The principles and practice of hip guidance articulations
G. K. Rose *
Based on a paper presented at the 6th. Scientific Meeting of the United Kingdom National Member Society, ISPO, Guildford, April, 1978.
Many complete paraplegic patients, ranging from adults with traumatic lesions to children with spina bifida cystica, are given a variety of orthoses in an attempt to produce locomotion on an empirical basis with little or no understanding of the mechanical principles involved. Not surprisingly large numbers fail to achieve this and even greater numbers abandon the exhausting, slow ambulation produced after long rehabilitation, in favour of the wheelchair.
Swivel walking devices (Motloch and Elliot, 1966) were used for the first time for this type of patient in Shrewsbury in 1967 (Edbrooke, 1970). The fundamental mechanics had been worked out by Spielrein in 1963 for an amelic case and were confirmed for these patients by Rose and Henshaw (1972). Despite the undoubted advantages of this type of apparatus and in particular the fact that it can be used in time coincident with normal developmental stages and leaves the hands free, the dynamic cosmesis is not always accepted by the patients or parents.
To provide rational, reciprocal ambulation the basic question, "how could a patient paralyzed in both legs ambulate?" was partially answered in 1974 (Rose). With the knees and ankles stabilized in KAFO's (long leg braces) the following criteria were identified:—
That the hip must be placed ahead of the foot. This can be achieved either in hip hyperextension or in flexion (Fig. 1 ).
One foot must be raised from the ground by a combination of :—
2.1 Downward pressure by the ipsilateral arm and crutch.
2.2 Body sway to the contralateral side.
2.3 Provision of a dihedral wedge under the sole of the contralateral shoe.
In these circumstances, under the influence of gravity, the lifted leg will then swing forward and can be grounded. The process is now repeated for the other foot. It is then that problems arise. Due to the geometry of the whole system the back foot is likely to strike the front or even become trapped behind it. The second, even more inhibiting, complication, is "wind swept" fall out, that is to say the patient falling laterally with abduction of one hip and adduction of the other and it is very difficult for the patient to control this.
It became apparent, therefore, that guidance of the hip joint path was imperative. From a mechanical point of view maintenance of the legs in abduction could be achieved most satisfactorily by some form of strut, or struts, placed between the long leg braces, which would be in compression (Fig. 2-A). To provide forward motion other than the swivel mode this strut could then be articulated at each end (Fig. 2-B), however, it would then produce a highly variable and unpredictable curved track which is useless for guidance. To produce a predictable path, also curved and therefore of short step length, the advancing leg must rotate about the longitudinal axis of the other and this requires a complex mechanical tracking device, (Fig. 2-C) which is difficult to produce and maintain (Glory, 1972). For a pathway nearer to normal reciprocal gait a compression strut which allows elongation would be necessary (Fig. 2-D) and the problems of providing this seem insuperable. In addition such struts as have been tried produced considerable problems with clothing and "doff and don".
It was necessary, therefore, to adopt an open structural device with no compression element between the legs. It consisted of a very rigid body brace articulating through ball bearings to the leg braces, providing planar leg movement and resistance to adduction. With this the problem of foot collision and wind swept fall out were eliminated.
Horizontal rotation about each hip was also incidentally eliminated.
Such patients still suffered from an incapacitating disadvantage, namely posterior fall out. It is not possible for them to prevent this with crutches, and it was necessary, therefore, to limit flexion of the hip by means of a stop. It was not necessary to limit extension because this was done naturally by the hip joint itself.
The next problem was to discover the optimum degree of permitted flexion. The greater this is, the greater the potential stride length. Yet there is an absolute limit to this range, and this derives from the relationship of the centre of gravity of the patient plus walking device to the supporting foot. Considering a patient in the double stance phase, when, by the process described, the back leg is raised the situation may be as shown with the centre of gravity behind the support leg (Fig. 3-A). In such circumstances the patient will inevitably fall backwards and there is no reaction between crutch tip and floor which will permit effective function of the arm muscles. On the other hand, if the situation is as in Fig. 3-B when the arm muscles can contract effectively and draw the patient's body forward rotating over the support leg. The critical nature of this flexion range was well demonstrated by patients who could walk vigorously with the apparatus initially, but would remain either immobile or fall backwards if the flexion was increased a few degrees only due to wear of the joint.
It is customary for patients using this apparatus to start with a rollator as this has mechanical advantages. It may enable the centre of gravity to be placed just ahead of the line of the back feet and this certainly occurs in some patients (Fig. 3-C). On the other hand, because a rollator is heavier than a pair of crutches, the centre of gravity of the whole system is affected advantageously. This situation was established in a patient who walked confidently with a rollator but diffidently with crutches. When a two pound weight was attached to the end of each crutch just proximal to the rubber the patient was equally confident as with a rollator.
As the swing leg goes forward the centre of gravity of the whole system advances. Provided that it does so rapidly, the disadvantages of a centre of gravity just behind the support foot may be overcome inertially by a sharp jerk forward of the pelvis and backwards of the shoulder prior to the moment the swing leg leaves the ground. Two practical points therefore arise :—
That greater initial gravitational moment on the KAFO segment, when posterior to the hip, in order to overcome inherent frictional constraints of the orthosis may be achieved by the addition of relatively small, empirically determined, distally placed weights on the KAFO segments.
Where the patient has some difficulty in clearing the swing leg from the ground the rate of forward movement may be improved by the addition of a spring anteriorly to the articulation, the so called Assisted Hip Guidance Orthosis. The spring is loaded during that phase of walking when the hip is extended, without necessarily going into extension, and rapidly unloaded at the beginning of the swing phase. The results of such a spring in terms of heart rate and speed are shown in Fig. 4 .
The problem of clearing one foot has several components. An abduction hinge placed at the hip level has been used for several years (Herzoz and Sharrard, 1966) but it has the disadvantage compared with angulation at the sole/floor interface (Fig. 5 ) that, for the same clearance, the centre of gravity of the system in the second case moves further laterally towards or over the support area, whereas with the abduction hinge the movement is less. This means, therefore, that with angulation at the sole/floor interface greater downward pressure by the ipsilateral arm and crutch is necessary. In practice it is often the fatigue and discomfort of the arms which limits this type of walking. Angulation at the sole/floor interface is facilitated by a sole dihedral (Fig. 6 ).
Another adverse factor is any flexibility of the device. To achieve rigidity it has been necessary not only to make the body brace of rigid material but in the larger child to reinforce it and to re-design the sacral band to provide not only pressure against the sacrum but also rigid spacing of the articulations (Fig. 7 ). It is possible that in the future with the development of better materials, for example a combination of metal and carbon fibre, that equally rigid, lighter body braces may be produced.
The disadvantages of the open structure method of securing hip guidance have been indicated and the stresses on and around the articulations required very considerable strengthening of the design.
Consideration has to be given also to the optimum degree of adbuction of the legs. Clearly if the swing phase is to be unimpeded there must be no contact during this time between one leg brace and the other.
Considering the geometry involved and the fact that the objective of a patient using two crutches is to raise one foot from the ground just sufficient to allow it to swing forward, significant differences with different degrees of abduction can be demonstrated (Fig. 8 ). In the particular configuration chosen, where the leg length is twice the hip width, and the legs parallel, Fig. 8-A shows that to raise the foot a standard amount an angulation of 6.5 degrees occurs and that this was foundtoputan adduction moment on the hip articulation generated by the ground reaction force at the supporting foot. In Fig. 8-B with 10 degrees abduction the angulation required to produce the same lift of the foot is 4.0 degrees and this was found to put an abduction moment on the articulation. For 20 degrees abduction (Fig. 8-C) the angulation is 2.5 degrees with an increased abduction moment. From the point of view of minimizing stress on the articulations, optimum abduction would be that which puts the stance leg at right angles to the support surface and this is 5 degrees for this example (Fig. 8-D).
This is not, however, the only factor to be considered. It will be noted that in all the examples the rise in the centre of gravity is the same but that the lateral shift of the centre of gravity diminishes with increased leg abduction as does the angulation of the trunk (if it is considered as a rigid body). This means that the moment about the support leg increases with the abduction and implies that in these circumstances the contralateral crutch load has to be increased to produce the elevation. The design of the apparatus keeps the lumbar spine relatively rigid but movement of the trunk above this, including head and arms, can produce a small beneficial shift of the centre of gravity towards the stance leg reducing this moment. Probably small differences in shift of the centre of gravity are of no significance, having regard to the leverage advantage of the crutch in elevating the leg (Fig. 9-A).
These geometric principles, whilst precise, cannot be precisely applied in practice. It is, however, necessary that the maximum rigidity should exist at all levels of the orthosis and indeed it is common with problem cases that one finds some flexing occurring at several levels, in, above and below the articulation. If such flexibility exists, it affects both the stance and swing leg. The swing leg tends to fall inwards requiring greater lateral deviation of the body to elevate it from the ground and this in turn increases the stresses on the stance leg with increasing adduction there and again further lateral deviation of the whole patient. It is here that the relationship of the centre of gravity to the support area may become quite critical. In such circumstances it can pass lateral to the support area (Fig. 9-B). Whilst this is not a disaster, as falling is prevented by the use of the ipsilateral crutch and the leverage advantage may be as good as on the other side, it does have an inhibiting disturbance of rhythm. It seems that the ideal situation is, as in normal walking, that the centre of gravity moves towards a position vertically above the support foot but does not reach it, i.e. stable equilibrium.
In these circumstances almost all the energy used in lifting is returned, whereas if the centre of gravity passes beyond the support, extra energy has to be injected by the ispilateral arm before stability is restored. At this time the energy situation has not been quantified and it is probably the rhythmic one which is the more important. Observing patients with this device there is no doubt as to the superior progression achieved by those who never reach the unstable situation.
The problem of the relationship of the axes of the articulation to the ground has a number of conflicting considerations and some compromise must be made:—
From the point of view of maintaining the axes parallel to the ground when in swing phase a configuration as shown in Fig. 10 is necessary. This should apply the least constraint to forward motion.
Having regard to the options of setting the axes of the articulations together with the pathway of the swing leg (Fig. 11 ), the pathway produced by the adduction setting is the least attractive.
For these reasons, and for engineering practicalities, 5 degrees of abduction below the articulations has been chosen.
The question of in-toeing or out-toeing has to be considered: in general we have set the articulations parallel. They do not remain so, where there is an added rotational element to progression, but this seems to represent the best compromise.
The geometry here is of course highly simplistic and pays regard to few of the many dynamic factors which must be present. Where some assessment has been made of the dynamic stresses within orthoses these have been found to be very considerably greater than those calculated from the static conditions and there is no reason to suppose that this situation is not the same. Clearly it will be important in the near future to quantify these.
As had been indicated mobility of the spine above the body brace can affect the situation and clearly it is affected by abnormalities of the spine, particularly where the curvature has caused "decompensation", that is to say that the centre of gravity of the trunk and arm segments have shifted laterally in regard to the pelvis and hip joints. In such circumstances it will be easier for the patient to lift one leg than the other. Commonly patients when first put into this apparatus may find that they can clear one foot with ease and the other with difficulty which is usually the left. When all other factors have been checked and if necessary corrected for example asymmetry, flexibility of the orthosis, spinal decompensation or shortening of one leg etc., this difficulty may still remain. In such circumstances a week or two of training, concentrating on swaying from one foot to the other may be sufficient to put the matter right, but in other cases use of a spring assist will be necessary. At this time we have not been able to define precisely those factors which make a spring assist imperative. In general it is our custom to make the orthosis without the spring assist but if success is not achieved with a very short period of training, the spring is added and in such circumstances it has been found that the situation improves rapidly. Almost certainly one factor is the degree of tolerance of rhythmic angulation of the individual patient consequent upon the cerebral element of the lesion.
To date twenty-eight patients have been supplied with this device and of these six were fitted with spring assist.
The apparatus permits independent transfer, assumption and removal, ambulation at a reasonable speed in the region of 50 feet per minute over a variety of surfaces including for example a grass field.
As in all orthoses it is important that when supplied a check should be carried out so that time is not wasted on abortive training which cannot succeed because of mechanical or geometric deficiencies in the device. It is simple, therefore, with the patient standing to rock him in turn on to each side until the foot clears the ground. It soon becomes apparent whether the apparatus is sufficiently rigid and whether the angle of deviation can be tolerated bythe patient. As regards the permitted flexion range, 5 degrees of this is supplied initially. Once the patient is used to the apparatus then the possibility of increasing the range can be estimated by leaning the patient backwards whilst holding crutches until the balance point is found and this corresponds to the permitted flexion (Fig. 12 ).
My thanks are due to the kind co-operation of the staff of the Orthotic Research and Locomotor Assessment Unit, The Robert Jones and Agnes Hunt Orthopaedic Hospital, in the developing and testing of this orthosis.
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