High-yield overview
Pattern recognition for OA, RA, crystal and seronegative arthropathies on plain film
—OA Key Features
4cardinal signs
—RA Key Features
—Periarticular changes
—Gout Key Features
—Overhanging edge
Arthritis Pattern Recognition
Degenerative (OA): Weight-bearing, sclerosis, osteophytes
Inflammatory (RA): Symmetric, periarticular osteopenia, erosions
Crystal (Gout): Asymmetric, preserved density, soft tissue tophi
Seronegative: Asymmetric, enthesitis, sacroiliitis
Key: Distribution and associated features distinguish arthritis types
Critical Must-Knows
- OA: Joint space narrowing, osteophytes, subchondral sclerosis, cysts
- RA: Periarticular osteopenia, marginal erosions, symmetric
- Gout: Punched-out erosions with overhanging edges, preserved density
- Psoriatic: Pencil-in-cup, periostitis, DIP involvement
Clinical Pearls
- “OA vs inflammatory: Distribution and presence of osteopenia
- “Gull-wing sign: Erosive OA of hands
- “Rat-bite erosions: RA marginal erosions
- “Ivory phalanx: Psoriatic arthritis
Clinical Warning
Arthritis radiological signs are commonly tested. You must distinguish OA from inflammatory arthritis patterns, know the distribution of different arthropathies, and recognise classic signs like overhanging edges in gout and pencil-in-cup in psoriatic arthritis.
Mnemonic
DDEBSThe Five Questions for Any Arthritis Film
D
D = Distribution (which joints? symmetric or asymmetric?)
D
D = Density of bone (osteopenia favours RA; preserved favours crystal/seronegative)
E
E = Erosions (marginal, central, or punched-out with overhang?)
B
B = Bone production (osteophytes, periostitis, enthesophytes, ankylosis?)
S
S = Soft tissue (fusiform swelling, tophi, dactylitis?)
Hook:Answer these five questions in order before committing to a diagnosis - distribution and bone density alone separate most arthropathies.
Overview & Principles
Why pattern recognition matters
No single radiographic feature is diagnostic in isolation. Arthritis is diagnosed on plain film by reading the COMBINATION of distribution, bone density, joint-space change, erosion morphology and soft-tissue change. The same finding (e.g. an erosion) carries a different meaning depending on its site, margins and the surrounding bone. The classic 'signs' below are simply memorable shorthand for these combinations.
Degenerative pattern
Osteoarthritis: asymmetric, weight-bearing joints, preserved or increased (sclerotic) bone density, osteophytes, non-uniform joint-space loss, no marginal erosions (except erosive OA).
Inflammatory pattern
Rheumatoid arthritis: symmetric, MCP/PIP and wrist, periarticular OSTEOPENIA, uniform joint-space loss, marginal (bare-area) erosions, soft-tissue swelling.
Crystal pattern
Gout: asymmetric, 1st MTP/midfoot, PRESERVED bone density and joint space until late, punched-out erosions with overhanging edges, soft-tissue tophi. CPPD: chondrocalcinosis and OA in unusual joints.
Seronegative pattern
Spondyloarthropathies: asymmetric, DIP and entheseal/axial sites, bone PROLIFERATION (periostitis, enthesophytes, ankylosis) alongside erosion, dactylitis, sacroiliitis.
Osteoarthritis Signs
Mnemonic
LOSSOA Cardinal Signs
L
L = Loss of joint space (narrowing)
O
O = Osteophytes (bone spurs)
S
S = Subchondral sclerosis (eburnation)
S
S = Subchondral cysts (geodes)
Hook:Joint space narrowing is the hallmark; the other features develop as the disease progresses
| Joint | Distribution | Specific Signs |
|---|---|---|
| Hip | Superior (weight-bearing) | Superior migration, osteophyte ring |
| Knee | Medial greater than lateral usually | Bone-on-bone, varus/valgus deformity |
| Hand DIP/PIP | Heberden's/Bouchard's nodes | Osteophytes at DIP/PIP |
| 1st CMC | Thumb base arthritis | Radial subluxation, squaring |
| Spine | Facet, uncovertebral | Disc space narrowing, osteophytes |
Erosive OA
Erosive OA is an aggressive form affecting DIP and PIP joints with central erosions creating a 'gull-wing' or 'seagull' appearance. Distinguished from inflammatory arthritis by DIP involvement and lack of periarticular osteopenia.
Rheumatoid Arthritis Signs
| Sign | Description | Location |
|---|---|---|
| Periarticular osteopenia | Reduced density around joints | Early sign, juxta-articular |
| Symmetric joint space narrowing | Uniform narrowing (not just weight-bearing) | All compartments affected |
| Marginal erosions | 'Rat-bite' erosions at bare areas | MCP, PIP, MTP joints |
| Soft tissue swelling | Fusiform swelling around joints | Early sign |
| Subluxations/deformities | Ulnar drift, swan neck, boutonniere | Late disease |
| Atlantoaxial subluxation | AADI greater than 3mm | Cervical spine involvement |
RA Hand Distribution
Classic RA affects MCP and PIP joints (spares DIP). Compare with OA which affects DIP and 1st CMC. Wrist involvement is common in RA. The distribution is typically symmetric between hands.
Crystal Arthropathy Signs
| Sign | Description | Significance |
|---|---|---|
| Punched-out erosions | Well-defined erosions with sclerotic margins | Tophaceous deposits |
| Overhanging edges | Erosion edge overhangs (Martel sign) | Pathognomonic for gout |
| Preserved bone density | No periarticular osteopenia | Unlike RA |
| Soft tissue tophi | Dense soft tissue masses | May calcify |
| Asymmetric distribution | 1st MTP, midfoot, ankle | Podagra classic |
| Preserved joint space | Until late disease | Unlike OA/RA |
Seronegative Arthropathy Signs
| Condition | Classic Signs | Distribution |
|---|---|---|
| Psoriatic arthritis | Pencil-in-cup, ivory phalanx, periostitis | DIP, asymmetric, dactylitis |
| Ankylosing spondylitis | Bamboo spine, SI fusion, squaring | Axial, symmetric SI joints |
| Reactive arthritis | Asymmetric oligoarthritis, calcaneal spurs | Lower limb, entheses |
| Enteropathic | Similar to AS, less severe | Axial, peripheral can occur |
Pencil-in-Cup Deformity
Pathognomonic for psoriatic arthritis. Central erosion of the proximal phalanx (cup) with pointed distal phalanx (pencil) of the adjacent bone. Associated with arthritis mutilans in severe cases.
Clinical Imaging & Modality Selection
| Modality | Strengths | Best role in arthritis |
|---|---|---|
| Plain radiograph | Cheap, available, documents structural damage, weight-bearing views possible | First-line and baseline; pattern recognition and progression tracking |
| Ultrasound | Detects synovitis (power Doppler), early erosions, crystal deposition (double-contour sign, tophi) | Early inflammatory arthritis; crystal disease at the bedside |
| MRI | Most sensitive for synovitis, early erosion and bone marrow oedema (predicts future erosion) | Early RA, axial spondyloarthropathy (sacroiliitis), pre-radiographic disease |
| Dual-energy CT | Colour-codes monosodium urate deposition; quantifies tophus burden | Confirming and mapping gout, especially when aspiration is difficult |
| CT | Bony detail, complex/axial joints | Erosion characterisation, surgical planning, sacroiliac joints |
Radiographic technique pearls
Request weight-bearing views for lower-limb OA (non-weight-bearing films underestimate joint-space loss). Always image both hands and feet in suspected inflammatory arthritis - feet often show the earliest erosions (5th MTP) even when hands look normal. Compare with prior films to detect progression and possible repair.
Clinical Warning
A normal plain radiograph does NOT exclude early inflammatory arthritis. Radiographic erosions are a late, cumulative marker. If the clinical suspicion is high and films are normal, proceed to ultrasound or MRI rather than reassuring the patient.
Systematic Approach & Differential Diagnosis
| Feature | OA | RA | Gout | Psoriatic |
|---|---|---|---|---|
| Distribution | Weight-bearing, asymmetric | Symmetric, MCP/PIP | Asymmetric, 1st MTP | Asymmetric, DIP, ray pattern |
| Bone density | Normal/sclerotic | Periarticular osteopenia | Preserved | Preserved or proliferative |
| Erosions | No (except erosive OA) | Marginal, rat-bite | Punched-out, overhanging edge | Marginal + central |
| Osteophytes | Yes | Uncommon | Uncommon | Uncommon |
| Periostitis | No | No | No | Yes |
| Soft tissue | Minimal | Fusiform swelling | Tophi | Dactylitis (sausage digit) |
Mnemonic
ABCDESABCDES Systematic Radiograph Review
A
A = Alignment and Ankylosis (subluxation, deformity, fusion)
B
B = Bone density (periarticular osteopenia vs preserved/sclerotic)
C
C = Cartilage and Calcification (joint space, chondrocalcinosis, tophi)
D
D = Distribution (symmetric vs asymmetric, which joints, DIP vs MCP)
E
E = Erosions (marginal/rat-bite, central, punched-out with overhang)
S
S = Soft tissues (fusiform swelling, dactylitis, tophi)
Hook:The single most discriminating step in the viva is naming the DISTRIBUTION first (which joints, symmetry) and the BONE DENSITY second (osteopenia points to RA; preserved density points to crystal or seronegative disease).
Erosion morphology decides the diagnosis
Marginal erosions begin at the 'bare area' (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.
Areas of Uncertainty & Controversies
Plain film is a late marker
Radiographic erosions reflect cumulative, often irreversible damage and appear late. In early inflammatory arthritis, ultrasound (power Doppler synovitis, early erosions) and MRI (synovitis, bone marrow oedema as a predictor of future erosion) are far more sensitive. A normal radiograph never excludes early RA.
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.
Is the overhanging edge truly pathognomonic?
The Martel overhanging-edge sign is highly suggestive but not 100% specific for gout; similar para-articular erosions can rarely occur in other tophaceous or aggressive crystal/erosive processes. Crystal confirmation (polarised microscopy) or dual-energy CT remains the reference standard.
DECT pitfalls
Dual-energy CT is sensitive for established tophaceous gout but can miss early disease (low urate burden) and produces artefact (nail-bed, beam-hardening, submillimetre vascular) that may be misread as urate. It does not replace aspiration in early or diagnostically uncertain cases.
Evidence Base
Evidence
Kellgren & Lawrence - Radiological assessment of osteo-arthrosis
Key Findings:
- The original description of the five-point (grade 0-4) radiographic grading scale for osteoarthritis, based on osteophytes, joint-space narrowing, subchondral sclerosis and bone-end deformity. It became the global reference standard adopted by the WHO for epidemiological definition of radiographic OA (commonly grade 2 or greater).
Clinical implication: The LOSS features you describe in the viva map directly onto the KL grade; quoting KL grade 2 or above as the threshold for definite radiographic OA is the expected examiner-level answer.
Source: Ann Rheum Dis 1957
Evidence
Taylor et al. - CASPAR classification criteria for psoriatic arthritis
Key Findings:
- Prospective international study of 588 PsA cases and 536 controls. The CASPAR criteria (inflammatory articular disease plus 3 or more points) include the radiographic feature 'juxta-articular new bone formation' alongside psoriasis, dactylitis, nail dystrophy and rheumatoid-factor negativity. Specificity 0.987 and sensitivity 0.914.
Clinical implication: Juxta-articular new bone formation (periostitis/proliferation) is the radiographic limb of the validated PsA criteria - it is the feature that distinguishes psoriatic erosive disease from the purely destructive erosions of RA.
Source: Arthritis Rheum 2006
Evidence
Ogdie et al. - Imaging modalities for the classification of gout (SLR & meta-analysis)
Key Findings:
- Systematic review against monosodium-urate crystal confirmation as gold standard. Ultrasound double-contour sign pooled sensitivity 0.83 and specificity 0.76; ultrasound tophus sensitivity 0.65 and specificity 0.80; dual-energy CT sensitivity 0.87 and specificity 0.84. Most studies involved longstanding disease (mean duration over 7 years).
Clinical implication: Advanced imaging (ultrasound double-contour sign, DECT) supplements plain radiography but performs best in established disease; plain films and these modalities under-detect early gout, where aspiration remains definitive.
Source: Ann Rheum Dis 2014
Evidence
Yu et al. - Diagnostic accuracy of dual-energy CT in gout (meta-analysis)
Key Findings:
- Meta-analysis of seven studies of DECT against the diagnosis of gout. Pooled sensitivity 88% (95% CI 84-90) and specificity 90% (95% CI 85-93), with area under the summary ROC curve of 0.96.
Clinical implication: DECT is a reliable non-invasive method for confirming urate deposition and quantifying tophus burden, but its imperfect sensitivity means a negative DECT does not exclude gout, particularly early disease.
Source: Skeletal Radiol 2018
Evidence
van der Heijde - Plain X-rays in RA: overview of scoring methods
Key Findings:
- Comparative review of the major radiographic damage scoring systems in rheumatoid arthritis - the Larsen method, the Rau-Herborn modification, the Sharp method and the van der Heijde-modified Sharp method - which separately quantify erosions and joint-space narrowing across selected hand and foot joints. The modified Sharp method is the most sensitive for detecting change in clinical trials; the Larsen method is faster for large datasets.
Clinical implication: When asked how radiographic progression in RA is quantified in trials, the modified Sharp/van der Heijde and Larsen scores are the expected answers, scoring erosions and joint-space narrowing separately.
Source: Baillieres Clin Rheumatol 1996
Evidence
EULAR recommendations for the use of imaging in inflammatory arthritis
Key Findings:
- EULAR guidance positions conventional radiography as the baseline for documenting structural damage, while recommending ultrasound and MRI for earlier detection of synovitis, erosions and (on MRI) bone marrow oedema that predicts subsequent radiographic erosion.
Clinical implication: Examiners expect candidates to state that a normal radiograph does not exclude early inflammatory arthritis and that US/MRI are the first-line modalities for early disease.
Source: EULAR (Ann Rheum Dis)
Guidelines, Registries & Global Practice
| Condition | Approximate global burden | Demographics |
|---|---|---|
| Osteoarthritis | Leading cause of disability in older adults worldwide; knee/hip OA among the highest-ranked musculoskeletal contributors to global years lived with disability | Prevalence rises sharply after age 50; knee OA more common in women |
| Rheumatoid arthritis | Roughly 0.5-1% of adults across most populations | Female predominance (around 3:1); peak onset 30-50 years |
| Gout | Most common inflammatory arthritis in men; prevalence rising with metabolic syndrome and ageing | Male predominance; increases with age, renal impairment and diuretic use |
| Psoriatic arthritis | Develops in roughly a fifth to a third of people with psoriasis | Equal sex distribution; typically 30-50 years |
| Body | Position on radiographs vs advanced imaging |
|---|---|
| EULAR (Europe) | Radiography for baseline structural damage; ultrasound and MRI recommended for early diagnosis and detecting subclinical synovitis/erosions |
| ACR (US) | Supports advanced imaging (US/MRI) where plain films are normal but inflammatory arthritis is suspected; DECT endorsed as an option to confirm urate deposition in gout |
| BSR / NICE (UK) | Emphasise early referral and treat-to-target in RA; baseline hand/foot radiographs at diagnosis with imaging to support early treatment decisions |
| ACR/EULAR gout criteria | Validated classification incorporates imaging evidence of urate (ultrasound double-contour sign or DECT) and radiographic gout-related erosion as weighted items |
High-resource setting
Ready access to MRI, musculoskeletal ultrasound with power Doppler, and dual-energy CT supports very early diagnosis, subclinical synovitis detection and quantitative monitoring. Treat-to-target strategies are guided by combined clinical and imaging remission.
Limited-resource setting
Plain radiography and clinical pattern recognition remain the mainstay. Confident use of distribution, bone density and erosion morphology on a single film, plus joint aspiration with polarised microscopy for crystals, allows accurate diagnosis without advanced imaging - which is why these classic signs remain essential exam knowledge worldwide.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
Viva scenarioStandard
Clinical prompt
“A patient presents with a painful, swollen 1st MTP joint. X-ray shows well-defined erosions with overhanging edges and preserved bone density.”
Practical approach
The diagnosis is gout. The classic radiological features are: (1) Punched-out erosions - well-defined erosions with sclerotic margins from tophaceous deposits. (2) Overhanging edges (Martel sign) - the erosion edge characteristically overhangs, which is pathognomonic for gout. (3) Preserved bone density - unlike RA, there is no periarticular osteopenia. (4) Asymmetric distribution - typically affecting the 1st MTP (podagra), midfoot, and ankle. (5) Preserved joint space until late disease. (6) Soft tissue tophi - may be visible as dense soft tissue masses that can calcify. The combination of punched-out erosions with overhanging edges and preserved bone density is virtually diagnostic of gout.
Key clinical points
Overhanging edges (Martel sign) pathognomonic
Punched-out erosions with sclerotic margins
Preserved bone density (unlike RA)
1st MTP classic location (podagra)
Preserved joint space until late
Common pitfalls
Confusing with RA (RA has osteopenia)
Missing the overhanging edge
Not recognising soft tissue tophi
Further questions
“How would you differentiate gout from pseudogout radiologically? What other investigations would you request?”
Viva scenarioChallenging
Clinical prompt
“A patient with long-standing psoriasis has hand pain. X-ray shows DIP joint erosions with a 'pencil-in-cup' appearance and periostitis along the phalanges.”
Practical approach
This is psoriatic arthritis. The pencil-in-cup deformity is pathognomonic - the proximal phalanx is eroded centrally (cup) while the distal phalanx becomes pointed (pencil). Key differences from RA: (1) Distribution - psoriatic affects DIP joints (RA spares DIP), can have ray pattern (whole digit), and is often asymmetric. (2) Periostitis - fluffy periosteal new bone is seen in psoriatic but not RA. (3) Bone density - preserved or even proliferative in psoriatic, whereas RA has periarticular osteopenia. (4) Dactylitis - sausage digit from soft tissue and joint swelling is classic for psoriatic. (5) Enthesitis - bone proliferation at tendon insertions. (6) Nail changes - clinically important as nail disease correlates with joint disease.
Key clinical points
Pencil-in-cup pathognomonic for psoriatic
DIP involvement (RA spares DIP)
Periostitis present (absent in RA)
Preserved bone density (RA has osteopenia)
Asymmetric, ray pattern involvement
Common pitfalls
Confusing with RA (different distribution)
Missing the periostitis
Not looking for associated skin/nail changes
Further questions
“What is arthritis mutilans? What spine changes occur in psoriatic arthritis?”
Viva scenarioChallenging
Clinical prompt
“An elderly patient has hand X-rays showing joint space narrowing, osteophytes, and central erosions at the DIP joints with a 'gull-wing' appearance.”
Practical approach
This is erosive osteoarthritis (EOA), an aggressive inflammatory variant of OA primarily affecting the DIP and PIP joints of the hands. The 'gull-wing' or 'seagull' sign describes the central collapse and marginal proliferation at the DIP joints creating a shape resembling a seagull in flight. Differences from standard OA: (1) Erosions - central erosions are present (standard OA has no erosions). (2) Inflammatory features - more swelling, redness, and pain than typical OA. (3) Characteristic pattern - affects DIP and PIP with gull-wing deformity. (4) Progression - can lead to ankylosis. Differences from inflammatory arthritis: (1) No periarticular osteopenia. (2) DIP involvement (RA spares DIP). (3) No systemic features or positive serology.
Key clinical points
Erosive OA = aggressive OA with central erosions
Gull-wing sign at DIP joints
Affects DIP/PIP (like OA distribution)
No periarticular osteopenia (unlike RA)
Can progress to ankylosis
Common pitfalls
Misdiagnosing as inflammatory arthritis
Missing the central erosion pattern
Not recognising the gull-wing sign
Further questions
“How would you manage erosive OA? What is the prognosis compared to standard OA?”
Exam day cheat sheet
Arthritis Signs Quick Reference
OA Signs (LOSS)
- L = Loss of joint space
- O = Osteophytes
- S = Subchondral sclerosis
- S = Subchondral cysts
RA Signs
- Periarticular osteopenia
- Marginal erosions (rat-bite)
- Symmetric MCP/PIP involvement
- Soft tissue swelling
Gout Signs
- Overhanging edges (Martel sign)
- Punched-out erosions
- Preserved bone density
- Soft tissue tophi
Psoriatic Signs
- Pencil-in-cup deformity
- Periostitis
- DIP involvement
- Dactylitis (sausage digit)