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Ultrasound (US) is one of the best imaging techniques in musculoskeletal radiology because it is low in cost, has high spatial resolution, wide availability in hospitals, is well-tolerated by patients and is not biologically invasive, as it uses sound waves and non ionizing radiation, as in conventional radiology or computed tomography (CT). These features make ultrasound the ideal technique for the diagnosis and follow up of many pathologies and rheumatic syndromes and for the evaluation of the effects of therapy

Cartilage is a greatly specialized type of connective tissue, mainly composed of water (70–80% by wet weight). It is avascular and aneural. The solid component of cartilage is formed of cells (chondrocytes) that are scattered in a firm gel-like substance (extracellular matrix) consisting of collagen and proteoglycans.Collagen forms a network of fibrils, which resists the swelling pressure generated by the proteoglycans. In the musculoskeletal system there are two types of cartilage: hyaline and fibrocartilage. Compared to hyaline, fibrocartilage contains more collagen and is more resistant at tensile strength.Fibrocartilage is found in intervertebral disks, symphyses, glenoid labra, menisci, the round ligament of the femur, and at sites connecting tendons or ligaments to bones.Hyaline cartilage is the most common variety of cartilage. It is found in costal cartilage, epiphyseal plates and covering bones in joints (articular cartilage). The free surfaces of most hyaline cartilage (but not articular cartilage) are covered by a layer of fibrous connective tissue (perichondrium). Hyaline cartilage structure is not uniform (Fig. 3.1). Instead, it is stratified and divided into four zones: superficial, middle, deep, and calcified. The superficial zone, also called tangential zone, is considered the articular surface and is characterized by flattened chondrocytes, relatively low quantities of proteoglycan, and numerous thicker fibrils arranged parallel to the articular surface in order to resist tension. In articular cartilage this layer acts as a barrier because there is no perichondrium.The middle zone, or transitional zone, in contrast, has round chondrocytes, the highest level of proteoglycan among the four zones, and a random arrangement of collagen.The deep (radiate zone) is the thickest zone, characterized by collagen fibrils that are perpendicular to the underlying bone, acting as an anchor to prohibit separation of zones and in order to resist at torsional and compressive mechanical strength

Sonography has great potential for the non-invasive study of hyaline cartilage, as it can depict microscopic lesions to be demonstrated with a high spatial resolution. The main limit to the sonographic study of articular cartilage is the relatively limited dimensions of acoustic windows available for the visualization of the cartilage surfaces. The most frequent errors in the study of cartilage, especially at knee level, are linked to incorrect examination. The most frequent artifacts come out in suprapatellar panoramic views, as the cartilage profile of the femoral trochlea is not perpendicular to the direction of the US beam. An apparent loss in sharpness of the chondro-synovial margin of the cartilage and an apparent reduction or increase of the cartilage thickness are the main artifacts caused by incorrect technique [ 2 ].

The ability of US to make an accurate evaluation of soft tissue involvement in a wide range of diseases of the locomotor system has led to its increasing widespread use in the field of rheumatology [ 1 ]–[ 10 ]. Significant technological progress has been made over the last few years, generating ever more sophisticated and reliable ultrasound machinery. The high resolution is now such that real in vivo histological examination is now possible.The main reason for the relative lack of wide diffusion of its use amongst rheumatologists is that a long training period is necessary in order to acquire full operator independence

US permits accurate and reliable assessment of soft tissue involvement in rheumatic disease [ 3 ]–[ 3 ].Highresolution US with power Doppler equipment can detect even minimal morphostructural and perfusional changes within soft tissues [ 4 ]–[ 14 ], and may offer additional information for disease activity monitoring [ 15 ]–[ 24 ] (Figs. 6.1-6.6).

Needle aspiration of synovial fluid and intralesional injection of various compounds are very common procedures in rheumatological practice. Local steroid injection, in particular, is relatively simple and cost-effective and may be alternative or adjunctive to systemic drug therapy in several rheumatological conditions [ 1 ]–[ 5 ]. Both efficacy and side effects of the injection depend on the correct placement of the tip of the needle inside or around the lesion. Particular attention must be taken to avoid direct needle contact with nerves, tendons, articular cartilage and blood vessels [ 6 ]. Intra-articular and intra-lesional therapy is usually performed using palpation and bony landmarks for guidance. Conventional blind interventional procedures may be particularly problematic when a small and/or deep target has to be reached, or when an injection has to be carried out into a dry joint