TECHNICAL NOTE

DIRECT IN C’IVO TENDON

FORCE MEASUREMENT

SYSTEM

KAI-NAN AN,*? LAWRENCEBERGLUND: WILLIAM P. COONEY: EDMUND Y. S. CHAO* and NEBOJSAKOVACEVIC~ l

BiomechanicsLaboratory.Departmentof Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, MN 55905. U.S.A. and $ NK BiotechnicalEngineeringCo.. Minneapolis,MN 55426, U.S.A.

lNTRODUClION

Control boom

(senrwig ~lrment) Several types of transducersused to measuretendon and ligament forceshave been described in the literature (Salmons. 1969; Yager. 1972; Barnes and Pinder. 1974; Bright and Urbaniak. 1976; Walmsley et al.. 1978; Hoffer et al.. 1981; O’Donovan ef al.. 1982; Lewis et 01.. 1982; Sherilet al.. 1983; Whiting et a/.. 1984; Abraham and Loeb. 1985). However, with most of these designs, multiple parts must be reassembled at each use or the transducer may slip olT during use. In addition, the nature of the clcctric bridge used dctcs not allow the effect of temperature to be easily compensated. A new improved tendon force transducer system has been developed. with these technical problems in mind, to measure tendon tension in vitro or in uiw. This report describes the design considerations and calibration of this new rorce transducer which is now commercially available through NK Biotechnical E .,&ering Co.. Minneapolis, Minnesota. U.S.A.

Fig. I. The tendon force transducer is an S-shaped stainless steel frame with four strain gages attached to the surface of the central beam to sense the strain generated by torsion.

METHOD AND MATERIALS Design of tendon force

transducer

The tendon force transducer was designed based on the principle of sensing primarily the torsional strain in the transducer structure. The structure of the transducer is an S-shaped stainless steel frame. I.4 cm long, 0.8 cm wide, and 0.5 cm high. A total of four strain gages are fixed on the side surfaces of the central beam (Fig. 1). On each side, two strain gages,perpendicular to each other, arc located on the neutral axis and oriented 45’ from the neutral axis. With the transducer placed on a tendon, the longitudinal tensile force along the tendon generates a torque on the central beam of the S-shaped frame. The relationship between the tensile tendon force and the amount of torque, T. generated depends on the OR-set.h. from the longitudinal axis of the tendon (Fig. 2). This amount of torque then deforms the central beam and is sensedby the strain gages.Therefore. the larger the o&set. the more sensitive the transducer is to measure the tendon force. In other words, lor the same transducer, the larger the tendon, the greater the sensitivity of the transducer measurement. Signal conditioner The transducer has four strain gageswhich form a full four active arms in a Wheatstone bridge. This full bridge arrangcment provides higher resolution and allows full temperature Receioed in jinaljorm I8 May 1990. t Author to whom corrcspondcncc should be addressed.

F

F

t

t f-F.rinB=F.h/l Torque

l

f 1’F. h

Fig. 2. The sensitivity of the transducer measurement is dependent upon the off-set. h. of the tendon from the longitudinal axis. compensation lor ‘xero shift’. Strain gage conditioners, designed with optical isolation using the technology of photon coupler for patient safety, are used for gage excitation and signal amplification. The amplifiers are quipped with gain and o&et adjustments. For in uiuo application, sterilization or the transducer can be achieved by dry gas at 175” F maximum. Measurement

of tendon si:e

As described, the output of the force transducer is primarily a function of the tensile force in the tendon, but it is also related to tendon size (diameter or thickness). For in uiuo 1269

1270

Technical Note

testing it is important that the method to meaxure crosssectional area be rapid and simple. With this in mind, mcthcds were developed to measure the thickness and the cross-sectional area of the tendon to be studied. A spring loaded micron-meter was modified for the measumnent of tendon thickness.The tendon to be measured was squeezed between two plates 5.5 mm wide [Fig. 3(a)]. Throughout the range of possible tendon thickness mcaxuremcnt, an approximately log force was applied between these two plates. The method for tendon cross-sectional area measurement is adopted from that of Dr Paul W. Brand (Carville. LA). This device works on the principle of utilizing a rotational potentiometer to determine the space occupied by the tendon and is

spring Ioadcd to produce more consistent

of the force

REStJLlS

compression

(usually about 150 g throughout the possible range of mcasurcment) on the tendon during measurement [Fig. 3(b)]. The change in electrical resistance of the potentiometer from the closed position and with a tendon in place is simply measured with a digital ohm-meter. Calibration

load on the tmdoo the transducer signal output OK-set was zeroed. Tensile load was then applied at 1.5 lb s - 1up to 30 lb while tensile load VI transducer output was recorded. The tendons were kept moist with saline throughout all handling Amplification oftht transduar output signal was fixed at go0 for all the tests. Reproducibility of the force transducer measurement was also examined by repeated calibration tests following removal and replacement of the transducer. In addition, the eKect of tendon creep on the force measurement was also studied in the calibration test under a constant lbad of 15 lb for five min.

transducer

In order to determine the conversion factors of this newly designed Sshaped tendon force transducer and to undcrstand the characteristics of the transducer related to tendon size, fresh frozen human flexor tendons of diKerent size were used. Ten tendons were harvested at the wrist level from uncmbalmed specimens. Each tendon was numbered and the thickness and cross-sectional area were measured in the central portion of the tendon where an S-shaped force transducer was applied. The ends of the tendons were then clamped in the specially designed soft tissue fixtures on a material test machine. The tendon force transducer was placed on the tendon in the middle portion. With no tensile

Tendon size measurements Theoretically, the conversion factor of the tendon force transducer should be influenced by the size of the tendon. Two methods were thus developed to index the size of the tendon to be studied. For the group of Kexor tendons studied, it was found that tendon thickness measurements correlate well with cross-sectional area measurements (r=O.9). It implies that either method can be used for determining tendon size for calibration purposes. Tendon force

transducer

calibration

A typical curve generated from tensile force on the tendon vs the transducer output is shown in Fig. 4. All the curves are extremely linear, with no hysteresis. The zero shifts among each test averaged less than 0.7%. The data of applied force and transducer output are thus fitted to a line by using linear regression analysis. In general, the curves all fit well with correlation coefficients greater than 0.995. The slopes of the linear regressions give the calibration conversion factors for the force transducers in units of NV”. The calibration curve raults for each transducer are very reproducible. In fact, if the tests are repeated with the transducer left in place, the curves coincide exactly. On the other hand, if the transducer is removed, replaced, and the test repeated. the diKercnces in conversion factors averaged less than 2 %. In the creep test. it was found that the force transducer output signal decreased slightly with time but averaged less than 2 % throughout the time period of the test. From results of the ten tendons tcsted. the conversion factors of the tendon force transducers were established based on tendon size for the range of tendon force tasted

Fig. 3(a). Device for the measurement of tendon thickness.

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2

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Tronrducrr Fig. 3(b). Device for the measurement of tendon crossrctional area. The tendon is placed in the curved slot, and the area is indicated by the rotational potentiometer. The amount of compression on the tendon is controlled by the spring at the center of the device.

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.

4

output(V)

Fig. 4. Typical calibration curve of the load applied on the tendon from a material testing machine (vertical axis. N) and the output from the tendon force transducer (horizontal axis, V) shows a linear characteristic throughout the range of the , . .. . tendon Iofce ot Interest.

1271

Technical Note

2

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CFIN/V1’184-194

.CR(mm)\

Tendon ~IZOby cal~prr rradmgtmml Fig. 5. The conversion factor of the force transducer is related to the size of the tendon. (Fig 5). It is obvious that the larger the tendon size. the more sensitive the force transducer (i.e. more signal output for the same amount of tendon force). Again the relationship is quite linear. Linear regression analysis was then performed. providing a correlation coefficient of 0.9. Tbe regression analysis provided a correction of the conversion factor as a function of tendon size.

DISCUSSION The general design principles of the S-shaped tendon force transducer system presented in this report are, in fact, quite similar to those used for belt tension measurement in auto mechanics. Baskally, they involve converting the longitudinal tension into transverse load which can then be measured. This concept has been used in the past in the field of biomechanics for both tendons and ligaments. This particular tendon force transducer was designed with the following objectives in mind. The transducer had to be a one-piece structure, easy to use. allow secure attachment or plaamcnt on the tendon, and provide optimum resolution. In addition, in a controlled laboratory setting, the transducer should have. at least, a well established performance of linearity, hysteresis, repeatability, and zero shift and zero gain due to load and temperature. There are some fundamental structural and mechanical characteristics common to both the current design and earlier designs. In general, it is transferring mechanical energy into electrical output. By generating known stress on a structural element on which a strain gage is attached, one can utilize either the normal stressdue to uniaxial or bending loads or the shear stressdue to normal or torsional loads. In a given design, one stress becomes primary and the other non-primary, both being present most of the time during the loading of a transducer where a strain gage is attached. In a comparison of the typical mechanical characteristics of transduars, the primary load ofthc earlier design was the bending moment and the non-primary load was shear. On the other hand, the current design is based on the concept of primarily the torsional moment in the presena of shear and bending moment. The primary strain to be sensed is induced by the torsional load on the central beam, thus allowing the application of a special configuration of strain gages to be utilized as four active arms in a Wheatstone bridge. This is one of the major advantages compared to the traditional design. Electrically, there are numerous advantages with the full active Wheatstone bridge used in the current transduar which could not be so easily implemented in the earlier design. The full bridge not only provides higher resolution but also allows better temperature compensation for ‘zero shift’ than half and quarter bridges. It makes calibration of the transduar and experiments more repeatable and accurate. With such a design, insulation and protection of the

electronic components are better achieved. Mechanically. the one-piece structure makes installation of the transducer in test specimens simpkr and more reliable for calibration and analysis. The S-shaped design enables the transducer to be more securely attached to the tendon without slippage during use compared to some of the earlier E-shaped designs. it should also be remembered that when using this type of force transduar. it is necessary to have the tendon bent to fit the transduar. Theoretically, the larger the bend. the larger the olT-set. and thus, the more sensitive the transducer. However. with a larger bend of the tendon. there will be more disturbance of the tendon length. Depending on the relative disturbana of tendon length, various degrees of its etkct on the physiological nature of the tendon system will be encountered. An important example of this would be consideration of the length-tension relationship. In fact, this problem is more severe in ligament applications because of their relatively shorter length. Although good correlation of the transduar conversion factor as a function of tendon size has been obtained, for practical in vitro and in oiuoapplications of the transducers. if possible, in siru calibration is encouraged in order to make the measurement more precise. Furthermore. in the application of such transduan. impingement or abutment by neighboring anatomic structures should be minimized. Otherwise, the effect on the output signals due to such environmental conditions should be identified and isolated. Acknowledgement-This AR 17172.

study was funded by NIH

grant

REFERENCES Abraham, L. D. and Loeb. G. E. (1985) The distal hindlimb musculature of the cat. Patterns of normal use. Expl Bruin Res. S& 580-593. Barnes, G. R. G. and Pinder, D. N. (1974) In uiuo tendon tension and bone strain mcasurcmcnt and correlation. 1. Biomrchonics 1, X-42. Bright, D. S. and Urbaniak. J. B. (1976) Direct measurements of flexor tendon tension during active and passive digit motion and its application to Hcxor tendon surgery. rrons. ?2nd A. Aftg Orthopedic Rrs. Sue.. p. 240. Hoffer. J. A.. O’Donovan. M. J.. Pratt, C. A. and Loeb. G. E. (198 I) Discharge patterns of hindlimb motoncurons during normal cat locomotion. Science 213.466-468. Lewis. J. L.. Lew. W. D. and Schmidt, J. (1982) A note on the application and evaluation of the buckle transducer for knee ligament fora measurement. J. biomed. Engng 104, 125-128. O’Donovan. M. J., Printer. M. J., Dum. R. P. and Burke, R. E. (1982) Actions of FDL and FHL muscles in intact cats: functional dissociation between anatomical synergists. J. Neurophysiol. 47, I 126-l 143. Salmons. S. (1969) Report on the 8th International Confcrcna on Medical and Biological Engineering. Biomed. Engny 4.467-474. Sherif. M. H.. Gregor. R. J.. Liu. L. M., Roy, R. R. and Hager, C. L. (1983) Correlation of myoekctric activity and muscle force during selected cat tradmill locomotion. J. Biome&attics 16.691-701. Walmsley. B.. Hodgson. J. A. and Burke, R. E. (1978) Forces produced by medial gastrocncmius and soleus muscles during locomotion in freely moving cats. J. Neurophysiol. 41, 1203-1216. Whiting. W. C.. Gregor. R. J., Roy, R. R. and Edgerton, R. V. (1984) A technique for estimating mechanical work of individual muscles in the cat during treadmill locomotion. 1. Biomechanics 17, 685-694. Yager. J. G. (1972) The electromyogram as a predictor of muscle mechanical response in locomotion. Ph.D. dissertation, University of Tennessee. Memphis.

Direct in vivo tendon force measurement system.

TECHNICAL NOTE DIRECT IN C’IVO TENDON FORCE MEASUREMENT SYSTEM KAI-NAN AN,*? LAWRENCEBERGLUND: WILLIAM P. COONEY: EDMUND Y. S. CHAO* and NEBOJSAKO...
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