Measuring strain distributions in the tendon using confocal microscopy and finite elements

Tendon is a multilevel fibre composite material, which generates complex variable local strains in response to gross loading. The tendon cells are located in long rows between collagen fibres, where they respond to the local strain environment with the initiation of mechanotransduction pathways, to control matrix maintenance and remodelling. Knowledge of the local strain environment is thus an important component in understanding tenocyte mechanotransduction. Confocal microscopy has previously been utilized to Visualize the tendon matrix during loading, to investigate local strains. Using the cells as strain markers, their response during quasi-static loading or stress relaxation can be monitored. This Study presents a novel method of analysing the resulting images, to provide all overview of the strain environment throughout the tendon sample. The cell markers are used as nodes in a mesh of triangular elements and the mathematical approach adopted in the finite element method is used to find the strain distribution. The analysis highlighted widely variable strains within the matrix during stress relaxation, with strains far greater than those applied to the sample. Large shear strains were indicative of fibre sliding during the relaxation process, while large transverse strains were also recorded throughout the sample, highlighting an overall reduction in sample Volume. These data indicate an exudation of water from the samples during stress relaxation and Suggest that the tenocytes are subjected to significant compressive strains. This analysis technique has great potential for measuring deformation of small tissue samples and understanding interactions between external loads, the extracellular matrix, and cells. This mathematical approach is likely to have other applications in biomechanics and could give even better results in other tissues with a higher density of cells and a less discontinuous,train distribution.