A Knee Immobilizer is a brace that does exactly what its name implies: it immobilizes the knee joint and prevents it from moving. It is a long cloth brace that extends the length of the shin and thigh. There are metal support struts on the inside and outside part of the brace. Velcro straps hold the knee immobilizer in place.
If you suffer a fracture of the patella or upper shin bone, a knee immobilizer may be used to help keep things in place during healing. The benefit of an immobilizer as compared to a plaster cast is that it can be removed to allow gentle motion to occur during healing.
A knee immobilizer may also be used after ligament or tendon injury or surgery around the knee. It can help keep the knee straight to allow proper healing to occur, but it can also be removed for gentle motion to occur. Be sure to speak with your doctor or physical therapist to be certain that it is safe before removing your knee immobilizer.
Source - Safran, M., Stone, D., & Zachazewski, J. (2003). Instructions for sports medicine patients. Philadelphia: Saunders.