Part 23

The consequence of this is that the worker learns to turn the piece of work around with his sound hand. To get over this difficulty passive joints are inserted at the wrist, which allow the appliance to move when it is pressed against the piece of work and to take up the direction which suits the inclination of the forearm.

FIGS. 192 to 194. 1. Tram driver's and chauffeur's bell. 2. Method of use by a tram driver. 3. Management of a motor car lever. Pressure at the extremity and traction.]

One of the simplest mechanisms--and one of the oldest, because it was designed by Gripouilleau--is that of the agricultural labourer's ring, intended to grip and manipulate the handle of a wheelbarrow or a plough. The ring is mounted on a transverse axis and moves in a horseshoe which in its turn revolves on a shank which is screwed into the forearm. The ring is provided with a screw, which may be tightened upon the handle if desired, but which is, however, rarely used (Figs. 188 to 190).

The tram-driver's bell, represented in figure 192, is devised on the same principle. It moves on a transverse axis, and in figures 193 and 194 its utility in managing levers in driving a tram or a motor car will be easily seen. The rotation of the horseshoe on the axis of the forearm is not required.

The "cardan" joint also allows movement in every direction: it consists of two semicircles of metal, placed at right angles, each working around a transverse axis, these axes being united in the form of a cross. The construction and working will probably be understood without further explanation by a study of figures 195 to 198. The first two represent a system with a ball in the centre, which is well known commercially. The last two represent the simple universal joint generally used in orthopædic surgery. It is somewhat more cumbersome than the previous model.

The spade holder used at the agricultural centre at Limonest is mounted on a cardan (Fig. 199).

The joint can be fixed by a compression screw which is easily and quickly adjusted.

Where several tools are necessary each should be complete with its own universal joint.

Other methods of terminal passive articulations are--

(1) Ball joints which have already been described in connection with the fingers and which may be applied to the wrist.

(2) Bayonet joints which are only applicable to certain artificial hands which are not subjected to any great strain.

Figure 200 explains this mechanism as it is applied to the artificial hand represented in figure 148.

CHAPTER VIII

ARTIFICIAL LIMBS FOR AMPUTATION THROUGH THE ARM

In this chapter we shall deal only with amputation of the arm below the upper third, i.e. with cases in which the stump is long enough to transmit movements to the artificial limb. Amputation through the deltoid muscle must be considered in association with disarticulation of the shoulder.

Below the arm socket is attached an artificial limb which represents the elbow joint, forearm, and hand.

There are two types to be described:--

1. The artificial arm proper, which has the external shape of the natural limb.

2. The worker's arm, a terminal appliance in which outward appearance is not considered.

The considerations as to the arm socket and its attachment by a shoulder cap are the same for the two types of appliance.

Attachment and Arm Socket.--The surface over the acromion and clavicle is the only point from which support can be given to an appliance for an amputation through the arm; the attachment is made by means of a shoulder cap.

The general shape of this shoulder cap and its attachment by means of a strap passed under the opposite axilla are similar to those described for appliances for amputation through the forearm.

The larger the shoulder cap the more it extends forwards over the anterior wall of the axilla, upwards over the supra clavicular fossa, and backwards over the scapula, the more secure will be the support. The appliance is heavy and has no support other than the axillary strap. The latter has a tendency to ride upwards against the axilla where it exerts a pressure which may be uncomfortable. This may be relieved by attaching a vertical strap which is buttoned to the trouser belt.

But although this extensive enclosure of the thoracic region may not hinder the movements of the stump forwards and backwards, it must obviously interfere with the movement of abduction. No doubt this movement is the less important of the two, but we ought to try to preserve it as far as possible.

In short stumps we must abandon it. But if the stump is long and consequently has no tendency to escape from the socket, even if this slips down a little, movement may be retained by two methods.

The first consists in separating the arm socket from a large shoulder cap, and inserting a joint between (see page 87); but the appliance is then heavy and cumbersome. Moreover, although abduction can thus be easily attained, thrusting and pulling movements require a light appliance, and finally it is impossible to secure rotation.

It is possible, on the other hand, by means of the other method, which consists in ending the shoulder cap at a line continued vertically upwards from the thoracic margin of the axilla. If the straps are strong and carefully adjusted the result is better than with the fitting over the scapula, so that this appliance is preferable. We here illustrate a method of fitting the straps which we consider a good one. From the posterior part of the ordinary axillary strap, a Y-shaped branch passes to the upper border of the shoulder cap above and in front of the clavicle, this makes up for the small extent of the enclosure of the shoulder.

For the worker's arm a considerable enclosure without any joint is essential, in order to secure stability.