Microscopic characterisation of local strain field in healing tissue in the central third defect of mouse patellar tendon at early-phase of healing

ABS T R A C T Tendons exhibit a hierarchical collagen structure, wherein higher-level components, such as collagen fibres and fascicles, are elongated, slid, and rotated during macroscopic stretching. These mechanical behaviours of collagen fibres play important roles in stimulating tenocytes, imposing stretching, compression, and shear deformation. It was hypothesised that a lack of local fibre behaviours in healing tendon tissue may result in a limited application of mechanical stimuli to cells within the tissue, leading to incomplete recovery of tissue structure and functions in regenerated tendons. Therefore, the present study aimed to measure the microscopic strain field in the healing tendon tissue. A central third defect was created in the patellar tendon of mice, and the regenerated tissue in the defect was examined by tensile testing, collagen fibre analysis, and local strain mea-surement using confocal microscopy at 3 and 6 weeks after surgery. Healing tissue at 3 weeks exhibited a significantly lower strength and disorganised collagen fibre structure compared with the normal tendon. These characteristics at 6 weeks remained significantly different from those of the normal tendon. Moreover, the magnitude of local shear strain in the healing tissue under 4% tissue strain was significantly smaller than that in the normal tendon. Differences in the local strain field may be reflected in the cell nuclear shape and possibly the amount of mechanical stimuli applied to the cells during tendon deformation. Accordingly, restoration of a normal local mechanical environment in the healing tissue may be key to a better healing outcome of tendon injury.

Automated summary

The study measured the microscopic strain field in healing tendon tissue by creating a central third defect in the patellar tendon of mice and examining the regenerated tissue at 3 and 6 weeks after surgery. The healing tissue exhibited lower strength and disorganized collagen fiber structure compared to normal tendon at both time points, with a significantly smaller local shear strain under 4% tissue strain. These differences in the local strain field may impact cell nuclear shape and the mechanical stimuli applied during tendon deformation, highlighting the importance of restoring a normal local mechanical environment for better healing outcomes in tendon injuries.