Os Trigonum Syndrome

Written By: Chloe Wilson BSc(Hons) Physiotherapy
Reviewed By: FPE Medical Review Board

An Os Trigonum is a small, extra bone found at the back of the ankle.

Os Trigonum Syndrome typically causes pain and stiffness at the ankle which tends to be worse when pointing your toes.

There is often swelling and you may be able to feel a small lump behind the ankle which is often tender to touch. 

This extra bone formation is fairly rare and only occurs in approximately 5-10% of people.  Often it goes completely unnoticed, not causing any problems.  

However, when combined with an ankle injury or in athletes who sports require frequent ankle flexion e.g. ballet dancers and footballers, Os Trigonum Syndrome can develop. 

What Is An Os Trigonum?

An Os Trigonum is a small, accessory bone that forms at the back of the foot behind the ankle joint.

The talus bone forms part of the ankle joint.  As the talus grows during childhood, a small piece of bone develops just behind it, known as the Os Trigonum.  This usually happens around the ages of seven to eleven. 

Os Trigonum is a small, extra bone at the back of the heel

This small bone is initially joined to the talus by fibrous structures and within one to three years, usually fuses i.e. joins with the talus bone forming part of the lateral tubercle, a small lump on the talus. 

However, sometimes it fails to join the talus and remains a separate piece of bone.  It is usually small, less than one centimetre, and varies in shape from round to oval to triangular. 

Usually, if the bone fails to fuse it doesn’t cause any problems, but if the ankle is injured, either through a specific incident or recurrent trauma, Os Trigonum Syndrome can develop.  It is also known as posterior talar impingement.  

What Causes Os Trigonum Syndrome?

When the presence of the unfused bone is combined with an ankle injury, Os Trigonum Syndrome can develop.  It is usually caused by either:

Ballet dancers are prone to Os Trigonum Syndrome due to repetitive plantarflexion when en pointe
  • Over-Use: from repeated plantarflexion (foot pointing downwards).  Os Trigonum Syndrome commonly affects ballet dancers, runners and football players

  • Trauma: an ankle injury where the foot is forced into excessive plantarflexion

Either of these can cause what is known as a “nutcracker injury”, where the unfused Os Trigonum bone and surrounding soft tissues become wedged between the ankle and heel bones (tibia, talus and calcaneus).  The soft tissues become inflamed leading to pain and stiffness. It is the irritation of the soft tissues that causes Os Trigonum Syndrome.

There is no specific age or gender that is more susceptible to Os Trigonum Syndrome, it is purely related to activity.

Common Symptoms

The most common symptoms of Os Trigonum syndrome are:

  • Foot Pain: Usually at the back of the ankle. The pain tends to be worse when plantarflexing the foot (pointing the toes) or during the push-off phase of walking and eases with rest

  • Tenderness: It is usually tender to touch the area at the back and outer side of the ankle

  • Swelling: swelling may develop due to the inflammation of the soft tissues

  • Lump: sometimes you may be able to feel a small, hard lump near the Achilles tendon.  This is the unfused bone

The condition usually affects one foot, but in approximately one third of cases, both feet are affected. 

How Is It Diagnosed?

X-ray showing Os Trigonum

Your doctor will suspect this condition from your signs and symptoms.  They may want to confirm the diagnosis with an x-ray or MRI scan as the symptoms are often similar to other conditions such as Achilles tendonitis, peroneal tendonitis, tarsal tunnel syndrome or a fracture.  

It is really important to get an accurate diagnosis to ensure you get the correct treatment.

Treatment Options

The first line of treatment is to allow the soft tissues to heal so the inflammation can settle.  This is done through a combination of:

Ice can help to reduce the pain and swelling associated with Os Trigonum Syndrome
  • Rest: complete rest for four to six weeks from all activities that irritate the condition

  • Ice: regularly applying ice to the back of the ankle also helps reduce swelling and inflammation.  Visit the ice section to find out how to safely and effectively use ice therapy

  • Medication: your doctor may prescribe anti-inflammatory medications such as NSAIDs like ibuprofen
  • Immobilisation: you may be given a walking boot to wear to restrict ankle movement when you are up and about

  • Injections: Corticosteroid injections are sometime used to reduce pain and inflammation.  You can find out more about how they work and the common side effects in the injections section on out sister site

  • Exercises: Working on a rehab programme of stretching and strengthening exercises with a physiotherapist can also help.  Strengthening the ankle and foot muscles can help the talus to glide forwards slightly during plantarflexion which can help reduce the pressure on the Os Trigonum and thus preventing the irritation of the soft tissues
  • Surgery: If symptoms persist, surgery may be advised to remove the small piece of bone.  You will then have a course of physiotherapy.  The outcome is usually extremely good with full resolution of symptoms and you can usually return to sports after four to eight weeks.

What Else Can Help?

Os Trigonum Syndrome is only one possible cause of pain behind the ankle.  If this is not sounds quite like your problem, visit the heel pain section for other common causes of pain behind the ankle, or visit the foot pain diagnosis section for help working out what is wrong.

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Page Last Updated: 11/03/21
Next Review Due: 11/03/23