Part 11

This limitation of extension can be effected quite easily by the tension of a popliteal cord (see page 41. The knee in Marks leg), or by carrying the anterior border of the leg piece upwards in front of the thigh piece so that it impinges against the latter.

This method is not very good because it is noisy.

Moreover, the repeated impact against the leg piece may split the wood, so that if this method is adopted the stop must be reinforced by a binding of several layers of parchment.

We will first describe a mechanism the association of which with the extending sling will be seen on page 48.

α. To limit extension of the knee all that is necessary is to prolong the antero-posterior diameter of the knee bolt (which turns with the leg) by a horizontal wing, which engages with a corresponding notch in the femoral condyle. We show here (Figs. 50 and 51) a rather more complicated but still simple mechanism which is interesting because it can be combined with the action of the extending sling (see page 48).

It consists of a piece of metal curved on the flat, ending above in a cylinder through which the knee bolt passes, continued below into a cylindrical tail piece, which fits into a ring which is fixed inside the top of the calf. During flexion this plate moves in a median posterior window in the femoral condyle, becoming oblique at the same time as the tail piece sinks into the ring; during extension the tail piece rises in the ring and the interior flat surface engages against a corresponding groove in the femoral condyle (covered with leather to secure silence).

β. In the Marks knee an internal system of cords and springs serves at the same time both to limit extension and to produce an elastic extending force. It is a system which is fairly simple and much used.

1. Limitation of extension is secured by a U-shaped cord, the extremities of which are fixed to a wooden cross piece (T), fixed in the thigh piece three centimetres above the axis of the joint. The cords leave the thigh through two lateral openings in the back of the thigh piece, and the loop passes through a ring halfway down the calf.

2. The extending force consists in a coiled steel spring the mechanism of which is combined with that of this cord. The lower half of the spring is enclosed in a copper tube lined with chamois leather to secure silence; its upper half or rather more is coiled around a wooden pin, which terminates above in a head which is cup shaped: it will be seen (Fig. 57) that if pressure is made on this head the spring is shortened and under compression.

This spring is fixed below (by means of a tenon which allows antero-posterior movement) upon a bracket in the calf which is continuous with the ring through which passes the check cord. The cup-shaped upper end is in contact with a ball which projects from the upper surface of the thigh piece between the two openings for the check cord (Fig. 53). It will be seen that when the knee is flexed the spring, the head of which lies below the axis of the joint, will be compressed at the same time as the check cord is relaxed so that there is an elastic recoil tending to reproduce extension. The ball which rests on the top of the spring is fixed in such a manner as to be in the same horizontal plane as the axis of the knee: that is to say, it is in the same vertical plane as this axis when the knee is flexed to a right angle (Fig. 52). Therefore in this position the spring has no tendency to produce either extension or flexion, that is to say the mechanism is now at dead point, and when the patient is sitting flexion to the right angle is maintained without any effort.

FIG. 52.

FIG. 53.

FIGS. 54 to 57.

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).

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.--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.

This is an old French method used in Fouilloy's appliance, which has, however, only become generally used in the suspending braces of the American appliance.

When the patient raises the leg from the ground, the weight of the appliance makes it slip down the stump, tension is thus produced upon this strap and as a result the knee is extended. By an adroit movement of the shoulder this extension can be carried out actively.

When the limb rests upon the ground the weight of the body presses the stump down into the bucket, the tension on the strap is released and consequently the knee is free to flex.

On pages 44 to 48 will be found figures showing the principal points in this extending brace.

The braces, whether they have or have not an extending strap, may be constructed in three ways:

a. To ease the constant pressure exerted on the shoulders by the strap which is stretched by the weight of the artificial limb, the brace may be made of elastic like the ordinary trousers brace. But the limb they carry is heavy, so they rapidly become overstretched and it is difficult to keep them properly adjusted.

b. The stretching is naturally diminished if the upper part of the brace is not elastic but an elastic section is inserted in its lower third, in front and behind.

c. But the patients almost always say that better command of the limb is obtained with inelastic braces. If the strap is wide on the shoulder, the pressure is well borne, and the lower attachment may be made narrower, consisting of a leather thong (Fig. 64).