Active plantar-flexion above-knee prosthesis: concept and preliminary design
C. Rigas *
Abstract
Lower limb prostheses produced so far do not replace the function of the musculature, which provides the necessary amount and the proper distribution of energy to the ambulatory mechanism. Provision of external power to lower limb prostheses has been hindered by the fact that the large amount of energy needed at a lower limb necessitates the use of external power sources of unacceptably high mass and volume. Self energized mechanisms offer a promising alternative to the problem.
Normally a large part of energy output during locomotion appears at the ankle joint. It seems therefore desirable to develop an above-knee prosthesis featuring active plantar-flexion at the ankle. Energy may be offered by a self energized system. The system, described here, accumulates energy during the passive movements of the knee and ankle joints and stores it for later use at the ankle.
Introduction
Loss of a limb by amputation is accompanied by the loss of the corresponding supporting structures, the joints, the ligaments and the neuromuscular system, which provided power, control to and proprioception from the amputated part of the limb.
Artificial limbs consist of a more or less solid structure which replaces the skeletal structure of the missing limb. Depending on the level of amputation and the type of the artificial limb, the latter can involve joints, where motion is permitted or restricted to a certain extent. Upper artificial limbs involving joints are provided with external energy.
Lower artificial limbs produced in practice are not provided with energy. This is due to the fact that the large amounts of energy required by the lower limbs would introduce the need for heavy and bulky external power sources, which make the use of such lower limb prostheses impractical.
This forbidding disadvantage has been overcome by the introduction of the self-energized A/K prosthesis (Seliktar, 1971a, b, Rigas, 1978, Rigas et al, 1979). The self energized A/K prosthesis is virtually a peg-leg providing active telescopic shortening during the swing-phase of the artificial limb. The knee joint is only unlocked to permit sitting. The telescopic leg offers substantially improved stability during the stance phase and adequate ground clearance due to shortening without undue tilting or vaulting of the amputee as it swings forward. Energy required for the shortening of the leg is obtained from a pneumatic cylinder where it was stored during the stance phase, in the form of compressed air. The energy stored in the self energized system is not used to contribute to the propulsion of the amputee, it is used to facilitate an easier swing of the artificial leg.
The philosophy behind the present design is that substantially improved A/K prostheses could be produced if active plantar-flexion were provided at the ankle joint.
Concept
Energy studies of normal human gait have proved that about half of the energy output by each leg during locomotion comes from the active plantar-flexion of the ankle joint (Bresler and Berry, 1951). Nevertheless artificial legs produced so far include an ankle which is either a passive joint, or else is completely immobilized. It seems therefore reasonable that it would be a useful development in the A/K prostheses state of art to explore the effect of an actively plantar-flexing ankle on amputee gait. The idea of using external power, however attractive it may be in theory, does not offer realistic answers to the problem due to the low capacity to weight ratio of present day power sources. On the contrary, self energized mechanisms have proved to work, without imposing a high burden on the amputee. For a self-energized mechanism to provide adequate energy for an active plantar-flexion of the ankle, it should be capable of subtracting a proportional amount of energy from some other part of the system (amputee plus prosthesis). This can be achieved either by increasing the positive power output from that part of the system, or taking advantage of any power input, that is, power spent to produce controlled passive movement and lost as heat, if not subtracted and stored in useful form. Obviously the latter solution is desirable, since, in contrast to the former, it would not impose an increased work load requirement on the amputee. The possibility of extracting work from passive movement of joints is realistic. In normal human gait the knee acts primarily as a damping mechanism. The damping action of the knee is imitated in one way or another by all knee mechanisms employed in artificial legs. They transform energy imput at the knee to heat.
There are also parts of the action of the normal ankle joint where this action is passive. If, for the sake of a first approximation the knee and ankle joints of an A/K prosthesis, were considered to act in a way similar to the normal joints, then-in theory—the following approximate amounts of energy could be made available and stored in the self energized mechanisms (Bresler and Berry, 1951).
5.4 J during the initial small flexion of the knee, following heel contact
18.3 J during the major flexion of the knee at the end of the stance and the beginning of the swing phase of the artificial leg.
10.2 J during the extension of the knee prior to the new heel contact
1.3 J from the ankle, during its initial passive plantar-flexion, just after heel contact
7.4 J during the dorsi-flexion of the artificial ankle, after positioning the foot flat on the ground.
This is a total of 42.6 J. Active plantar-flexion of the ankle requires 29.8 J. In theory, therefore, more than enough energy can be extracted from the knee and ankle to facilitate active plantar-flexion of the ankle, without imposing extra work-load requirements on the amputee.
In practice, though, the power that can be available will be considerably less, because of the following two reasons:
The procedure of extracting energy from the joints, storing and re-using it will involve a certain amount of loss in the form of heat.
The mass and moment of inertia of the prosthetic shank and foot are smaller than the corresponding parameters of the normal shank and foot.
On the other hand this second reason has a positive effect on the amount of energy required by active plantar-flexion. This is so because the smaller the mass and moment of inertia of the shank and foot assembly, the smaller amount of energy will be required to facilitate its motion at push-off.
In any case, providing active plantar-flexion of the ankle is expected to be an advantageous feature, even if the amount of energy that can be made available from a self energized system is not sufficient to meet the energy requirements estimated on the basis of an amputee gait that is as close to normal as possible. This is so because:
Active plantar-flexion of the ankle can make available a certain amount of forward momentum to the amputee.
Active plantar-flexion of the ankle, however smaller than in normal gait, will contribute to the bending of the knee at the end of stance phase and the forward acceleration of the thigh, thus relieving the hip joint of part of its work load.
Because of the contribution of the ankle plantar-flexion, described above, higher mass and moment of inertia values could be tolerated than with conventional A/K prostheses. This would lead to proportionally higher energy levels of the new leg during its swing phase. Since a considerable part of this energy flows back to the strider's opposite leg and body during deceleration of the swinging leg (Bresler and Berry, 1951; Ralston and Lukin, 1969) it follows that a more energetic "pull" mechanism could be effected. This again would decrease the requirements by the "push" mechanism of the contralateral leg and would make gait more symmetric.
The importance of the "pull" mechanism has been pointed out before. Inman (1968) reports that deceleration of the swinging leg contributes to locomotion eight units of work, while push-off contributes five units. On the basis of this estimate he explains why the amputee usually does not like a prosthesis that is too light.
Active plantar-flexion of the ankle can reduce the effort and work required by the amputee and increase gait symmetry. In addition it would reduce the impact of the artificial leg on the ground at heel strike and make gait smoother.
Preliminary design
The self-energized system
An active plantar-flexion A/K prosthesis is sketched in Fig. 1. It is an "open system" consisting of four compression cylinders, two pistons, a storage chamber, seven "one-way" valves and three "on-off" valves. Following heel contact the foot plantar-flexes under the action of the ground reaction forces. Piston P2 is pulled downwards, air is compressed in cylinder LC2 and forced to enter the storage chamber S through valve V3. During this process air is admitted into cylinder LC1 from the atmosphere through the non-return valve V1. At the same time the moment at the knee joint forces the knee to bend. Piston PI is forced downwards. Air is compressed in cylinder UC2. A flexion of the knee of about 15 degrees will cause a pressure build-up in UC2 that will prevent any further flexion and hence provide a stable knee. At the same time air flows into UC1 through valve V6.
After the foot-flat position, the foot begins to dorsi-flex. Piston P2 moves upwards. Air is compressed in cylinder LC1 and forced to enter the storage chamber S through valve V4, while atmospheric air flows into cylinder LC2 through valve V2. (At this instant air in the storage chamber is already under pressure, built up during the previous stage. The same applies also to the following stages of compression. To make sure that compression can be achieved although air is already under pressure in the storage chamber S, the final design must provide for the proper sizes of storage chamber, compression cylinders, pistons and levers forcing pistons to move).
A small flexion of the toes will cause valves V8, V9, V10 to open. Air that was compressed in cylinder UC2 can now enter the storage chamber S. Pressure drops in UC2, the knee can flex further and piston P1 forces more air into S. At the same time compressed air from S enters the lower cylinder LC1 and forces piston P2 to move downwards, thus causing plantar-llexion. After toe-off valves V8, V9, V10 come back to their "off" position, spring S1 brings the foot to the neutral position. Inertia causes the knee to flex further, but excessive foot rise is prevented by pressure built up in cylinder UC2. Following maximum knee flexion, knee extension is aided by air pressure in this cylinder. While the knee is extending, air is compressed in cylinder UC1 and forced to enter the storage chamber S through valve V7, until the knee reaches its fully extended position and is ready for the next cycle. Extension of the knee may be aided by a spring.
Control
Control of the self energized mechanism is achieved through three "on-off Valves, V8, V9, and V10. For the greater part of the cycle these valves are at the "off position. They are triggered to the "on" position when the toes begin to flex. Flexion of the toes exerts a pull on cord d, and this brings valves V8, V9, and V10 to the "on" position. They return to their "off" position under the action of a spring built into each one of them, when the foot leaves the ground and the toes come to their neutral position, thus relieving cord d of tension. Hydraulic, electrical or other triggering mechanisms could be employed instead of the present mechanical one.
The amount of foot-rise after toe-off is controlled by the amount and the rate of pressure built-up in cylinder UC2. This parameter, in turn, can be changed by proper angular adjustment of cam C.
To achieve full bending of the knee for sitting the amputee will require to transmit part of the ground reaction force to the toes of the prosthetic foot, causing flexion of the latter.
References:
- Bresler, B., Berry, F. R. (1951). Energy and power in the leg during normal level walking. Berkeley C.A.: University of California. Prosthetic Devices Research Project (Series II, Issue 15).
- Inman, V. T. (1968). Conservation of energy in ambulation. Bull. Prosthet. Res. 10-9. 26-35.
- Ralston, H. J., Lukin, L. (1969). Energy levels of human body segments during level walking. Ergonomics, 12, 39-46.
- Rigas, C. (1978). Voluntarily controlled telescopic above-knee prosthesis PhD Thesis.-Glasgow: University of Strathclyde, Bioengineering Unit.
- Rigas. C. Solomonidis, S. E., Bermf., N., Kenedi, R. M. (1979). New unconventional prosthesis for above-knee amputees. In: Kenedi, R. M., Paul, J. P., Hughes, J. (eds) Disability-London: Macmillan, 375-384.
- Seliktar, R. (1971a). Telescopic artificial leg: basic concepts and device development. PhD Thesis- Glasgow: University of Strathclyde. Bioengineering Unit.
- Seliktar, R. (1971b). Self energized power system for above-knee prostheses. J. Biomech. 4, 431-435.
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