Pain in the outer part of the mid-foot is common in runners and sportspeople. However, one rare but essential cause of lateral mid-foot pain is os peroneum syndrome. So, how do we diagnose os peroneum syndrome, and what can we do about it?

What is os peroneum syndrome? 

The os peroneum is a small bone within one of the tendons in the outer ankle called the peroneus longus. This small bone is only found in one out of 5 people or less. Bilateral ossicles occur in 60% of patients. We don’t know why some people develop an os peroneum. Sometimes, this small bone can cause pain outside the mid-foot called os peroneum syndrome.

Os peroneum syndrome causes  

Generally, problems with the os peroneum can occur from acute injury or overuse. For example, a fall or twist on the outer ankle can lead to a crack through this small bone. Alternatively, overload of the ankle leads to bone stress in the os peroneum or swelling of the peroneus tendon next to this small bone.

Diagnosis 

X-ray of os peroneum

Generally, a painful os peroneum causes pain and swelling in the outer ankle. In addition, the cuboid bone, which lies next to the os peroneum, is often tender to touch. Also, pain is reproduced by turning the foot outwards, called eversion.

Often, we use imaging to confirm the diagnosis and exclude other causes. Firstly, an X-ray shows the os peroneum as normal, bipartite, or fractured. If the bone is displaced away from its normal position, the peroneus longus tendon has torn. Secondly, an ultrasound reveals the small bone and surrounding problems of the peroneus longus tendon. Finally, MRI shows acute swelling or fracture of the os peroneum and peroneus longus tendonitis.

MRI of inflamed os peroneum

Other causes of mid-foot pain to think about  

In general, other causes of a painful mid-foot include peroneal tenosynovitis, a bone break of the cuboid, the base of the 5th metatarsal or calcaneus, and lateral ligament injury.

Os peroneum syndrome treatment 

Generally, we start with simple treatments. We suggest a Cam Walker boot for 3-4 weeks to rest the os peroneum. Anti-inflammatories such as ibuprofen and orthotics can help reduce the outer foot load.

In more complex cases, a cortisone injection into the peroneal tendon sheath next to the os peroneum can reduce swelling and improve symptoms. Finally, surgery to remove the bone or repair the tendon sheath might be necessary if other treatments fail.

More on injeciton for Os Peroneum Syndrome 

We reserve injections for cases that fail simple treatments. The needle is directed toward the peroneus longus tendon sheath next to the os peroneum. We think that cortisone reduces inflammation and pain from the os peroneum. Generally, we believe that injecting with ultrasound improves the accuracy and effectiveness of soft tissue injections and makes them safer.

Related conditions: 

Dr. Masci is a specialist sport doctor in London. 

He specialises in muscle, tendon and joint injuries.