Our question, therefore, amounts to this: Is the use of the fingernail a requirement of law? The fact of the matter is that the use of the fingernail to tear the membrane is not required in the older law, and is not even mentioned. There is no mention of it in the Mishnah or in either Talmud. The first mention of it is in Maimonides (Hil. Milah II, 2), who merely says that periah is done with the fingernail (b'ziporon). Not one of the commentators and particularly not Joseph Caro in his Kesev Mishnah, who is careful about such things, mentions a single earlier source from which Maimonides may have drawn the fact that the fingernail is to be used. The only fairly early source is not an halachic one but a midrashic one. In the Yalkut to Psalm 35:10, "All my bones proclaim, etc." there is an enumeration how the head, the mouth and the eyes, etc., participate in the worship of God; and in this listing is included, "and the fingernail, also, to perform the periah." The Shulchan Aruch, Yore Deah 264 #3, copies Maimonides' statement that periah is done with the fingernail. But neither of these says that if it is done otherwise it is not a valid periah. As a matter of fact, there is a responsum of Hai Gaon to the effect that periah was generally done with a knife. The responsum is cited in Ozer Ha'-Gaonim, Shabbos, #407. There Hai Gaon says that in Babylon it was a custom to draw taut both the foreskin and the undermembrane simultaneously and to cut them both with one cut of the knife. Hence, under army conditions, we may consider it permissible if the surgeon does not use his fingernail to tear the membrane but uses the scissors. Of course, it does not follow that this is the preferable method. Good mohelim are very skillful. Their tearing of the membrane and their folding it back is often done much more quickly than the surgeon can do it with his instruments. It may well be that the old-fashioned method is better, but certainly the surgical method is permissible according to the law,provided, of course, that after the slitting of the membrane, it is completely retracted.