Classic Radiological Signs: Arthritis

Read the pattern, not the sign. No single radiographic feature is diagnostic in isolation. Arthritis is diagnosed on plain film from the combination of distribution, bone density, joint-space change, erosion morphology and soft-tissue change, and the classic signs below are memorable shorthand for these combinations.

The same finding means different things. An erosion, for example, carries a different meaning depending on its site, its margins and the surrounding bone.

Five questions, in order. Answer them before committing to a diagnosis:

1. Distribution - which joints, and symmetric or asymmetric?
2. Bone density - periarticular osteopenia favours RA; preserved density favours crystal or seronegative disease
3. Erosions - marginal, central, or punched-out with an overhanging edge?
4. Bone production - osteophytes, periostitis, enthesophytes, ankylosis?
5. Soft tissue - fusiform swelling, tophi, dactylitis?

Distribution first, density second. These two alone separate most arthropathies, and naming the distribution first and the bone density second is the single most discriminating step in the viva. Alignment (subluxation, deformity, fusion) and cartilage and calcification (the joint space, chondrocalcinosis, tophi) complete the review.

Four patterns. Four patterns cover most cases: degenerative (osteoarthritis), inflammatory (rheumatoid arthritis), crystal (gout and CPPD) and seronegative (the spondyloarthropathies). Each has its own section below, and the table in the systematic approach sets the main patterns side by side.

The degenerative pattern. Osteoarthritis affects the weight-bearing joints, asymmetrically. Bone density is preserved or increased (sclerotic), the joint space is lost non-uniformly and osteophytes form. There are no erosions, except in erosive OA, where they are central.

Weight-bearing views. Request weight-bearing films for lower-limb OA. Non-weight-bearing films underestimate joint-space loss.

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Erosive OA. An aggressive form of OA affecting the DIP and PIP joints. Its central erosions create the gull-wing or seagull appearance. DIP involvement and the lack of periarticular osteopenia distinguish it from inflammatory arthritis.

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The inflammatory pattern. Classic RA affects the MCP and PIP joints and spares the DIP joints; OA, by contrast, affects the DIP and the 1st CMC. Wrist involvement is common in RA, and the distribution is typically symmetric between the hands.

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The seronegative pattern. The spondyloarthropathies are asymmetric and favour the DIP joints and the entheseal and axial sites. Their signature is bone proliferation (periostitis, enthesophytes, ankylosis) alongside erosion, with dactylitis and sacroiliitis. The symmetric sacroiliac involvement of ankylosing spondylitis in the table below is the exception to the asymmetry.

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Pencil-in-cup. Pathognomonic for psoriatic arthritis. Central erosion of the proximal phalanx forms the cup, and the pointed distal phalanx of the adjacent bone forms the pencil. In severe cases it is associated with arthritis mutilans.

The spine. The figures below set the marginal syndesmophytes of ankylosing spondylitis against the coarse, non-marginal paravertebral ossification of psoriatic spondyloarthropathy.

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The don't-miss monoarthritis. Septic arthritis is the critical differential for an acute monoarthritis, and the main alternative to gout. Its hallmark is rapid progression, over days to weeks rather than the months to years of the other arthropathies.

A normal film does not exclude it. As with the inflammatory pattern, the early radiograph can be normal. Septic arthritis is a clinical and aspiration diagnosis, never excluded by a normal film.

Aspirate. Any suspected septic joint must be aspirated. Ultrasound or MRI confirms an effusion and guides urgent washout.

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Tuberculous versus pyogenic. Tuberculous arthritis is more indolent and classically gives the Phemister triad:

• Juxta-articular osteoporosis
• Peripherally located bony erosions
• Gradual joint-space narrowing

Because the narrowing is gradual, the joint space is relatively preserved for longer than in rapidly destructive pyogenic infection.

The painless, destroyed joint. A neuropathic (Charcot) joint is strikingly destroyed yet relatively painless, with bone density preserved or increased. That combination of gross destruction, maintained density and little pain separates it from septic arthritis, where the joint is painful and osteopenic.

Causes. It follows loss of protective sensation:

• Diabetes mellitus, the commonest - the foot and midfoot, and the Lisfranc (tarsometatarsal) joints

• Tabes dorsalis - knee and hip

• Syringomyelia - shoulder and upper limb

• Spinal cord injury

  Mnemonic

  6 D'sNeuropathic Joint — the 6 D's

  D

  Density preserved or increased (not osteopenic)

  D

  Destruction of the articular surfaces

  D

  Dislocation / subluxation

  D

  Debris (intra-articular loose bodies and fragments)

  D

  Disorganisation (deranged, jumbled joint architecture)

  D

  Distension (effusion) — with gross Deformity

Atrophic and hypertrophic forms. Two forms are described:

• Atrophic (resorptive) - non-weight-bearing and upper-limb joints, with bone resorption and a tapered 'licked candy-stick' or 'surgical amputation' appearance
• Hypertrophic (productive) - weight-bearing and lower-limb joints, with sclerosis, osteophytes, fragmentation and debris

Mixed patterns are common in the diabetic foot.

Erosion morphology decides the diagnosis. Where the erosion starts and what surrounds it points to the disease:

• Marginal erosions begin at the bare area, the intracapsular bone not covered by cartilage, and are typical of RA
• Central erosions arise within the subchondral plate and suggest erosive OA
• Periarticular punched-out erosions with sclerotic margins and an overhanging edge of cortex (Martel sign) are characteristic of gout
• Erosion with adjacent fluffy bone proliferation favours a seronegative spondyloarthropathy

How far to trust crystal imaging. Against crystal confirmation, the ultrasound double-contour sign pools at 0.83 sensitivity and 0.76 specificity, and an ultrasound tophus at 0.65 and 0.80, so a negative scan is weak evidence. Pooled DECT sensitivity is 87-88% and specificity 84-90% (summary ROC 0.96) against crystal-confirmed gout. That is good, but it is the sensitivity that matters here, because a negative DECT does not exclude gout.

Two caveats compound it. Nearly all of the DECT studies enrolled longstanding disease (mean duration over 7 years), so the published accuracy overstates performance in the early presentation, where the diagnosis is genuinely difficult and the urate burden is low. DECT also generates artefact (nail-bed, beam-hardening, submillimetre vascular) that can be misread as urate. It does not replace aspiration in early or diagnostically uncertain cases.

Kellgren-Lawrence limitations. The KL grade emphasises osteophytes and weights joint-space narrowing inconsistently, correlates only moderately with symptoms, and has limited responsiveness for tracking progression. Quantitative joint-space width and MRI-based scores (e.g. semi-quantitative cartilage/BML scoring) are preferred for OA trials, though KL remains the pragmatic clinical standard.