Finite element modeling of human Achilles tendon: impact of subtendon modeling and fiber direction

The Achilles tendon is the thickest tendon in the human body, and it is composed of three subtendons connected to the calf muscles (soleus, medial and lateral gastrocnemius). This study aimed to evaluate the importance of explicitly modeling subtendons and collagen fiber orientation on the predicted strain distribution within tendon tissue during mechanical loading. Magnetic resonance images from seven subjects were used to create subject-specific finite element models. The segmented tendons were modeled in two ways: the entire tendon as one structure, then dividing it into the three subtendons. A fiber-reinforced poro-visco-hyperelastic material model was used to describe the mechanical response. The collagen fibers were first oriented in the proximal-distal direction, then twisted following the subtendon orientation. Principal strains were analyzed, assessing the impact of subject-specific geometry on strain magnitudes, localization and distributions. Average strains were similar across subjects (coefficient of variation = 18%), but location and magnitude of peak strains were subject-specific. While models including the subtendons predicted similar average strain values to those without, local heterogeneity increased substantially in the subtendon model (Full-Width at Half-Maximum increased from 0.03 to 0.08). Altering the collagen fiber orientation shifted the localization of higher strains (Full-Width at Half-Maximum = 0.11). In conclusion, both subtendons and realistic fiber orientation should be considered when modeling the mechanical behavior in the Achilles tendon as it strongly affects the strain distribution within the tendon. Accurate prediction of strain localization and distribution is important for further studies of tendon mechanobiology and for designing more effective rehabilitation protocols.