Computational modeling of mechanobiology in intact and healing rat Achilles tendon

Tendons fulfill an important musculoskeletal function by enabling energy-efficient force transmission between muscles and bones. The tendon is a collagen-rich connective tissue that adapts to mechanical loading through mechanobiological processes. The tendon contains a hierarchical collagen fiber structure that displays complex mechanical behaviour by storing and dissipating energy. Current understanding of how tendon properties adapt to short and long-term mechanical loading is limited, but is key to prevent tendon disease and design optimal rehabilitation protocols after tendon rupture. Recently, an increasing amount of small animal experiments have investigated how intact and healing tendons adapt in vivo upon different mechanical loading regimens. Yet, limited numerical models have investigated tendon mechanobiology; even though existing modeling tools from other research fields are available and the amount of experimental data for validation is growing. The aim of this thesis was to investigate the mechanobiology of intact and healing tendon by utilizing and developing advanced numerical models. First, a 3D finite element framework was used to determine the constitutive viscoelastic material properties of intact healthy tendons in rats. The material properties were fitted to experimental data from rats that were subjected to two loading regimens, i.e. free cage activity (full loading) and reduced loading, for five weeks. The resulting material properties showed strong differences in both elastic and damping properties of the collagen between the rats that were subjected to full or reduced loading. Using this material model, a finite element mechanobiological healing framework for Achilles tendons was developed. The adaptive healing model investigated how principal strain and cell infiltration can govern tissue regeneration. The tendon model was stimulated with different levels of external loading, mimicking physiological and sub-physiological load levels explored in animal experiments. Model predictions of ...