Gout and crystal arthritis

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 24 October 2026 · Level: Intermediate

Crystal arthropathies are caused by crystals deposited in and around joints. The two main types are gout (monosodium urate crystals) and calcium pyrophosphate deposition disease (CPPD, which includes acute "pseudogout"). A third, basic calcium phosphate (hydroxyapatite) deposition, causes calcific tendinitis. Each looks different on X-ray. The most important practical point is that early gout usually has a normal X-ray, and an acutely hot joint must have septic arthritis excluded by aspiration, whatever the film shows. Crystals and infection can coexist.

1. Gout: the signs

Gout typically causes attacks of intense joint pain and swelling. X-ray changes appear only after years of recurrent attacks or persistent high urate levels, as urate deposits (tophi) build up and erode bone.

• Punched-out erosions with sclerotic margins, often a little away from the joint itself (juxta-articular).
• Overhanging edges: a lip of bone that curves over the erosion, sometimes called the Martel sign or "rat-bite" erosion. This is the most characteristic feature.
• Joint space preserved until late in the disease.
• Bone density preserved: no periarticular osteopenia.
• Tophi: lumpy soft-tissue masses, denser than surrounding soft tissue, sometimes calcified, often next to erosions.
• Asymmetric involvement of several joints in chronic disease.

2. Where gout appears

• The first metatarsophalangeal (MTP) joint is the classic site (podagra).
• Other foot and ankle joints, the knee, the hand and wrist.
• The olecranon bursa at the elbow, and the helix of the ear (seen clinically, not on X-ray).
• Bursae and tendons, where tophi form soft-tissue masses.

Reading a foot film for gout

Start with the soft tissues before the bones. Look around the first MTP joint, the midfoot and the dorsum of the foot for lumpy, slightly dense swellings that do not follow the smooth outline of normal soft tissue. Then trace the cortex of each metatarsal head and phalangeal base, looking for a notch with a crisp sclerotic edge. Check whether the joint space is still preserved, and whether bone density next to the joint is normal: both point towards gout rather than rheumatoid arthritis. Finally, compare with any previous films. Gouty erosions enlarge slowly over years, whereas infection can destroy a joint within weeks. If you see a mixture of chronic gout and a new, ill-defined area of destruction, do not assume it is all gout: infection or a fracture may be superimposed, especially in a patient with diabetes or an ulcer over the joint.

3. Imaging beyond the plain film

• Joint aspiration with polarised light microscopy is the reference test for gout: needle-shaped, negatively birefringent urate crystals.
• Ultrasound can show the double contour sign, a bright line of urate coating the surface of the cartilage, as well as tophi and erosions.
• Dual-energy CT can map urate deposits in and around joints and is used in specialist centres.
• The 2015 ACR/EULAR gout classification criteria include imaging evidence (ultrasound or dual-energy CT urate deposition, and radiographic erosions) as supporting items.

Plain films are mainly useful in established gout, to assess damage and to exclude other causes such as fracture or infection.

4. CPPD: chondrocalcinosis

Chondrocalcinosis means calcification of cartilage. It is the radiographic hallmark of CPPD and becomes more common with age; it can also be found in people without symptoms.

• Hyaline (articular) cartilage: a thin line of calcification running parallel to and slightly separate from the bone surface, best seen in the knee.
• Fibrocartilage: fluffy or triangular calcification in the knee menisci, the triangular fibrocartilage complex (TFCC) of the wrist (just beyond the ulnar head), and the pubic symphysis.
• Calcification may also appear in joint capsules, tendons and ligaments.

A good screening set if CPPD is suspected is the knees (AP), the wrists (PA) and the pelvis (AP), because these are the commonest sites of chondrocalcinosis.

Acute CPP crystal arthritis ("pseudogout") causes a hot, swollen joint, most often the knee or wrist, in older adults. Aspiration shows rhomboid-shaped, weakly positively birefringent crystals, and is also needed to exclude sepsis.

5. Pyrophosphate arthropathy

Long-standing CPPD causes a degenerative arthritis that looks like OA, but in unusual places:

• The radiocarpal joint of the wrist, which can progress to scapholunate advanced collapse (SLAC wrist).
• The patellofemoral joint, sometimes in isolation.
• The second and third MCP joints, with hook-like osteophytes.
• The shoulder and elbow.

Joint space loss, prominent sclerosis and large subchondral cysts are common, and osteophytes may be variable. When it appears in a younger patient, or alongside hook osteophytes at the MCP joints, consider an underlying metabolic cause: haemochromatosis, hyperparathyroidism, hypomagnesaemia and hypophosphatasia (mnemonics vary).

Compare with primary OA in osteoarthritis on X-ray.

6. Calcific tendinitis (hydroxyapatite)

Deposits of basic calcium phosphate (hydroxyapatite) form dense, homogeneous, cloud-like or oval calcifications in tendons, without the linear pattern of chondrocalcinosis.

• The commonest site is the supraspinatus tendon of the shoulder, just above the greater tuberosity.
• Other sites include the hip (gluteal tendons), the longus colli tendon in the neck and the wrist.
• An acute attack can cause severe pain; during resorption the calcification may look less well defined.

Shoulder views are covered in shoulder dislocation and shoulder and clavicle fractures.

7. Pitfalls and mimics

• Normal X-ray in acute gout: a first attack usually shows only soft-tissue swelling. The diagnosis is clinical and by aspiration.
• Chondrocalcinosis is not proof of the cause of an acute hot joint. It is common in older people, and septic arthritis can coexist with crystals.
• Vascular calcification (in the popliteal or digital arteries) can mimic chondrocalcinosis. Vessels run in tubular lines away from the joint surface.
• Loose bodies and meniscal ossicles may mimic calcified cartilage.
• Rheumatoid erosions are marginal and ill-defined, with osteopenia and uniform narrowing; psoriatic erosions have fluffy new bone. Gout has sharp, sclerotic, overhanging margins. See inflammatory arthritis.
• Tophi versus tumour: a tophus can look like a soft-tissue mass eroding bone. Its location and the history usually help; if unsure, seek specialist review (see bone tumours).

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	Overhanging edges, preserved joint space, tophi; chondrocalcinosis in the knee and wrist; the crystals seen on aspiration.
Nurse / practitioner	A hot joint needs urgent assessment and aspiration, not just anti-inflammatories.
Junior doctor	Do not let a normal X-ray or a little chondrocalcinosis stop you from aspirating a hot joint.
Radiographer	Good soft-tissue detail on foot and hand films; the knees, wrists and pelvis screening set for suspected CPPD.
Advanced / FRCR	Pyrophosphate arthropathy in unusual joints, haemochromatosis patterns, and dual-energy CT for urate.

Practise on bone and joint X-rays →

References

1. Neogi T, et al. 2015 Gout classification criteria: an American College of Rheumatology/European League Against Rheumatism collaborative initiative. Ann Rheum Dis 2015;74(10):1789–1798.
2. Zhang W, et al. European League Against Rheumatism recommendations for calcium pyrophosphate deposition. Part I: terminology and diagnosis. Ann Rheum Dis 2011;70(4):563–570.
3. Radiology Masterclass. Arthritis gallery. radiologymasterclass.co.uk
4. Radiopaedia. Gout. radiopaedia.org
5. National Institute for Health and Care Excellence. Gout: diagnosis and management (NG219). nice.org.uk

For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.