How do you differentiate between an avulsion fracture and an os peroneum?

Differentiating Between an Avulsion Fracture and Os Peroneum

The key to differentiating between an avulsion fracture and os peroneum is through radiographic assessment of fragment separation, displacement, and clinical context, with fragment separation of 6mm or more strongly suggesting an avulsion fracture rather than a normal os peroneum. 1

Initial Imaging Assessment

• Standard radiography with three views (anteroposterior, lateral, and mortise/oblique) should be the initial imaging modality for suspected foot or ankle trauma 2, 3
• On radiographs, an os peroneum typically appears as a sesamoid bone within the peroneus longus tendon near the lateral aspect of the cuboid 4
• Normal os peroneum location ranges from 7mm proximal to 8mm distal to the calcaneocuboid joint on lateral radiographs and from 9mm proximal to 8mm distal on oblique radiographs 1

Key Differentiating Features

Fragment Separation and Displacement

• Fragment separation of 6mm or more strongly suggests an avulsion fracture with associated full-thickness peroneus longus tendon tear 1
• Fragment separation of 2mm or less may represent either a non-displaced avulsion fracture or normal bipartite os peroneum 1
• Proximal displacement of the fragment by 10mm or more on lateral radiograph or 20mm or more on oblique radiograph indicates avulsion fracture with tendon disruption 1

Clinical Context

• Avulsion fractures typically present with acute onset of pain following trauma, inversion injury, or strong muscle contraction 4, 5
• Os peroneum is a normal anatomical variant present in approximately 26% of the population 4
• Pain localized to the lateral foot with history of direct trauma, inversion injury, or chronic overuse may suggest fracture of a pre-existing os peroneum 4

Advanced Imaging

When Initial Radiographs Are Equivocal

• CT without IV contrast can provide detailed assessment of osseous structures when radiographs are inconclusive 2
• MRI without IV contrast is superior for evaluating associated soft tissue injuries, particularly peroneus longus tendon integrity 2
• Ultrasound can be valuable for dynamic assessment and characterization of associated tendon injuries 2, 1

Specific Findings on Advanced Imaging

• MRI can detect bone marrow edema patterns in acute fractures, which would not be present in a normal os peroneum 2
• Ultrasound can demonstrate peroneus longus tendon disruption, partial tears, or tendinosis associated with os peroneum fractures 1
• CT can help identify subtle fracture lines not visible on conventional radiography 2

Common Pitfalls and Caveats

• Bipartite os peroneum (normal variant) may be misinterpreted as a fracture, but fragment separation is typically 2mm or less 1
• Minimally displaced fractures of the os peroneum may be overlooked on initial radiographs 6, 4
• Chronic pain following conservative treatment of an os peroneum fracture may indicate non-union requiring surgical intervention 7
• Lateral ankle pain following inversion injury may be misdiagnosed as a simple ankle sprain when an os peroneum fracture is present 4, 5

Management Implications

• Accurate differentiation is crucial as management differs between the two conditions:
  • Non-displaced or minimally displaced os peroneum fractures may be treated conservatively with restricted weightbearing and physical therapy 6, 4
  • Avulsion fractures with significant displacement or associated tendon injuries may require surgical intervention 5, 7
• Follow-up imaging is recommended to ensure proper healing and alignment 3