THE WALKING CALLIPER

 

By

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JOHN CHARNLEY M.B., B.Sc. Manc., F.R.C.S.

LECTURER IN ORTHOPAEDICS, UNIVERSITY OF MANCHESTER

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THE younger generation of orthopaedic surgeons tends to neglect the study of the splint-maker’s craft; the commercial splint-maker, isolated from the stream of modern ideas, perpetuates traditional types unreached by appraisal or condemnation. In some quarters the design of a splint is a matter of dogma, and scientific fact avails little in argument. Tendencies of this kind are illustrated in the calliper splint, the design of which is here selected for a critical examination.

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There seems to be no dispute that H.O. Thomas invented the walking calliper splint, as is shown in contemporary English and American references. This splint was a natural development from the Thomas bed " knee-splint," and it is interesting to recall the egotistical but prophetic words regarding this splint written by Thomas in 1890 :

 

" This appliance is destined before long to have a permanent place in our surgical armamentaria, and to occupy first place in its own field of action as soon as the correct construction and mode of action are generally known. A badly constructed or wrongly made instrument may do good work in the hands of an ingenious surgeon. But, like a master musician, the master surgeon will only be able to exhibit more skill when in possession of a well and correctly made instrument."

 

But since his time many of the details of construction regarded as important by its inventor appear to have been forgotten, and many different forms of the splint have grown up having widely different shapes ; these variations are the subject of this paper.

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VARIATIONS IN RING DESIGN

 

The most important element in design which governs the function of weight-bearing is the shape of the ring of the calliper.

 

Though it is customary to refer to the calliper as weight-bearing, it is more correctly only a weight relieving splint. No apparatus in which the toes can exert pressure against the ground can ever be  completely weight-bearing ; the only splint which truly fulfilled this function was the patten-ended calliper used by Thomas for tuberculous disease of the ankle but now obsolete.

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For the purpose of description, it is convenient to divide calliper rings into two classes : (1) rings modified from the Thomas pattern ; and (2) rings deviating from the Thomas pattern.

 

1     Modifications of the Thomas Pattern

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The feature common to this group is the complete metal ring. The principal modes of this type are illustrated in fig. 1 (to eliminate confusion a nomenclature has here been coined for those types not already dignified with a particular name)

 

(1) Authentic Thomas.-Under this heading two types of rings are to be considered ; these can be termed the " adapted bed-splint " and the "Thomas calliper."

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The adapted bed-splint was made from the bed-splint by cutting off its end and inserting the splint into the heel of the boot. This Thomas was able to do " in less time than it took for the blacksmith to blow up his fire."

 

The ring of this splint was a simple ovoid, the sidebars were attached at opposite ends of the transverse diameter (fig. la). The splint was thus symmetrical and reversible.

 

The Thomas calliper (fig. la) was an irregular ovoid. the ring was asymmetrically placed on the sidebars and inclined to the long axis of the splint in two planes. The design of this ring is considered in detail below.

 

(2) Proprietary I -This is the most common commercial shape in current use. At a casual glance it appears indistinguishable from the ring of a bed-splint, but closer examination shows that the sidebars are attached slightly behind the transverse diameter.

 

The ring of the splint is symmetrical and ovoid. The essential feature is a shallow V-shaped dip on the inner part of the ring, its purpose being to relieve pressure from the perineum, on the supposition that the tuber ischia can be carried on the elevated portion of the ring (fig. lb).

 

In practice the tuber ischia does not stay on that part of the ring where the maker hopes it will, but gravitates to the lowest level of the ring ; this leads to the very common error of internal rotation of the whole limb (in-toeing), because the patient finds the position of greatest comfort by internally rotating the splint to bring the dip in the ring to lie under the tuber ischia.

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(3) Proprietary II - This is probably the next most common of the commercial shapes current in England. As in the preceding type, the ring is at first glance a simple ovoid with the sidebars mounted slightly behind the transverse diameter.

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The essential feature of this ring is that the posteromedial segment of the ring is higher than the anteromedial (fig. Ic). This is another attempt to raise the region of the tuber ischia above the general level of the ring.

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In this ring, as in the preceding, there is the same tendency for the tuber ischia to slide down the inclined surface of the ring to reach the lowest point, and thus for the splint to in-toe.

 

Some makers have tried to counter this tendency by fashioning a localised recess in the metal ring to retain the tuber ischia at the desired point. This well-intended notion is rendered useless when the padding is applied, in any case this ring requires careful and patient fitting not possible in a cheap appliance.

 

(4) Ischial Seat.-This is a local indentation of the ring at the posteromedial segment, seen only in the plan view, and intended to increase the purchase of the ring under the tuber ischia (fig. ld).

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At first sight this seems a commendable practice ; in fact, it is often of doubtful value. The fitting of the ring of a calliper to the tuber ischia is a far more difficult proposition than the fitting of an amputation prosthesis, because in the above-knee amputation the hamstring muscles, which pass vertically downwards from the tuber ischia, are extremely wasted, and often a visible ledge is offered under the tuber ischia for the reception of the prosthesis. But, in muscular young adults, the fitting of a calliper by true tuber-bearing is virtually impossible, and in these cases, in the standing position, the tuber ischia may be palpable only with some difficulty.

 

It is significant that Thomas never referred to the tuber ischia either by name or by implication in his description of 1890. He constantly referred to the fitting of the ring to the groin.

 

If a ring is made with a local indentation at the site of the tuber ischia, and if this ring is thrust under the tuber when the hamstrings are relaxed, contraction of these muscles will cause the ring to be rejected from under the artificial ledge (fig. 2).

 

It appears, then, that the tuber ischia, in the case of the weight-relieving calliper takes only a fraction of the body weight. A large part of the weight is efficiently taken on the ring through the fibro-fatty fold of the buttock and the lower border of the gluteus maximus when in contraction. The tuber ischia merely offers an important landmark for the accurate fitting of the ring.

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(5) Triangular.-The shape of this ring is roughly three cornered when seen in plan-view. The base of the triangle lies in the perineum, and the apex at the great trochanter.

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The abductor tendons lie in the anteromedial corner, the tuber ischia at the posteromedial corner, and the great trochanter at the lateral corner (fig. le).

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This shape conforms with standard practice in the design of amputation prostheses. In use these rings present no outstanding advantage over the simple ovoid ring ; the common site of maximal tenderness in a calliper-the region of the abductor tendons-is not notably abolished. Unless the ring is very carefully fitted the tuber, ischia falls through the posteromedial corner.

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(6) British Army Adjustable Calliper.-This splint (fig. If) was designed by me and accepted as the standard Army calliper by the War Office to fulfil a special purpose-i.e., the fitting of large numbers of adult patients at low cost and with elimination of delay by the use of stock parts.

 

The ring is a simple ovoid, and the essential feature is that the circumference can be adjusted to fit the root of the limb.

 

I believe that better weight-relief can be obtained with a simple ovoid ring which fits the circumference of the thigh closely than with any anatomically designed ring which fits the root of the limb rather loosely.

 

In commercial callipers (the vast majority for hospital patients being delivered from stock) the happy combination of anatomical shape and correct fit is almost too rare to be seriously considered. In my opinion contemporary calliper rings are usually too loose; a perfect ring can only be drawn to the root of the limb with a slight effort, though it must not constrict. -

 

Rings Deviating from Thomas Type

 

None of the callipers in this group possesses the intact rigid metal ring which has been taken as the characteristic of the preceding types.

 

(1) Block Leather (fig. 3a).-The weight-bearing surface of this pattern is a moulded leather corset like that of an amputation prosthesis. It offers the most perfect anatomical ring but obviously is more expensive and difficult to manufacture than any of the preceding patterns.

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This ideal ring has its own sphere of usefulness for which there is no substitute and is often needed when all other types have failed or proved uncomfortable. For this reason, no further reference will be made to this design.

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(2) Half Ring with Soft Front.-In this type the metal ring is incomplete in front and the gap is bridged by a leather strap and buckle (fig. 3b).

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This ring is sometimes used when an intact metal ring will not slide past a deformity in the thigh. Some prefer this ring because it is comfortable, and the patient can easily apply and remove it without assistance.

 

The weight-relieving function of this ring is usually considered in England to be inferior to that of the intact metal ring, though it is difficult to assemble convincing scientific facts to support this claim. It is, perhaps, unfair to say that the patient may be tempted to slacken the strap to secure relief from discomfort without consent of the surgeon. The dogmatists are satisfied that Thomas experimented with several modifications of this type but discarded them all in favour of the -intact ring.

 

In practical tests I have found that the soft-fronted ring seems to encourage flexion of the hip, which allows the tuber ischia to slip off the ring  posteriorly ; the intact ring appears to offer a much more certain weight-bearing surface.

 

In cases where weight-relief is not the primary function there can be little doubt of the superiority of the half-ring.

 

The commonest use of this ring would therefore be in polio myelitis, where the calliper is used merely as a long knee-cage to stabilise the knee with deficient quadriceps muscles.

 

(3) Galland (1936) Ischial Seat.-This type is not seen in England: it consists principally of an oval pad accommodated under the tuber ischia and dispensing with a true ring (fig. 3c).

 

(4) Perineal Crutch.-This is essentially a padded leather strap hanging in a loop from a fixed point on the rigid splint in the region of the great trochanter. The dependent loop passes under the perineum (after the fashion of the groin strap in the Jones abduction frame), and the efficacy of the  weight-relief can be adjusted by the tightness of the strap (fig. 3d).

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This principle was followed in the now obsolete splints used for the ambulant treatment of tuberculous disease of the hip and knee by traction.

 

 THE THOMAS CALLIPER

 

Variations in the shape of the " authentic " Thomas ring after the death of its inventors are of great significance because they cast doubt on the true value of many subtleties of ring design which have been accepted without criticism.

 

McMurray (1943) describes the Thomas calliper in the following words:

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" The ring is not ovoid; the back of the ring is flattened, and the two lateral bars are attached behind the middle point of its circumference."

 

This would make the ring shown in fig. la that for the right side (McMurray 1946).

 

The original description by Thomas (1890) is ambiguous on this point, and it is impossible to determine for which side the illustrated splint was intended; nor can it be deduced from indirect evidence in the text.

 

The American surgeon John Ridlon, who was a close friend of Thomas and spent some time in Liverpool studying Thomas’s methods at first hand, described the calliper in the Transactions of the American Orthopaedic Association of 1893 in the following words and used the same woodcuts which illustrated Thomas’s article :

 

" The ring is an irregular ovoid, flattened in front, and drawn out at the posterior and inner portions.... The ring slopes from without inward, and from before backwards in such a way that the point ... upon which rests the tuber ischia is the lowest part of the ring."

 

This indicates that the ring shown in fig. la is that for the left side.

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Further evidence to support Ridlon’s description as the one Thomas intended has been encountered in a patient, aged 75, seen at the Shropshire Orthopaedic Hospital under the care of Mr. H. Osmond-Clarke. This patient had his original calliper applied at the age of 4 years by H. 0. Thomas, and subsequent changes of splint were made by Robert Jones. The present calliper (fig. 4) had been used continuously for over fifty years.

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The sidebars were in front of the transverse diameter, the ring was flat in front, and the lowest part of the ring was posterior.

 

Finally, an inspection of the shape of the human thigh leaves no doubt that Thomas was adapting the position of the sidebars to lie in the line of the shaft of the femur. Fig. 5 is a tracing taken from a photograph showing that the line of the femur lies at the junction of the anterior and middle thirds of a horizontal line from the front of the thigh to the gluteal fold just below the tuberischii.

 

The posterior position of the sidebars, which is more common in current designs, is probably a better mechanical design than the anatomical splint because the body-weight is taken more directly in the line of the sidebars and a lighter splint can be used with equal stiffness.

 

However, it is interesting to observe the principle that the position of the sidebars in relation to the ring is purely arbitrary, and that the splint will function "back to front " within wide limits.

 

From such evidence it is clear that experienced surgeons have used this splint successfully (1) with the front higher or lower than the back, (2) with the front or the back flat, and (3) with the sidebars attached in front or behind the transverse diameter. These facts support my opinion that adequate weight-relief can be obtained by a slim, ovoid ring, provided that the circumference of the ring fits closely and comfortably the circumference of the root of the limb.

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Inclination of Ring.- Like ring shape, the angle of inclination of the ring appears to be arbitrary.

 

Theoretically, Thomas’s pattern, with the tuber ischia at the lowest part of the ring posteriorly, is sound: but in practice this renders the ring very painful at the higher point near the pubis. The high posterior and low anterior ring suffers from the defects described under Proprietary II. For this reason, the ring is best placed horizontally in the anteroposterior plane.

 

Current patterns vary in steepness of inclination from without inwards, from the very steep ring of Thomas (45 deg above the horizontal) to the almost flat ring. Some inclination seems to be an advantage as helping to retain the weight bearing surface towards the inner half of the ring. If the ring is as steep as 45 deg, it tends to cut into the pubis and to cause pain in its steep upward and outward curve at this point. For these reasons I prefer an inclination of not more than 30 deg. above the horizontal.

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Heel Socket.-The ideal site for the heel socket would be concentric with the axis of the ankle joint.

 

Ankle movement would thus be possible without any displacement of the lower end of the tibia forwards or backwards (fig. 6).

 

In default of the concentric axis the common ideal is to place the socket vertically below the axis of the ankle joint. In an ordinary shoe this point lies in the leading edge of the heel, and therefore the posterior position of the socket, Fig. 5-Diagram showing that line of femur lies at junction of anterior and middle thirds of horizontal line from front of thigh to gluteal fold just below tuber ischia. as usually used, lies some 1/2-3/4 in. behind the vertical.

 

Practical tests have shown me that the wearer of a calliper is unable to detect variations in the position of the socket from as much as 3/4 in. in  front to 3/4 in. behind the vertical. The anterior position of the socket is superior so far as ease of fitting is concerned and has the advantage that it lies in the line of the centre of gravity, which passes anterior to the ankle joint.

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Of far greater importance than the mere site of the ankle socket is the decision whether to make it mobile or fixed. In the treatment of delayed union of the bones of the leg (to which the calliper has an important place if correctly used) a very common mistake is seen in the use of a mobile heel socket. Ridlon (1891) insisted that in this condition the ankle joint must be fixed. It can be appreciated from fig. 6 how a mobile ankle socket not concentric with the axis of the ankle-joint must allow considerable angulation at a tibial fracture.

 

With a fixed heel socket a well-fitting calliper splint allows the delayed union to take longitudinal stress protected from lateral angulation just as adequately as any plaster cast and with the advantage that the nutrition of the limb is superior in a calliper to that of a limb encased in plaster.

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TWO PRACTICAL HINTS

 

A weight-relieving calliper is never comfortable before the skin of the groin has become accustomed to the presence of the ring. Two practical hints, trivial in themselves, can sometimes determine the difference between success and failure.

 

The first point is the raising about 3/4 in. of the heel of the good leg. Remarkable benefit can often be derived from this simple aid.

 

The second point lies in instructing the patient to learn the trick of sitting back on to the ring, thus keeping the hip extended. Patients who complain of intolerable pain in the region of the abductor tendons in the groin will usually be found walking with the trunk sloping forward and with the hip in slight flexion.

 

SUMMARY

 

The variations in calliper design are described, and deductions drawn from them.

 

The calliper is much more useful than plaster in the later stages of fracture treatment.

 

I wish to thank Prof. Harry Platt for his advice and criticism during the writing of this paper, and Prof. T. P.McMurray, with whom it has been discussed. The experimental work was done with the  generous facilities offered by the R.E.M.E. of the M.E.F. and was only made possible by the interest shown by Brigadier D. C. Munro, consultant surgeon to the Army, and by Mr. Rowley Bristow and Mr. Philip Wiles, late orthopaedic consultants to the Army and the M.E.F. respectively.

 

REFERENCES

 

Galland, W. I. (1936) J. Bone Jt Surg. 18, 790.

 

McMurray, T. P. (1943) A Practice of Orthopaedic Surgery, London; (1946) Brit. med. J. i, 872.

 

Ridlon, J. (1891) Med. Rec. 39, 130.

 

Thomas, H. O. (1890) Contributions to Surgery and Medicine, part VII, London.