Advice on gaining weight & muscle mass!?

Alexis

I was diagnosed with bone cancer in 2013, I dropped from 118 down to 69-70 lbs. I am currently fluctuating between 107-110. I lost a lot of muscle mass in my leg due to my total knee replacement not bending properly. I am trying to gain weight and get back to 118 lbs. What are some gaining techniques and leg muscle building exercises where bending is not required??