If you pull a chicken leg out of its socket, you'll see a fibrous white ligament. That's the equivalent of the ligamentum teres in the human. It helps hold the head of the femur (thigh bone) in the hip socket.
Traumatic or twisting injuries can cause this ligament to tear. Hip dislocation can stretch it to the tearing point, too.
Doctors don't have a test to help them find this type of tear. In fact, it wasn't until arthroscopic surgery came along that they even knew it occurs as often as it does. Now that we know it's a problem, more studies will be done to find easier ways to diagnose it.