There are a few possibilities to the cause of the pain like patellar dislocation, chondromalacia patellae, infections/ inflammations, arthritis, bursitis, tendonitis, cyst formation, iliotibial band syndrome, osgood-schlatter disease, bone growths/ tumours, nerve impingement/ pressure fractures, injuries to the internal knee ligaments/ menisci etc; just to name a few. Without a detailed history, physical examination and probably a few tests it would be difficult to point out a single cause. I would suggest consulting an orthopedician for an evaluation. After a cause is diagnosed, it can be managed accordingly.
Hope this helps.
Was the other one patella femoral chondrosis that is what it sound like to me. I found the best thing was to find a chiropractor who knows how to treat knees. (And I have had a lot of p.t. for my knee that helped some, but not as much as seeing a good chiropractor.
I hope this helps,