O&P Library > POI > 1979, Vol 3, Num 2 > pp. 68 - 72


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Socket fabrication

J. Van Rolleghem*
X. Berteele*

Based on a paper presented at the ISPO International Course on Above-knee Prosthetics, Rungsted, November, 1978.


Even after a great many years the best method of making a suitable above-knee socket is still open to discussion and, despite all the different theories that have been introduced, a clear and satisfactory method has not been established.

It is also surprising to find that, with all the modern progress in surgical techniques, there are still a large number of above-knee amputations performed even when it is known that amputation at this level is a great handicap for persons of all ages.

Immediate or early fitting of a prosthesis has many advantages if it is applied by a specialized team. However, the shape and height of the cast socket are very important, manufacturing has to be carried out with a view to the socket which will be used after healing, it should not deform the stump by causing, for example, skin folds in the upper part of the stump. The same conditions must be observed when fitting a pneumatic thigh socket.

It is of prime importance that, after amputation, deformation and oedema of the stump is controlled; it is our practice to give each amputee a leaflet informing them of the advantages of good stump preparation to obtain a healthy undeformed stump. Not only do we inform the amputee of this technique, but also the nursing staff of hospitals, clinics, and nursing homes with the hope that this information will help in preparing the amputee for the future fitting of a prosthesis.

Treatment is started as soon as the stump is healed. If the stump has persistent oedema, a "JOBST" extremity pump, is used which applies equal pressure to the stump. The patient is fitted at the earliest possible time using the actual prosthesis with a temporary socket, which is later replaced by the definitive socket.

The first stage of making a satisfactory above-knee prosthesis is to produce a socket which gives maximum contact and control of movement between the stump and the prosthesis.

Formerly it was necessary to use a waist belt or other suspension. Then, years ago, we used high density polyethylene sockets with front opening; these have since been abandoned as they caused deformation of the stump. In addition, they were rather uncomfortable because of their bad suspension which made good re-education impossible.

It has been proved that if the stump is satisfactorily fitted with a suitable form of above-knee socket, contact between the wall and the stump gives greater active force and control, which is very important during walking. If only a tuber brim is fitted, great loss is incurred by not using the distal end which is indispensable for good control.

It is of great importance that, when an above-knee stump is in a total weight bearing position, the maximum surface of the stump is employed to evenly distribute the pressure by providing total stump contact.

Since amputation techniques are continuously improving maximum contact can be exerted on the stump and everything possible done to take advantage of the smallest stump area to increase the contact surface. Indeed, when a stump is under total weight bearing, that is the way to spread and distribute the support and the load.

It is necessary to make allowances for the muscle groups so that their function may not be impaired when fitting them into the socket.

We cannot reject the possibilities of weight bearing on the ischial tuberosity, a bone which is not ideal because of its position and shape, but is however, the only bony bearing point. It would be quite wrong to think that it is possible to make a correct cast by using only the hands because they are quite unreliable instruments.

Above-knee brim

We would now like to describe an above-knee brim, the form of which has come about partly by tradition and study of the literature, and partly by the fruits of experience. A series of above-knee brims based on the quadrilateral concept (Fig. 1 ) are used to make sockets. The medial edge of the brim is nearly the same height as the ischial seat and the anterior edge is sufficiently high to obtain a counter pressure to stop the ischial tuberosity sliding off the seat.

It is very important to note that the height of the casting brim is kept as low as possible so as not to change the shape of the stump when casting. There is a selection of sizes based on the mediolateral dimension. The lowest brim is 2cm medial height and 4 to 5cm lateral, the highest brim being 5 to 6cm medial, and 10-12cm lateral.

Suspension system

Each above-knee brim has four points of suspension which enables us to obtain good contact and correct suspension of the stump. The first is at the medial posterior aspect of the brim, the second at the medial anterior aspect, the third laterally at the position of the gluteus maximus, and the fourth at the rectus femoris.

The suspension system (Fig. 2 ) consists of 4 cables with hooks whose length can be regulated. The ischial seat is kept in the right position by means of this suspension. Normally we use a series of 13 left and right brims, but for short stumps or for cases that have considerable muscular atrophy, there are a further 8 sets of brims left and right, which provide more accentuated antero-posterior pressure and greater support on the ischial seat. In addition, plaster brims that can be modified are used in exceptional cases.

Casting procedure

The patient stands in the suspension appliance, a tubular stockinette is then placed on the stump (Fig. 3 , left), followed by the application of a suitably sized tuber bearing brim. The four cables of the suspension appliance are attached to the brim and traction is applied to give the patient weight bearing. The suspension is adjusted to give a horizontal tuber seat. In exceptional cases flexion of the stump can be corrected. The cast is then taken with 20cm quick setting plaster bandages. Whilethe cast is drying the femur is fixed by hand (Fig. 3 , right) to avoid pelvic tilt and antero-posterior pseudarthrosis of the femur in the socket. All necessary measurements are taken. The vertical lines are marked on the cast, the number of the tuber brim is recorded and the negative cast is removed from the stump and sent for pouring of the positive cast.

A specially constructed chair (Fig. 4 ) is used for geriatric patients or double amputees for whom it is impossible to take the cast in a standing position. The chair is constructed on a metal tubular pylon which can be raised or lowered by a hydraulic lift, it can also be rotated; this enables the patient to lift himself from the wheelchair straight into the casting chair.

The back of the casting chair and the two adjustable arm-supports hold the patient in a vertical and horizontal position. The seat of the chair is made in two halves, which permits a right or left stump to be placed vertically for cast taking.

As there are four suspension cables fitted to this chair, the same method is employed as in the case of a patient in a standing position. Traction can be regulated to the nearest millimetre. This method has the great advantage that the patient is perfectly relaxed and the cast can be made with the amputee in a sitting position.

When the cast has been made the tuber brim and the negative cast are separated and the cast goes to the casting shop where a further series of brims with the same profile are used for pouring the positive.

The brim is pushed into the negative (Fig. 5 ), each brim has an 8cm collar. The positive cast is then poured using a tube to allow the resin fabrication work.

The brim can easily be removed by the lateral opening and at this point the measurement chart is consulted with special reference to the physical condition of the stump. For example;

  1. swollen postoperative stumps,

  2. weak muscular conditions,

  3. strong muscular sections,

  4. bony protuberance,

  5. bad distal end of the stump,

  6. stump with retraction or muscular atrophy, (unfortunately only a small percentage of myoplastic stumps are available),

  7. position of the stump, flexion, adduction, abduction or hyperextension.

All this information is noted to help with the correction of the positive cast which is the job of a qualified technician who provides maximum contact with the stump surface while still allowing natural contraction of the muscle groups. For a normal stump the total area is altered by 3% except at the ring level. We take into account the necessary support to the femur and the space required for the adductor group. For stumps which will not tolerate distal support a soft contact pad is used. Space for the pad is provided by adding 2cm to the distal area of the positive. After the necessary correction has been made, the cast is rubbed down and smoothed off.

Socket fabrication

The cast is now ready for making the plastic socket, and we proceed by isolating the plaster cast with a P.V.C. sock. We then apply 6-8 thicknesses of poliamide felt near the tuber brim, and 2 thicknesses at the distal end of the cast, we then build up with glass fibre and finish off with nylon tubular material. The use of poliamide felt is very important because it can be easily polished. It is essential to polish the inside of the socket with sand paper as soon as the resin has polymerized. When the resin comes in contact with the P.V.C. it removes the product softening the material and this is harmful to the epidermis and provokes the so-called allergies. Polishing the socket avoids such a problem.

The prosthesis is then aligned and small modifications of the socket are made to suit each patient; for instance, adaptation of the proximal edges to prevent an eventual painful point.

Contact is intimate for juvenile amputees and firm stumps, but intermediate for geriatric stumps where filling with polyurethane balls or injected silicone is used.

This method of making above-knee sockets (Fig. 6 ) has been employed for many years now and more than 3,000 cases have been treated.

Many geriatric patients have flexed stumps (Fig. 7 , left); they are helped by correcting the positioning in the modular system (Fig. 7 , right). Below the knee joint, an "S" shaped tube is fitted to place the foot in a backward position to give greater stability.

It is imperative however, that the tuber bearing ischial seat is horizontal to stop any internal rotation. This gives very good functional and aesthetic results.

Donning the prosthesis

It is almost impossible to enter a stump into a total contact socket using a tubular sock without using very complicated instruments, which are unsuitable for old people. A solution to this problem is possible for all patients. The patient sits on a normal chair, he wraps a long cord (1cm diameter) round his stump (Fig. 8 , top), starting at the top of the thigh and winds it, not too tightly, round and round to the distal end. He passes the cord through the valve opening and pushes the stump into the socket. He places one hand on the knee and pulls the limb towards him; with the other hand he pulls out the cord (Fig. 8 , bottom). The total length of the stump is then introduced into the socket without any skin pinching between stump and socket. When the patient stands the foot is in correct external rotation.

Socket modifications

The prosthesis fitted to a primary amputee is temporary because the stump is in a transitional state for four to six months or more, depending on each particular case. The shape of the stump will change and in general it shrinks. It is therefore necessary to re-examine the primary amputee every three to four weeks to find out if physical changes have taken place; if so, the necessary modifications must be carried out immediately. Leaving the patient without his limb at this stage is not good practice.

For socket and limb modifications we use a technical data chart on which the socket is divided according to the clock face to identify positions for modification to the socket. Modification of a socket after physical changes have taken place gives better contact, control and function of the limb, and also helps vascularization.

Atrophy of the rectus femoris presents many difficulties. By modifying only on the lateral wall the stump is forced toward the medial side of the socket and displaces the ischial seat; this hinders the dynamic and static function of the limb.

When stump atrophy is present, attention should be given not only to the upper part of the stump but also to the lower distal end because it has lost its terminal support, resulting in lack of control and function of the limb.

Modifications should be made to the anterior and posterior parts of the socket so as to preserve the muscular function. To carry out this correction, we use "Flexo" cork sheets of 3-5mm thickness. Because of the flexibility of this material it can be applied to curves and is easily shaped and rubbed down to a smooth surface. The cork is sealed with a coat of resin so that there is no skin irritation. Scarpa's triangle is given the necessary frontal support when carrying out these modifications.

It is of great importance during the postoperative period of 4-6 months to make regular inspections and note the development of the stump.

This data enables us to decide when the stump has stabilized so that a new cast and definitive socket can be made.

At that time it is possible to rectify the alignment of the prosthesis in collaboration with the patient, who will have recovered his body image.

In the case of gross anatomical changes in the stump a third socket can be manufactured if required.

O&P Library > POI > 1979, Vol 3, Num 2 > pp. 68 - 72

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