Well, without a clinical evaluation it would be difficult to determine the cause of your symptoms. Possibilities that may need to be considered include patellar dislocation, chondromalacia, infections/ inflammations, arthritis, bursitis, tendonitis, cyst formation, bone growths/ tumours, nerve impingement/ compression, fractures, injuries to the internal knee ligaments/ menisci etc. It would still be possible to do an ACL repair if indicated, knee replacement is usually considered as the last resort in most cases. I would suggest consulting an orthopedician for a detailed evaluation and suggestion of an appropriate management plan
Hope this helps.