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A mathematical model was developed for an implantable force transducer to be inserted within the midsubstance of a ligament or tendon. The model was generated by performing both equilibrium and strain-displacement analyses on a metallic, curved beam structure placed within a parallel-fibered tissue. The analysis permitted the transverse pressure acting between the device and fibers to be calculated along with peak device strain and sensitivity (ratio of strain output to axial tissue force). Transducer pressure and transducer strain were expressed in terms of nondimensionalized design factors. A parametric analysis of the key design factors was then performed. The transverse pressure was shown to vary little for large changes in these factors whereas device strain changed markedly. The analysis was verified by a bench test on an example device. Such a model permits a proposed design to be evaluated without having to conduct costly experiments.

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Local surface strains in bone-fascicle-bone subunits from human patellar tendon and anterior and posterior cruciate ligaments were measured between markers using low-speed photography during low rate subfailure testing. A simple stress-strain relationship of the power form was found to describe the bone-to-bone responses up to four percent strain for all three tissue types examined. The regional material behavior were best fit using an inverted strain-stress relationship, however. The power model, fitted to the experimental data, conformed to the expected stress-strain relationship better than either the quadratic or cubic models. With few exceptions, for a given stress, the strains near the proximal and distal bone ends were not significantly different from each other, but were significantly higher than the strains in the tissue midregions. Local strain patterns generally varied among subunits from the same tissue.

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The fascicle material properties in bone-fascicle-bone units were determined for the anterior and posterior cruciate ligaments (ACL, PCL), the lateral collateral ligament (LCL) and the patellar tendon (PT) from three young human donor knees. Groups of fascicles from each tissue were isolated with intact bone ends and failed at a high strain rate in a saline bath at 37 degrees C. In each knee tested the load related material properties (linear modulus, maximum stress and energy density to maximum stress) for the patellar tendon were significantly larger than corresponding values for the cruciate and collateral ligaments. Bundles from different ligaments in the same knee were similar to each other in their mechanical behavior. In addition, no significant differences were present in the maximum strains recorded for any of the four tissue types examined. The results presented have implications in studies of ligament injury. They are also important in the design and use of synthetic and biological ligament replacements and in tissue and whole knee modeling.

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The objectives of this study are twofold. First, to further develop the understanding of the relationship between the observed mechanical response and changes in the crimp pattern in human patellar tendon bone units. This is accomplished through the use of a specially constructed test frame and microscope system that permits observation and measurement of the crimp patterns as a function of load. Second, the results of the experimental study are used to develop a constitutive equation that includes spatial variation in the crimp pattern. The results of both the experimental and analytical study imply that local strain in the proximity of the attachment site is significantly larger than the strain in the central region of the tendon. The experimental and histological results are for specimens taken from four human bone-patellar tendon-bone units.

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An experimental program was devised to determine whether the canine anterior cruciate ligament (ACL), with an apparent helical twist of its component fiber bundles, could support a torque during axial loading to failure. At the point of first significant failure, the anterior cruciate was found to develop an average maximum torque of 122 +/- 26 N-mm (mean +/- SEM) for the nine tension-torsion tests performed. A nearly linear axial force-torque curve was also observed where the average slope of all tests was 5.2 +/- 0.2 mm-1 (mean +/- SEM). The experimental axial force data was then used to determine material parameters in the constitutive equation for the fascicle and underlying bone. A nonlinear ligament model based on this response was found to reproduce the axial force data adequately and to reasonably predict the observed ligament torque over the entire subfailure loading range. The presence of a ligament torque during axial loading has implications in the design and selection of a ligament substitute. Such a substitute has already been examined in the canine in another study. The results also suggest more refined experiments which could relate the mechanical properties of a ligament to its detailed macrostructure and insertion site geometries.

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It has been observed that the fascicles in the canine anterior cruciate ligament do not follow a straight line path between the tibial and femoral attachment sites, but rather their path is approximately helical about the ligament axis. Thus, a mathematical model for the mechanical response of a ligament was developed from the observed geometrical configuration of the fascicle. The principal result of the model indicated that a torque must always accompany the force during elongation of a ligament. The relationships between the ligament and fascicle variables are also examined in detail.

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