Part 10

The appliance is made entirely of wood; it is strong and light.

Nothing need be added to the description already given of the fitting and method of attachment of the thigh piece, which ends below in a curved "condyle," which fits into the top of the leg piece. It is transfixed by a metal bolt, from each end of which a metal plate descends and is riveted into a corresponding groove in the leg. This forms the axle which rotates in the thigh piece when the knee flexes or extends. Flexion of the knee is free. Extension is stopped just short of the straight line (see p. 16).

The bucket and the condylar portion are made of two separate pieces of wood.

The hole through which the bolt passes being cut in soft wood (willow or lime), must be strengthened by a cylinder of metal, of leather, or of harder wood (beech or service tree) in which the axle revolves.

The foot is in equinus at an angle of 25° to 30° so that the heel is 2 or 3 centimetres from the ground (the usual height of the heel of a boot). The piece of wood which forms the instep and which is continuous with the leg stops at a point corresponding to the middle of the metatarsus, and is only half the thickness of the foot. The rest of the foot is shaped of indiarubber stuck on to the instep piece; the wood and rubber being enclosed in a sheath of leather.

The foot should also point slightly outwards, as in the normal standing position.

To ascertain whether the limb is built so as to ensure equilibrium, a thread is stretched against its side so as to pass through the axes of the knee and ankle joints, if this cuts the ischial bearing point at its centre the equilibrium of the patient is assured. Equilibrium will be better still if the cord lies entirely behind the ischial bearing point, leaving in front of it the greater part of the thigh piece. The best method of ascertaining if the foot is properly mounted is to hold the limb in front of one by the thigh piece, with the knee bent at a right angle; it can then be seen whether the foot turns outwards at the correct angle.

It is not necessary to say anything more about the shape of the thigh piece (page 17).

The metal bolt which transfixes the knee must not allow any play; the hole through which it passes must be lined with hard wood or leather.

The indiarubber sole should be reinforced with several layers of canvas incorporated in the rubber, as the latter if not so reinforced perishes and cracks.

The appliance must further be examined after it is applied. The level of the iliac spines must be compared: the spine on the side of the amputation should be 2 cm. below that of the sound side.

Examine the position of the point of the foot. Make the patient sit down, see if the knees are on the same horizontal plane; if the sound knee is the higher the leg piece is too short. The foot being fixed in the equinus position the patient must wear boots while the examination is being carried out.

B. Mechanism of walking.--In walking, a step being taken with the artificial leg, the toe of the foot is the last to leave the ground, the heel being raised and the knee straight. The limb is swung forward and raised by flexion of the hip: active flexion of the knee is impossible, but passive flexion occurs, owing to the weight of the leg piece, as the thigh is raised.

At this moment the leg piece is vertical, forming an angle with the thigh, from this position it must pass into one in which it is oblique forwards and downwards, in a straight line with the thigh, so that the knee may be fully extended when weight is again borne by the limb as the foot meets the ground. If at this moment the knee is flexed the limb will double up under the weight of the body.

The first contact of the limb with the ground should be at the heel with, as we have already said, the knee extended. Afterwards as the limb, which at first points obliquely forward and downwards, passes into the vertical position in which it must be at the period when it bears the whole of the weight, this complete extension becomes locked and transforms the limb into a rigid column.

This is brought about as explained on page 48 by mounting the foot in equinus, and we must here describe the methods by which the commencement of the movement of extension may be communicated to the leg so that the heel may be the first part of the foot to touch the ground.

These methods may be termed knee extending mechanisms. They assist the passive action of the weight of the leg.

In fact the recurrence of extension is brought about by a pendulum movement of the leg, which, at first oblique downwards and backwards, swings into a downward and forward obliquity. But this movement is slow (the pendulum which marks one second is one metre long) and incomplete. The patient can make it complete with a little instruction, by extending the thigh slightly as soon as the foot touches the ground.

This may be sufficient if the stump is long; the leverage is good, and while the hip is being flexed a swing can be given to the thigh piece which accentuates the pendulum movement of the leg.

But with a short stump some special mechanism is essential to make sure that extension will be complete, otherwise the patient will be obliged to walk with short and calculated steps, to wait whilst the pendulum action produces extension of his knee and allows him to put weight upon his foot.

C. Mechanism for starting extension of the knee during the time the leg is swinging.--There are two methods which are generally combined:

1. Elastic traction by an artificial muscle.

2. The extending sling.

1. Artificial muscle.--The action of an artificial muscle made of elastic (noiseless) or of a coiled steel spring, is easily understood.

(a) The simplest method (that which is commonly used for infantile paralysis affecting the quadriceps) consists in fixing an elastic band divided into two slips, one on either side of the patella between the front of the thigh and of the leg, about the middle of each. (This is represented in figure 98 in our convertable leg.)

(b) When the apparatus includes the regular artificial knee the makers generally place this mechanism in the interior of the thigh and leg pieces, using methods which are often very ingenious. Of these we illustrate some on pages 40 onwards, with an explanatory description.

In describing these mechanisms, which may be called intra-condylar, it is necessary to speak at the same time of the stop to limit extension because, as will be seen, it is combined with the extending spring.

We have already said that rigidity in extension when the limb is vertical is essential, but whilst it is necessary for extension to be complete at this moment it is also necessary to prevent the knee being forced into the hyperextended position, as this would quickly strain the joint and render the limb useless.