The cryo-jaw, a clamp designed for in vitro rheology studies of horse digital flexor tendons

The cryo-jaw, a clamp designed for in vitro rheology studies of horse digital flexor tendons

TECHNICAL NOTE THE CRYO-JAW, A CLAMP DESIGNED FOR IN VITRO RHEOLOGY OF HORSE DIGITAL FLEXOR TENDONS* STUDIES Abstract-A clamp designed for holdin...

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TECHNICAL NOTE

THE CRYO-JAW,

A CLAMP DESIGNED FOR IN VITRO RHEOLOGY OF HORSE DIGITAL FLEXOR TENDONS*

STUDIES

Abstract-A

clamp designed for holding tendons in forceieiongation studies is described. No slippage occurred when tensile forces up to 13,800N were applied to horses digital flexor tendons fixed in this clamp

INTRODUCTION

Essential for in r+rro rheology studies of tendon is the availability of a clamp which can hold the tendon rigidly at high loads without damaging it. The great variety of designs published in previous papers reflects the difficulties in this respect (Abrahams. 1967; Arnold. 1972; Benedict er al., 1%8; Cohen et al., 1976: Elliott, 1967; Partington er al., 1963; Viidik, 1967). These difficulties arise from the very low friction between the material of the devicesand wet soft collagenous tissues. This requires a special device to prevent the tendon from slipping out of the clamp when loaded. One of the solutions is to compress the ends in the clamp; however, the tissue is then markedly deformed and rapidly damaged. An additional disadvantage of this solution is that the initial length of the load-bearing fibres will be changed in different ways for different fibres. Friction will be increased by using serrated jaws. However, high compression remains necessary and this results in the transection of many collagenous fibres. The large deformations produced on the ends of the tendons by compression can be reduced by entwining the tendon ends with thread or copper wire (Benedict et al., 1968; Cohen et al.. 1976; Elliot, 1967). In such devices however, the central fibres may not fully contribute to load bearing. The use of the bony attachment of tendons andligaments forms a good alternative, for bones can be fixed easily in several ways (Alm er al.. 1974; Lochner et a/.. 1980; Viidik. 1969). Unfortunately there are many experiments with tendons where the bony attachment is missing, at least on one side. In the present paper a clamp is described by which an excellent grip is achieved without major deformations or damage of the tendon by freezing the clamp and the clamped part of the tendon. High loads can be applied by this device so that the tendons are disrupted but do not slip from the clamp. DESCRDWON

The clamp. as presented in Fig. 1, is made of brass. which ‘Receirzed for publication

1 March 1982. 619

has a high heat capacity and is easy in working up. Two indented plates (A and B), between which the tendon is placed, can be pressed together with four MS screws (Fig. 1). The indentations are 1 mm deep and 2mm wide with interstices of 3 mm. Plate B forms one wall of a cavity (C) which is closed at the opposite side by a plate (D) with eight M4 screws. Plate D has two openings (E) with diameters ot 4mm. An Ml6 plug (F) with a narrow canal (OSmm diameter) is screwed into plate D. Freezing temperatures are achieved by expansion of liquid CO* in compartment C. Liquid CO, enters the compartment (C) through the narrow canal in plug F and waste CO2 gas and snow can escape via the openings (E) in plate D. The clampcan be connected with a draw bench by an Ml8 screw-threaded steel bar, inserted at G.

RESULTSANDDISCUSSION The major problem in clamping a fresh tendon is the low friction between the tendon and the surface of the jaw. Therefore high pressure is applied in order to obtain enough friction to prevent the tendon from slipping out of the clamp. As a result of this pressure water is squeezed out of the compressed tendon and this forms a water film which decreases friction. High compression further causes a large deformation of the clamped part of the tendon. As a result of this deformation the initial length of the 8bres will no longer be equal. The load will therefore be distributed unevenly over all fibres. Moreover. friction between mutual collagen fibres is probably lower than friction between superficial fibres and the clamp. This might result in unnoticed slipping of the inner tibres. The outer fibres are thereby more loaded and therefore more stretched than the inner ones. Force/ elongation curves which are unrepresentative of the whole tendon will be the inevitable result. In addition. the irreversible displacement of the inner fibres will cause a restitutional elongation at zero load after a cyclic extension. When deformation is reduced by entwining the ends of

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Technical Note

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Fig. 1. The figure presents the cryo-jaw in lateral view. front view and back view successively. Dimensions are given in mm. For explanation of the symbols, see description in the text. the tendons with thread or wire, the possibility of slipping of the inner flbres will increase, since the contact area between clamp and fibres decreases in this way. In our clamping device these difficulties have been overcome. Initially only moderate pressure is applied in order to get an impression of the indentations of the plates into the clamped part of the fresh tendon. Since the initial pressing force is relatively low, the deformation of the tendon by the pressure of the plates is only moderate. Then the clamp and the clamped part of the tendon are frozen. After freezing the compressive forces are increased without increasing the deformation of the tendon visibly. Since the tendon is frozen up to the undeformed zone a few mm from the clamp. it is assumed that the initial length of the libres is not disturbed by this deformation. Furthermore, displacements of the inner fibres of the frozen tendon with respect to each other can be neglected. Thus tensile forces are likely to be evenly distributed over all fibres in the remaining fresh part of the tendon. The frozen tendon part is not easily damaged by compressive forces and the frozen profile of the tendon is gripped tightly in the jaws of the clamp. Cutting of the fibres is prevented by carefully rounding off all edges of the jaw borders and the indentations. It is essential to prevent thawing of the clamped part of the tendon during experiments, since then slippage from the clamp will occur at loads above 3000N. When slippage occurs, the tendon will be damaged locally and rendered unsuitable for further experiments. To prevent thawing, good insulation of the clamp is indispensable and a temperature indicator may be helpful. The major part of the tendon remains at room temperature, since the temperature gradient of the tendon near the clamp is very steep. Room temperatures were already reached at 6cm from the clamp. Therefore freezing a part of the tendon will not interfere with rheology measurements at sites on the tendon above 6cm from the clamp. Ultimate loads from 8200 N to 13,800 N with an average of 10,200 N were applied to fourteen digital flexor tendon preparations of horses. At these loads the tendons were ruptured or the phalanges, used to fix the dista1 end of the tendon, fractured, but slippage did not occur. Such tremendous forces could not be achieved with previous devices.

Acknowledgements-The authors wish to acknowledge the facilities and helpful advices they received at the Instituut T.N.O. voor Bouwmaterialen en Bouwkonstrukties, Rijswijk, The Netherlands. Department of Veterinary Anatomy State University of Utrecht Yalelaan 1 Utrecht The Netherlands

D. J. RIEMERSA H. C. SCHAMHARDT

REFERENCES Abrahams, M. (1%7) Mechanical behaviour of tendon in vitro. Med. biol. Engng 5, &33-443. Alm, A., Ekstrom, H. and Stromberg, B. (1974) Tensile strength of the anterior cruciate ligament in the dog. Acta chir. stand. Suppl. 445, 15-23. Arnold, G. (1972) Mechanische Eigenschaften von Sehnen. Verb. Anat. Ges., Zagreb 1971,499-504. Benedict, J. V., Walker, L. B. and Harris, E. H. (1968) Stress-strain characteristics and tensile strength of unembalmed human tendon. 1. Biomechanics 1,53-63. Cohen. R. E., Hooley, C. J. and McCrum, N. G. (1976) V&o-elastic creep of collagenous tissue. J. Biomcchanics 9, 175-W. Elliott, D H. (1967) The biomechanical properties of tendon in relation to muscular strength. Ann. phys. Med. 9, 1-7. Lochner, F. K., Milne. D. W., Mills, E. J. and Groom, J. J. (1980) In vivo and in vitro measurement of tendon strain in the horse. Am. 1. vet. Res. 41, 1929-1937. Partington, F. R. and Wood, G. C. (1963) The role of noncollagenous components in the mechanical behaviour of tendon libres. Biochim. biophys. Acta 69,485-495. Viidik, A. (1%7) Experimental evaluation of the tensile strength of isolated rabbit tendons. Biomed. Engng 2, 64-67. Viidik, A. (1969) Tensile strength properties of Achilles tendon systems in trained and untrained rabbits. Acta orthop. stand. 40,261-272.