They call it the anterolateral ligament - or A-L-L. It runs alongside the C-L-L ligament on the outside of the knee.
It seems to play a big role in stability.
The surgeons say injuries to the A-L-L can explain why some people who've had surgical repairs to their A-C-L - another key ligament - still feel their knees give way.
It may also explain why some people who tear their A-C-L once, tear it again
That's common in football, basketball, and soccer players, who pivot on their feet a lot.
In fact, a few years ago, the New York Giants had 4 players who had torn their A-C-L ligaments twice.
The surgeons are working on a way to diagnose, and fix A-L-L injuries, in the belief that it will help people with A-C-L injuries.