Principle of Syndesmotic Screws
Syndesmotic screws function to stabilize the distal tibiofibular joint by maintaining anatomic reduction of the syndesmosis while the injured ligaments heal, preventing pathologic widening of the ankle mortise that would otherwise lead to joint instability and post-traumatic arthritis.
Biomechanical Function
The syndesmotic screw serves as a temporary rigid fixation device that:
- Maintains anatomic reduction of the tibiofibular relationship during the critical healing period of the anterior and posterior tibiofibular ligaments and interosseous membrane 1, 2
- Prevents diastasis (pathologic widening) of the ankle mortise that occurs when syndesmotic ligaments are disrupted 3
- Restores contact forces between the talus and fibula that are reduced after syndesmotic injury 3
- Stabilizes the ankle mortise to allow proper load distribution across the tibiotalar joint during weight-bearing 1
Clinical Rationale for Use
Syndesmotic injuries represent unstable injuries requiring operative fixation when the syndesmosis is disrupted 4. The key principle is that:
- Early fixation and stabilization are superior to non-treatment or delayed treatment for unstable syndesmotic injuries 4
- MRI is the reference standard for determining the grade (1,2, or 3) of syndesmotic ligament injury, which is critical for treatment planning 5, 6
- Screw fixation remains the gold standard treatment for syndesmotic injury 4
Technical Principles of Screw Placement
Screw Configuration Options
Lag screw technique is biomechanically superior to tricortical screws with clamp reduction:
- Lag screws (3.5mm or 4.5mm) engaging both tibial cortices maintain significantly higher compression forces (112-131N) compared to tricortical screws with clamp reduction (23N after clamp release) 1
- Tricortical screws show transient compression that drops markedly when the reduction clamp is released, suggesting less reliable maintenance of syndesmotic reduction 1
- Both configurations successfully reduce ankle clear space, but lag screws maintain reduction more effectively during cyclic loading 1
Optimal Screw Position
- Placement 1.5 to 3.5 cm above the tibiotalar joint line is standard 2
- Ideal angle is 18.8° ± 5.6° from the perpendicular line of the second toe when the ankle is in neutral position with the second toe positioned anteriorly 7
- This angle is less than conventional methods currently used and provides more anatomically accurate reduction 7
Important Limitations and Considerations
Physiologic Micro-Movement
A critical caveat is that syndesmotic screws eliminate normal physiologic motion between the tibia and fibula:
- Motion between tibia and fibula develops despite screw fixation, as evidenced by lucent lines developing around the screw in two-thirds of patients before removal 2
- Screw fixation decreases joint range of motion and changes stress distribution around the ankle in non-physiologic ways 3
- The screw increases stress on the crural interosseous membrane 3
Complications and Sequelae
- Calcification of the interosseous membrane occurs in some patients, with progression to distal tibiofibular synostosis in others 2
- Screw breakage is rare when patients are mobilized before removal 2
- Long-term fixation is not physiologically suitable for the ankle joint and should be time-limited 3
Common Pitfalls to Avoid
- Failing to recognize syndesmotic injury in ankle fractures—use the crossed-leg test (pressure on medial knee producing syndesmotic pain) for clinical diagnosis 5, 8
- Inadequate assessment of injury stability—MRI should be used to grade the injury and guide treatment decisions 5, 6
- Malposition of screws due to improper angle—use the 18.8° angle referenced to the second toe rather than conventional estimates 7
- Prolonged screw retention—the screw should be removed after ligament healing is complete, as it restricts normal ankle biomechanics 3
- Using tricortical screws with clamp reduction alone—lag screw technique provides more reliable sustained compression 1