The fibrous structure of the Achilles tendon enthesis in mice and its deformation under load

At many tendon-bone insertions the tendon fibers pass through unmineralized and mineralized fibrocartilage before they dissociate at the interface with bone. The fibrocartilages are thought to mediate between tendon and bone. The aims of this project are (1) to understand which aspects of the micromechanics of unmineralized fibrocartilage contribute to a homogeneous distribution of stress along the transition, and (2) to identify mechanisms within tendon and fibrocartilage that homogenize stress across the tendon. The Achilles tendon enthesis in mice is the object of research of this project. In the first part, the 3D fibrous structure is analyzed. A workflow for connective tissue imaging, fiber tracking and fiber analysis is developed. The fibers are shown to follow helical courses with an increasing helical angle towards the insertion. They exhibit low levels of branching. Deep fibers inserting at the protrusion exhibit lower reserve lengths than superficial fibers. The fibers of the protrusion seem to correspond to a subtendon of other muscle bellies than the superficial fibers. In the distal tendon the fibers are curved towards the interface with bone. The second part is a study of the 3D deformation of entheses using a custom tensile testing device that is mounted in a synchrotron beamline for deformation imaging. The curvature of the distal tendon around the Tuber calcanei leads to a different loading of this region as compared to the proximal tendon. The mechanical behavior of these regions is compared with regard to stress, strain and volume losses. Higher strains are found along fibers of the distal tendon. Higher volume losses in the distal region can be explained by the compression against the Tuber calcanei. Mechanisms leading to stress maxima near the insertion occur under sustained loads. Future deformation studies have to examine the dynamic load case. Critical parameters for deformation tests with higher temporal and spatial resolutions are deduced.