Three-dimensional deformation of the Achilles tendon insertion in mice - The entheseal fibrocartilage as a water cushion

The insertion of the Achilles tendon to the Calcaneus transmits forces between soft and hard tissue. From an adaptionist perspective, the insertional tissues are expected to distribute stress homogeneously, in spite of the abrupt change in tissue elastic behavior. But it remains yet unclear, whether and how they achieve those properties. The collagen fibers follow approximately straight courses along the tendon. Before they insert, the fibers curve over the posterior Calcaneus surface while passing through an unmineralized fibrocartilage. According to their curvature, they can exert pressure on the extrafibrillar matrix which is rich in water. Therefore the fibrocartilage is expected to lose volume under pressure. We compared the deformation of the proximal tendon and the distal fibrocartilage within three-dimensional images of relaxed and loaded Achilles tendon insertions from mice [Mus musculus, strain C57BL/6J, n = 3], acquired using synchrotron radiation-based micro-computed tomography with propagation-based phase-contrast. Forces were measured during the entire period of the experiment.The tomographic images are the first to render the three-dimensional deformation of intact tendon insertions. The permanent stretch of the experiment leads to a relative volume loss of 20 % in the unmineralized fibrocartilage, which is twice as high as in the tendon proper. The scans of the loaded insertions reveal that pressed out liquid accumulates in the retrocalcaneal bursa. These findings contribute to the interpretation of the fibrocartilage as a poroelastic composite inhibiting fast water loss by its low permeability. Therefore, under dynamic loads the fibers could exert pressure on a “water cushion”