How can I determine whether a syndesmotic sprain is stable?

Determining Stability of Syndesmotic Sprains

To determine if a syndesmotic sprain is stable, perform a squeeze test and assess for deltoid ligament injury—a positive squeeze test makes instability 9.5 times more likely, and concomitant deltoid injury makes it 11 times more likely, indicating the need for surgical intervention rather than conservative management 1.

Clinical Assessment Algorithm

Initial Physical Examination

Start with these specific clinical maneuvers:

• Squeeze test: Compress the tibia and fibula at mid-calf level. Pain at the syndesmosis indicates likely instability 1
• Palpate for deltoid ligament tenderness: Medial ankle tenderness suggests combined injury, which strongly predicts instability 1
• Weight-bearing status: Inability to bear weight four steps immediately after injury raises concern for instability 2
• External rotation stress: Pain with external rotation of the foot suggests syndesmotic involvement 2

Key clinical predictors of instability:

• Positive squeeze test (9.5× increased likelihood of instability)
• Deltoid ligament injury (11× increased likelihood of instability)
• Combined anterior-inferior tibiofibular ligament (AITFL) and deltoid injury predicts delayed return to sport 1

Important Clinical Distinction

If the anterior talofibular ligament (ATFL) is also injured, the syndesmosis is LESS likely to be unstable 1. This indicates a different injury mechanism and typically allows earlier return to activity.

Imaging Strategy

Initial Imaging

Weight-bearing radiographs (anteroposterior, lateral, and mortise views) should be obtained first 3. Look for:

• Increased tibiofibular clear space (>4 mm suggests instability)
• Medial clear space widening
• Tibiofibular overlap changes

Advanced Imaging When Radiographs Are Inconclusive

MRI has 100% accuracy for detecting tibiofibular ligament tears 4 and should be obtained when:

• Clinical examination suggests instability but radiographs are normal
• You need to assess the extent of ligament injury (AITFL, deltoid, interosseous membrane)
• Persistent symptoms beyond 1 week warrant evaluation 5

MRI findings indicating instability:

• Complete AITFL tear
• Concomitant deltoid ligament injury
• Interosseous membrane disruption

Emerging Diagnostic Tools

Ultrasound can detect syndesmotic instability without stress application 6. After AITFL injury, the tibiofibular distance increases significantly even without external rotation stress (from 4.9 mm to 6.7 mm) 6.

Weight-bearing CT with 3D volume measurements up to 5 cm above the tibial plafond shows 95.8% sensitivity and 83.3% specificity for detecting subtle syndesmotic instability 7. This is particularly useful for cases that are difficult to diagnose clinically.

Classification for Treatment Decisions

Grade IIa (Stable)

• Negative squeeze test
• No deltoid injury
• Can bear weight
• Treatment: Conservative with boot and rehabilitation
• Expected return to sport: ~45 days 1

Grade IIb (Dynamically Unstable)

• Positive squeeze test
• Deltoid ligament injury present
• Combined AITFL and deltoid injury on MRI
• Treatment: Arthroscopic assessment and surgical stabilization
• Expected return to sport: ~64 days 1

Common Pitfalls to Avoid

1. Don't rely solely on initial examination: Swelling and pain within 48 hours can limit accuracy. Re-examine at 4-5 days post-injury when physical examination sensitivity (84%) and specificity (96%) are optimized 8

2. Don't miss the deltoid component: Isolated syndesmotic assessment without evaluating the deltoid ligament will miss a critical predictor of instability 1

3. Don't assume all syndesmotic injuries are unstable: The presence of ATFL injury actually suggests a MORE stable syndesmotic injury with better prognosis 1

4. External rotation stress is most revealing: Under loading, external rotation of the foot is the most responsive position for detecting isolated syndesmotic instability, showing both increased clear space and posterior fibular translation 9