An orthotic joint, and the orthosis with which it is used, for the protective treatment of ligamentous injuries or deficiencies, or as a supplement to total joint replacement. The joint includes coacting guide members that are both slidable and pivotal with respect to each other and, when assembled and supported in pairs on opposite sides of a patient's knee, allow freedom of movement of the knee except for those movements that would normally be controlled by the ligament or ligaments requiring protection. An arrangement of straps connect each pair of guide members and are tensioned (and untensioned) in sequence to perform operations in conjunction with the articulating surfaces of the members that would otherwise be demanded from the protected ligament(s). The orthosis includes a system of interfacial elements and suspension elements that engage the upper and lower leg portions and effectively maintain the orthotic joints in proper positions to restrain abnormal knee movements or, in the case of prosthetic joint replacement, to prevent movements that do not conform with the prescribed action of the replaced joint.