Articular cartilage optical properties vary with tissue depth and integrity

Articular cartilage, the soft tissue covering the ends of articulating bones, plays a crucial role in facilitating smooth joint movement. This specialized

connective tissue is avascular and lacks nerve supply, which limits its ability to heal following injury. The traumatic damage to articular cartilage often

goes undetected in standard clinical assessments, making early detection of degeneration essential. Identifying and monitoring articular cartilage health

could enable timely interventions to prevent the progression to post-traumatic osteoarthritis (PTOA) – an irreversible condition characterized by cartilage erosion, joint pain, and diminished mobility. Currently, the evaluation of cartilage injuries is primarily conducted through arthroscopy, relying on subjective visual assessments and manual palpation, which are inherently variable.

This thesis investigates the optical properties of articular cartilage, particularly in the context of osteoarthritis and other degenerative diseases. Articular cartilage exhibits unique structural and compositional characteristics that influence its optical behavior. However, it is not fully understood how these properties vary with tissue depth and integrity. Study I addressed the limited understanding of the variation in optical properties along articular cartilage depth and its correlation with the tissue’s integrity. By exploring the depth-wise differences in optical characteristics, this study aims to establish a link between these properties and the underlying tissue structure and composition, providing insights into the implications for cartilage health assessment. Study II highlighted a notable gap in the literature regarding the quantitative analysis of the refractive index (RI) in relation to tissue structure and composition. By examining these relationships, this research enhances our understanding of how optical properties can reflect changes in cartilage integrity, particularly in pathological conditions. Study III explored the relationship between the RI of human articular cartilage and its biomechanical properties, an area that has seen limited scientific investigation. This study seeks to elucidate how alterations in the tissue’s biomechanical characteristics can be reflected in its RI, thus establishing RI as a potential non-invasive biomarker for assessing cartilage mechanical competency.

In conclusion, this thesis provides valuable insights into the optical properties of articular cartilage, addressing critical gaps in knowledge and

offering promising avenues for the development of advanced diagnostic tools. By linking optical and biomechanical properties, this research lays the

groundwork for improved clinical assessments of cartilage health and targeted therapeutic strategies for degenerative diseases.

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 fat 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 fling and then glued to the bottom of the measurement fask, flled 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. Based on the instantaneous modulus, the samples were divided into two groups using a

threshold of 4 MPa: healthy (n=22) and degenerated (n=28) group.

Keywords:

Optical properties, articular cartilage, osteoarthritis, refractive index, biomechanical properties.