The 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. Occasionally, it can cause ankle pain during sport, also known as posterior ankle impingement. So, how do we recognise Os Trigonum syndrome, and what can we do about it?
What is an Os Trigonum?
The os trigonum forms when one area of bone fails to fuse with the rest. During fetal growth, an extra bone forms behind the talus bone. When this bone fails to fuse with the talus, there is an additional small bone in the ankle.
Approximately 1 in 4 of the general population has this extra bone. This extra bone is similar to an appendix in most people – it doesn’t typically cause a problem. However, like the appendix, it can cause pain after an ankle twist or in certain sports such as ballet, running, or football.
Os Trigonum syndrome diagnosis
Generally, Os Trigonum causes pinching at the back of the ankle. This pinching occurs when you point your foot, such as a ballet dancer assuming a pointe position or a football player striking a ball. Typically, pain is felt at the back of the ankle, called posterior ankle impingement. Often, pain from posterior ankle impingement is confused with pain from Achilles tendonitis. Sometimes, the Os Trignum can become displaced or fractured after twisting the ankle, leading to pinching and posterior ankle impingement.
It is essential to understand that pain occurs for two primary reasons: bony pinching of the trigonum on the ankle and soft tissue impingement due to inflammation at the back of the ankle.
It is vital to get a doctor to confirm the diagnosis. Typically, we perform a posterior ankle impingement test, which can cause ankle pain. We also test the long tendon at the back of the ankle, known as the flexor hallucis longus tendon, to determine if it is inflamed.
In general, an X-ray is helpful to show the Os Trigonum at the back of the ankle. Sometimes, an MRI shows swelling between the os trigonum and the back of the ankle. Additionally, MRI can help exclude other potential causes of pain in the back of the ankle, such as damage to the ankle joint cartilage or Achilles tendonitis. However, just having an os trigonum on an X-ray or MRI scan does not mean you have pain or need an operation. For example, a recent study found that an os trigonum does not necessarily correlate with ankle pain.
What are the options for the treatment?
Generally, starting with simple steps rather than more invasive treatments would be best.
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 plantar flexion (pointing the toe)
- Soft tissue massage to the calf to reduce tightness in the ankle
- Joint mobilisation of the subtalar joint
Other options include a cortisone injection directed to the back of the ankle joint.
Finally, removal is an option in cases that fail simple treatments. Most surgery is generally done with a keyhole, reducing recovery time. Also, other problems, such as joint swelling or cartilage injuries, can be treated simultaneously.
Overall, it is crucial to understand that Os Trigonum removal involves a long recovery time, often 3-6 months, and should not be taken lightly. Also, removing the os trigonum might increase the risk of arthritis from an uncovered calcaneus at the back of the subtalar joint. A study examining outcomes after os trigonum excision found a 13-14% complication rate, primarily due to the development of postoperative subtalar arthritis.
More about an Os Trigonum Injection
Generally, a cortisone injection into the back of the ankle is a good option for pain arising from posterior ankle impingement. Typically, we place the needle between the Os Trigonum and the back of the talus. We can use the posterior or lateral subtalar joint approach. Cortisone is a potent anti-inflammatory drug that reduces inflammation and pain. However, it is essential to guide the cortisone into the exact spot with ultrasound. Also, using ultrasound requires excellent skills and should only be done by a doctor with expertise in ultrasound-guided injections.
Other frequently asked questions:
Apart from Os trigonum, are there other causes of posterior ankle impingement?
Yes. Other causes of posterior ankle pain include Steida process (elongated posterior talar process), spur formation, loose bodies, joint inflammation and tendon tenosynovitis.
What is the success of conservative management in posterior ankle impingement?
Studies suggest 60% of patients respond to conservative treatment of physiotherapy and injections.
Is Os trigonum hereditary?
An os trigonum forms as a congenital abnormality, meaning it is present at birth. About 15% of the general population has an Os trigonum, and we think that some of these cases have a direct hereditary cause.
Final word from Sportdoctorlondon about Os Trigomum Syndrome
Often, Os Trigonum syndrome is misdiagnosed as Achilles tendonitis. It is a common cause of pain in the back of the ankle. A cortisone injection should be tried first, followed by an excision. You should see a Sports and Exercise Medicine doctor for a correct diagnosis and treatment plan.
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