Thread by @Rheumat_Aravind

🧵 Enthesial Organ & Enthesitis: The Overlooked Core of Spondyloarthritis 📌

Understanding the enthesis is key to decoding SpA, PsA, dactylitis, and axial pain. It’s more than a tendon-bone junction.
Let’s explore why.
#Rheumatology #SpA #PsoriaticArthritis #MedTwitter @IhabFathiSulima @Janetbirdope @RheumNow @DrAkhilX @nileshnolkha

1️⃣ What is an Enthesis?
An enthesis is the site where tendons, ligaments, fascia, or joint capsules attach to bone.
🧩 Types:
• Fibrous enthesis – direct attachment
• Fibrocartilaginous enthesis – layered zones (e.g., Achilles, patellar tendon)

2️⃣ The Enthesial Organ: More Than a Point of Attachment
🧠 Concept by Benjamin & McGonagle
Includes:
• Enthesis
• Adjacent bone
• Bursa
• Fat pad
• Fibrocartilage
• Synovium
• Periosteum
These work together to distribute mechanical load.

3️⃣ Why is this Important?
🔍 Explains clinical & imaging findings:
• Bone marrow edema on MRI
• Synovitis near entheses
• Dactylitis as a result of pulley enthesitis
Enthesitis = inflammation of the whole enthesial unit, not just the tendon tip.

4️⃣ Pathogenesis: The Mechanoinflammatory Interface
⚠️ Repetitive microtrauma → immune activation in HLA-B27+ or PsA-prone patients
🧬 Cytokines involved:
• TNF-α
• IL-17A
• IL-23
Result = osteitis, bone proliferation, and ankylosis

5️⃣ Clinical Sites of Enthesitis
📍 Commonly affected:
• Achilles insertion
• Plantar fascia
• Patellar tendon (proximal/distal)
• Iliac crest, ischial tuberosity
• Costochondral junctions
• Epicondyles
Pain may mimic tendinopathy or overuse syndromes.

6️⃣ Clinical Clues to Enthesitis
✅ Localized tenderness over bony insertions
✅ Morning stiffness
✅ Pain on pressure, stress, or use
🧠 May occur with normal inflammatory markers

7️⃣ Imaging Clues
🟡 Ultrasound (First-line):
• Hypoechoic thickening
• Doppler activity = active enthesitis
• Calcification or erosions in chronic disease

🔵 MRI (Deeper insights):
• Bone marrow edema
• Enthesial + peri-entheseal inflammation
• Great for deep or pelvic entheses

8️⃣ Histology of Enthesitis
🧬 Early:
• Inflammatory infiltrates (T cells, macrophages)
• Neoangiogenesis
🧬 Late:
• Fibrosis
• Bone remodeling and new bone formation

9️⃣ Role in Disease Pathogenesis
🎯 Enthesitis is the primary lesion in:
• Ankylosing spondylitis
• Psoriatic arthritis
• Reactive arthritis
• Enteropathic arthritis
Also seen in:
• SAPHO syndrome
• Occasionally in SLE/RA (mechanical overuse)

🔟 Enthesitis = Driver of Dactylitis
“Sausage digit” =
• Flexor tenosynovitis
• Pulley enthesitis
• Edema of soft tissue
→ This supports the enthesitis-first hypothesis in PsA

1️⃣1️⃣ Enthesitis Indices
📊 Used in clinical trials and practice:
• LEI – Leeds Enthesitis Index (6 sites)
• MASES – Maastricht AS Enthesitis Score (13 sites)
• SPARCC – MRI-based scoring tool

1️⃣2️⃣ Management of Enthesitis
💊 Treatment approach:
• NSAIDs – Symptom relief
• Anti-TNF agents – First-line biologics
• IL-17 inhibitors – Great for axial + peripheral disease
• JAK inhibitors – Emerging evidence
⚠️ Local steroids: Useful, but avoid direct tendon injection

📌 Key Takeaway
The enthesis is not an isolated attachment.
It is a functional organ, central to the pathogenesis of SpA and PsA.
👉 Recognizing and treating enthesitis early prevents irreversible damage.

🔥 Save, share, and revisit this thread when managing spondyloarthritis.
Enthesitis may be subtle, but it’s not insignificant.

#RheumTwitter #Spondyloarthritis #Enthesitis #Ultrasound #RheumatologyEducation #MSK