This paper addresses the mechanical design and control of a new active stance-control knee-ankle-foot orthosis. The orthosis is intended to provide gait assistance for incomplete spinal cord injured patients with functional hip muscles, but partially denervated knee and ankle muscles. This device consists of a passive compliant joint that constrains ankle plantar flexion, along with a powered knee unit that prevents knee flexion during stance and controls flexion-extension during swing. For this purpose, the knee joint incorporates a controllable mechanical locking system and an electrical DC motor. Based on human walking biomechanics, a hybrid control model is proposed. This model takes into account the parameters of the orthosis and the characteristics of the gait cycle, which is divided in eight different phases. A fractional order controller is designed following decision based control techniques.