Rashi (DH "Ha l'Amchuyai Gavra") explains that the law of a Kameah working three times is similar to the Halachah in Bava Kama (37a) of an ox who gores animals. Presumably the laws of Chazakah are the same for both Halachos. Therefore it seems that just as an ox who has a chance to gore, and does not gore, interrupts his Chazakah of goring (see Bava Kama ibid.), so too a Kameah which does not work three times in a row has its Chazakah interrupted when it does not work. It is possible that Rashi may even apply the law of "Seirugin" -- "intervals" to Kameah as well. If a Kameah works the first, third, and fifth time it is tried, it is possible it is considered Mumchah for the seventh time it is tried, just like an ox who gores in a similar pattern is considered a Muad the seventh time. [It should be noted that some Rishonim, such as the RITVA, argue on Rashi and say that this Gemara has nothing to do with the Gemara in Bava Kama].

It would seem that this should also apply somewhat to medical treatments, meaning that if a particular doctor has success with a certain type of treatment three times, it would seem to be that he is capable of performing this treatment. However, just because the patients survive does not mean that the doctor has succssfully performed the operation. For example, the IGROS MOSHE (Y.D. 1:174) discusses heart transplant surgery when there was one doctor who had performed an operation on a patient who lived for half a year. Rav Moshe calls this operation a probable failure, being that many doctors have said that the patient will probably soon die. It is very difficult to give precise guidelines for when a new procedure could be called successful.

Kol Tuv,

Yaakov Montrose