Enthesitis

Terminology

• Enthesis: Insertion of tendon/ligament/joint capsule into bone ± adjacent fibrocartilage.
• Enthesopathy: Structural abnormalities at the enthesis (can be enthesitis (inflammatory) or mechanical, or degeneration)
• Enthesitis: Inflammatory change at the enthesis (hallmark feature of Spondyloarthropathy), subset of enthesopathy
• Enthesophyte: Bony spurs at insertion
  • Inflammatory (SpA-type): fluffy/ill-defined due to periostitis
  • Degenerative/mechanical: more sharply marginated along the line of pull.

Clinical

• Localized pain and tenderness at enthesis.
• Worse with use; inflammatory types may have morning stiffness/night pain
• Common sites:
  • Heel: Achilles, plantar fascia
  • Knee: patellar tendon, quadriceps tendon
  • Pelvis:
    • Hamstrings → ischial tuberosity
    • Adductors → pubis
    • Gluteal tendons→ greater trochanter
• Often coexists with other manifestations of SpA
  • Sacroiliitis
  • Dactylitis
  • Peripheral arthritis

Imaging Features

X-ray

• Often normal early
• Later:
  • Peri-entheseal erosions
  • Periostitis (fluffy new bone formation)
  • Enthesophytes
  • ± Adjacent osteosclerosis

Ultrasound

General tendinopathy signs at or just proximal to enthesis

• Thickening/fusiform enlargement
• Hypoechoic change (beware Anisotropy as a pitfall)
  Signs more specific to enthesitis
• Increased Doppler signal at or just deep to the bony insertion.
• Bony changes: erosions, small enthesophytes.
• Adjacent bursitis or soft-tissue oedema

MRI

• Oedema and enhancement at insertion ± adjacent bone marrow
• Subentheseal bone marrow oedema is very typical in SpA
• Associated findings:
  • Bursitis
  • Tendinopathy/partial tears
• Chronic findings:
  • Sclerosis
  • Fat metaplasia
  • Enthesophytes
  • Ankylosis (end-stage)

Buzzwords

• “Bone proliferation at enthesis” → SpA-type enthesitis
• “Retrocalcaneal bursitis” → Achilles enthesitis
• “Osgood-Schlatter disease"
  • Traction apophysitis at tibial tuberosity → enthesopathy of patellar tendon origin
  • Typically adolescent, athletic, often self-limiting.
• “Hot heel sign”
  • MRI: bone marrow and soft-tissue oedema at Achilles/plantar fascia insertions.
  • Think inflammatory heel pain, esp. in axSpA