An os trigonum is a small extra (accessory) piece of bone in the back of the ankle. While up to 15% of people have this extra bone, it usually does not cause any symptoms. It may be present only on one side but can be found in both feet. An os trigonum does not move as it attached by thick tissue to the talus bone just behind the ankle joint.
Os trigonum syndrome is the term used when it becomes painful. This is due to the piece of bone and surrounding tissue becoming irritated and inflamed. It may happen as a result of a single injury or due to repetitive/overuse injury such as pointing the
toes in dancers, downhill running, or frequent kicking seen in soccer players. It has been termed a "nutcracker injury" as the os trigonum is compressed like a nut when the toes are pointed down.
Frequently, people experience a deep achy pain at the back of the ankle and pain when pointing the toes. It also can be associated with pain when simply moving the big toe because the big toe tendon runs close to the os trigonum at the back of the ankle. Often symptoms are worse when patients are active and improve with rest. As it is quite deep at the back of the ankle, swelling is not common.
Patients are usually successfully treated with rest, ice, anti-inflammatory medication, and occasionally immobilization in a walking boot. Sometimes an injection of steroid medications will be considered to relieve the pain and settle the inflammation. If pain persists despite these therapies, surgery can be considered. This simply involves removal of the bone and inflamed tissue which can be performed either by an open procedure or arthroscopically using two small incisions.
Complete healing usually takes 4-6 weeks but depending on their activity level some people find they can have discomfort for several months. If surgery is performed, patients usually wear a surgical boot for 2-4 weeks to let the tissues heal. Activities can be resumed as symptoms allow, although the ability to run and perform jumping activities comfortably may take 3-6 months.
All surgeries come with possible complications, including the risks associated with anesthesia, infection, damage to nerves and blood vessels, and bleeding or blood clots. Also, there are two main sensory nerves on either side at the back of the ankle
which are at slight risk, but the surgical approach generally avoids them.
Did this occur because I broke a bone in my ankle?
No. The os trigonum is a naturally occurring bone in up to 15 percent of people who are walking around with no symptoms in this area. It is joined to the rest of the ankle bone by
a thick but slightly flexible cartilage. Some people can develop pain if they damage this attachment. As noted above, this damage can occur either with repetitive small injuries or by one big injury. There can be a broken bone in this same area, but
generally it looks quite different on X-ray.
Does the removal of this bone affect my athletic ability?
No studies to date show an impact on athletic ability. Most people improve because they no longer have the pain, and most people get back to full athletic activities. It can
take several months to get there to allow for slow healing.
Original article by Jason Bariteau, MD
Last reviewed by Raymond Walls, MD, 2020
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