Articular cartilage optical properties in the near-infrared (NIR) spectral range vary with depth and tissue integrity

Abstract

Optical properties of biological tissues in the NIR spectral range have demonstrated significant potential for in vivo diagnostic applications and are critical parameters for modelling light interaction in biological tissues. This study aims to investigate the optical properties of articular cartilage as a function of tissue depth and integrity. The results suggest consistent wavelength-dependent variation in optical properties between cartilage depth-wise zones, as well as between healthy and degenerated tissue. Also, statistically significant differences (p<0.05) in both optical properties were observed between the different cartilage depth-wise zones and as a result of tissue degeneration. When taken into account, the outcome of this study could enable accurate modelling of light interaction in cartilage matrix and could provide useful diagnostic information on cartilage integrity.

In order to investigate cartilage functional integrity, biomechanical testing was conducted on the B-specimens. Instantaneous modulus of the samples was determined using a Mach-1 micromechanical testing system (V500css, Biomomentum Inc., Laval, Canada). The equipment comprises of a cylindrical indenter with flat face (diameter = 1 mm) and multi-axis force transducer (Nano17, ATI Industrial Automation Inc., Apex, USA). The small size of the indenter was chosen to reduce the impact of the subchondral bone on the measured stress. Before measurement, the subchondral bone end of the samples was made parallel to the articular surface by filing and then glued to the bottom of the measurement flask, filled with PBS. 12.5 kPa pre-stress and 5 cycles of pre-conditioning loadings (strain amplitude = 2%) were applied to ensure perfect indenter-cartilage surface contact before measurement. The stress-relaxation protocol comprised of 3 steps with 5% strain amplitude and 100% strain rate, with a relaxation period of 15 minutes between steps. To determine the instantaneous modulus, the ratio of the peak stress (at the 2nd step) and the strain amplitude was calculated. Cartilage was assumed to be elastic and isotropic during this protocol; thus, instantaneous modulus was calculated using solution described in and by assuming Poisson’s ratio of 0.5 during instantaneous loading.