The subchondral region bone plate is located deep to the articular surface, separated only by a thin layer of calcified articular cartilage (Fig. 10.2). When there is loss of the overlying cartilage, as in osteoarthritis, the subchondral bone is directly exposed to the stresses across the joint with subsequent trabecular collapse, flattening and eburnation (Fig. 10.3). Cartilage loss is discussed in greater detail in Chap.​ 3 and the Outerbridge classification of cartilage loss is outlined (see Table 3.​5). The subchondral regions in areas of lower stress become increasingly vascularized. This stimulates endochondral ossification with new bone formation, osteophytes (Fig. 10.4). They are usually marginally growing as an extension of the joint margin. New bone formation may also occur centrally in areas of full cartilage loss and appear as new bone formation at the cortical surface, often with an irregular margin.

Subchondral cysts develop between the deformed trabeculae in areas of eburnation (Fig. 10.5). They are of variable size. The overlying cortex may be intact or may have a focal defect allowing communication of the cyst with the joint space. Prominent subchondral cyst formation raises possibility of underlying crystal disease. Joint space loss has long been attributed to cartilage thickness; however, in the knee, meniscal degenerative changes can also contribute to narrowing. Marked joint space narrowing is seen in advanced disease (Fig. 10.6).

On radiographs eburnation is noted as subchondral sclerosis and may demonstrate flattening and collapse of its surface. Osteophytes are noted as new bone forming at the periphery of the joint space. Subchondral cysts are variable in size, have a thin sclerotic margin, and may demonstrate communication with the joint space, e.g., intra-articular gas may extend into the cyst. The overlying cortex is usually intact or demonstrates a focal defect. Occasionally it may be difficult to differentiate a subchondral cyst from an erosion if there is collapse of the cyst overlying cortical margin. Subchondral cysts are however associated with joint space loss, subchondral sclerosis, and osteophytosis. Additional studies other than radiographs are rarely required, and the above changes however can be detailed on other imaging modalities when performed for alternative indications.

CT demonstrates exquisite detail of subchondral changes as described in the above radiographic findings; however, it is rarely required in the assessment of OA. CT is occasionally used for assessment of intra-articular loose bodies and residual bone mass, e.g., glenoid, prior to joint replacement (Fig. 10.7).

Subchondral sclerosis on MRI is of low SI on both T1- and T2-weighted sequences. Subchondral cysts may contain proteinaceous material or joint fluid if they communicate with the joint and are of high SI on T2 and usually low SI on TI. MRI is the gold imaging standard in assessing cartilage. Cartilage abnormalities include changes in signal intensity, fibrillation cartilage surface, fissuring eventually extending full-depth cartilage, and areas of full-thickness cartilage loss (Fig. 10.8). These are often the sites of associated subchondral cyst formation and bone marrow edema. MRI of joints also allows visualization of the soft tissues including ligaments, capsule, synovium, and bone marrow changes. It should be stressed however that although MRI is useful in detecting lesions that are not possible with radiographs, they may not add to the clinical picture when plain radiographs demonstrate osteoarthritic changes. Further imaging beyond radiographs in clinical practice should be considered when the results will affect the management of the patient.

There is an array of studies using MRI as a semi- and quantitative imaging tools in the assessment of osteoarthritis. These studies are focused predominantly on the knee, hand, and hip. The required high-resolution imaging required for detailed cartilage review is mainly confined to research studies at this time due both to the extended time required to acquire and interpret these sequences and whether these sequences affect clinical management beyond currently performed sequences. Ultrasound is used predominantly for assessment of joint fluid and synovitis. Ultrasound can also demonstrate associated changes such as joint space loss, cartilage thinning and loss in the periphery of superficial joints, cortical irregularity, and osteophytosis. Nuclear medicine is generally not indicated.

The description of osteoarthritis will be subdivided into primary OA of large and small joints with typical examples provided.