I Have A (best Word To Describe It) "dimple" On

I have a (best word to describe it) "dimple" on my patella that has gradually gotten bigger over the course of about 3 years. I have had undiagnosable problems with my knee before. The dimple is above the middle of the patella and my knee sometimes feels weaker than my other knee. I have been to physical therapy many times. I have been perscribed steriods. Nothing helps. I'm only 18! I have vitamin D deficiency. Could this be the source of my problem? Please help. It feels like my knee is deteriorating.