= uncommon autoimmune disease involving synovium + ligamentous attachments (= enthesopathy)with propensity for sacroiliitis / spondylitis classified as seronegative spondyloarthropathy 6/c

Frequency: 20% of patients with psoriasis (peripheral arthritis in 5%, sacroiliitis in 29%, peripheral arthritis + sacroiliitis in 10%)

Path: synovial inflammation (less prominent than in rheumatoid arthritis) with early fibrosis of proliferative synovium; bony proliferation at joint margins / tendon insertions / subperiosteum

Age of onset: 30–50 years; rare under the age of 13 years; M = F

Types:

1. true psoriatic arthritis (31%)
2. psoriatic arthritis resembling rheumatoid arthritis (38%)
3. concomitant rheumatoid + psoriatic arthritis (31%)

• plaque psoriasis = sharply demarcated erythematous plaques with thick white, laminated (“micaceous”) scale
  • Skin rash precedes / develops simultaneously with onset of arthritis in 85%!
  • Arthritis may antedate dermatological changes by an interval of up to 20 years!
• pitting, discoloration, hyperkeratosis, subungual separation, ridging of nails (in 80%)
• positive HLA-B27 in 80%
• negative rheumatoid factor (= seronegative spondyloarthropathy)

Location: usually asymmetric + oligoarticular; upper >lower extremities, sacroiliac joints, spine

Distribution: widely variable terminal interphalangeal joints, ray distribution, unilateral polyarticular

• NO / minimal juxtaarticular osteoporosis (early stage); frequent osteoporosis (later stages)
• marginal erosions in joints
• periarticular new bone formation (frequent)
• intraarticular osseous excrescences
• enthesitis (especially of calcaneus)
• Hand + foot
  Target area: DIP, PIP, MCP
  • CLASSIC “sausage digit” (40%) = nodular / irregular soft-tissue swelling of tendon sheath of entire digit ← dactylitis
    Path: flexor tendon tenosynovitis
  • increased tenosynovial fluid
  • solid hyperemic nodules at color Doppler
  • destruction of interphalangeal joint of 1^st toe with exuberant periosteal reaction + bony proliferation at distal phalangeal base (PATHOGNOMONIC)
  • destruction of distal interphalangeal joints (erosive polyarthritis) + osseous resorption
  • “pencil-in-cup” deformity = erosions with ill-defined margins + adjacent proliferation of periosteal new bone (CHARACTERISTIC)
  • bony ankylosis (10%)
  • ivory phalanx = sclerosis of terminal phalanx (28%)
  • poorly defined diffuse new bone formation at attachment of Achilles tendon + plantar aponeurosis
  • erosions at superior / posterior margin of calcaneus (20%)
  • acroosteolysis (occasionally)
• Axial skeleton
  • “floating” osteophyte = large bulky vertically oriented paravertebral soft-tissue ossification (AP view):
    • ill-defined excrescence sweeping across the diskovertebral junction from midportion of one vertebra to the next
    
    Location: lower cervical, thoracic, upper lumbar spine; asymmetric / unilateral
  • squaring of vertebrae in lumbar region
  • sacroiliitis (40%) = (most commonly) bilateral + asymmetric sacroiliac joint widening, increased density, fusion
  • apophyseal joint narrowing + sclerosis
  • atlantoaxial subluxation + odontoid abnormalities

DDx:

1. Reiter syndrome (affects mostly lower extremity)
2. Ankylosing spondylitis
3. Rheumatoid arthritis (bilaterally symmetric well-defined erosions, juxtaarticular osteoporosis, no new bone formation)