Investigating Achilles tendon adaptation to mechanical load: a computational model integrating collagen fibre orientation heterogeneity.

Tendons are known to adapt their structural and mechanical properties in response to mechanical loading, but the precise mechanisms underlying this adaptation remain poorly understood. A previous study on rat Achilles tendons compared the effect of unloading (Botox injections and orthosis) with free cage activity (full loading) and revealed that unloading impaired the mechanical response and resulted in more dispersed collagen fibre orientations. The current study investigates tendon mechanobiology by integrating this experimental fibre data into a finite element model. The aim is to evaluate whether the altered mechanical response after unloading results from changes in collagen fibre orientation, tendon geometry, or material properties. Collagen fibre orientation analysis was performed based on phase-contrast enhanced synchrotron X-ray tomography images. Two levels of collagen fibre orientation detail were implemented into the finite element model: 1) global fibre orientation analysis that averaged fibre directions across the entire tendon and 2) local orientation analysis that introduced spatial heterogeneity by incorporating element-specific fibre distributions. Our results indicate that the impaired mechanical response in unloaded tendons is mainly due to changes in fibre orientation distribution and geometry. The local collagen orientation analysis showed a lower overall force response, but did not alter the relative differences between fully loaded and unloaded tendons. Incorporating the increased heterogeneity may still be important for future studies of tendon mechanobiology. The established framework provides a robust tool for exploring tendon biomechanics, capturing detailed fibre information, and offering valuable insights into tendon adaptation under various conditions.