A standard test for laboratory animal bone

A standard test for laboratory animal bone

J Bl~~nischonicr.Vol.4.pp. 155-158.PergamonPress. 1971. RintcdinGreaI Britain TECHNICAL A STANDARD TEST FOR LABORATORY INTRODUCTION have recognize...

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J Bl~~nischonicr.Vol.4.pp. 155-158.PergamonPress. 1971. RintcdinGreaI

Britain

TECHNICAL A STANDARD

TEST FOR LABORATORY

INTRODUCTION have recognized the need to determine the mechanical strength of long bones for more than a century. (Wertheim. 1847: Rauber, 1876). These tests have been conducted not only for the purposes of determining the normal mechanical strength of bones. but also for purposes of determining the strength of bone as an end point in experimental biochemical and endocrinological programs. The need for such tests is especially prevalent when investigating such phenomena as fracture healing, dietary, metabolic. or endocrine alterations in animal bone. Since the variety of possible mechanical tests to which long bones may be subjected is virtually infinite, it has been difficult. if not impossible. for researchers to compare data on the strength of whole bones obtained by many of the previously used techniques. In addition to variations in test configurations, variation of test duration will lead to significant changes in the observed results. Studies have been undertaken to show the variation of whole bone and bone tissue properties with respect to loading rates and strain rates. (Smith and Walmsley. 1957: McElhaney and Byars. 1965; But-stein and Frankel. 1968; Sedlin. 1965; Sammarco et al.) These studies have all demonstrated the great variability in strength and rigidity characteristics that can be produced by variation of the time factor in testing. For example, human tibias can absorb 45 per cent more energy when broken at strain rates equivalent to trauma than when the bones were broken over a period of several minutes (Frankel and Burstein, 1965). Data which do not include controlled loading rates cannot be reproduced or be evaluated with respect to normal and traumatic loading rates on bones. The purpose of this paper is to describe the technique and instrumentation for a proposed standard method of mechanical testing of long bones. INVESTIGATORS

METHOD A study was undertaken to determine the most suitable configuration of loading of the bones. Five criteria were chosen against which each loading configuration would be judged. These criteria were: ( 1) The loading configuration should produce fractures similar to clinical fractures. (2) The loading configuration would have to subject the bone to equally severe loading conditions at every section along its length. so as to be able to identify weak sections. (3) The loading mode must not be critically dependent *Received

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NOTE ANIMAL

BONE

upon bone geometry, in particular bone length. in terms of the severity of its effect. (4) The loading configuration must allow control of the rate of application of bads, so that the time conditions produced in the test will be reproducible and preferably representative of those of normal trauma. (5) The loading configuration should result in a test apparatus which is relatively inexpensive and may be operated by persons with only ordinary skills in handling such mechanical equipment. Axial compression or tension as a loading mode is not satisfactory as it does not meet criteria 1 or 5. While long bones are certainly loaded in axial compression. they seldom fail because of this loading. A bending loading configuration using two supports and a single load point is not satisfactory. since it fails to meet criteria 2 or 5 (Fig. 1). In this loading configiiration. I

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Posltlon MAXIMUM STRESSES

Position MAXIMUM STRESSES

Fig, 1. For a beam with a central load and two supports. the bending moment varies from zero at the ends to a maximum at the point of load application. When two symmetrical loads are applied the moment is constant between the loads. The value of the moment is. however. dependent upon the bone dimensions.

1970.

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TECHNICAL

that section which is directly below the loading point is subjected to the maximum moment. All other sections are subjected to lesser moments of varying degree. If, instead of a single loading point, two symmetrically placed loading points are used. the region between the loading points is subjected to constant bending moment. However, this loading conftguration does not meet criteria 3 or 5. The only loading configuration which will satisfy all criteria is torsion. BIOM&cIuNICAL CONSIDBBATIONS The important mechanical parameters of bone as structural members are its strength and rigidity. Strength may be further divided into load capacity and energy storage capacity. Rigidity is a measure of the resistance to deformation under load. Torsional load capacity is an important functional characteristic of long bones. This quantity is defined as the maximum amount of torque which may be applied axially to the bone before fracture occurs. Load capacity is measured in cmdyn or in.-lb. Rigidity is the ratio of the applied torque and the resulting angular deformation of the bone. Rigidity is expressed as cm-kg,/deg.* A convenient method for determining these two parameters, is to plot the angular deformation as a function of the applied torque (Fig 2). If such a curve is produced up to the fracture point, then the load strength and rigidity can be determined. The area under the curve represents the energy absorbed by the bone before failure.

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NOTE ranged from 35 to 1000 cm-kg,. Using the same apparatus, the maximum angular deformation suffered by each bone before fracture occurred was measured. The maximum values were found to be approximately I S-25”. Calculations were performed to determine the loading interval normally encountered in traumatic injuries. Cases of ski fractures, bumper fractures, and similar incidents were studied. Using the velocity of impact and the rigidity of the bone, time durations for loading of between 0.05 set and O-IOset were obtained. INSl’BtJMBNTATION The torque transducer chosen for this design was Reaction Torque Sensor Model 2 121; 1000 in-lb capacity Manufactured by: Lebow Associates Oak Park, Michigan. This transducer is one of a family of transducers with torque capacities from 3.5 to 350,000 cm-kg,. Thus, with slight modification to the design. a broad range of specimens can be tested. The I I50 cm-kg, model was chosen for the prototype, since it allows testing of animal bones ranging in size from rabbit to large dog bones. This transducer requires either a well regulated d.c. voltage supply and a dc. amplifier, or a cartier wave amplifier to obtain an output voltage proportional to the input torque. The transducer will deflect only ,OG” under full torque input. Therefore, it will not significantly affect the stiffness measurement of the bone specimens. The angular displacement transducer is an Angular Position Transducer Model 33-03 Brush Metripak Manufactured by: Brush Instruments Cleveland, Ohio.

Fig 2. The torque-angular deformation curve displays most of the mechanical properties of the torsionally loaded bone. The maximum torque capacity, maximum angular deflection, stiffness (slope of curve), and energy absorption may be obtained. Preliminary studies were undertaken to determine the design criteria which must be met by the torsional load apparatus (Franked and Burstein, 1965). A dead weight torsional loading device was constructed. in which common laboratory animal long bones and human bones were tested. Torques required for fracture *A kilogram force (kg,) is the weight of a I kg mass.

The range ,of ‘this transducer is & 20” (40’ total) with & IS” (36” total):of linear motion. This transducer oroduces a high level ‘d.c. output proportional to-its angular displacement, and requires only I IOV ac. input. It has extremely low inertia and is therefore suitable for measurements at high angular acceleration. The area under the load deflection curve, which represents the energy to fracture, may be obtained with either electronic or graphical integration. The output data from the torsion tester is in the form of a plot of torsional load vs. angular deformation, up to the point of fracture (Fig. 3). Angular deformation vs. time is also available in graphical form, so that loading rates may be calculated. The torsional testing machine is compatible with common recording instrumentation. The tester is compatible with a cathode ray tube oscilloscope. The oscilloscope may be of the plug-in vertical driver type, with one general purpose a.c.-d.c. vertical amplifier and one carrier amplifier. The resulting design of the testing machine is shown in Fig 4.

. Fig. 3. Data is obtained in this graphical form.

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TECHNICAL

NOTE

Fig. 4. The components of a Standard Torsion Testing machine are: B. Tail stock: C. Pendulum; D. Dog clutch in pendulum; E. Dog clutch shaft: F.* Rotating grips; G. Angular deformation transducer; H.* grips; I.* Torque transducers; J.* Self calibration system:. *Two sizes A pendulum loading system was chosen since it requires no external power input. This eliminates the most expensive feature found in commercial testing machines and at the same time guarantees the compatibility of the testing machine in any laboratory. The pendulum forces an angular deformation in the specimen, which in turn develops a resistive torque. The deformation is imparted about a fixed axis, but the distribution of stress, and hence the direction of the moment, is determined by the bone geometry. This type of test is highly repeatable since identical geometric distortions are forced in each bone. Specimen deformation occurs only after the pendulum reaches a position near the bottom of its swing. The time duration over which the deformation is applied is controlled by the length of the pendulum, since the maximum angular defiection of the specimen is fixed. Actually, the deforming time varies inversely with the square root of the pendulum length. To allow the pendulum to traverse a 30” arc at the bottom of its swing in a time of 0.075 set, a 23 cm pendulum length is required. The weight of the pendulum was chosen by noting how much energy was needed to fracture the largest bone. A value of 140 kg,. cm of energy is felt to he conservative, since this is the

A. Frame; on rotating Stationary shown,

energy strength of large dog bones. In order to keep the deforming velocity nearly constant, the pendulum was designed to weigh 14kg,. Since it displaced a total of 46 cm, it would develop 644 kg,.cm maximum kinetic energy at the lower portion of its fall. It would therefore be able to fracture animal bones with the loss of less than 22 per cent of its energy. Since the energy loss is related to the square of the velocity decrease, velocity should remain constant to within 6 per cent of its average value. This variation would be expected only on the largest dog bones. Rabbit bones would cause only 2 per cent variation of velocity. Thus. reasonably constant strain rates would be achieved. The tailstock. which houses the torque transducer, may be moved to accommodate various size bones. When smaller bone sizes. such as rat bones, are to be tested an adjustment is also provided on the pendulum grip (Fig. 4). Gripping is achieved by bedding the condyles in epoxy. (Plastic steel. Devcon type B), acrylic, (Acralite, Acralite Company, Incorporated) or any stiff potting material. A tapered gripping device is provided on both the torque transducer and the pendulum to eliminate free motion while the bones are loaded. Various size grips

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allow the testing of all laboratory animal and cadaver specimens. A built-in calibration device is provided which electronically simulates predetermined values of torque. This alleviates the need for calibration using weights.

Acknowledgemenrs-This research was supported in part by a grant from the Orthopaedic Research and Education Foundation. Biomechanics Laboratory Division of Orthopaedic Surgery Case Western Reserve University Cleveland Ohio 44106, USA.

A. H. BURSTEIN V. H. FRANKEL

,REFERENCES Burstein, A. H. and Franked, V. H. (1968) The viscoelastic properties of some biological materials. Ann. N. Y. Acad. Sci. 146.158-165. Frankel, V. H. and Burstein, A. H. (1965) Load Capacity

NOTE of Tubular bone. Biomechanics and Related BioEngineering Topics, (edited by R M. Kennedi) Chap. 32.381-396, Pergamon, Oxford. McElhaney. J. H., and Byars. E. F. (1965) Dynamic response of biological materials. Am. Cot. Mech. Engineers Pub; 65-WA/HUF-9. Rat&%. A. (1876) Elasticittit und Festigkeit der Knochen, Engehnann. Leipzig. Sammarco. G. J., But-stein, A. H., Davis, W. L. and Frankel, V. H. (1971) The biomechanics of torsional fmctures: The effect of loading on ultimate properties. J. Biomechanics4,113-117. Sedlin, E. D. (1965) A rheologic model for cortical bone. a study of the physical properties of human femoral samples, Acta orthop. stand., Suppl. 83. Smith, J. W., and Walmsley, R. (1957) Elastic aftereffect, plasticity and fatigue in bone., J. Anatomy 91. 603-604, 1957. Wertheim. M. G. (1847 Memoire sur l’elasticite et la cohesion des principaux tissus du corps humain. Annls Chim. Phys. 21.385-4 14.