Joint Radiology & Arthritis Imaging

Plain radiographs remain the cheapest, most accessible and most frequently tested modality for arthritis in NEET PG. The examiner's favourite trick is a single AP radiograph of the hand, knee, foot or pelvis with a one-line clinical vignette — your job is pattern recognition. Master the "soft-tissue, alignment, bone, cartilage" search and the disease-specific signatures below.

The ABCDES systematic search

Every arthritis film should be read in a fixed order so nothing is missed. The classic radiologic mnemonic is ABCDE'S:

• A — Alignment (subluxation, deformity, ankylosis)
• B — Bone mineralisation & Bony erosions (periarticular osteopenia vs preserved density)
• C — Cartilage / joint space (uniform vs non-uniform loss)
• D — Distribution (which joints, symmetry, axial vs peripheral)
• E — Erosions (marginal, central, periarticular, "punched-out")
• S — Soft tissue & Special features (swelling, tophi, calcification, periostitis)

High-yield: Two features decide most MCQs at a glance — bone density (osteopenia in RA, preserved/sclerotic in OA and gout) and distribution (symmetric small-joint MCP/PIP in RA, weight-bearing DIP/first CMC/hip/knee in OA).

Rheumatoid arthritis (RA) — the proliferative, erosive, symmetric arthropathy

Pannus (inflamed hypertrophic synovium) is the destructive engine. It erodes bone at the "bare areas" — the intracapsular zones not protected by articular cartilage — producing marginal erosions. Hyperaemia of chronic inflammation causes periarticular (juxta-articular) osteopenia, the earliest plain-film sign.

Radiographic features (in order of appearance)

1. Fusiform soft-tissue swelling around MCP/PIP → earliest change.
2. Periarticular osteopenia → earliest bony change.
3. Uniform/concentric joint-space narrowing (pannus destroys whole cartilage surface evenly).
4. Marginal erosions at bare areas (radial side of metacarpal heads classically).
5. Late deformities: ulnar deviation at MCP, swan-neck, boutonnière, Z-thumb, "main en lorgnette" (opera-glass hand) in arthritis mutilans.

Distribution and named signs

• Hands: MCP and PIP, sparing the DIP (opposite of OA/psoriasis). Bilateral and symmetric.
• Wrist: ulnar styloid erosion, scapholunate dissociation (Terry-Thomas sign), carpal crowding.
• Cervical spine: atlantoaxial subluxation — atlanto-dental interval >3 mm in adults (>5 mm in children) on flexion lateral view; a feared anaesthetic/intubation hazard.
• Feet: MTP erosions, often the earliest erosions in the body are at the 5th MTP head.

High-yield: RA = proliferative erosive arthritis with osteopenia + symmetric MCP/PIP involvement + uniform joint-space loss, characteristically DIP-sparing. No new bone formation, no osteophytes, no sclerosis (unless secondary OA supervenes).

Osteoarthritis (OA) — the degenerative, productive arthropathy

OA is "wear-and-tear" with attempted repair. Hence the radiology is productive (new bone) rather than destructive/osteopenic.

The four cardinal signs

Feature	Mechanism	Radiographic appearance
Non-uniform joint-space narrowing	Maximal cartilage loss at weight-bearing surface	Medial knee compartment narrowing; superior hip
Subchondral sclerosis	Eburnation of stressed bone	Increased density beneath cartilage
Subchondral cysts (geodes)	Synovial fluid intrusion via microfractures	Lucent rounded lesions
Osteophytes	Reparative new bone at margins	Beak-like spurs at joint edges

Distribution and eponyms

• Hands: DIP > PIP, and the first carpometacarpal (CMC) / trapezio-scaphoid joint. DIP osteophytes = Heberden nodes; PIP osteophytes = Bouchard nodes.
• Erosive (inflammatory) OA: central erosions of IP joints → "gull-wing" appearance, classically in middle-aged women.
• Hip: superolateral joint-space narrowing (vs axial/concentric in RA). Buttressing of the femoral neck.
• Knee: medial compartment narrowing → varus; weight-bearing/standing views are mandatory because supine films underestimate narrowing.
• Spine: disc-space narrowing, osteophytes, vacuum phenomenon, facet OA.

High-yield: OA = preserved bone density + non-uniform narrowing + sclerosis + osteophytes + cysts, targeting DIP, 1st CMC, hip, knee, spine. Bone density and absence of osteopenia separate it instantly from RA.

Gout — the erosive arthritis that "preserves" the joint

Chronic tophaceous gout deposits monosodium urate (MSU) crystals para-articularly. Tophi are radiolucent (urate is not radio-opaque unless calcified) but produce a characteristic dense soft-tissue mass that pressure-erodes adjacent bone.

Classic radiographic signature

• Punched-out / "rat-bite" erosions with sclerotic margins and an overhanging edge (Martel sign / overhanging margin sign) — the pathognomonic feature.
• Erosions are periarticular or even intra-osseous, often located away from the joint margin.
• Preserved joint space until late and no periarticular osteopenia (key contrast with RA).
• Asymmetric, eccentric soft-tissue tophi, sometimes with faint calcification.
• Classic site: first MTP (podagra), also tarsus, knee, hands, olecranon bursa.

High-yield: Gout = punched-out erosions with overhanging sclerotic margins + preserved joint space + maintained bone density + lumpy soft-tissue tophi. "Overhanging edge" is the single most examined eponymous sign.

Dual-energy CT (DECT) colour-codes urate deposits (green) and is the modern non-invasive confirmatory imaging when polarised microscopy is unavailable.

The big three at a glance

Parameter	Rheumatoid arthritis	Osteoarthritis	Gout
Process	Inflammatory, proliferative pannus	Degenerative, productive	Crystal deposition
Bone density	Periarticular osteopenia	Preserved / sclerotic	Preserved
Joint space	Uniform narrowing	Non-uniform narrowing	Preserved till late
Erosions	Marginal (bare area)	Absent (unless erosive OA)	Punched-out, overhanging edge
New bone	Absent	Osteophytes, sclerosis	Sclerotic erosion margins
Hand target	MCP, PIP (spares DIP)	DIP, PIP, 1st CMC	1st MTP, asymmetric
Symmetry	Symmetric	Often symmetric	Asymmetric
Soft tissue	Fusiform swelling	Minimal	Tophi

Diagnostic flow for a hand film: Look at bone density → osteopenic? think RA; preserved? think OA/gout → which row of joints? MCP/PIP = RA, DIP = OA → erosion character? marginal = RA, punched-out overhanging = gout, central gull-wing = erosive OA → new bone present? yes = OA.

Seronegative spondyloarthropathies & sacroiliitis

This group (ankylosing spondylitis, psoriatic, reactive, enteropathic arthritis) shares enthesitis, new bone formation, HLA-B27 association and sacroiliitis.

Ankylosing spondylitis (AS)

• Sacroiliitis is the radiological hallmark and usually the first finding — bilateral and symmetric.
• Spine: squaring of vertebral bodies (Romanus lesion / "shiny corners"), syndesmophytes (thin, vertical, marginal) → "bamboo spine"; "dagger sign" (ossified supraspinous/interspinous ligaments) and "trolley-track sign" (ossified ligaments + facet joints, three vertical lines).
• Complication: carrot-stick / chalk-stick fracture through the rigid spine after minor trauma.

Modified New York grading of sacroiliitis (radiographic)

Grade	Findings
0	Normal
1	Suspicious changes (blurring of margins)
2	Minimal sclerosis, some erosions; joint width normal
3	Definite sclerosis + erosions + widening/narrowing, partial ankylosis
4	Total ankylosis (fusion)

High-yield: Modified New York definite radiographic sacroiliitis = grade ≥2 bilateral OR grade 3–4 unilateral. MRI (STIR/fat-suppressed showing bone marrow oedema) detects active sacroiliitis years before the X-ray and underpins the ASAS criteria for axial spondyloarthritis.

Psoriatic arthritis (PsA) — the great mimic

• DIP predilection, asymmetric.
• "Pencil-in-cup" deformity (whittled proximal phalanx in expanded cup of distal bone) and arthritis mutilans / opera-glass hand.
• Erosion with adjacent proliferation ("mouse-ear" / fluffy periostitis) — destruction plus new bone, unlike RA.
• Ivory phalanx, acro-osteolysis, and bulky, asymmetric non-marginal syndesmophytes (paravertebral ossification) in axial disease.

High-yield: Reactive arthritis and PsA produce asymmetric, fluffy non-marginal syndesmophytes, whereas AS produces thin, symmetric marginal syndesmophytes.

Other named arthropathies to recognise

Septic arthritis

Rapid uniform joint-space loss, periarticular osteopenia, soft-tissue swelling and possible subchondral bone destruction. Radiographs lag clinically — MRI (effusion, marrow oedema) and joint aspiration are the real investigations of choice; never delay aspiration for imaging.

Neuropathic (Charcot) joint

The 6 D's: Destruction, Dislocation, Density increase, Debris, Disorganisation, Distension. Florid destruction with surprisingly little pain. Tarsometatarsal (Lisfranc) joint in diabetics; knee in tabes dorsalis; shoulder in syringomyelia.

CPPD (pseudogout)

Chondrocalcinosis — linear calcification of fibrocartilage (knee menisci, triangular fibrocartilage of wrist, symphysis pubis). May produce an OA-like picture in unusual joints (MCP, wrist, patellofemoral) — "OA where OA shouldn't be."

Haemophilic arthropathy

Recurrent haemarthrosis → dense effusions, epiphyseal overgrowth, widened intercondylar notch of the knee, squared patella, secondary OA. Knee, elbow, ankle.

Juvenile idiopathic arthritis (JIA)

Epiphyseal overgrowth/ballooning, gracile (slender) diaphyses, periostitis, early growth-plate fusion, and cervical apophyseal fusion. Ankylosis is more common than in adult RA.

Investigation of choice — what beats plain film

• Earliest erosions / synovitis (RA): MRI with contrast or musculoskeletal ultrasound with power Doppler (detects synovitis, erosions and effusion before radiographs).
• Active sacroiliitis: MRI (bone-marrow oedema on STIR) — the modality of choice in early axial SpA.
• Gout confirmation without aspiration: dual-energy CT.
• Septic arthritis / osteomyelitis: MRI, but diagnosis is by joint aspiration (definitive).
• Crystal identification (gout vs CPPD): polarised light microscopy of synovial fluid (needle-shaped, negatively birefringent MSU vs rhomboid, weakly positively birefringent CPPD) — the true gold standard, not imaging.

High-yield: "Investigation of choice for early erosions" = MRI/USG; "for active sacroiliitis" = MRI; "to confirm crystal arthritis" = synovial fluid polarised microscopy (gout) or DECT for urate mapping.

Key differentials by single radiographic clue

• DIP involvement → OA (Heberden), psoriatic arthritis, erosive OA — never classic RA.
• Periarticular osteopenia + symmetric MCP/PIP → RA.
• Overhanging margin erosion → gout.
• Pencil-in-cup → psoriatic arthritis.
• Bamboo spine + bilateral sacroiliitis → ankylosing spondylitis.
• Chondrocalcinosis → CPPD/pseudogout (also hyperparathyroidism, haemochromatosis, hypophosphatasia — mnemonic "3 H's").
• Gull-wing erosions → erosive (inflammatory) OA.
• 6 D's of joint destruction → neuropathic (Charcot) joint.

Recently asked / exam angle

• A radiograph of the hand showing marginal erosions with periarticular osteopenia at the MCP joints, sparing DIP → diagnosis rheumatoid arthritis; earliest sign asked = periarticular osteopenia / fusiform soft-tissue swelling.
• Overhanging edge / Martel sign repeatedly asked as the X-ray feature of chronic tophaceous gout; pairs with "punched-out erosions with preserved joint space."
• Modified New York criteria — grade of sacroiliitis is a recurring single-best-answer (definite = grade ≥2 bilateral). MRI showing bone-marrow oedema = earliest/active sacroiliitis.
• Pencil-in-cup deformity matched to psoriatic arthritis; arthritis mutilans / opera-glass hand as the severe form.
• Atlantoaxial subluxation in RA — anaesthesia MCQs about airway/intubation risk; ADI >3 mm.
• Chondrocalcinosis on knee film → CPPD, with image-based "name the deposit site" (menisci, TFCC, symphysis pubis).
• Image-based "non-uniform vs uniform joint-space narrowing" to separate OA from RA — a classic two-image comparison MCQ.
• Bamboo spine, dagger sign, trolley-track sign, shiny corners (Romanus) matched to AS.
• Standing/weight-bearing knee view as the correct method to assess OA narrowing.

Rapid revision

1. RA = periarticular osteopenia + symmetric MCP/PIP + uniform narrowing + marginal erosions, spares DIP.
2. OA = non-uniform narrowing + sclerosis + osteophytes + cysts, normal bone density; targets DIP, 1st CMC, hip, knee.
3. Gout = punched-out erosion with overhanging sclerotic margin (Martel sign), preserved joint space, tophi.
4. Heberden = DIP, Bouchard = PIP nodes of OA.
5. Earliest RA bony change = periarticular osteopenia; earliest RA erosion often 5th MTP.
6. Atlantoaxial subluxation in RA: ADI >3 mm in adults — intubation hazard.
7. Sacroiliitis = first sign of AS, bilateral & symmetric; definite radiographic = grade ≥2 bilateral (modified New York).
8. MRI (bone-marrow oedema on STIR) detects active sacroiliitis before X-ray — best for early axial SpA.
9. Bamboo spine = thin marginal syndesmophytes; dagger and trolley-track signs = ossified ligaments/facets.
10. Pencil-in-cup + asymmetric DIP + fluffy periostitis = psoriatic arthritis.
11. Chondrocalcinosis (menisci, TFCC, symphysis pubis) = CPPD; OA in odd joints (MCP/wrist).
12. Charcot joint = 6 D's; gold standard for crystal diagnosis = polarised microscopy; DECT maps urate in gout.