Figs. 66 to 68.
This mode of attachment has the advantage that when the limb is swung the movement does not take place upon the shoulders—which easily become chafed by the ordinary suspenders—but upon the pulleys upon which the leather thongs work.
The mechanism shown in [figures 69 to 71] is interesting. When the metal lever moves around the axis of the knee joint, its lower end and the pulleys at the upper end travel in opposite directions: in flexion the pulleys move downwards and forwards, the lower end upwards and backwards; in extension they move in the opposite direction. Therefore when the limb is swung and the knee bends ([Fig. 71]), the thigh piece drops of its own weight, the braces tighten, raise the pulley and consequently make the lower end of the lever move downwards and forwards, thus extending the knee joint.
Figs. 69 to 71.
D. Mechanism to secure rigidity of the knee during weight hearing.—During the time that the healthy limb is raised from the ground and carried forwards there must be complete rigidity of the artificial limb in the extended position. This is secured by mounting the foot in the equinus position. When it has been swung forwards, in taking a step, the limb comes in contact with the ground heel first; then, as the leg becomes vertical the entire sole lies flat on the ground; if the foot is in equinus this position is only possible with the knee hyperextended, or with full extension it may be possible for a very short period. So that it is the weight of the body that locks the limb in the extended position, the sole of the foot sloping obliquely downwards and forwards; and the weight being taken on the toe. There is always a tendency to hyperextension, and to avoid straining the limb in this direction (as occurs in a living knee which is forced into the position of genu recurvatum by a talipes equinus) it is as well, as we have already said, to oppose it by some passive resistance, either in the form of a simple popliteal check cord or by a stop fixed to the front of the leg.
Figs. 72 to 75.
In Figure 72 the foot is fixed, the weight comes upon the point of the foot, and pressure upon the axis AB tends to close the angle ABC, i.e. to produce a genu recurvatum, and so to lock the knee in extension. If the foot is articulated, equilibrium is secured in the same way. Figures 73 to 75 are intended to show that in order that the axis ABC may not be vertical (Fig. 73) the axis B of the knee must be behind the perineal concavity in the bucket, and it is better if at the same time the axis of the ankle joint C is carried forward.