Osteoarthritis (OA) is a degenerative joint disease that results in the degradation and loss of articular cartilage. The pathogenesis of OA is not fully understood, however, structural alterations to the underlying subchondral bone (SB) have shown to precede articular cartilage (AC) degeneration and may play an important role in the initiation and progression of the disease. The hypothesis of this PhD is that structural changes in SB precede those in the AC, which subsequently alters strain distribution in the overlying cartilage during the initiation and development of OA. The primary aim of this project is to combine techniques of high-resolution X-ray Computed Tomography (XCT) with Digital Volume Correlation (DVC), to assess 3D strain patterns across the osteochondral interface. Since it was difficult to visualise AC using lab-based CT systems without the use of Contrast-Enhancing Staining Agents (CESAs), this project compared and mechanically evaluated the effect of CESAs. Nanoindentation and micromechanical probing were used to investigate the mechanical properties of AC and SB, before and after staining with either a 1:​2 hafnium-substituted Wells-Dawson polyoxometalate (Hf-WD POM in PBS) or Phosphotungstic acid (PTA) in 70% ethanol. Using nanoindentation, changes to AC and SB in fresh unfixed, unembedded tissues during the early stages of OA progression, were also compared to immunohistochemical markers of OA.

Using a novel nanoindentation method in unstained tissues, site-specific mechanical changes (in terms of the reduced modulus) throughout the osteochondral depth in a Dunkin-Hartley (DH) guinea pig model of spontaneous OA, was measured and compared with the localisation of immunohistochemical markers of OA. In unstained tissue, stiffening of the deep zone of AC and a 35% reduction in stiffness of SB during early OA was found. In severe OA, there was a significantly lower reduced modulus throughout the superficial and middle zones, where increased ADAMTS 4 & 5 staining, and proteoglycan loss was also detected. Conversely, the severe OA age-group had significantly increased SB stiffness (up to 110%).

PTA-staining in 70% ethanol caused a significant (~6-fold) increase in AC stiffness, whereas Hf-WD POM-stained specimens had mechanical properties similar to pre-stain tissue. Hf-WD POM was, therefore, identified as the most suitable contrast agent for subsequent DVC analysis. DVC analysis showed higher strain (141.7% higher εp3 & 98.2% higher γ) was present in tibial AC in severe OA compared to non-OA specimens in DH whole joints. Additionally, more strain was transferred into mineralised regions of calcified cartilage and SB (23% higher εp3 & 55% higher γ strain) in these specimens. Residual strain distribution was seen in regions throughout AC and SB in ‘healthy’ pre-OA tissues and shifted in location and intensity towards the AC surface where histological signs of glycosaminoglycan (GAG) loss and superficial fibrillation were apparent. Strain magnitude further increased (50% higher peak ɛp3 & 25% higher γ strains) and extended into the AC as OA progressed.

These results partially support the hypothesis of the role of SB in early OA pathology, since mechanical changes in both the SB and deep zone of AC may cause a shift strain distribution to the superficial zones of AC, where degeneration initiates histologically. Biomechanical changes in combination with high strain at the articular surface increase in magnitude and extend into the deeper layers of AC as OA progresses.