Pain in the outer part of the midfoot is common among runners and other athletes. One rare but important 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?
Os peroneum syndrome is one of several peroneal tendon problems — but it sits further forward, in the outer mid-foot.
What is os peroneum syndrome?
The os peroneum is a small bone (an ossicle) within the peroneus longus tendon, near the outer mid-foot. It’s found in about one in five people or fewer, and, when present, it’s in both feet in about 60% of people. We don’t know why some people develop one. When this small bone becomes painful, we call it os peroneum syndrome.
Causes of os peroneum syndrome
Problems with the os peroneum follow either acute injury or overuse. A fall or twist on the outer ankle can crack the small bone. Alternatively, overloading the ankle can lead to stress on the os peroneum or swelling of the peroneus longus tendon adjacent to it.
Diagnosis
A painful os peroneum causes pain and swelling in the outer foot. The cuboid bone, which lies next to the os peroneum, is often tender, and pain is reproduced by turning the foot outward (eversion).
We usually use imaging to confirm the diagnosis and exclude other causes. An X-ray shows the os peroneum as normal (bipartite) or fractured; if the bone is displaced from its normal position, the peroneus longus tendon is likely torn. Ultrasound shows the small bone and any surrounding peroneus longus tendon problem, and MRI shows acute swelling or fracture of the os peroneum and any peroneus longus tendonitis.
Other causes of mid-foot pain to consider
Other causes of a painful outer mid-foot include
- a bone bruise or stress fracture of the cuboid, the base of the 5th metatarsal, or the calcaneus,
Os peroneum syndrome treatment
We start with simple treatments. Reducing sport or running activity helps symptoms, and in severe cases, a Cam Walker boot for 3–4 weeks can rest the os peroneum. Anti-inflammatories such as ibuprofen and orthotics to reduce the load on the outer foot also help.
In more complex cases, a cortisone injection into the peroneal tendon sheath next to the os peroneum can reduce swelling and improve symptoms. If other treatments fail, surgery to remove the bone or repair the tendon sheath may be needed.
More on injections for os peroneum syndrome
We reserve injections for cases that fail simple treatment. The needle is directed towards the peroneus longus tendon sheath next to the os peroneum, where cortisone reduces inflammation and pain. We use ultrasound guidance to improve the accuracy, effectiveness, and safety of the injection. See a practitioner experienced in injecting tendons under ultrasound.
Frequently asked questions about os peroneum syndrome
Where is the os peroneum pain felt?
On the outer part of the mid-foot, slightly forward of the ankle, often tenderness over the nearby cuboid bone. Pain is typically reproduced by turning the foot outward.
How is os peroneum syndrome diagnosed?
From the pattern of outer mid-foot pain and tenderness, confirmed with imaging. An X-ray shows whether the os peroneum is normal, bipartite, or fractured; ultrasound and MRI assess the bone and the surrounding peroneus longus tendon. A displaced os peroneum suggests a peroneus longus tendon tear.
Does os peroneum syndrome need surgery?
Rarely. Most cases settle with rest, a boot, anti-inflammatories, orthotics, and — if needed — an ultrasound-guided cortisone injection. Surgery (removing the bone or repairing the tendon) is reserved for cases that fail everything else or for cases where the tendon is torn.
Is os peroneum syndrome the same as peroneal tendonitis?
They’re related but different. Peroneal tendonitis is inflammation of the peroneal tendons behind the outer ankle; os peroneum syndrome involves the small bone within the peroneus longus tendon further forward, in the outer mid-foot.
How long does os peroneum syndrome take to settle?
With load reduction and a period in a boot where needed, many cases improve over several weeks. A fracture or an associated tendon tear takes longer to heal and may require an injection or surgery.
Final word from Sport Doctor London about os peroneum syndrome
Os peroneum syndrome is a rare cause of pain on the outer side of the midfoot. If you’re a runner with persistent outer mid-foot pain, see a doctor experienced in making this diagnosis — ultrasound expertise helps both confirm it and guide an injection if needed.
To book a one-stop foot and ankle assessment with Dr Masci in London, contact the team here or call +44 (0) 203 488 0350.
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