Jo Mar makes an important point. Remember in all the paperwork you fill out for ssdi it is important not only to state what disease or condition you have but how that particular condition affects your ability to work. for example, how it affect ability to stand, sit, bend, kneel, climb, reach, concentrate. fatigue, balance etc. whatever it affects. this also goes for whatever medications you are taking and also if you have pain and cant sleep, or concentrate, or have to take frequent naps. So for your knee you would explain how that inparticular limits you in being able to work and then go through each condition you have plus medication.