Modelling light interaction in healthy and degenerated articular cartilage

This master thesis investigates the sensitivity of near -infrared light propagat ion in articular cartilage to its health and integrity. Samples used in this study were collected from a local abattoir. Bovine knees joints (n=15) were harvested within the week of slaughter. Cartilage samples were extracted from the upper and lower regions of the patellar lateral and medial facets. Each sample was cut further into two sections (A and B), sections A were carried out for measuring optical properties, and the B sections were carried for biomechanical measurements. Reflectance and transmittance of the samples in the spectral range 600 -2000 nm were measured using a spectrometer with a single integrating sphere. Following the optical measurements, absorption and scattering coefficients (µa, µs ) were estimated using the inverse adding doubling (IAD) technique. The thickness of the cartilage sections was measured visually with an optical microscope. Fluence values were estimated by solving the radiative transport equation (RTE) using the Monte-Carlo solver ValoMC. Cartilage sample is modeled as a rectangular mesh with width (10 mm), and height equivalent to sample thickness. Samples were divided based on their mechanical moduli into three groups representing three different levels of cartilage integrity. The fluence value of the three groups was investigated with in cartilage zones, superficial zone (SZ), middle zone (MZ), and deep zone (DZ) . Grouping was based on instantaneous modulus and equilibrium modulus separately. The fluence values captured discrepancies between samples having high mechanical properties ( Eins, 8.198±2.8 Mpa, and Eeq, 2.26± 1.08 Mpa) and the ones with low mechanical properties ( Eins, 2.275±0.63 Mpa, and E eq, 0.58±0.169 Mpa). In particular, light distribution showed higher intensity and penetration depth within cartilage samples with high mechanical properties, while light propagation in samples with low mechanical properties exhibited low intensity within the cartilage and poor penetration depth. Light propagation in the spectral range of 600 -1000 nm showed the most sensitivity to dis parity in the samples’ biomechanical properties , whereas light propagation in the spectral range of 1000 -2000 nm showed lower sensitivity to this biomarker. Moreover, light propagation in DZ showed more robust sensitivity to its biomechanical properties than light propagation in SZ and MZ.