Biomechanical Aspects of Cumulative Trauma to Tendons and Tendon Sheaths.

Abstract

Cumulative trauma disorders of the tendons and tendons sheaths in the h and and wrist have been associated with continuous repetitive tasks. This investigation was designed to examine the biomechanical aspects of tendon-tendon sheath articulations during physiologic usage patterns. The specific objectives were to: (1)describe the biomechanical response of the tendon-tendon sheath composite to normal patterns of h and usage, (2)design a protocol for measuring the mechanical properties of the tendon-tendon sheath composite as it articulates through an osseofibrous canal and , (3)derive a constitutive equation relating h and usage patterns to cumulative strain in the tissues and discuss the etiology of tendon-tendon sheath disorders related to cumulative strain. The extrinsic flexor and extensor tendons were modelled as viscoelastic belts wrapped around anatomical pulleys and surrounded by synovial sheaths. Load transmission from the tendons to the invaginating sheaths and retaining ligaments, is dependent on tendon loading parameters and h and and wrist position. 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 h and s ranging in age from 55 to 72 years. By attaching specially designed clip strain gage transducers on the tendons just proximal and distal to an undisrupted carpal tunnel, the interactions of the tendons, tendon sheaths and retinacula were measured. The elastic and viscous response of the tendons to step stresses were found to fit fractional power functions of stress and time respectively ((epsilon)('elastic) = A(sigma)('.495); (epsilon)('viscous) = Kt('.409)). The elastic response of the female tendons tested in this study was significantly stiffer than the males. The viscous creep was dependent on sex, digit, and measurement location. A significant and quantifiable decrease in strain (a range of 0 to .75% strain) from the proximal to the distal tendon segment was found to be a function of wrist deviation and load. The decrease in strain (increase in shear traction force) was significantly greater in flexion and extension than in neutral position. The strain decrease was also significantly greater in flexion than in extension. Using a form of Fung's quasi-linear viscoelastic heredity integral and experimentally determined material properties of the tendon composite, the mean response to periodic cyclic loading was predicted well. The results of this study indicate that an accumulation of strain (creep phenomena) does occur in the tendinous tissues during physiologic loading. The results of the study also provide evidence that the accumulation of strain in collagenous tissues is an important etiological factor of chronic disorders of tendons and tendon sheaths. This study supports the hypothesis that the tissues are strain limited, and suggests that increases in strain during physiologic cyclic loading are a result of the collagen fibrils interactions with the protein ground substance matrix; specifically the crimping and uncrimping of the fibrils. The mechanisms leading to permanent deformations are, therefore, proposed to be a compromise in the biochemical conditions sustaining the crimp configurations.