The os trigonum is a small extra bone at the back of the ankle joint, present in roughly 10–15% of people. Occasionally, it causes ankle pain during sport — a form of posterior ankle impingement. So how do we recognise os trigonum syndrome, and what can we do about it? This page specifically covers the os trigonum. For the broader condition and its other causes, see posterior ankle impingement.
What is an os trigonum?
The os trigonum forms when one area of bone fails to fuse with the rest. During growth, an extra centre of bone develops behind the talus. When it doesn’t fuse with the talus, it remains as a separate small bone at the back of the ankle.
Around 10–15% of people have this extra bone. For most, it’s like the appendix — it sits there and causes no problem. But like the appendix, it can also become painful after an ankle twist or in sports such as ballet, running, and football.
How do we diagnose os trigonum syndrome?
An os trigonum can cause pinching at the back of the ankle when you point the foot, for example, a ballet dancer going into pointe or a footballer striking a ball. The pain is at the back of the ankle, a form of posterior ankle impingement, and is often mistaken for Achilles tendonitis. Sometimes this extra bone is displaced or fractured by an ankle twist, which triggers the pinching.
Pain arises for two reasons: bony pinching of the os trigonum against the ankle, and soft-tissue impingement from inflammation at the back of the ankle.
Getting a doctor to confirm the diagnosis matters. We perform the posterior ankle impingement test, and we also test the long tendon at the back of the ankle — the flexor hallucis longus — to see if it’s inflamed.
An X-ray shows the os trigonum at the back of the ankle. An MRI can show swelling between the os trigonum and the back of the ankle and help exclude other causes, such as cartilage damage or Achilles tendonitis. Importantly, simply having this extra bone on a scan doesn’t mean it’s the source of pain — one study found an os trigonum doesn’t necessarily correlate with ankle pain. The clinical picture must fit.
What are the treatment options?
We start simple, before anything invasive.
Simple treatments include:
- Physiotherapy to improve calf and foot strength, balance, and mobility
- Anti-inflammatory tablets such as ibuprofen
- Taping the ankle to limit full plantarflexion (pointing the toe)
- Soft-tissue massage to the calf to reduce ankle tightness
- Joint mobilisation of the subtalar joint
If these don’t help, a cortisone injection directed to the back of the ankle is the next step (covered below).
Finally, surgical removal is an option for cases that fail simple treatment. Most surgery is keyhole, which shortens recovery time, and other problems, such as joint swelling or cartilage injuries, can be treated at the same time. But removal isn’t to be taken lightly: recovery is often 3–6 months, and removing the os trigonum may increase the risk of arthritis due to an uncovered calcaneus at the back of the subtalar joint. One study of os trigonum excision reported a 13–14% complication rate, primarily due to postoperative subtalar arthritis.
More about an os trigonum injection
A cortisone injection into the back of the ankle is a good option. We place the needle between the os trigonum and the back of the talus, using a posterior or lateral subtalar approach. Cortisone is a potent anti-inflammatory that reduces inflammation and pain. It’s essential to guide the injection to the exact spot with ultrasound. This area is deep and technically demanding, so it should only be done by a doctor with expertise in ultrasound-guided injection.
Frequently asked questions about os trigonum syndrome
Apart from an os trigonum, what else causes posterior ankle impingement?
Other causes include a Stieda process (an elongated posterior talar process), bony spurs, loose bodies, joint inflammation, and tendon tenosynovitis. See our posterior ankle impingement page for the full list.
How successful is conservative treatment for os trigonum syndrome?
Studies suggest athat bout 60% of patients respond to conservative treatment with physiotherapy and injections, so most avoid surgery.
Is an os trigonum hereditary?
An os trigonum is congenital — present from birth. Around 10–15% of people have one, and some cases appear to have a hereditary basis.
Do I need surgery if a scan shows an os trigonum?
No. Many people have an os trigonum with no pain at all, so the scan alone never justifies surgery. We only consider removal when it is genuinely the cause of pain and simple treatments, plus an injection, have failed.
Can an os trigonum be fractured?
Yes. An ankle twist can displace or fracture an os trigonum, which then pinches at the back of the ankle and causes posterior ankle impingement. Imaging confirms it.
Final word from Sport Doctor London about os trigonum syndrome
Os trigonum syndrome is often misdiagnosed as Achilles tendonitis, and it’s a common cause of pain at the back of the ankle. A cortisone injection is usually tried first, with excision reserved for stubborn cases. See a Sport and Exercise Medicine doctor for an accurate diagnosis and treatment plan.
To book an ankle assessment with Dr Masci in London, contact the team here or call +44 (0) 203 488 0350.
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