It has nothing to do with a rib. That is one possible procedure, but TOS procedures date back a long way, and there are many types. I did an entire article on it for Call To The Pen and talked with a guy at the Mayo Clinic about the surgery. It's the result of baseball's over-indulgence on velocity, though unlike with Tommy John, this can be life threatening, as in the case with J.R. Richard, who was one of the first "major" big leaguers to have the surgery.