It's the ligament in the knee, Stilly. In Meggie's case, we were fortunate that the ligament had not actually broken yet, just frayed and thinned and stretched. When the ligament snaps it can mean that the patella, the kneecap, also has to be attached in place, which is not only a longer surgery and longer recuperation and more expensive but more painful.

Incidentally, the replacement 'ligament' is actually fishing line! In little dogs like mine rather than imbedding the line they hook the line over the bone and wait for scar tissue and muscle to help hold it. Until then, the line is vulnerable to re-breaking. Eventually it will become stronger than the congenital one.