Hie, fyi, i do share the same problem as you.
Before i started practicing aikido last 5 years, i'm not able to
fully straightened and fully bend my right knee. My coy doctor
informed me that my knee is deformed.
My sensei and leader of the dojo were kind enuff to go easy on me
due to my injury. ( escaped knee walking, rolls and are allowed to
sit cross legged) As time goes by, it hepls to stretch and
strengthens the knee.
Thus, after a year, my knee got better. I'm able to fully
straightened and bend without any problems. And i've been doing all
the techniques, be it high falls and rolls without any
problem.
I'll highly recommend you this art and trust me, within a year or
less, your knee will get better. 