Knee Pain, Swelling, Locking After Previous Accident. Taken Physiotherapy. What Could It Be?

hi there. a year and a half ago i had an accident in which i severely damaged my knees. luckily i had no broken bones or anything however within minutes of the accident my knees swelled and turned purple from the damage. since then ive had mri scans and xrays but nothing has showed up yet i still receive constant pain... i cant walk for more than half hour due to pain and if i walk long amounts of time my knes will swell and lock... ive had pshyio but still no improvement, what could it be? also ive been asked to advice if i would qualify for dissability living allowance for mobility issues? please help as i dont know what to do anymore. thank you. kimberley millman