Ideas on the suspension of the below-knee prosthesis
S. Grevsten *
Comparative studies with the PTB-suction prosthesis and the ordinary PTB prosthesis have provided opportunities to analyze and understand some functional differences in the suspension of below-knee prostheses. In a roentgenological study of the stump-socket contact significantly less vertical displacement in the PTB-suction prosthesis has been shown. A study of the pressure variations in the suction socket verified the theory of the necessary pressure gradient in the PTB-suction socket. Added negative pressure by muscle action was also observed. An electromyographic study suggested a muscular activity pattern in the suction prosthesis similar to that of a normal leg, as a contrast to the ordinary PTB prosthesis where simultaneous contractions of antagonistic muscles seemed to be the usual pattern. The latter is interpreted as a defence reaction. An optimal prosthesis has been deduced. It shall have a soft prosthetic socket with a perfect fit, a suction, adhesion and friction fixation and a pressure gradient between the skin and the socket wall with minimum compression distally. The plaster casting shall be done in a downward modelling manner making available space for soft tissue without compression.
The basic purpose of any artificial limb is to enable the amputee to perform everyday activities in an easy, natural and comfortable manner (Radcliffe, 1955). The introduction of the Patellar-Tendon-Bearing (PTB) prosthesis (Radcliffe, 1961) represented a considerable advance in the prosthetic treatment of the below-knee amputee. The PTB, in general, has been a functionally successful prosthesis which has fulfilled the above purpose. However it is not problem-free for some wearers who have different types of sensitive skin or are used to heavy physical activity. These difficulties have demanded an alternative to the PTB (Bakalim, 1965; Eriksson and Lemperg, 1969).
Various attempts have been made in different centres to counteract the circumstances leading to skin sores. It would seem a natural solution to reduce the pressure on the tender parts of the stump and distribute the load to other less easily injured parts. A soft liner inserted in the socket may sometimes bring simple relief to a tender stump.
Sockets specially constructed with this aim have been produced, one of them being the PTB air cushion socket (Wilson et al., 1968). Here, a sealed air chamber between the inner and outer sleeves of the socket provides support and distributes the load by virtue of tension in the elastic inner sleeve and through compression of the air. Another approach to this problem has led to a new design of socket for the below-knee prosthesis (Renström et al., 1974). For a closer fit of the socket against the stump it has been constructed with a socket bottom consisting of a floating pad filled with water and connected to a pivot on the leg shell. Thus the socket is meant to be self adjusting to the circumferential and longitudinal variations of the stump.
Various auxiliary suspension aids such as thigh lacers, special straps and condylar wedges have been developed with the aim of improving the anchorage of the prosthesis and at the same time diminishing the piston action between the stump and socket (Wilson, 1969). The PTS prosthesis (Pierquin, 1964) and the KBM prosthesis (Kuhn, 1966) are such constructions. The proximal brim of the PTS is extended to cover the patella and the medial and lateral condyles of the femur thus providing better lateral stability. The KBM socket is also extended over the femoral condyles but does not cover the patella. Suspension of the KBM socket is provided by a wedge inserted between the medial lip and the femoral condyle. A recent simple device adaptable to any knee prosthesis that does not have mechanical joints or straps attached is the rubber sleeve suspension (Chino et al., 1975; Giacinto, 1976). These improvements in the suspension of the PTB prosthesis are all directed towards the effects of the piston action, namely soft tissue movements and the effect on sensitive pressure points.
Many improvements on the PTB prosthesis have dealt with only one of these two components of the piston effect. An ideal solution for the patient, however, must take into account both the tissue movements and the distribution of the load. The well known above-knee suction-type prosthesis seemed the most natural model to use as a basis for an optimal socket construction for below-knee stumps with such problems as those mentioned above (Radcliffe, 1955). In a suction-type prosthesis the skin is immobilized and the load is distributed over its surface.
A trial with PTB-suction prosthesis was initiated in 1969 (Grevsten and Marsh, 1971) and two different sockets have been developed. One has a total rigid socket construction (Fig. 1 ). The other is a semi-rigid socket (Fig. 2) which is detachable and is put on separately (Fig. 3 left). It is then put into a below-knee leg construction (Fig. 3 centre and right). Both are called PTB-suctiou prostheses.
One problem with this prosthesis that was evident from the start was that of too much load and pressure being applied to the distal parts of the soft tissues. A relieving measure was therefore introduced—the socket was made wide enough distally to receive down-stretched soft tissue. If the soft tissues were pulled down, pre-stretched into the available space, they would be ready to retract and therefore in a position to reduce the pressure on the end of the stump and also distribute the load and pressure from the end of the stump in a proximal direction. The plaster casting is most important when constructing a PTB-suction prosthesis. One plaster cast is seldom enough. The plaster cast must be made in a downward modelling fashion to create the possibility of pre-stretching the soft tissue distally, thus achieving the gradient pressure which avoids disastrous pressure effects on the distal parts of the stump. In this matter the importance of frank and intimate contact between the prosthetist and the orthopaedic surgeon when dealing with the amputee's problems cannot be over-stressed.
Different studies with the PTB-suction prosthesis and the ordinary PTB prosthesis have been made to analyze and understand their function. In a roentgenological study the stump movements in simulated walking were analyzed (Grevsten and Eriksson, 1975). It was shown that the vertical displacement (Fig. 4 ) in stance phase was on average 11.3 mm less in the suction prosthesis than in the PTB prosthesis. No air was seen in the socket of the suction prosthesis in any of the simulated stride movements but this too was always the case in the ordinary PTB prosthesis. It was also shown that with the suction prosthesis the main sagittal compression of anterior soft tissues of the stump occurs when the prosthesis is suspended and at double support. This is compatible with the line of action of the floor reaction force acting posterior to the knee making a relaxed knee flex (Radcliffe, 1962).
A study of the pressure variations (Pearson et al., 1974), in the suction socket verified a theoretical idea of a pressure gradient in the socket. Further added negative pressure due to muscle action, beside the pressure induced by the swing phase load of the weight of the prosthesis, was also observed. These analyses were made through a mathematical model of assumed forces acting on the below-knee stump. From this model a formula was derived giving predicted values for the pressure in stance phase and swing phase to be compared with the observed values.
This study also illustrated an interesting relationship between the intercavity pressure in the socket and the blood pressure in the stump. When walking, the arterial and venous blood pressures are oscillatingly overlapped by the pressure variation in the PTB-suction prosthesis, accounting for a pumping effect which assists the normal pumping action of the below-knee muscles and thus improves the circulation (Wilson, 1969).
In an electromyographic study of the residual muscles of the amputation stump while walking with ordinary PTB and PTB-suction prosthesis a muscular activity pattern in the suction prosthesis similar to that of a normal leg is suggested (Grevsten and Ståhlberg, 1975). Presumably the normal muscle activity is a spinal reflex easily elicited as long as no strong irregular sensory input is present. Walking with the PTB prosthesis often shows simultaneous contractions of antagonistic muscles which were assumed to be a defence reaction to the piston action resulting from the loose suspension of the PTB prosthesis.
From 1969 to 1976, 46 patients were fitted with PTB-suction prosthesis and 27 patients were converted to it (Grevsten, 1977). They represent the patients with the stump problems initially pointed out. In this group 12 patients were observed with healing of skin lesions while walking with the prosthesis. A further 6 patients with extreme excess of soft tissue (more than 30 mm) distal to the bony end of the stump and with great problems when using the PTB prosthesis became the best users of the suction prosthesis.
The PTB-suction prosthesis was introduced for those wearers of the ordinary PTB prosthesis who often sustained contact sores and skin irritation. It is seen in the clinical presentation that in such cases the skin lesions generally disappear in the PTB-suction prosthesis. There are still some groups of patients who are left unsatisfied, however, as all below-knee amputees are not able to use this suction model. With very short stumps fitting the prosthesis is a physical impossibility, and with too slender and bony stumps the problems for the prosthetist in fitting the socket to obtain sufficient sealing, preventing air being sucked in during activity, are too great. The PTB-suction prosthesis is, however, very practicable for stumps longer than 125 mm and with a reasonable amount of soft tissue. If these two criteria are fulfilled this prosthesis comprises a prosthetic therapy and an alternative for stumps with areas of sensory disturbances, frail skin, a tendency to contract sores or eczema and/or ischaemic problems.
The PTB-suction prosthesis is also cosmetically favourable, which has been particularly appreciated by the female amputees.
An optimal prosthetic suspension should both minimize stump movements and distribute the load evenly over the stump. The above studies have shown that the PTB-suction prosthesis fulfils the conditions for such an optimal prosthetic suspension which the ordinary PTB prosthesis generally does not do although, as a rule, it functions satisfactorily. Thus when the ordinary PTB prosthesis results in skin problems for the wearer there are good reasons to consider the PTB-suction model.
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