OS Peroneum - Left & Right Foot, Location, Ultrasound, Management

OS Peroneum:

• What is OS Peroneum?
• Left
• Right
• Location
• Ultrasound
• Management

What is OS Peroneum?

OS peroneum is a small accessory sesamoid bone found within the peroneus longus tendon of the foot. It is considered a normal anatomical variant and is present in a significant percentage of the population. The os peroneum plays a mechanical role by reducing friction and improving the leverage of the peroneus longus tendon as it curves around the lateral aspect of the foot. In many individuals, it remains completely asymptomatic and is discovered incidentally on imaging performed for other reasons.

Clinical importance arises when the os peroneum becomes involved in injury or degeneration of the peroneus longus tendon. Conditions such as fractures of the os peroneum, tendon tears, or chronic overuse can lead to lateral foot pain, collectively referred to as painful os peroneum syndrome. Understanding the normal anatomy and variations of the os peroneum is essential for accurate diagnosis and appropriate management, especially in patients presenting with unexplained lateral foot discomfort.

Left Foot

When the os peroneum is present in the left foot, it follows the same anatomical and functional principles as in the right foot. Symptoms related to the left os peroneum typically include pain along the outer border of the foot, especially during walking, running, or activities involving ankle eversion. Athletes and individuals with repetitive stress on the foot may be more prone to developing symptoms on the left side.

Imaging findings on the left side may show a single or multipartite os peroneum. It is important to distinguish a multipartite os peroneum from an acute fracture, as management differs significantly. Correlating imaging with clinical symptoms helps clinicians determine whether the left-sided os peroneum is an incidental finding or the true source of pain requiring treatment.

Right Foot

The right os peroneum is more frequently symptomatic in right-dominant individuals due to increased mechanical load. Pain on the lateral aspect of the right foot may worsen with push-off during gait, climbing stairs, or prolonged standing. In some cases, acute trauma such as ankle inversion injury can cause fracture or displacement of the os peroneum on the right side.

Right-sided os peroneum pathology may also be associated with peroneus longus tendon tears, tenosynovitis, or chronic degeneration. Identifying whether the pain originates from the bone itself or the surrounding tendon is crucial. Advanced imaging, including ultrasound or MRI, is often used to evaluate the right os peroneum and guide treatment decisions.

Location

The os peroneum is located within the peroneus longus tendon as it passes along the lateral side of the foot. Specifically, it is found near the cuboid bone, at the plantar-lateral aspect of the midfoot. This location allows the peroneus longus tendon to change direction smoothly as it courses under the foot toward its insertion on the first metatarsal and medial cuneiform.

Because of its position, the os peroneum is subjected to significant mechanical stress during walking and running. This explains why it can become symptomatic in cases of overuse or trauma. Understanding its exact location helps clinicians differentiate os peroneum-related pain from other causes of lateral foot pain, such as cuboid syndrome or fifth metatarsal injuries.

Ultrasound

Ultrasound is a valuable imaging tool for evaluating the os peroneum and associated peroneus longus tendon pathology. On ultrasound, the os peroneum appears as a hyperechoic structure with posterior acoustic shadowing within the tendon. Dynamic ultrasound examination allows real-time assessment of tendon movement and detection of tears or subluxation.

Ultrasound is particularly useful for distinguishing between a fractured os peroneum and a multipartite variant. It can also identify surrounding soft tissue inflammation, tendon thickening, or fluid accumulation. Because ultrasound is non-invasive, cost-effective, and widely available, it is often the first-line imaging modality in patients with suspected os peroneum-related pain.

Management

Management of os peroneum conditions depends on symptom severity and underlying pathology. Conservative treatment is the first approach and includes rest, activity modification, nonsteroidal anti-inflammatory drugs (NSAIDs), physical therapy, and footwear modification. Immobilization with a walking boot may be recommended in acute or painful cases.

If conservative measures fail or if there is significant tendon rupture or displaced fracture, surgical intervention may be considered. Surgical options include excision of the os peroneum with tendon repair or reconstruction. Early diagnosis and appropriate management are key to preventing chronic pain and restoring normal foot function.