The Marks knee.
Figs. 52 and 53.—O, knee bolt. T, cross piece of wood, situated in the extended position above the knee bolt, in the flexed position behind it. C, bracket fixed halfway up the interior of the calf.
A U-shaped cord a passes through a hole in the bracket C and is attached at each end to the cross piece T; it limits extension. The two ends of the word enter the thigh piece by two apertures in the posterior surface, between which is fixed a metal ball which projects 2 cms. The extending spring is the rod b which is fixed to this ball and to a socket in the upper surface of the bracket. Figs. 54 to 57 show the parts of this spring: a tube, a spiral spring, and a rod with cup-shaped head. When the spring is in the tube and the rod in the spring ([Fig. 57]), it will be seen that pressure upon the head of the rod increases the tension of the spring.
In the knee shown in [figures 58 and 59] the extending mechanism is as follows. Directly behind the axis of the joint is a metal crossbar, upon which fits the grooved upper extremity of a piece of wood, the other end of which rests (like a lance) in a pocket which is suspended in the leg piece by an elastic band (the latter being kept stretched to a greater or less extent by a lace which emerges from the calf).
Figs. 58 and 59.—Elastic spring for extending the knee.
The elastic being slightly stretched when the knee is extended, it will be seen that the crossbar turning round the axis of the knee becomes lowered as the knee flexes, so that the elastic is stretched and consequently opposes flexion; but when the knee is bent to a right angle the axis of the joint, the crossbar and the wooden rod are in the same vertical line; the mechanism is at a dead point just as we have already seen in the Marks knee, and the tension on the elastic presses the leg directly downwards without tending either to flex or to extend it.
Leather pads deaden the noise of the impact.
Extension is limited, as will be seen by comparing [figures 58 and 59], by the vertical wooden rod meeting flat surfaces in the thigh and leg pieces simultaneously.
3. Extending slings.[6]—To the sling which passes over the shoulder on the side of the artificial limb, is attached a strap which passes down in front of the thigh piece and is attached to the upper third of the leg.