Osteoarthritis is an extremely common degenerative disorder of the joints, and is a major cause of morbidity worldwide. Its incidence increases with age, but is also influenced by a number of genetic, metabolic and occupational factors. The principal radiographic features of osteoarthritis are shown in Box 26.1. The earliest change is the development of osteophytes – bony outgrowths usually at the joint margins. Subchondral bone sclerosis and bone cysts also occur relatively early. In more advanced disease, cartilage compression and degeneration lead to narrowing and eventually loss of the ‘joint space’ (actually cartilage, being radiolucent and appearing black) – see Figure 26.1. Eventually, bone remodelling and attrition result in flattening or deformities of the articular bone surfaces.

As well as aiding with diagnosis, X‐rays also provide information on severity. This is particularly useful in assessing the need for arthroplasty (joint replacement) – see Figure 26.2.

Radio‐opaque loose bodies may also be seen and are a relatively common finding on plain radiographs of the knee. They represent fragments of cartilage or bone which have broken free within the joint. They characteristically cause the symptom of mechanical ‘locking’ of the affected joint. They can sometimes be removed by arthroscopy.

Chondrocalcinosis is the radiological finding of calcific deposits within cartilage, usually in older patients. Common sites include the knee (Figure 26.3) and the triangular fibrocartilage at the wrist. In the correct clinical context, the presence of chondrocalcinosis can help to confirm a diagnosis of calcium pyrophosphate deposition (CPPD) disease. However, it can also be an incidental finding and is not necessarily of clinical significance.

Rheumatoid arthritis is the most common form of inflammatory arthritis. If not adequately treated, it can be highly erosive and lead to significant joint damage and deformity. The radiographic features of rheumatoid arthritis are shown in Box 26.2. Early features include soft tissue swelling, juxta‐articular (periarticular) osteopenia and the development of periarticular bone erosions (Figure 26.4). As with osteoarthritis, more advanced disease gives rise to narrowing and loss of the ‘joint space’ (cartilage). Ankylosis (fusion) of the joints may occur, most characteristically at the carpal bones. Joint subluxation leads to the classic deformities associated with rheumatoid arthritis (Figure 26.5). The most common patterns are radial deviation of the hand at the carpometacarpal joints and ulnar deviation of the fingers at the metacarpophalangeal joints.

Patients with suspected rheumatoid arthritis should undergo plain radiographs of both hands and both feet. The films of the feet are essential, even in patients who do not complain of foot symptoms. This is because the earliest erosive changes often occur in the joints of the feet (most commonly at the fifth metatarsophalangeal joint) and early inflammation may be subclinical.

Usually, serial radiographs are taken every 1–2 years, to ensure that there is no radiological progression. If new erosions develop, a patient’s treatment may need to be reviewed even if their disease appears clinically controlled.

Fortunately, due to the modern approach of early diagnosis and aggressive treatment, as well as powerful novel therapies (tumour necrosis factor alpha inhibitors and other biologic agents), there are now very few patients with severe or disabling joint damage. It is likely that there will continue to be a small number of patients in whom this cannot be avoided, due to severe disease phenotype or medications being either contraindicated or poorly tolerated.

Psoriatic arthritis also has the potential to be highly erosive in some patients. The radiographic features of psoriatic arthritis are similar to those of rheumatoid arthritis, and in some cases the two diseases can be difficult to distinguish.

Arthritis mutilans is a rare but highly destructive small joint arthropathy which can occur in either rheumatoid or psoriatic arthritis. In arthritis mutilans, joint destruction is so severe that the joint architecture is completely lost, resulting in ‘telescoping’ of the affected digit. The plain radiograph shows a characteristic ‘pencil‐in‐cup’ appearance (Figure 26.6).

Gout is another potentially erosive arthropathy. In gout, erosions tend to be juxta‐articular (para‐articular) rather than within the joint itself (Figure 26.7). The most classic site for these changes is the first metatarsophalangeal joint (usually in patients who have suffered recurrent episodes of podagra) but gout can occur in any peripheral joint. Tophi are another radiographic feature of gout.

Plain radiography is also useful in the diagnosis of axial spondylarthropathy (ankylosing spondylitis and related disorders). The radiological features of axial spondylarthropathy are shown in Box 26.3. Axial inflammation often (but not always) starts at the base of the spine and progressively ascends. For this reason, patients are usually assessed initially via plain radiographs of the lumbar spine and sacroiliac joints (Figures 26.8a and 26.8b). Vertebral squaring is seen on lateral views of the spine and is the earliest abnormality, caused by erosions at the corners of the vertebral bodies. Bone sclerosis due to repair at these sites gives rise to a ‘shiny corner’ appearance (Romanus lesions). In more advanced disease, there is ossification of the longitudinal ligaments and the outer fibres of the intervertebral discs, resulting in the formation of syndesmophytes. Eventually, this process of progressive fusion (ankylosis) spreads to involve the entire spine. The ‘bulges’ seen radiographically due to syndesmophytosis at each intervertebral level give a characteristic ‘bamboo’ appearance.