Tues Nazir 52

The Gemara says, although the spine (with ribs) has a stronger Tumah than regular bones. However, if most of the ribs are broken or disconnected (even if all the pieces are lying there together) they no longer have the same Tumah, and receive the halcha as regular bones.

Tosfos points out: we see in the Gemara in Chulin that compares Treifos to Tumah of the spine. (Over there it compares how many vertebrates needs to be missing that the spine doesn't make Tumas Ohel also renders an animal missing that many to be a Treifa.) So too here breaking most of an animal's ribs render it a Treifa. We see a Gemara like this, that to render a Treifa it needs to break most of the ribs of both sides of the animal. If the ribs get dettached, then it's a Treifa if done to most of one side of the ribs.