Structure of tendon causes highly optical anisotropic properties and transport

Tendons are highly anisotropic tissues that exhibit distinct optical properties depending on the direction of light propagation relative to their fiber orientation. Understanding these variations and how to modify them through optical clearing techniques is beneficial for many light-based applications, including photobiomodulation therapy.

To quantify how tendon optical transport depends on fiber orientation and wavelength, comparing light propagation parallel and perpendicular to the tendon's long axis, the axis along which collagen fibers are primarily aligned, and to evaluate glycerol-based optical clearing for increasing light penetration.

The reduced scattering coefficient ( ${\mu }_s^{\text{'}}$ ) was measured from 400 to 1600 nm in tendon samples oriented parallel to the tendon's fiber axis (transverse slices) and perpendicular (longitudinal slices) to it. Diffuse reflectance and total transmittance were measured using an integrating sphere and spectrophotometer, and optical coefficients were derived through a theoretical Monte Carlo model. Angular scattering measurements at 633 nm were performed to characterize forward scattering behavior. Power transmission was measured in centimeter-scale tendon sections, and Monte Carlo simulations using the measured optical properties were used to model the transmission experiment and compare orientation-dependent penetration. Measurements were conducted in samples treated with saline or glycerol (10% and 60%). Beam profiler images were also captured to assess light distribution.

Longitudinal tendon sections exhibited consistently higher ${\mu }_s^{\text{'}}$ values than transverse sections. At 800 nm, ${\mu }_s^{\text{'}}$ was $9.01{\mathrm{mm}}^{-1}$ (longitudinal) versus $0.57{\mathrm{mm}}^{-1}$ (transverse). Immersion in 60% glycerol greatly reduced ${\mu }_s^{\text{'}}$ in both orientations and increased transmitted power. Beam profiler images showed higher intensity in fascicles and fibers relative to the surrounding matrix. This structural pattern diminished with 60% glycerol treatment.

Tendons have strong anisotropic optical properties that favor optical propagation along their fiber axis. Glycerol increases light penetration depths, though further research is needed to confirm safe concentrations for clinical use, as 10% glycerol proved insufficient.