Knee injury

While standing on a chair to loosen a vertical blind, three months ago, I fell onto my concrete patio. I remember a popping noise as I hit the ground. Both knees were injured, primarily my right, which began to swell to twice its size within 30 minutes of the fall.After a trip to an emergency clinic, a brace was placed on my knee for support. At that time, walking was impossible because of the severe pain.
The doctor took x-rays and found no fracture. I followed up, three days later, by going to the hospital emergency room and was advised that there was probably internal damage to the knee – specifically the ligament and/or the meniscus.
The brace was changed to a static model, which I was advised to wear for 24 hours per day for the following three weeks, at which time I was to see an orthopedist.Following my visit to the surgeon’s office I was advised that, unless an MRI and surgery were performed, my knee would continue to be unstable and that the knee could ‘give out’ while performing my day-to-day activities.Because the knee is still weak, and the muscles atrophied somewhat, the knee ‘collapses’ from time-to-time, and I find it impossible to strengthen the muscles, as I used to prior to the accident, by walking regularly.Please advise if there are any exercises I can perform that will assist me in strengthening the muscles in the legs/knees that will give me the support I need. Thank you so much.