Gaining Insight into the Deformation of Achilles Tendon Entheses in Mice

Understanding the biomechanics of tendon entheses is fundamental for surgical repair and tissue engineering, but also relevant in biomimetics and palaeontology. 3D imaging is becoming increasingly important in the examination of soft tissue deformation. But entheses are particularly difficult objects for micro-computed tomography because they exhibit extreme differences in X-ray attenuation. In this article, the ex vivo examination of Achilles tendon entheses from mice using a combination of tensile tests and synchrotron radiation-based micro-computed tomography is reported. Two groups of specimens with different water content are compared with regard to strains and volume changes in the more proximal free tendon and the distal tendon that wraps around the Tuber calcanei. Tomograms of relaxed and deformed entheses are recorded with propagation-based phase contrast. The tissue structure is rendered in sufficient detail to enable manual tracking of patterns along the tendon, as well as 3D optical flow analysis in a suitable pair of tomograms. High water content is found to increase strain and to change the strain distribution among proximal and distal tendon. In both groups, the volume changes are higher in the distal than in the proximal tendon. These results support the existence of a compliant zone near the insertion. They also show that the humidity of the specimen environment has to be controlled. Necessary steps to extend the automatic tracking of tissue displacements to all force steps are discussed.