The stifle is equivalent to our knee joint. The patella (kneecap) in a horse is the same as ours, and when it locks it's because the ligament did something it shouldn't have and kept the leg locked. The locking itself isn't a problem, that's how horses sleep standing up. The problem is that it won't unlock when it should. My horse has it, it's really bad this year and we're thinking about giving in and getting the surgery after the competition season is over. It's one of the major reasons we gelded him besides the fact we didn't plan on breeding anyway and were well aware that he was not stallion quality. As Lyn pointed out you can certainly still use and even compete successfully with a horse who has the condition, you just shouldn't breed them.
Leia