The interface between the body and the above-knee prosthesis
G. Holmgren *
Based on a paper presented at the ISPO International Course on Above-knee Prosthetics, Rungsted, November 1978.
When fitting a prosthesis to an amputee, the primary task is to try to compensate as much as possible for his functional loss, although sometimes only a cosmetic compensation is possible.
A functional compensation is possible only if the remaining forces and the movements of the hip and the stump can be transmitted to the prosthesis.
Transmission of forces and movements
Functionally the leg prosthesis is a mechanical extension of the amputated leg and it serves as a component in the locomotion system. When standing and walking, considerable forces are involved. The prosthesis therefore has to be designed and aligned to be able to accept these forces and to stabilize and respond to the needs of the amputee.
The normal leg is connected to the rest of the locomotion system by the skeleton. As it is not possible to fit a prosthesis in a similar way, the prosthesis has to be connected to the skeleton via the soft tissues of the stump. These soft tissues can be deformed and displaced due to external forces. This decreases the stability and rigidity of the fixation of the prosthesis. The result is loss of energy, discomfort and distortion of the gait pattern.
It is therefore not possible to utilize the qualities of the prosthesis and its alignment if the fixation to the stump is not optimized.
The prosthesis is fitted to the stump by means of a rigid socket, or rather to the deformable and mobile soft tissues covering the remaining skeleton (Fig. 1 ).
The stability of the transmission of forces and movements therefore depends on how successfully the soft tissues can be utilized to reduce relative movements between the femur and the socket.
The prognosis for a good result is improved if the stump is well prepared and firm. If myoplastic or osteoplastic techniques have been used, i.e. the distal ends of the cut muscles have been anchored to the distal end of the stump, the result is usually much better than if this has not been done.
The stability of the fixation of the prosthesis also greatly depends on how the prosthesis is suspended.
The most efficient suspension and fixation is achieved by means of total contact sockets utilizing suction (suction sockets).
When fitting such a prosthesis, the stump is purposely deformed in order to achieve a pretension of the surrounding skin. This pretension increases the stability and reduces relative movements between the stump and the socket.
This technique, however, demands that the amputee is able to put the prosthesis on properly which may sometimes be difficult for elderly patients.
Mediolateral stabilization of pelvis
The negative consequence of instability is most obvious as far as the mediolateral stabilization of the pelvis is concerned.
Due to the forces from the abductors, the femur, which lacks rigid skeleton connection to the foot and ground, tends to move in the lateral direction. If the socket is not in proper contact with the ischial tuberosity, it also tends to move in the vertical direction.
The absence of a rigid skeleton connection also results in the fact that the floor reaction between the prosthesis and the ground cannot efficiently be transmitted to the femur. Consequently the physiological hip joint is unable to serve, in a strict sense, as an origin and a support for the stabilization of the pelvis when the prosthesis is weightbearing.
This can be reasonably compensated for by connecting the socket to the ischial tuberosity. If this has been successfully achieved, the ischial tuberosity serves as a support-point during the latter portion of the stance phase, allowing the amputee to utilize his hip muscles for balancing the pelvis mediolaterally.
The result of the abductor forces depends on how much the relative movements between femur and the socket can be restricted. It has already been mentioned that the flexibility of the soft tissues of the stump limits the stabilizing of the femur in the socket and consequently the stabilization of the pelvis. There are, however, other limiting factors.
When stabilizing the pelvis, the femur tends to rotate in the socket due to the forces from the abductors. The reaction forces against this turning effect occur in the lateral-distal and the medio-proximal positions of the stump (Fig. 2 ). If the tissues in those areas are less able to accept the resulting pressures, this reduces the possibility to stabilize the pelvis in a proper manner.
From elementary mechanics it is also known that the shorter the stump the higher will be the reaction forces due to the stabilizing moment. The pressure on the lateral contact surface also increases as the surface area itself decreases when the stump is shorter. Amputees who have short AK stumps consequently tend to tilt their bodies towards the amputated side to reduce the pressure on the stump surfaces involved.
In spite of all the various theories and methods used to fit an AK prosthesis to the body there is no disagreement that a proper configuration and "matching" of the socket to the stump is of utmost importance in each individual case.
It is also agreed that it is not possible to correct for an improperly fitted socket by means of any advanced knee and foot mechanisms. If the socket is functionally well fitted and comfortable, the rest of the leg could very well be a simple pylon.
During weightbearing, when the opposite leg swings, a prosthesis with mobile knee and ankle joints works as a stick. It is only in the latter portion of the weightbearing phase that the limitation against dorsiflexion of prosthetic foot and the characteristics of polycentric knee joints provide a different function.
Functionally, therefore, knee and foot joints mainly contribute to the possibilities of improving the gait pattern.
Experience and investigations also indicate that most of the daily problems, when using the prosthesis, are related to the fitting of the prosthesis to the body.
Practical considerations on suction sockets
If the volume of the socket is smaller than the volume of the stump, there will frequently be oedema, fatigue sensation, impaired sensibility, and tissue damage (Fig. 3 ). If the socket is too narrow, it may also be difficult to provide necessary contact between the socket and the ischial tuberosity.
As the suction effect contributes to the suspension of the socket, it is required, for the purpose of sealing, that the skin is sufficiently stretched when the stump is moved into the socket. Therefore there has to be such a pressure between the skin and the socket walls that sufficient friction and sealing is established.
The increased stability should not only be looked at as a result of the suction effect itself but of its capability to maintain the pre-tension of the soft tissues, which is developed when the stump is pulled into the socket. This pre-tension of the stump tissues contracts the compression force of the tissues at the weightbearing areas.
If the socket is not sufficiently sealed to the stump the suction will be lost. In an effort to make sure of sufficient sealing, the socket volume is sometimes made too small, which creates stumps problems and also impairs the function.
The elastic properties of the soft tissues vary, which calls for individual analysis of each case (Fig. 4 ). Thus the flexibility of the soft tissues may affect the final circumference at different levels and the depth of the socket.
The dynamic requirements put certain demands upon the proximal configuration of the socket. The quadrilateral socket shape is determined by considering the function of the four major hip muscle groups.
The proximal shape also considers the relationship of the socket to the ischial tuberosity and the inferior ramus of the pubis.
As the physiological hip joint is situated proximally and laterally to the inferior ramus of the pubis and the ischial tuberosity, the stump must be properly mediolaterally aligned when casting. The basic consideration in this respect is that the distance between the distal end of the stump and inferior ramus of the pubis decreases, and also to some extent the distance to the ischial tuberosity, as the stump adducts.
The above is an attempt to focus on the main factors that have to be considered when constructing the interface between the body and the prosthesis. Of course, any aid that improves the control of volumes and pressures and can simulate the functionally necessary deformations of the soft tissues is of great value.
For the past decade we have applied the following techniques in producing the socket for the AK amputee:
The casting procedure is illustrated in Fig. 5, Fig. 6, Fig. 7 and Fig. 8 .
A reproducable control of volume, compression and simulation of soft tissue deformation is achieved by using the elastic bag, developed at the BRADU under the supervision of Dr. Redhead (1973), although the application of the plaster of Paris and some important steps of the procedure have been modified.
The proximal configuration of the socket is based on the basic work by Radcliffe (1955).
The quadrilateral shape of the socket and the alignment to the ischial tuberosity is achieved through palpation by the hands and not by using standardized brims. It is our view that by doing this we improve the conditions for individual considerations of differences in muscle prominence and soft tissue conditions. Working this way we have found that the angle between the anterior and the posterior wall is frequently different from case to case.
Radcliffe, C. W. (1955). Functional considerations in the fitting of above-knee prostheses. In Selected Articles from Artificial Limbs, January 1954- Spring 1966, 5-30. Robert E. Kreiger Publishing Co. Inc., Huntington, N.Y.
Redhead, R. (1973). Above-knee amputation stumps and their prosthetic management using total surface bearing self-suspending sockets. Ph.D. Thesis, Department of Mechanical Engineering, University of Surrey.