|dc.description.abstracteng||Osteochondral unit is emerging as a focal point in osteoarthritis (OA) research, with subchondral bone (SB) attracting more attention as a critical contributing factor to the initiation and progression of OA. In particular, evaluation of the SB microarchitecture has become vital for a deeper understanding of its intimate interrelationship with articular cartilage (AC), for unravelling OA pathogenesis, and for developing future clinical interventions. The aim of this thesis was to investigate the role of the SB (micro)architecture in joint physiology and pathophysiology by developing methods based on high-resolution imaging to quantitatively describe the SB microarchitecture in maturation, normal physiological human joint, early idiopathic OA, and early and late traumatic OA. Specifically, I established a protocol to systematically define, extract, and scan small cartilage-bone cylinders at a resolution of 1.2 µm, which allowed me to detect a complex microchannel network that connected the medullary cavity to the basal cartilage (CMMC), and to create and quantify an unprecedentedly detailed 3D map of the SB microarchitecture in healthy and early OA human femoral heads. Furthermore, a segmentation method based on a modified Seeded Region Growing technique was developed to semi-automatically calculate the thickness of the calcified cartilage layer in the traumatic OA model.
A layer-by-layer analysis of the SB during maturation revealed intricate developmental changes of the microarchitecture and mineralization. In particular, it was found that calcified cartilage should be considered as a prime focus of investigation owing to its steep mineralization profile and dynamic composition characteristics. The CMMC demonstrated maturation-dependent morphological changes, being small and abundant in young ages while less frequent and enlarged in early adulthood. By profiling the CMMC in normal physiological human femoral heads, a strong association with loading areas was found. In general, the CMMC were small, circular, and with high local density in the load-bearing region, while intermittent, irregularly-shaped, and significantly enlarged in the non-load-bearing region and the peripheral rim of the joint. The unresolved contradictions in the literature such as whether CMMC advance through the calcified cartilage layer, and if they are inherently a pathological feature or not were answered and discussed thoroughly. Likewise, I showed why traditional histology / immunocytochemistry methods based on coronal sectioning of the cartilage-bone unit are lacking for examining the complex, spatial distribution of the CMMC.
SB thinning was observed in early idiopathic OA as a localized occurrence linked with areas of partial chondral defect. Moreover, an increased porosity of the SB was detected compared to the healthy group, which was mainly induced by enlargement of the CMMC. Based on these observations, a new hypothesis regarding the role of SB microarchitecture was introduced that may also explain why early AC degradation does not predominantly occur at the load-bearing areas. Additionally, I discuss the idea of a novel therapeutic invention based on the concept of CMMC that can potentially address today’s insufficiencies of common treatment methods such as microfracturing. I could also show several (micro)structural changes of the SB in a traumatic murine OA model. In particular, while investigating the role of sensory neuropeptides Substance P and αCGRP in tissue maintenance. A lack of αCGRP led to a distinct bone phenotype with a marked SB sclerosis. DMM surgery aggravated SB sclerosis in both Substance P -/- and αCGRP -/- models, while osteophyte formation and an calcified cartilage thickness increase were detected as compensatory reactions to trauma. Likewise, increased heterotopic ossification of the meniscus was observed due to the strong imbalanced expression of the neuropeptides. It is therefore concluded that the AC-SB interface is even more intricate than originally thought, while a revised model for the osteochondral junction is proposed.||de