Joints

Septic arthritis —including tuberculosis. Marked juxtaarticular osteopenia with loss of definition of subchondral bone plate.

Trauma —evidence of fracture or lipohaemarthrosis.

Osteoarthritis (OA) —marginal osteophytes, subchondral sclerosis and/or cysts, joint space narrowing. Commonly involves weight-bearing joints.

Calcium pyrophosphate deposition disease (CPPD) —chondrocalcinosis is characteristic; most commonly seen in knee, wrist, symphysis pubis and intervertebral discs. Arthropathy mimics OA but has a characteristic distribution: radiocarpal, patellofemoral, C1/2. Large clustered subchondral cysts are common.

Gout —well-defined juxtaarticular erosions + periarticular soft tissue mass ± calcification (tophi—highly suggestive). The joint space and bone density are relatively preserved. Most commonly involves the hallux MTPJ; other sites include intertarsal, ankle and knee.

Neuropathic (Charcot) arthropathy —seven Ds: joint d egeneration, d estruction, d islocation, d eformity, d ebris (loose bodies), d istension (effusion) and increased d ensity (subchondral sclerosis). Due to reduced pain sensation and proprioception, most commonly seen in diabetics. The joint involved suggests the underlying cause:

• (a)

  Ankle/foot — diabetes , alcoholism, dysraphism, leprosy, congenital insensitivity to pain (young patients, multiple involved joints), amyloidosis, neurosyphilis.

• (b)

  Knee —steroid injection, alcoholism, congenital insensitivity to pain, neurosyphilis.

• (c)

  Hip —alcoholism, steroid injection, neurosyphilis.

• (d)

  Spine —spinal cord injury, diabetes, syrinx (C-spine), neurosyphilis (L-spine).

• (e)

  Shoulder/elbow —syrinx.

• (f)

  Wrist —diabetes, syrinx.

• (g)

  Small joints of hand/feet —diabetes, leprosy, congenital insensitivity to pain.

Avascular necrosis —e.g. in hip. Can mimic OA but changes mainly involve one side of the joint with relative preservation of the joint space.

Monoarticular presentation of a usually polyarticular arthritis —see Section 3.2 .

Viral —typically hips in children, a.k.a. ‘transient synovitis’.

Pigmented villonodular synovitis (PVNS) —well-defined erosions + synovial nodules on MRI showing low T2 signal and blooming on the gradient echo due to haemosiderin. Most cases occur in the knee; less commonly other large joints. A localized form limited to Hoffa's fat pad can also occur.

Synovial osteochondromatosis —multiple intraarticular loose bodies + calcification. Can be primary (uniform in size) or secondary (different sizes + coexistent OA).

Rheumatoid arthritis* —proximal and symmetric distribution, especially MCPJs and carpus; other sites include feet (especially 5th MTPJ), knees, ankles, elbows and shoulders. Features include synovitis, juxtaarticular osteopenia, joint space narrowing, periarticular erosions, subluxation and ankylosis.

Juvenile idiopathic arthritis* —presents in childhood. Common joints include hips, knees, ankles, elbows, hands and feet, C-spine. Features include soft tissue swelling, juxtaarticular osteopenia, periostitis, epiphyseal overgrowth, joint effusions and subluxation (e.g. carpus, hip, C1/2). Periarticular erosions, joint space narrowing and ankylosis are late features.

Seronegative spondyloarthropathies —enthesitis + proliferative entheseal erosions with preserved bone density; classically asymmetrical, oligoarticular involvement of peripheral joints.

• (a)

  Psoriatic arthritis —usually asymmetric and distal (especially distal interphalangeal joints). Can present with dactylitis (‘sausage digit’). Occasionally symmetrical, resembling rheumatoid arthritis. Pencil-in-cup appearance and arthritis mutilans in severe disease. Acroosteolysis and interphalangeal joint ankylosis may be seen and are suggestive.

• (b)

  Chronic reactive arthritis* —usually seen in young adults, following an STI or gut infection. Similar in appearance to psoriatic arthritis but the hands are typically spared. Commonly involves the feet, SIJs and spine. Calcaneal enthesitis (+ erosions) is suggestive. Dactylitis may also be seen.

• (c)

  Ankylosing spondylitis* —typically involves SIJs and the spine. Other joints include hips, shoulders and knees.

• (d)

  Enteropathic arthritis —associated with IBD. Distribution is similar to ankylosing spondylitis.

Osteoarthritis —e.g. hands; involves interphalangeal joints, thumb carpometacarpal and triscaphoid joints. The erosive form causes central erosions resulting in a ‘gull-wing’ deformity ± ankylosis.

Neuropathic arthropathy .

Gout .

CPPD .

Haemochromatosis* —resembles CPPD but has a predilection for the index and middle finger MCPJs with hook-like osteophytes.