Infrared spectroscopic and biomechanical characterization of human articular cartilage and chondrocyte microenvironment

ABSTRACT

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Osteoarthritis (OA) is a degenerative joint disease that leads to joint pain and loss of mobility. It affects an estimated 57 million people in western Europe, an increase of 54% in the past 30 years. One of the signs of the development of OA is the progressive loss of articular cartilage, the soft tissue that protects articulating joints and allows for joint lubrication. Despite numerous studies, understanding of OA mechanisms at the cell and tissue

level is still limited. The pericellular matrix (PCM), a narrow region around cells, regulates mechanical and chemical cues to the cells to maintain cartilage health and overall cartilage mechanical properties. During OA, compositional alteration occurs in cartilage PCM and extracellular matrix (ECM). However, how PCM changes across the tissue depth during OA is still unknown. Articular cartilage tissue also exhibits site-specific mechanical properties. Yet, how cartilage mechanical properties change in each joint site during OA progression is not well known. A deeper understanding of OA alterations in the PCM and the tissue level changes in each joint site could facilitate the development of novel disease intervention methods.

Alterations in cartilage properties are known to occur even before they become visible in radiographic images during medical examination. In cases

of joint injuries, arthroscopy, a minimally invasive joint repair procedure, is required. During arthroscopic surgeries, surgeons can visually and manually

inspect cartilage surfaces for minor defects not observed in radiographi images and intervene early enough. However, this method is also subjective;

hence, there is a need to augment the current arthroscopic technique to improve sensitivity to early degenerative changes in cartilage.

This thesis aims to evaluate the changes in the collagen content of cartilage cell microenvironment and site-specific biomechanical changes at

different stages of OA. High-resolution Fourier Transform Infrared (FTIR) microspectroscopy was utilized to study changes in the collagen content of

the knee cartilage cell microenvironment (study I), while biomechanical testing was performed to assess site-specific mechanical properties at

different joint sites (study II). Furthermore, we developed a machine learning (ML) assisted visible and near-infrared spectroscopy (Vis-NIRS) classification method to identify normal from degenerated human knee cartilage (study III) to enhance objective and minimally invasive cartilage diagnostics.

The results revealed that cartilage PCM and ECM of the upper middle zone (10–20 % of tissue thickness measured from the surface) were altered

in OA, while the more superficial and deep zones were not altered. In the upper middle zone, early OA changes in collagen content first occurred in

the PCM before advancing to the ECM in advanced OA. In donor-matched samples extracted from different joint sites, equilibrium, instantaneous, and

dynamic moduli of the femoral cartilage were similar in normal and early OA tissue. These properties in the tibial and trochlear cartilage were smaller in

early OA compared to normal tissue. Furthermore, ML classifiers developed based on Vis-NIRS spectra and histological scores were able to differentiate

normal from early degenerated tissue.

To conclude, this thesis provides insights into cartilage cell-level  compositional changes and site-specific biomechanical alteration during OA

progression. Furthermore, the thesis establishes Vis-NIRS spectroscopy as a technique to objectively identify early tissue deterioration, especially during

joint repair surgery. The information in the thesis would be valuable for developing effective diagnostic and therapeutic strategies for better OA

management. Additionally, the biomechanical and spectroscopic data from the thesis could enhance current tissue engineering and computational

modelling methods.