Endoscopic cellular microscopy for in vivo biomechanical assessment of tendon function

This study explores a novel method to quantify in vivo soft tissue biomechanics from endoscopic confocal fluorescence microscope images of externally loaded biological tissues. A custom algorithm based on normalized cross-correlation is used to track fluorescently labeled cells within soft tissue structures as they deform. Cellular displacements are subsequently reduced to tissue strains by deriving the spatial gradient of the spline smoothed cellular displacement field. The relative performance of the tracking method is verified using a synthetic dataset with known underlying deformation. In biological application of the method, tissue strains are measured in the Achilles tendon of an anesthetized mouse. Over repeated trials, structural strain in the tendon (i.e., the relative change in distance between cells located at view field extremes) is 20.3+/-3.1%, thus establishing the reproducibility of the loading protocol. Analysis of local tendon tissue strains reveal primary engineering strains in the tissue to range from 5 to 55%, signifying a highly inhomogeneous strain state, with complex relative motions of neighboring tendon substructures. In summary, the current work establishes a baseline for a promising experimental method, and demonstrates its technical feasibility.