An os trigonum is a small extra bone that sits behind the ankle, just off the back of the talus (the bone that connects your foot to your leg). About 9% of people have one, and most never know it’s there. It only becomes a problem when it gets pinched during certain movements, causing pain in the back of the ankle.
Where the Os Trigonum Sits
During childhood, the back of the talus bone develops a bony projection that normally fuses to the main bone as you grow. In some people, that piece never fuses. It remains as a separate little bone connected to the talus by fibrous tissue. This unfused fragment is the os trigonum.
The name comes from its triangular shape, formed by three surfaces: one facing the talus in front, one facing the heel bone below, and one in back where a ligament attaches it to the fibula. It sits just to the outside of the groove that houses the tendon controlling your big toe, which is why that tendon sometimes gets caught up in os trigonum problems.
About one-third of people who have an os trigonum have it in both ankles.
Why It Causes Pain
The os trigonum becomes symptomatic when it gets repeatedly compressed between the talus and the heel bone. This happens during forceful pointing of the foot (plantar flexion), which narrows the space at the back of the ankle. Think of it like a nutcracker: the two larger bones squeeze the small os trigonum between them.
Two patterns typically trigger this compression. The first is a single acute injury, like a forced hyperextension of the foot during a fall or tackle. The second, more common in athletes, is chronic repetitive microtrauma from activities that demand repeated toe-pointing. Over time, the soft tissues around the os trigonum become inflamed and thickened, worsening the compression and irritating nearby tendons and ligaments.
Who Gets Symptoms
Ballet dancers are the classic example because pointe work demands extreme plantar flexion with every step. But soccer players and basketball players are also at higher risk, since kicking and jumping load the back of the ankle in similar ways. Symptoms often first appear during adolescence, when the ossification process is still underway and the bone is most vulnerable to stress. These groups tend to get diagnosed more frequently than the general population simply because they push their ankles into the range of motion that provokes symptoms.
What It Feels Like
The hallmark symptom is a deep, aching pain at the back of the ankle, usually on the outer side. It worsens when you point your toes hard, push off while walking or running, or go up on your toes. You may notice tenderness when pressing the soft area between your Achilles tendon and the ankle bone. Some people also feel a vague stiffness or a sense that something is “catching” in the back of the ankle. The pain tends to build gradually with activity and ease with rest, though an acute injury can bring it on suddenly.
How It’s Diagnosed
A standard lateral (side-view) X-ray of the ankle can reveal an os trigonum as a separate bone fragment behind the talus. X-rays detect it in roughly 8% of feet. MRI picks it up more often, in about 24% of feet scanned, because it can also show the surrounding soft tissue inflammation, bone marrow swelling, and tendon irritation that confirm the os trigonum is actually causing symptoms rather than sitting there quietly.
One diagnostic challenge is distinguishing an os trigonum (which you’ve had since childhood) from a fresh fracture of the back of the talus. The key difference on imaging is the edges: an os trigonum has smooth, rounded borders from years of existing as a separate bone, while a fracture shows jagged, irregular edges. Clinical context matters too. If you’ve had a sudden injury and the pain is new, your doctor will look more carefully for fracture signs.
Conservative Treatment
Most people with os trigonum syndrome improve without surgery. The initial goal is calming the inflammation. That means rest from the activity triggering pain, ice several times a day, and anti-inflammatory medications. Avoiding positions that force the foot into extreme plantar flexion is critical during this phase. Some people need a walking boot to immobilize the ankle and let the irritated tissues settle down.
Once the acute inflammation is under control, physical therapy focuses on soft tissue work, stretching, and joint mobilization in the foot and lower leg. Rehabilitation typically starts with non-weight-bearing exercises and gradually progresses to balance drills and isometric strengthening. The goal is building stability around the ankle so the os trigonum encounters less stress during movement.
For people with more significant pain that isn’t responding to rest and therapy, a corticosteroid injection into the area around the os trigonum can reduce inflammation enough to allow progress in rehab. This is often done under ultrasound guidance to ensure accurate placement.
When Surgery Is Needed
If several months of conservative care don’t resolve symptoms, surgical removal of the os trigonum is the next step. The bone is simply excised, since it isn’t structurally necessary. Two approaches exist: open surgery through a small incision, and endoscopic (keyhole) surgery using a camera and small instruments through two tiny portals.
A randomized controlled study with five-year follow-up compared the two approaches in athletes. Both produced good results in terms of pain relief and function. But the differences in recovery were striking. Athletes who had endoscopic surgery returned to training in about 4.5 weeks, compared to nearly 10 weeks for open surgery. Return to their previous competitive level took about 7 weeks with the endoscopic approach versus nearly 12 weeks with open surgery. The complication rate was also dramatically lower: roughly 4% for endoscopic versus 23% for open.
What Recovery Looks Like
After surgical removal, you can put light pressure on the foot (touch weight-bearing) for the first two weeks. By weeks two to three, most people can bear full weight. Physical therapy starts a few weeks after the operation and focuses on restoring range of motion, strength, and balance. Low-impact daily activities resume within the first month or so. Getting back to vigorous sports like soccer or rugby typically takes two to three months, depending on how the ankle responds to progressive loading.
For the majority of people, removal of the os trigonum resolves the pain permanently, since the source of the impingement is gone. The ankle functions normally without the extra bone.