Syndesmosis Injury: Differential Diagnosis, Symptoms, and Mimicking Conditions

What is a differential diagnosis of syndesmosis?

A differential diagnosis of syndesmosis involves systematically distinguishing an injury to the distal tibiofibular joint (syndesmosis) from other ankle and lower leg conditions that present with similar pain and symptoms, ensuring an accurate diagnosis and appropriate treatment strategy.

Understanding the Syndesmosis

The syndesmosis is a critical fibrous joint located at the distal end of the tibia and fibula, just above the ankle joint. It comprises several strong ligaments:

• Anterior Inferior Tibiofibular Ligament (AITFL): Connects the anterior aspects of the tibia and fibula.
• Posterior Inferior Tibiofibular Ligament (PITFL): Connects the posterior aspects of the tibia and fibula.
• Interosseous Ligament (IOL): A strong, thick continuation of the interosseous membrane, running between the tibia and fibula.
• Transverse Ligament (TL): A deep portion of the PITFL, connecting the posterior malleoli.

These ligaments work collectively to maintain the stability of the ankle mortise, preventing excessive separation and rotation of the tibia and fibula, which is crucial for proper ankle function during weight-bearing activities. A syndesmosis injury, often referred to as a "high ankle sprain," typically results from forceful external rotation of the foot on a planted leg, often combined with dorsiflexion.

What is Differential Diagnosis?

Differential diagnosis is a systematic clinical process used by healthcare professionals to identify the most likely condition causing a patient's symptoms from a list of possibilities. It involves:

1. Gathering information: Detailed patient history, mechanism of injury, symptom characteristics.
2. Physical examination: Palpation, range of motion assessment, strength testing, and specific orthopedic special tests.
3. Diagnostic imaging: X-rays, MRI, or CT scans to visualize structures and rule out or confirm conditions.
4. Eliminating less likely conditions: Based on the gathered evidence, progressively narrowing down the list of potential diagnoses.

For syndesmosis injuries, this process is particularly vital because their symptoms can overlap significantly with more common ankle sprains and other lower leg pathologies, leading to misdiagnosis and potentially inadequate treatment if not properly differentiated.

Key Symptoms of Syndesmosis Injury

While a definitive diagnosis requires clinical examination, common symptoms associated with a syndesmosis injury include:

• Pain location: Typically felt higher up the ankle, above the malleoli, often described as deep pain in the anterior or anterolateral aspect of the lower leg.
• Aggravating factors: Significant pain with ankle dorsiflexion (especially forced dorsiflexion), external rotation of the foot, push-off during gait, and single-leg stance.
• Swelling and bruising: May be present, often localized around the distal tibiofibular joint.
• Difficulty with weight-bearing: Patients often report pain and instability, leading to a limping gait.
• Palpation tenderness: Direct tenderness over the AITFL, IOL, or PITFL, located superior to the ankle joint line.

Conditions Mimicking Syndesmosis Injury

The following conditions are commonly considered in the differential diagnosis of suspected syndesmosis injury:

• Lateral Ankle Sprain (Inversion Sprain):
  • Differentiation: This is the most common ankle injury, involving the anterior talofibular ligament (ATFL), calcaneofibular ligament (CFL), and posterior talofibular ligament (PTFL). Pain is typically located inferior to the lateral malleolus, often with a history of an inversion mechanism. Specific tests (anterior drawer, talar tilt) will be positive for lateral ligament laxity, while syndesmosis tests will be negative.
• Medial Ankle Sprain (Eversion Sprain):
  • Differentiation: Involves the strong deltoid ligament complex on the medial side of the ankle. Pain is localized inferior to the medial malleolus, often with a history of an eversion mechanism. While less common in isolation, it can occur concurrently with syndesmosis injuries, especially with significant external rotation forces.
• Ankle Fracture:
  • Differentiation: Fractures of the distal fibula, tibia (malleoli), or talus can present with acute pain, swelling, and inability to bear weight. Ottawa Ankle Rules are used to screen for the need for X-rays, which are essential to rule out bony injury. Syndesmosis injuries can occur with associated fractures (Maisonneuve fracture, Pilon fracture).
• Peroneal Tendinopathy or Subluxation:
  • Differentiation: Involves the tendons running behind the lateral malleolus (peroneus longus and brevis). Pain is typically posterior or inferior to the lateral malleolus, often exacerbated by eversion and plantarflexion. Subluxation may present with a snapping sensation.
• Achilles Tendinopathy or Rupture:
  • Differentiation: Pain is located at the posterior aspect of the heel or calf. A rupture presents with acute, sharp pain and a "pop," followed by inability to plantarflex (positive Thompson test). Tendinopathy causes gradual onset pain, often worse with activity.
• Tibialis Anterior Tendinopathy:
  • Differentiation: Pain along the anterior aspect of the shin or ankle, particularly with dorsiflexion. Tenderness is typically over the tibialis anterior tendon.
• Stress Fracture:
  • Differentiation: Can occur in the distal tibia, fibula, or talus. Presents with gradual onset pain, often worse with activity and relieved by rest. Point tenderness over the bone is common. X-rays may initially be normal, with MRI being more sensitive.
• Osteochondral Lesion of the Talus (OCLT):
  • Differentiation: Damage to the cartilage and underlying bone of the talus. Presents with deep ankle pain, often vague, and can include clicking, locking, or catching sensations, especially with weight-bearing. MRI is the diagnostic imaging of choice.
• Tarsal Tunnel Syndrome:
  • Differentiation: Entrapment of the posterior tibial nerve. Presents with burning pain, numbness, or tingling in the sole of the foot or toes. Pain is typically not localized to the syndesmosis and often has a neuropathic quality.

Clinical Examination and Diagnostic Tools

A thorough evaluation is paramount for accurate differential diagnosis:

Subjective History

• Mechanism of injury: Was there an external rotation and/or forced dorsiflexion component?
• Location of pain: Ask the patient to point precisely to the area of maximal pain.
• Aggravating/alleviating factors: What makes the pain better or worse?
• Functional limitations: How does it affect walking, stairs, sports?

Objective Examination

• Observation: Assess for swelling, bruising, and deformity.
• Palpation:
  • Syndesmosis: Tenderness directly over the AITFL, PITFL, and interosseous membrane.
  • Lateral ligaments: Tenderness over ATFL, CFL.
  • Medial ligaments: Tenderness over deltoid ligament.
  • Bony structures: Palpate malleoli, tibia, fibula, talus.
  • Tendons: Peroneal, tibialis anterior, Achilles tendons.
• Range of Motion (ROM): Assess active and passive ankle dorsiflexion, plantarflexion, inversion, eversion. Pain with forced dorsiflexion and external rotation is suggestive of syndesmosis involvement.
• Strength Testing: Assess plantarflexors, dorsiflexors, invertors, evertors.
• Special Tests for Syndesmosis:
  • Squeeze Test (Distal Tibiofibular Compression Test): Compression of the tibia and fibula together, typically about halfway up the calf. Pain elicited distally at the syndesmosis indicates a positive test.
  • External Rotation Test (Kleiger's Test): With the knee flexed to 90 degrees, the examiner stabilizes the tibia and externally rotates the foot. Pain at the syndesmosis suggests injury.
  • Fibular Translation Test: Stabilize the tibia and attempt to translate the fibula anteriorly and posteriorly at the distal joint. Increased translation or pain indicates instability.
  • Cotton Test: Assesses lateral talar shift by applying a lateral translation force to the foot.

Diagnostic Imaging

• X-rays:
  • Standard views: Anteroposterior (AP), lateral, and mortise views are essential to rule out fractures.
  • Weight-bearing views: Can reveal subtle widening of the syndesmosis (tibiofibular clear space, tibiofibular overlap) that might not be apparent on non-weight-bearing images.
  • Stress views: May be performed to assess instability under stress, though often painful.
• Magnetic Resonance Imaging (MRI):
  • Gold standard for soft tissue: Provides detailed images of ligaments (syndesmosis, lateral, medial), tendons, cartilage, and bone marrow edema. Highly effective in confirming syndesmosis injury and ruling out other soft tissue pathologies.
• Computed Tomography (CT):
  • Excellent for bony detail: Useful for assessing subtle fractures, osteochondral lesions, or assessing the tibiofibular clear space more precisely than X-ray, particularly in cases of suspected bony impingement.

Why Accurate Differential Diagnosis Matters

An accurate differential diagnosis of syndesmosis injuries is paramount for several reasons:

• Appropriate Treatment: Misdiagnosis can lead to inappropriate or delayed treatment. Syndesmosis injuries often require a longer recovery period and sometimes surgical intervention, unlike typical lateral ankle sprains.
• Optimized Rehabilitation: Knowing the specific structures involved guides the rehabilitation protocol, ensuring appropriate loading, protection, and progressive strengthening.
• Prevention of Chronic Instability: Undiagnosed or poorly managed syndesmosis injuries can result in chronic ankle pain, instability, and functional limitations due to persistent diastasis (widening) of the tibiofibular joint.
• Return to Activity: A correct diagnosis allows for a more realistic prognosis and timeline for return to sport or daily activities, minimizing re-injury risk.

In conclusion, distinguishing a syndesmosis injury from other ankle pathologies requires a meticulous approach combining a detailed patient history, a thorough physical examination, and judicious use of diagnostic imaging. This comprehensive process is crucial for effective management and optimal patient outcomes.