Knee hyper-extension - Ligament, Cartilage damage or something else?

Hi All,

I play football at a very high level. I've just returned from the USA where I've been playing. I'm only 21 but I've had some problems in the past with my ankles. I've worked hard on strengthening my legs and core, and I haven't had an injury that has stopped me from training for more than a few days for nearly two years now.

I was playing in an eight a side game two days ago (on a hard old style astro turf which I despise). An opposing player had the ball with his back to me, and I managed to nip in front of him and took a touch on the ball with my left foot. My leg was extended and and straight because I had to stretch a little to move the ball forward. He reacted slowly and thought he still had control of the ball, so he moved to kick the ball, but instead kicked the back of my foot which was already about six inches off the ground about to touch the ball. This resulted in my left leg hyper extending and knocking me off my feet onto the floor.

I didn't feel too much pain, and was able to get up and start jogging slowly. I thought it may have just been a knock and I could run it off. I didn't feel comfortable so I stopped playing.

I'm a bit worried because I can't locate the pain when sitting down i.e pressing into the affected area. I can walk and jog fine, but it feels a bit unstable, and I am walking with a very slight limp. The only time I have pain is when I stand up straight, lock the left leg/knee straight and take my other leg off the ground. It feels like a sharp pain, that turns into an ache after a few seconds.

The reason I am worried is because I can't feel any pain when walking, going up the stairs, cycling, or light jogging. It sounds like they're good signs but the sharp pain I am getting when standing on one leg is concerning me. The pain is on the inside of the knee and I think it's located at the top of the inside.

Any suggestions/help would be great. Hopefully it's just a small tear or knock, and I'll be able to resume my pre pre-season training in the next few days. I've just got the small trampoline out, and I'm going to do some light one legged work on that.