What does a medial clear space less than 4 mm indicate about ankle stability?

Medial Clear Space <4 mm Indicates Ankle Stability

A medial clear space (MCS) measurement of less than 4 mm on ankle radiographs confirms that the ankle mortise is stable and the deltoid ligament complex is intact, indicating no significant medial-sided injury requiring surgical intervention.

Understanding the Measurement

The medial clear space is the radiographic distance between the medial border of the talus and the lateral border of the medial malleolus on a mortise view. This measurement serves as a critical proxy for assessing the integrity of the deltoid ligament and overall ankle mortise stability.

Key Threshold Values

• <4 mm = Stable ankle: The deltoid ligament is intact and the ankle mortise maintains proper alignment 1, 2
• 4-5 mm = Borderline zone: Instability should only be considered if lateral talar shift exceeds 2 mm on stress examination 1
• ≥5 mm = Unstable ankle: Indicates deltoid ligament disruption requiring surgical consideration 3

Clinical Context and Application

Normal Physiologic Values

Research on uninjured ankles demonstrates that:

• Mean MCS in healthy ankles is approximately 3.1-3.3 mm 1, 4
• The MCS should be equal to or less than the superior clear space (which averages 3.6 mm) 5
• No physiologically healthy ankle widens beyond 4-5 mm even under external rotation stress 1

When to Measure

The MCS measurement becomes critical in:

• Isolated lateral malleolar (fibular) fractures where medial ligamentous injury must be excluded
• Weber B and C fractures where deltoid competency determines operative vs. non-operative management
• Ankle fracture-dislocations where initial MCS >9 mm predicts failure of closed reduction 6

Measurement Technique

For optimal accuracy: Measure the MCS halfway between the superior medial talar dome and the tip of the medial malleolus, perpendicular to the tibiotalar joint 7. This method demonstrates the best agreement with CT measurements and excellent interobserver reliability (ICC 0.82-0.97).

Common Pitfall

The superior clear space is typically 0.9 mm wider than the MCS 5. If the MCS exceeds the superior clear space, this suggests medial instability even if the absolute MCS value appears borderline.

Stress Radiography Considerations

When the initial MCS is borderline (3-4 mm) and medial injury is suspected:

• Weightbearing radiographs are more reliable than gravity stress views for determining true stability 8
• Gravity stress radiographs tend to overestimate instability and may lead to unnecessary surgical treatment 8
• If stress testing is performed, dorsiflexion with external rotation stress is most predictive, with ≥5 mm indicating deltoid disruption 3

Fracture Pattern Correlation

MCS widening correlates with injury severity:

• Weber A fractures: Mean MCS 3.3 mm (typically stable)
• Weber B fractures: Mean MCS 4.3 mm (variable stability)
• Weber C fractures: Mean MCS 5.7 mm (frequently unstable) 4

Additional medial or posterior malleolar fractures significantly increase MCS widening beyond the lateral malleolar fracture pattern alone 4.

Clinical Decision-Making Algorithm

1. MCS <4 mm: Ankle is stable → Non-operative management appropriate
2. MCS 4-5 mm: Check for lateral talar shift >2 mm on stress views → If present, consider operative treatment
3. MCS ≥5 mm: Ankle is unstable → Operative fixation indicated
4. Initial MCS >9 mm in fracture-dislocations: High risk of secondary displacement → Consider external fixation or acute internal fixation 6