## 8 Conclusions  
Articular cartilage is a highly specialised connective tissue with a unique structure. The mechanical properties of AC result from the complex organisation of the extracellular matrix, in which type II collagen and proteoglycans play an important role, ensuring the appropriate stiffness and elasticity of the tissue. The mechanical properties of AC play a key role in ensuring its correct biomechanical functions, as the cartilage's function is to provide movement with minimal friction and to transmit and dissipate mechanical loads. Osteoarthritis leads to a progressive deterioration of these properties, as well as the structure of the AC. In the course of OA, the AC structure degrades, which significantly changes its mechanical properties, such as a decrease in the modulus of elasticity, an increase in the coefficient of friction, or a loss of structural integrity. As a result, the cushioning and protective functions of the joint cartilage deteriorate. These changes are caused by mechanical and biochemical factors that affect the degeneration of collagen and proteoglycans, as well as disturbances in cellular homeostasis. Evaluation of the mechanical properties of AC is possible via classical as well as alternative diagnostic methods. Typical diagnostic methods mainly provide information on the thickness and morphology of the AC. MRI is characterised by high imaging resolution, making it possible to detect cartilage defects, while visible lesions are only seen at an advanced stage of OA. Arthroscopy is an invaluable method for direct assessment, but its main limitation is its invasiveness, which can result in numerous complications. A quick and accessible method is ultrasound, although this method cannot be used to assess the full structure of the AC, which reduces its diagnostic quality [34]. In addition, the assessment is dependent on the radiologist performing the examination. Enhanced MRI techniques allow indirect imaging of AC matrix composition. The T2 relaxation time is related to the collagen network structure and water content, while T1\(\rho\) shows sensitivity to the proteoglycan content. T1\(\rho\) can detect early biochemical changes in OA before the changes are visible on T2 [382]. [PERSON] et al. [383] showed that T1 \(\rho\) and T2 relaxations are related to AC elasticity depending on tissue type. Elastographic techniques, on the other hand, allow in vivo measurements of AC stiffness. Ex vivo tests have confirmed the effect of proteoglycan degradation on lowering the velocity of wave propagation in articular cartilage, reflecting a reduction in its stiffness [345]. [PERSON] et al. [345] demonstrated that with shear wave ultrasound elastography, it is possible to distinguish typical mechanical changes for different stages of AC damage. In addition, in vivo MRE tests indicated an increase in stiffness as the regenerated AC matured [339]. Vibroarthography provides dynamic data and can show even subtle biomechanical abnormalities of the AC. While the literature describes the higher diagnostic sensitivity of VAG in contrast to ultrasound, the lack of standardised testing protocols prevents this method from being implemented in everyday clinical practice [34]. [PERSON] et al. [322] emphasised the importance of the mechanical and tribological properties of AC in the context of the development and treatment of OA. The effectiveness of repair methods, such as autologous chondrocyte implantation or tissue engineering techniques, is dependent on the reconstruction of the structure and restoration of the appropriate mechanical properties of the tissue. Analysis of vibroacoustic signals can therefore be a valuable tool for monitoring the effectiveness of therapy and for assessing the progression of the disease. A method to assess the chemical composition of the AC matrix, such as the collagen or proteoglycan content, is Raman spectroscopy. Preliminary studies show that there is a correlation between Raman spectral markers and mechanical parameters of AC.