The invention relates to an orthosis comprising an articulated structure (1) (2) arranged in such a way as to be fixed to the thigh and to the leg on either side of the knee. According to the invention the orthosis comprises restraining means in the form of straps (4) (5) (6) and (7) coupled to said structure and able to surround the thigh and the leg leaving the patellar area free and to be crossed and placed one over the other on the rear side facing the cutaneous surface of the popliteal fossa (D) at the same height as the centre of rotation of the knee in order to prevent the front part of the tibia from sliding forwards each strap (4) (5) (6) and (7) being held in a rigid guide (9) on the leg and the thigh and being held together by a non extendible sleeve where they cross over at the popliteal fossa (D). The first strap (4) that is the shortest is arranged on the outside as far back as possible and the last strap that is the longest is arranged on the inside as far forward as possible facing the cutaneous surface of the popliteal fossa.
