Os Trigonum is a small extra bone that sits at the back of the ankle joint. This extra bone is present in 5-15% of the general population. In some cases, an Os Trigonum can cause ankle pain during sport. How do we recognise this condition and what can we do about it?

What is an Os Trigonum? 

os trigonum

Os Trigonum forms when one area of bone fails to fuse with the rest of the bone. during growth. In the ankle, this extra bone forms behind the talus bone. Approximately 1 in 5 of the general population has an Os Trigonum. In most people, this extra bone is like an appendix – it doesn’t cause a problem. However, similar to the appendix, it can cause pain after an ankle twist or in certain sports such as ballet, running, or football.


Generally, Os Trigonum causes pinching at the back of the ankle. This pinching occurs when you point your foot such as a ballet dancer who assumes a pointe position or a footballer striking a ball. Typically, pain is felt at the back of the ankle and is called posterior impingement. Often, pain from posterior impingement is confused with pain from Achilles tendonitis. Sometimes, after twisting the ankle, the Os Trignum can move or break leading to pinching and posterior impingement.

It is important to get a doctor to confirm the diagnosis. Typically, we perform a ‘posterior impingement test’, which brings on ankle pain.

In general, X-ray is useful to show the Os Trigonum at the back of the ankle. Sometimes, MRI is used to show swelling between the Os Trigonum and back of the ankle. In addition, MRI can exclude other causes of pain at the back of the ankle such as ankle joint cartilage damage or Achilles tendonitis.

os trigonum on X-ray

What are the options for the treatment of Os Trigonum syndrome?

Generally, you should always start with simple steps rather than more invasive treatments for posterior impingement of the ankle.

Simple treatments include the following:

  • physiotherapy to improve calf and foot strength, balance, and mobility
  • anti-inflammatory tablets such as ibuprofen
  • taping of the ankle to stop movement into full plantarflexion (pointing the toe)
  • soft tissue massage to the calf to reduce tightness in the ankle

Other options include an Os Trigonum cortisone injection directed to the back of the ankle joint.

Finally, in cases that fail simple treatments, Os Trigonum removal is an option. In general, most surgery is done with key-hole reducing recovery time. Also, other problems such as joint swelling or cartilage injuries can be treated at the same time. Overall, it is important to understand that Os Trigonum removal involves a long recovery time – often 3-6 months- and should not be taken lightly.

More about an Os Trigonum cortisone injection

Generally, a cortisone injection into the back of the ankle is a good option for pain arising from posterior impingement. Typically, we place the needle between the Os Trigonum and the back of the talus. Cortisone is a powerful anti-inflammatory drug that reduces inflammation and pain. However, it is important to guide the cortisone into the exact spot with ultrasound. Also, using ultrasound requires great skill and should only be done by a doctor with expertise in ultrasound-guided injections.

Final word from Sportdoctorlondon 

Often, Os Trigonum syndrome is misdiagnosed as Achilles tendonitis. Overall, it is a common cause of pain in the back of the ankle. An Os Trigonum cortisone injection should be tried first followed by Os Trigonum excision. You should see a Sports and Exercise Medicine doctor to get a correct diagnosis and treatment plan.

Other specific foot and ankle conditions:

Dr. Masci is a specialist sport doctor in London. 

He specialises in muscle, tendon and joint injuries.

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