Abstract: A prosthetic ligamentary device consists of one or two longitudinal elements of relatively inelastic material and an element of relatively elastic material. When the longitudinal elment (s) is or are subjected longitudinally to an anatomical tensile force, it or they extend (s) to a limited degree by transversely compressing the relatively elastic element, until the force is incapable of compressing the latter element further, whereupon the device becomes inextensible by the force. Two devices can be combined to form a cruciate ligament prosthesis for a knee joint.