If you have torn your ACL, you might benefit from a derotation brace. It goes from about mid-thigh to mid-calf and is made of sturdy metal and plastic. It can provide some support and keep your knee more stable.
Talk to your doctor about different types of braces. Most of the time for a derotation brace you have to be measured and it has to be custom ordered for your leg.
Hope this helps - comment back with other questions.