J. Biomechanics Vol. 20, No. 1. pp. l-6. 1987. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Printed in Great Britain 0021-9290/X7 S3.00 + .OO CI 1987 Pergamon Journals Ltd ANALYSIS OF CUMULATIVE STRAIN IN TENDONS AND TENDON SHEATHS STEVENA.GOLDSTEIN*,THOMASJ.ARMSTRONG$', DON B.CHAFFIN~ and LARRY S. MATTHEWS* *Biomechanics, Trauma and Sports Medicine Laboratory, Department of Surgery, Section of Orthopaedic Surgery, University of Michigan, Ann Arbor, MI, tThe Center for Ergonomics, and SDepartment of Environmental and Industrial Health, University of Michigan, Ann Arbor, MI. U.S.A. zyxwvutsrqponmlkji Abstract-Twenty-five fresh frozen flexor digitorum profundus tendons stratified by sex were subjected to uniaxial step stress and cyclic loads in twelve intact human cadaver hands. By attaching specially designed clip strain gage transducers on tendons just proximal and distal to an undisrupted carpal tunnel, the interactions of the tendons, tendon sheath and retinacula were measured. The elastic and viscous response of the tendon composites to step stresses were found to fit fractional power functions of stress and time respectively. A significant and quantifiable decrease in strain from the proximal to the distal tendon segment was found to be a function of wrist deviation. The results indicate that an accumulation of strain does occur in tendinous tissues during physiologic loading. INTRODUCTION Past studies have shown that cumulative trauma disorders of the upper extremities such as tenosyno- vitis, tendonitis, bursitis, and carpal tunnel syndrome can be caused, precipitated, or aggravated by repeated exertions with certain hand and wrist positions- particularly in combination with forceful exertions. These findings were based on both epidemiological and biomechanical studies of manual work activities. The epidemiological studies have shown that mor- bidity patterns are associated with repeated exertions and certain job-related attributes (Wilson and Wilson, 1957; Thompson et al., 1951; Kendall, 1960; Eichoff, 1927; Armstrong and Chaffin, 1979a; Cannon et al., 1981; Muckart, 1964; Hymovich and Lindholm, 1966). Biomechanical studies have illustrated that work ac- tivities involving forceful exertions in combination with certain postures can produce stress concentra- tions on the tendons and adjacent tissues that cor- respond with the sites of injury (Armstrong and Chaffin, 1979b; Castelli et al., 1980; Meachim and Roberts, 1969; Muckart, 1964; Eichoff, 1927). These studies treated the tendons and adjacent structures as perfectly elastic, frictionless materials; however, ten- dinous tissues have been shown to demonstrate visco- elastic properties. Consequently, repeated exertions may produce progressive elongation or creep of the tendons and tendon sheaths. In a study conducted by Rais (1961), morphological tissue changes resembling peritendinitis crepitans were experimentally induced in rabbits by over exercising the hind limb. The most significant factor related to the severity of tissue abnormalities was the duration of hyperfunction. This work was the first to correlate time and load characteristics to a subsequent cumulative trauma injury. __ Received June 1983; zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA in revised form 22 May 1986. Based on these studies, it is hypothesized that the viscoelastic creep responses of the tendons or tendon sheaths is an important etiological factor in cumulative trauma disorders. These disorders are hypothesized to be a physiological response to zyxwvutsrqponmlkjihgfedcbaZYXW cumulative strain de- veloped in the tendons, tendon sheaths, or retaining ligaments that form the anatomical pulleys. The purpose of this study was to investigate the local biomechanical aspects of tendon-tendon sheath func- tion. Specifically, to determine whether the tendons and tendon sheaths exhibit viscoelastic properties under simulated physiologic loading conditions. MATERIALS AND METHODS A procedure was developed to measure strains in tendons under simulated physiological loads in uiuo in cadaver hands. Twelve hands from seven subjects, four females and three males, between the ages of 55 and 72, were tested. Twenty-five fresh frozen flexor digitorum profundus tendons were subjected to uniaxial step stress and cyclic loads at physiologic levels. The hands were cut from the cadaver forearm approximately 8 cm from the distal wrist crease. Careful dissection of the hands was performed to isolate the profundus tendons to digits two, three and four. The carpal tunnel and common flexor sheath were carefully preserved. Proximal and distal to the common sheath, 4 cm segments of superficialis ten- dons two, three and four were carefully resected to expose the profundus tendons. The surface of the profundus tendons proximal and distal to the carpal tunnel were prepared and clip strain gages made from beryllium copper with two 120 Q resistive gages were attached to each tendon by passing sutures through the bulk of the tendon and using a cynoacrylic adhesive on the surface. A schematic of a prepared specimen is