What is a Large Os Peroneum?
A large os peroneum is an enlarged sesamoid bone embedded within the peroneus longus tendon near the cuboid bone in the lateral foot, which can cause cavovarus foot deformity and is associated with increased risk of tendon pathology.
Anatomic Definition and Clinical Significance
The os peroneum is a small accessory sesamoid bone located inside the peroneus longus tendon, positioned close to the cuboid bone in the lateral midfoot 1. It occurs in approximately 6.6% of feet, making it one of the most common accessory bones in the human foot 2.
When the os peroneum is "large," it has specific biomechanical and pathological implications:
Biomechanical Impact: A large os peroneum increases the mechanical advantage and power of the peroneus longus muscle, leading to a forefoot-driven, hindfoot varus deformity 3. Specifically, patients with a large os peroneum demonstrate on average 7.7° more apex dorsal angulation of Meary's angle (lateral talo-first metatarsal angle) compared to those without an os peroneum, and their kite angle (AP talocalcaneal angle) is 4.2° more varus 3.
Pathological Association: Large os peronea are prone to fracture, enlargement, and entrapment at the cuboid tunnel, which can cause complex, irreparable tears of the peroneus longus tendon 4. The enlarged bone can become fibrotic and adhered to surrounding tissues, preventing normal movement through the cuboid tunnel 4.
Clinical Presentation
Painful Os Peroneum Syndrome (POPS) is the primary clinical manifestation, characterized by:
- Pain localized on the lateral aspect of the cuboid area 1, 5
- Can result from acute trauma (ankle sprains) or chronic overuse 1
- May present with tenosynovitis or discontinuity of the peroneus longus tendon 5
Pathological Patterns
The os peroneum can be involved in three types of peroneus longus tendon tears based on location 1:
- Type I: Tears proximal to the os peroneum
- Type II: Tears at the level of the os peroneum
- Type III: Tears distal to the os peroneum
Each type presents with different changes in os peroneum morphology or position, which aids in diagnosis.
Diagnostic Imaging
The os peroneum is readily identified on:
- Plain radiographs (lateral foot views)
- Ultrasound for soft tissue evaluation
- MRI for comprehensive assessment of tendon pathology, edema, and surrounding soft tissue involvement 5, 6
Clinical Caveat
A large os peroneum is not merely an incidental finding—it represents a biomechanical risk factor for cavovarus foot deformity in patients without neurologic disease 3 and significantly increases the risk of peroneus longus tendon pathology requiring surgical intervention 4.