What is the appearance of an Achilles tendon disruption on X-ray (X-ray)

Appearance of Achilles Tendon Disruption on X-ray

X-ray is not the primary diagnostic imaging modality for Achilles tendon rupture, but it can provide valuable indirect signs that suggest disruption when properly evaluated. 1

Key Radiographic Findings on Lateral Ankle X-ray

Primary Indirect Signs

• Kager's Fat Pad Sign - Disruption of the normal triangular shape of Kager's fat pad with abnormal soft tissue density, showing the highest sensitivity (86.4%) and specificity (91.6%) among radiographic findings 2
• Tibio-First Metatarsal Angle - A more neutral or dorsiflexed ankle position with angle ≤100° (compared to normal 125° in intact tendons), with sensitivity of 61.7% and specificity of 78.9% 3, 2
• Tibiocalcaneal Angle - Altered angle between the tibia and calcaneus, with sensitivity of 65.6% and specificity of 56.2% 2

Mechanism

• The ankle adopts a more dorsiflexed position in Achilles tendon rupture due to loss of plantar flexion tension from the gastrocnemius-soleus complex 3
• This altered biomechanical relationship is visible on properly positioned lateral ankle radiographs 3, 2

Limitations of X-ray for Achilles Tendon Evaluation

• X-ray is not routinely recommended as the primary diagnostic tool for suspected Achilles tendon abnormality 1
• Up to 20% of Achilles tendon ruptures are misdiagnosed at initial presentation, often as ankle sprains 4, 2
• X-rays cannot directly visualize the tendon structure or evaluate the extent of disruption 1

Recommended Imaging Approach

Initial Evaluation

• Radiographs should be obtained to rule out fractures using Ottawa Ankle Rules if clinically indicated 1
• When examining lateral ankle radiographs, clinicians should specifically look for Kager's fat pad disruption as the most reliable radiographic sign 2

Definitive Imaging

• Ultrasound is recommended as the first-line imaging modality for suspected Achilles tendon rupture:

  • High sensitivity (94.8%) and specificity (98.7%) for complete ruptures 4
  • Can differentiate full-thickness from partial-thickness tears with 92% accuracy 1
  • Allows dynamic assessment and is more cost-effective than MRI 1

• MRI should be reserved for cases with equivocal clinical findings or when planning complex surgical interventions:

  • Excellent sensitivity (>90%) for diagnosing tendon tears 1
  • Detected 26 of 27 cases of tendinosis and partial rupture in one study 1
  • Can identify associated injuries that may affect treatment decisions 1

Clinical Correlation

• Radiographic findings should always be correlated with clinical examination, which remains the gold standard for diagnosis 1
• Physical examination should include two or more of the following tests:
  • Clinical Thompson test (Simonds squeeze test)
  • Decreased ankle plantar flexion strength
  • Presence of a palpable gap
  • Increased passive ankle dorsiflexion 1

Pitfalls and Caveats

• Relying solely on X-rays can lead to missed diagnoses of Achilles tendon ruptures 4, 2
• Proper positioning for lateral ankle radiographs is essential for accurate assessment of the indirect signs 3
• Ultrasound results are more dependent on operator skill and expertise compared to MRI or CT 1
• Even with normal radiographs, clinical suspicion should prompt further imaging with ultrasound or MRI 5