Exam Detail:

Key Revision Focus: Anatomy of the Sinus Tarsi contents (Cervical Ligament vs Interosseous Talocalcaneal Ligament). The etiology is typically Subtalar Instability following inversion injury. Diagnostic injection is key.

1. Clinical Overview

Sinus Tarsi Syndrome (STS) is a clinical entity characterized by persistent anterolateral ankle pain and a sensation of instability, typically resulting from synovitis and fibrosis within the sinus tarsi space following an inversion injury. It is essentially the manifestation of Chronic Subtalar Joint Instability.

Clinical Pearl:

The "Eye of the Storm": The Sinus Tarsi is the pivot point for hindfoot motion. Pain here is usually due to excessive motion (instability), not checking. If a patient feels "unsafe" on uneven ground despite a stable ankle (ATFL intact), suspect the Subtalar Joint (Sinus Tarsi).

Key Concepts

1. Anatomy: The Sinus Tarsi is a conical tunnel between the talus neck and the calcaneus.
   • Lateral Opening: Wide.
   • Medial Exit: Tarsal Canal (Posterior to sustenaculum tali).
2. Ligaments (The Stabilizers):
   • Cervical Ligament: Strongest. Limits Inversion. Located laterally.
   • Interosseous Talocalcaneal Ligament (ITCL): In the Tarsal Canal. Limits Eversion. V-shaped.
   • Roots of Retinaculum: Inferior Extensor Retinaculum inserts here.
3. Pathology: An inversion sprain tears the ATFL and often the Cervical Ligament. This leads to rotary instability of the STJ -> Chronic Synovitis -> Scar tissue (Fibrosis) in the sinus.
4. Diagnostic Block: Relief of pain after injection of Lidocaine into the sinus tarsi is the gold standard for diagnosis.

Clinical Pearls

• Deep Paint: Patients describe the pain as "Deep inside" or "Feeling a hole".
• Uneven Ground: The classic complaint. Walking on flat surfaces is fine; cobbles or grass cause apprehension.
• The "Hissing" MRI: T1 images showing replacement of normal fat (bright) with scar/fluid (dark/grey) is the "Obliteration of Fat Sign".

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2. Epidemiology

• Prevalence: Common sequel to "Ankle Sprains" (up to 30% of chronic pain).
• Demographics: History of significant inversion injury (70% of cases).
• Associated Conditions:
  • Pes Planus (Impringement laterally).
  • Rheumatoid Arthritis (Inflammatory synovitis).
  • Gout.

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3. Pathophysiology

Mechanism of Injury

• Traumatic (70%): Supination/Inversion. The STJ opens up, tearing the Cervical Ligament. Bleeding occurs in the sinus -> Fibrosis.
• Non-Traumatic (30%):
  • Pes Planus (Flatfoot): Excessive valgus leads to lateral impingement of the talus on the calcaneus, crushing the contents of the sinus (Impringement STS).

The Cycle of Instability

1. Ligament tear (Cervical/ITCL).
2. increased STJ motion (micro-subluxation).
3. Synovial irritation.
4. Chronic inflammation and neural proliferation (free nerve endings abound in the sinus).
5. Proprioceptive deficit -> Further instability.

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4. Clinical Presentation

• Pain: Localized to the lateral opening of the sinus tarsi (just anterior to the lateral malleolus).
• Aggravation: Walking on uneven ground, cutting movements.
• Subjective Instability: "My ankle feels weak" (even if mechanical testing is stable).
• Stiffness: In the morning (inflammatory).

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5. Clinical Examination

1. Look:
   • Fullness in the sinus tarsi (loss of the normal depression).
   • Hindfoot alignment (Valgus? Varus?).
2. Feel:
   • Point Tenderness: Exquisite pain on deep pressure into the sinus opening.
3. Move:
   • Subtalar ROM: Inversion/Eversion usually causing pain at extremes.
   • Anatomical Instability Test: Use the "Drawer test" for the STJ (stabilize tibia, move calcaneus medially/laterally). Often difficult to detect clinically.
4. Proprioception: Single leg stance test (positive Romberg/wobble).

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6. Investigations

Diagnostic Injection (The Gold Standard)

• Inject 2-4ml of 1% Lidocaine into the sinus tarsi.
• Patient walks for 15 mins.
• Positive Test: Complete resolution of pain. Confirms the generator is within the sinus.

MRI

• Normal: Sinus tarsi contains bright fat on T1.
• Pathology:
  • Fat Obliteration: Replacement of bright fat with low/intermediate signal (scar/fluid).
  • Ligament Disruption: Absence of the black bands of Cervical/ITCL.
  • Synovitis: High signal on T2.

X-ray

• Usually normal.
• Rule out Tarsal Coalition or OA.
• Stress Broden View: Can show subtalar opening (rarely done).

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7. Management

ASCII Algorithm:

          Sinus Tarsi Pain
                 ↓
┌─────────────────────────────────┐
│     CONFIRM DIAGNOSIS           │
│ - MRI (Fat obliteration)        │
│ - Local Anesthetic Block (Pos)  │
└────────────────┬────────────────┘
                 ↓
┌────────────────┴────────────────┐
│      CONSERVATIVE  (3-6 mo)     │
│ 1. CSI Injection (Therapeutic)  │
│ 2. Physio (Proprioception)      │
│ 3. Bracing (Ankle Brace)        │
│ 4. Orthotics (Correct Flatfoot) │
└────────────────┬────────────────┘
                 ↓
           Failure?
                 ↓
┌─────────────────────────────────┐
│          SURGICAL               │
├────────────────┬────────────────┤
│   OPEN / SCOP  │   SALVAGE      │
│  DEBRIDEMENT   │                │
├────────────────┤   STJ FUSION   │
│ - Remove scar  │                │
│ - Synovectomy  │ (If instabilty │
│ - Reconstruction?  severe/OA)   │
└─────────────────────────────────┘

1. Conservative Management

• Corticosteroid Injections (CSI): Highly effective for inflammatory synovitis. Can act as a cure by breaking the cycle of inflammation.
• Physical Therapy: Focus on peroneal strengthening and proprioception (wobble board).
• Orthotics:
  • If Flatfoot -> Medial Arch Support (lifts talus, opens sinus).
  • If Varus -> Lateral Wedge.

2. Surgical Management

A. Arthroscopic / Open Debridement

• Procedure: Cleaning the sinus.
  • Synovectomy.
  • Removal of fibrotic fat pad ("Hofmann's Ligament" - not a real ligament, just scar).
• Outcome: Good relief of pain in 70-80%. Does NOT fix instability.

B. Ligament Reconstruction

• Reconstruction of the Cervical Ligament using tendon graft (Gracilis).
• Technically demanding. Rare.

C. Subtalar Arthroereisis (Implant)

• Procedure: Inserting a titanium screw/plug into the sinus tarsi to block eversion.
• Mechanism: Acts as a doorstop. Prevents excessive motion.
• Controversy: High rate of removal due to pain. Popular in Europe/Peds flexible flatfoot.

D. Subtalar Fusion (Arthrodesis)

• The Definitive Fix.
• Indicated if gross instability or degeneration present.
• Sacrifices motion for stability/pain relief.

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8. Complications

• Sural Nerve Injury: Nearby (lateral branch).
• Persistent Instability: If only debridement done on an unstable joint.
• Post-operative Stiffness: Loss of eversion (walking on uneven ground remains hard).

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9. Prognosis & Outcomes

• Conservative management effective in >60%.
• Debridement effective for pain but recurrence of instability common.
• Fusion successful but limits function.

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10. Evidence & Guidelines

Guidelines

• AOFAS: Diagnostic block recommended before surgery.

Landmark Trials

• O'Connor (1958): First described "Sinus Tarsi Syndrome". Proposed removal of fat pad.
• Lee et al: Arthroscopic debridement results.
  • Result: 85% good/excellent results in patients with confirmed synovitis on MRI.

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11. Patient Explanation

What is the Sinus Tarsi?

It is a small tunnel between your ankle bones (talus and heel bone). It is usually filled with fat and important ligaments that tell your brain where your foot is (balance) and stabilize standing.

What is the Syndrome?

After a sprain, the ligaments inside tore and bled. Instead of healing normally, the tunnel filled with scar tissue and inflammation. Now, every time you twist slightly (uneven ground), the scar gets pinched and hurts.

Treatment

We usually start by injecting a strong anti-inflammatory (steroid) into the tunnel to calm the swelling. Physiotherapy helps retrain the balance nerves. If that fails, we can clean out the scar tissue with keyhole surgery.

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12. References

1. O'Connor D. Sinus tarsi syndrome. A clinical entity. J Bone Joint Surg Am. 1958;40:720. (Original Description).
2. Lee KB, et al. Arthroscopic treatment of sinus tarsi syndrome: clinical results and review of the literature. Arthroscopy. 2008.
3. Klein MA, Spreitzer AM. MR imaging of the subtalar joint: synovitis, cystic lesions, and fluid collections. Magn Reson Imaging Clin N Am. 2017.
4. Helgeson K. Examination and intervention for sinus tarsi syndrome. North Am J Sports Phys Ther. 2009.
5. Kuwada GT. Long-term retrospective analysis of the treatment of sinus tarsi syndrome. J Foot Ankle Surg. 1994.
6. Giorgini RJ, et al. The sinus tarsi syndrome. J Am Podiatr Med Assoc. 2007.
7. Pisani G, et al. Sinus tarsi syndrome and subtalar joint instability. Clin Podiatr Med Surg. 2005.
8. Frey C, et al. Arthroscopic evaluation of the subtalar joint: does sinus tarsi syndrome exist? Foot Ankle Int. 1999.
9. Lektrakul N, et al. Tarsal canal and sinus tarsi: anatomy and MR imaging pathology. Radiographics. 2003.
10. Choudhary S, et al. Sinus tarsi syndrome. Br J Hosp Med (Lond). 2013.

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13. Examination Focus

Common Exam Questions (FRCS/Boards)

1. What ligaments are in the Sinus Tarsi? (Answer: Cervical Ligament and Interosseous Talocalcaneal Ligament / ITCL).
2. Which ligament limits inversion? (Answer: Cervical Ligament).
3. What is the classic MRI finding? (Answer: Obliteration of the fat signal on T1).
4. Confirm the diagnosis? (Answer: Lidocaine block into the sinus).
5. Differentiate from Ankle Sprain? (Answer: Tenderness is anterior to the malleolus for ATFL, but deep inside the sinus/inferior for STS. Instability feels different).

Viva "Buzzwords"

• "Obliteration of Fat"
• "Cervical Ligament"
• "Subtalar Instability"
• "Diagnostic Block"
• "Proprioceptive Deficit"

Common Pitfalls

• Operating without a block: Never debride unless you proved the pain source.
• Missing Tarsal Coalition: A coalition can present similarly with rigid sinus/pain. Check X-rays.
• Confusing with Peroneal pathology: They are neighbors. Palpate carefully.