Factors Affecting Human Achilles Sub-tendon Mechanical Behaviour and the Clinical Application Potential

The human Achilles tendon is composed of three twisted sub-tendons, arising from soleus and gastrocnemii muscles, but the interaction between these sub-tendons during body movements is largely unknown. The most widely reported phenomenon is the non-uniform intra-tendinous displacements during movements; however, how this non-uniformity contributes to the overall tendon mechanical behaviour has not been thoroughly studied. This thesis aims to investigate the potential factors affecting this intra-tendinous displacement non-uniformity, by conducting mechanical and material characterisation and in silico modelling on Achilles sub-tendons, and to discover possibilities to translate these findings into clinical applications. The study conducted on the equine bifurcated tendon-ligament model demonstrated that differences in the material and mechanical properties at the fascicle and sub-tendon levels may be associated with displacement non-uniformity. Human Achilles sub-tendon material properties were not statistically different but a greater cross-sectional area, failure force and displacement were found in the soleus than gastrocnemii sub-tendons. These results suggest that the material and mechanical properties of sub-tendons may affect the overall tendon level mechanical behaviour. Finite element analysis on three reconstructed Achilles sub-tendon models identified that reduced inter-sub-tendon sliding, as reported previously in aged tendons, reduced displacement non-uniformity. Comparing the results from different models further suggested that the inherent tendon morphology, with different length, twist, and sub-tendon arrangements, could result in different tendon level displacements and stress distributions. The results from Raman spectroscopy on both equine and human tendon tissues demonstrated the ability to differentiate tendon samples with known compositional differences based on spectral features alone, suggesting that Raman spectroscopy has the potential for rapid analysis of tendon compositions in the future. In conclusion, this thesis has identified three factors that could affect the overall Achilles tendon mechanical behaviour and proposed potential approaches for future clinical applications to improve tendon healthcare