In my case, I was born with a condition called patella alta, which consists of high-riding knee caps. Compare the x-rays of a patient with patella alta and a normal knee:
Above: patella alta
Anyway, when the knee caps sit too high, they aren't positioned correctly in the trochlear groove, which keeps them in place. Thus, people with patella alta usually dislocate their knees frequently. My knees dislocate laterally- to the outside of the leg- and they dislocate a lot. My caps don't sit as high as those in the top x-ray image, so as a result, they stick out a lot and look really knobbly.
My patella alta actually gives me pretty hyperextension (not a crazy amount or anything), so as a young dancer, I always thought I had the ideal legs for ballet. Then, when I started training every day as a teenager, the dislocations started. Ideal legs?? Think again, girl.
Doctors and physiotherapists told me the same thing: I needed to strengthen my vastis medialis muscles (the muscles on the medial (inside) side of the knee). So I did, for several years, but nothing changed. One day, I dislocated my knee in my sleep and woke up to a knee cap out of place and swelling that looked like a balloon was stuck inside my leg. It wasn't until then that I was referred to an internationally renowned knee surgeon and sports injury specialist in Toronto, Dr. John Cameron, also known as 'the knee cap guy.' The first thing he said when he saw my x-rays was, "Those are some high knee caps. I don't even need to examine you."
I am having surgery on both of my legs in about a year, 4 months apart. Patella alta is corrected by reattaching the patellar tendon (the tendon connecting the patella and shin bone) lower on the tibia (shin bone) with metal pins. Only one of Dr. Cameron's patients has ever dislocated a knee after this surgery, and that was because she had a very shallow trochlear groove, so her patella was still unstable. I am feeling very positive, but that will probably change as the first surgery date comes around!