What is the acceptable range for mortise widening in a displaced distal fibular fracture?

Acceptable Range for Mortise Widening in Displaced Distal Fibular Fractures

A medial clear space of less than 4 mm is considered the acceptable range for mortise widening in a displaced distal fibular fracture, confirming stability of the ankle joint. 1

Anatomical Considerations and Significance

The ankle mortise is a critical structure for ankle joint stability. When evaluating displaced distal fibular fractures, understanding the acceptable parameters for mortise widening is essential for proper treatment decisions:

• The medial clear space measurement represents the distance between the medial malleolus and the talus
• Widening beyond 4 mm indicates instability of the ankle joint that may require surgical intervention
• Mortise widening directly impacts tibiotalar contact area - widening by just 1 mm can decrease tibiotalar joint contact area by 42%, potentially leading to instability and early osteoarthritis 2

Diagnostic Evaluation

Proper radiographic assessment is crucial for accurate measurement of mortise widening:

• Standard three-view radiographic protocol should include:

  • Anteroposterior view
  • Lateral view
  • Mortise view (internal rotation view)
  • These should include visualization of the base of the fifth metatarsal 1

• Weight-bearing radiographs provide the most valuable information for assessing stability, particularly with fractures of uncertain stability 1

• The gravity stress view is more reliable and easier to perform than manual stress views in supination-external rotation injuries with suspected deltoid ligament disruption 1

Treatment Algorithm Based on Mortise Widening

1. Medial clear space <4 mm: Confirms stability and may be suitable for non-operative management 1

2. Medial clear space ≥4 mm: Indicates instability that typically requires surgical intervention

3. Additional factors suggesting instability:

   • Medial tenderness, bruising, or swelling
   • Fibular fracture above the syndesmosis
   • Bi- or trimalleolar fractures
   • Open fracture
   • High-energy fracture injury 1

Surgical Considerations

When surgical intervention is indicated due to unacceptable mortise widening:

• Restoration of fibular length is critical for normal ankle joint kinematics 3
• Care must be taken with syndesmotic screw placement, as overtightening can cause paradoxical widening of the lateral clear space
• Syndesmotic screws placed farther from the plafond (5 cm vs. 2 cm) cause less lateral clear space widening (0.34 mm/mm vs. 0.76 mm/mm) 4
• In elderly patients with displaced distal fibular fractures, minimally invasive intramedullary nailing may be considered as an alternative to traditional ORIF 5

Common Pitfalls and Caveats

1. Radiographic positioning: Improper positioning can lead to inaccurate assessment of mortise widening

2. Syndesmotic overtightening: Excessive compression of the fibula toward the tibia during syndesmotic fixation can paradoxically widen the lateral clear space 4

3. Rotational assessment: CT evaluation may be necessary in complex cases, as both supination-external rotation and pronation-external rotation fractures typically result in external rotation of the distal fibular fragment 6

4. Age considerations: While age 65 is often used as a threshold for surgical decision-making in fracture care, functional demand should be the primary consideration rather than chronological age 1

5. Follow-up imaging: Regular radiographic evaluation is essential to ensure proper alignment during healing, though evidence suggests that reducing the frequency of follow-up radiographs may not negatively impact outcomes 1