A drop-out cast is a splint for improving knee extension.
Features of the drop-out cast
This is basically a cylinder which can be slipped on and off the leg. The thigh is held firm in the cylinder. Below the knee, the front bit is cut away exposing the knee and leg. By placing pads under the ankle, the knee can be pushed into extension.
Case series utilizing drop-out casting for the treatment of knee joint extension motion loss following anterior cruciate ligament reconstruction. Logerstedt D and Sennett BJ. J Orthop Sports Phys Ther. 2007 Jul;37(7):404-11.