A few weeks ago I was in Sha'arei Tzedek with R. Moishele Weissberg (who i am training by to become a mohel) and an older guy walked over dressed in shorts and sandals (not looking very well put together). He said that he had a question, but not about milah, rather about a chicken (Moishele spent over 25 years as a shochet). The question was that he has a chicken who is missing a leg at the knee and he wants to know if the chicken is kosher, because he wants to keep it alive and eat the eggs. Moishele didn't know the answer, so he called another shochet who said that it depends on the tzomet ha'gidin, whether or not they are in tact.
I didn't fully grasp the importance of the tzomet ha'gidin at that point. However, the gemara says that in treifos one cannot compare one thing to another based on the assumption that the rules should always be consistent and logical. The gemara says
אין אומרין בטריפות זו דומה לזו, שהרי חותכה מכאן ומתה חותכה מכאן וחיה
Rashi (unlike tosafos in baba basra 130b who says this is referring to a hole in the spleen) says that so long as no bones are broken, if the meat of the chicken is cut off the bones, if it is above the tzomes ha'gidin it is kasher, but on the tzomes ha'gidin it is a treifa. This proves that the laws of treifos aren't intuitive because it could be a treifa when less is missing, and kasher when more is missing.
Rashi at the beginning of the perek 42a also made this point by saying that there is no rationale when it comes to treifos because it is all a halacha l'moshe mi'sinai. It is for this reason that the chazon ish explains that the ability to keep a treifa alive would not change it's status as being a treifa since the list of 18 treifos are halacha l'moshe misinai.