In reply to Gentleman: Go see a good physio if you haven't already. Get them (or a podiatrist) to check your gait as well. I was diagnosed with it by a GP when I was younger and played a lot of footy, a few years later when it flared up again (combination of cycling to work and returning to footy) I saw a physio who noticed flattened arches in my feet were pulling my knees inwards. Getting custom made insoles really made a difference. That and some exercises to strengthen the muscles supporting the knees. Never had a problem with footy after that and now only notice a bit of ache walking down steep mountains (which anyone of my fitness level probably gets!) Your case may be completely different, but I would recommend getting someone to look at the bio-mechanical aspect. Good Luck!