Experimental and analytical development of a poroelastic finite element model for tendon

Mechanical models which allow the mechanical response of tendon to be predicted and quantified are important in the development and assessment of orthopaedic reconstruction techniques. In the first study an autogenous patellar tendon ACL reconstruction was performed in a goat model in order to gain first hand insight into the assessment of reconstruction techniques. Extensive tendon and fat pad proliferation were observed along with significant reductions in the biomechanical properties of the host tendon. An existing mechanical model was used to obtain a description of the tensile response of the tissue. While these data helped explain some of the clinical complications documented in the reconstructed joint, they did not describe the role of the fluid within the healing tissue. Experimental evidence suggests that the tensile behavior of tendon is a function of the collagen structure of the tissue and the tissue hydration. The models currently available do not offer a means by which the hydration effects might be explicitly explored. In order to study potential influences of water content on tendon tensile response a finite element model of a subfascicle (a microstructural element of tendon) was constructed in the second study. The collagen fiber morphology reflected in the model interacted with the interfibrillar matrix to produce behaviors similar to those seen in tendon and ligament during tensile, cyclic, and relaxation experiments conducted by others. Although this model exhibited mechanical responses which were similar to those observed in whole tendon and ligament, it was preliminary in nature and as such contained some undesirable compromises. In the third study a more detailed description of the subfascicular microstructure was incorporated into the model. This model was shown to exhibit reasonable relaxation and tensile responses as well as a realistic, positive pressure profile throughout the subfasicle. In the fourth study experiments were performed to support the development of the subfascicle model and its extension to whole tendon. The experimental data suggested that small portions of tendon exhibit a higher tensile modulus, a slower rate of relaxation and a lower amount of relaxation in comparison to larger specimens from the same location in the same tendon. In the fifth study the subfascicle model was able to match subfascicle relaxation and constant strain rate tensile responses as described in the previous experimental study. In addition, a fascicle model, consisting of two subfascicles surrounded by epitenon, was created to investigate potential interactions between subfascicles and the connective tissue membrane. This analysis suggested that the presence of connective tissues in tendon may play an important role in defining the whole tendon relaxation response. In the final study the subfascicle model was utilized in the development of a recruitment model tendon. This work suggested that subfascicle organization within a tendon specimen also plays a role in the development of the relaxation response. These studies highlight the importance of the collagen microstructure in the development of the time varying responses of tendon.