What Causes Weakness In Knees After A Knee Surgery?

I have had surgery four times on my knee cap ( left), each time the staff at the hospital has bent my knee and undone the surgery. Now the knee gives way frequently and if i can not catch to something, i fall. Is there anything that can be done to help? I can not walk without the use of a walker because of falling. I am in a lot of pain. Thank you. my e-mail is YYYY@YYYY