Above-knee prosthetic alignment
J. S. Taylor *
Based on a paper presented at the ISPO International Course on Above-knee Prosthetics, Rungsted, November, 1978.
The alignment of an above-knee prosthesis is defined as the position and orientation of the socket, knee joint and foot relative to each other.
There are three procedures which are performed during the fitting and fabrication of most above-knee prostheses, namely bench, static and dynamic alignment.
Bench alignment, which is the assembly of the various components can be done without the aid of any special devices. The other two procedures require the aid of a device or devices if optimum placement of the components is to be achieved.
To enable this to be done the alignment device must provide sufficient angular or tilt adjustment of the socket in two planes and also linear shifts in two planes of the socket, knee and foot in relation to each other.
The principal systems of alignment can be grouped into two categories;
angular and linear adjustments at one level—neutral or vertical pylon tube,
angular adjustments at more than one level—non-neutral pylon tube.
Irrespective of the principle adopted, however, the required configuration of the finally aligned components can be the same.
The availability of alignment devices for the above-knee amputee is of fairly recent origin, previously, in most countries, above-knee prostheses were constructed of wood or light metal and simply bolted together in some manner whilst trial fittings and adjustments were made.
An example of this is the light metal leg (Fig. 1 , left) common in most parts of the United Kingdom, this prosthesis is constructed to measurements of the patient taken by the prosthetist and sent on a prescription form to the fabrication factory.
If the requirements for alignment, as outlined earlier, are considered to be necessary, then this arrangement suffers the following disadvantages as far as alignment is concerned;
tilt adjustments of socket difficult and often inadequate,
rotation of the foot including rotation of the knee difficult,
provision of length adjustment below the knee not readily available,
length adjustment above the knee requires a considerable amount of restructuring,
linear shifts almost non-existent.
However, it must be stressed that these prostheses are very skilfully assembled by highly competent craftsmen to an established alignment configuration which, in many cases, is close to the alignments obtainable.using the new techniques. The problem which remains with this arrangement is that one can never be sure if the best possible alignment has been arrived at, whereas an alignment unit with a wide range of adjustments gives the prosthetist an infinite number of alignment configurations to choose from.
One of the early alignment units for the above-knee prosthesis, is the above-knee adjustable leg (Radcliffe, 1951) shown in Fig. 1 , right. Designed at' the University of California, Berkeley, it allows a wide range of adjustment;
+or-15° mediolateral tilt of the socket
+or—20° internal-external rotation
+or—19 mm of anteroposterior shift
+or—24 mm mediolateral shift.
The position of the knee extension stop can be varied through a range of 25°.
At the level below the knee internal and external rotation of the foot and length adjustments can be varied. All adjustments can be easily carried out and are independent of each other. The unit also incorporates a swing phase constant friction unit, and is one of the neutral or vertical pylon units. Disadvantages of the unit are that with a plastic socket and SACH foot it can weigh upwards of 4 kg and the friction device is often troublesome.
The AKAL with its wide range of adjustment and colour coding of adjustment screws is very suitable as a teaching aid, and can be found in most prosthetic teaching establishments.
Another system using a vertical pylon is incorporated in the more modern British Modular Assembly (Blatchford) prostheses where the alignment unit is part of the final structure of the prosthesis (Fig. 2 ).
The unit allows linear shifts of approximately 25mm mediolaterally and anteroposteriorly and approximately 12° angular adjustment and rotation. Length adjustments and/or rotations are possible at two levels below the knee. There is a special attachment available for use with plastic sockets as well as struts for attaching metal sockets. The alignment coupling nuts are finally fixed by means of a torque spanner to 145 lb/N. This unit has an adequate range of adjustment, but requires a lot of experience to be able to use it efficiently and easily. The centre fixation bolt of the socket controls tilt and also rotation and when loosened to make an adjustment there is always the danger of losing alignment.
With experience this unit can be handled well and the structure when secured forms a very rigid and safe component, also, being part of the final prosthesis, the alignment can be altered easily should the need arise.
It is probably true to say that a vertical pylon system of alignment is the easiest to learn as it is necessary to concentrate only on the pylon tube to establish whether the load is being transmitted to the ground in the correct manner.
The non-neutral pylon system on the other hand needs a greater degree of concentration, at least initially, as it takes some time to adjust to the structure between foot and knee and between knee and socket inclined at considerable angles.
Early alignment units included the Winnipeg Modular System (Foort and Hobson, 1967) which used a pair of wedge discs placed on top of the foot and below the knee and also a pair above the knee. This arrangement was inadequate as the wedges were not independent of each other and optimum alignment was almost impossible.
The arrangement used in the Otto Bock modular system employs a non-neutral pylon incorporating alignment units below—and above-knee with independent adjustment on both planes. These alignment units are part of the structure of the completed prosthesis and are very easy for the prosthetist to handle, they can be used independently or in conjunction with the alignment and balancing apparatus available with the Bock Modular System (Fig. 3 ).
The alignment unit provides for accurate assembly for all components up to and including the socket attachment block. The balancing apparatus is designed to allow the prosthetist and patient to align the socket independently of the remainder of the leg, to prove the socket fit and make adjustments before final assembly. Accurate and consistent bench alignment is easily and quickly done, using conventional as well as modular components.
With good equipment to provide accurate bench alignment, the need for alignment units with large adjustment capability is greatly reduced and a reduction in dimensions is possible.
The Staros Gardner unit is a relatively small unit which provides all the slide and tilt adjustment necessary. This unit can be used at below-knee level as well as above-knee, it is capable of being used with modular systems as well as conventional prostheses and is transferred out of the final structure.
The question is often asked, why do we need to have alignment units? We can certainly provide limbs without them, but there always remains uncertainty as to whether the best arrangement has been achieved or not.
Alignment devices can eliminate this uncertainty and from experience a system is preferred in which the alignment unit is incorporated in the completed prosthesis; this allows quick and corrective adjustments to be made routinely at our primary amputee review periods. Although many of these adjustments are fairly minimal in range they add up to a great deal in energy saving and improved gait, in many cases at no extra cost.
A final word of caution—alignment units must not be used to iron out ill-fitting sockets—correct stump/socket interface contact must always be established first.
Foort, J., Hobson, D. A. (1967). Modular adjustable prostheses for above-knee amputees. Manitoba Rehabilitation Hospital Pordu, Winnipeg.
Radcliffe, C. W. (1951) Use of the adjustable knee and alignment jig for the alignment of above-knee prostheses. University of California (Berkeley), Prosthetic Devices Research Project, Report to the Advisory Committee on Artificial Limbs, National Research Council.