## 1 Introduction  
Articular cartilage (AC) is a smooth, elastic, and glassy connective tissue that covers the interacting bony surfaces in diarthrodial joints [1, 2, 3, 4]. It is one of the most vulnerable elements in the human body to overload and degenerative changes [3]. Articular cartilage includes proteoglycans and collagen, organised into zones containing articular chondrocytes [4, 5, 6]. The main function of AC is to facilitate load transfer with a low friction coefficient, thereby protecting the subchondral bone [7]. It also provides wear resistance as well as shock absorption of up to eight times the body weight [4; 8]. Due to constant exposure to mechanical stress, it is susceptible to wear and tear and traumatic injuries. As it is a non-vascular tissue, its ability to regenerate itself is limited [9]. To a large extent, musculoskeletal abnormalities are associated with articular cartilage injuries [2]. In most cases, once articular cartilage is damaged, it is not possible to fully recover its structure, function and also its biomechanical properties, resulting in degeneration and ultimately osteoarthritis (OA) [7; 10; 11]. People affected by joint cartilage damage struggle with impaired movement of the limbs and trunk, pain and reduced quality of life. Due to the ageing population, more and more people are affected by this problem [12]. Surgical joint replacement is often used to restore cartilage function. However, this solution is associated with the risk of complications and residual pain [13]. A key element in the early detection and monitoring of osteoarthritis progression, a key element is the assessment of AC status. Typical diagnostic methods used in clinical practice include magnetic resonance imaging (MRI), ultrasound, and computed tomography with arthrography (CT-arthrography) [14; 15; 16; 17]. MRI is the most commonly used method to assess the condition and structure of the AC. It is characterised by its ability to assess the volume and thickness, as well as integrity of the AC layers, and is a non-invasive method with high spatial and contrast resolution [16; 18]. On the other hand, it also has its limitations related to the high cost, especially of advanced imaging protocols, long examination times and sensitivity to patient movement. With MRI, there is only an indirect assessment of the biomechanical properties of the AC [19; 20]. CT arthrography is a particularly useful method in joints that are difficult to access for MRI and has a shorter examination time [21]. The method provides a very good visualisation of the AC surface. However, it requires the administration of contrast into the joint cavity and exposure of the patient to ionising radiation, which may be associated with additional pain, infection or allergic reactions, and limits the possibility of repeated examinations [22]. In addition, assessment of the biomechanical properties of the cartilage is not possible. The diagnosis of degenerative changes according to the Kellgren-Lawrence scale is based on radiography (X-ray) [23]. Factors indirectly assessing the quality of AC are the presence of osteophytes, joint stenosis, joint surface deformities and subchondral bone hypertrophy [24]. The advantages of this imaging modality are low cost, high availability, and low patient exposure to ionising radiation. However, one of its biggest disadvantages is the lack of imaging of the articular cartilage, which only shows the bony changes that result from cartilage damage. Conventional radiography also does not provide information about the surrounding soft tissues [23; 25; 26; 27]. Ultrasound is a fast, widely available and low-cost imaging modality that allows real-time assessment of the AC surface [17; 28]. Unfortunately, it is not useful in assessing the internal structures of the AC, as well as its biomechanical properties, due to its low resolution [28; 29]. The examination performed also mainly depends on the experience and skill of the operator [17].