A force-line visualization system
A. B. Wilson, Jr. *
C. Pritham *
T. Cook *
Based on a paper presented at the ISPO International Course on Above-knee Prosthetics, Rungsted, November, 1978.
Alignment of artificial legs and lower-limb orthoses can be carried out properly only if the prosthetist and orthotist have a thorough understanding of the force system involved in the man-machine combination at rest and in action.
Radcliffe (1954) and others have set forth the theory of alignment that is used universally by prosthetists and orthotists in applying lower-limb prostheses and orthoses. These theories have been substantiated in the laboratory using very expensive equipment and in practice. However, to arrive at an optimum alignment in clinical practice a great deal of skill is involved since until now it has not been practical to establish the location of the weightline and its magnitude, especially under dynamic conditions.
To rectify this situation, a team at the Rehabilitaion Engineering Center, Moss Rehabilitation Hospital—Temple University— Drexel University led by Thomas Cook has devised a relatively inexpensive method that makes it possible to superimpose the weightline vector on the patient (or a picture of the patient) under both static and dynamic conditions (Fig. 1 ).
Force plates, 150cm Longx30cm wide using paired strain gauges are used to measure the three orthogonal components of the ground reaction force under each leg as the subject traverses the central portion of a walkway 7m long.
The force plate uses a strain-gauge in each corner to measure vertical load, and gauges in the horizontal plane to measure the longitudinal and lateral shear force components. The centre of pressure is determined by an analogue division of the difference between the vertical gauges on opposing ends of the plate by the total from all vertical gauges. The ratio is scaled properly so that a given voltage represents a particular co-ordinate of the centre of pressure. This voltage is one of three needed for the force vector display circuitry. The other voltages are derived from the sums of voltage values from the vertical and shear gauges.
The display is basically a modified Lissajous figure and is generated by simultaneous application of sine waves having amplitudes proportional to the vertical and horizontal components of force. The common sine wave source ensures that a single-line trace is generated. In each axis, amplitude modulation is accomplished by an analogue mutliplier. Correct horizontal positioning of the vector is obtained by summing the shear-force-modulated sine wave with the centre of pressure voltage. In order to ensure that the resulting vector display has a common origin and does not appear to go under the floor the sine wave must be unipolar, and a bias voltage is required.
The outputs of this rather simple circuitry are channelled to an oscilloscope which provides a real-time visualization of the floor reaction force which can be related to the patient's body segment configuration at any instant in time by the use of an optical beam splitter. This allows the superimposition of two images, thus making it possible to view and photograph simultaneously the patient walking on the force plate and the real-time display of the ground reaction force vector. A major technical question to be considered is that of display screen size. In order for the composite image to be in focus and for the vector to appear "life-size," the display screen must be relatively large. Also, the displayed line must be intense enough to provide good contrast against the subjects' skin. Three different display screen systems have been used during the various development stages of this technique.
The final and most satisfactory system developed to date uses an X-Y optical scanner and low power laser to project a life-size vector on a screen. The display has good intensity and contrast. A sine wave frequency of 300-400 Hz can be used so that a variety of photographic shutter speeds are possible. The fact that the display can be made as large as the force plate area makes possible the use of a variety of focal length lenses without recalibration of the display. Thus, one can obtain a more detailed record of a particular joint-force relationship by using a longer focal length lens or of a complete body picture by using a lens of wider angle.
The equipment is so new that we have not yet accumulated sufficient data in above-knee prosthetics to be useful in improving alignment procedures. However we have made a preliminary experiment with one very active subject who is 22 years of age and regularly uses a Henschke-Mauch Swing 'n' Stance unit. The object of the experiment was to take a look at the changes that take place at extreme ranges of adjustment.
The subject was provided with a University of California adjustable leg. What was considered to be optimum adjustment was obtained and recorded. Adjustments to provide extreme degrees of knee flexion and initial extension, anterior and posterior position of the axis of the knee bolt, and medial and lateral positioning of the knee bolt were introduced and the results recorded. The results are shown in Fig. 2 and Fig. 3 .
No surprises are revealed in this preliminary study, but the results demonstrate the potential of the instrumentation in research and in clinical practice.
It is hoped that the research effort, which will include energy expenditure, will result in a significant refinement of the present methods of aligning artificial legs and lower-limb orthoses.
Radcliffe, C. W. (1954). Alignment of the above-knee artificial limb. In: Human Limbs and their Substitutes, Klopsteg, P.E. and Wilson, P.D., 676-692, Hafner Publishing Co., New York and London.