All well to put it on the psycho side but do you feel your examination or examination in general is sufficient to conclude that it is likely a psychological problem?
Physical examination by X-ray cannot conclude anything about soft tissue damage (although it might indicate) and further investigation by p.e. Wikipedia reference-linkMRI is necessary. How about pain, muscle waiste (should be remarkable after 2 years, no vastus medialis left by now) Injuries like Wikipedia reference-linkmeniscus cannot be excluded at this point.
Depression is an indication that the mind might play tricks and contribute to the problem. Have you tried with resistance to flexion?