diagram of location of os trigonum

What is an Os Trigonum?

This is a piece of bone at the back of the ankle joint. It can cause pain at the back of the ankle. This is known as posterior ankle impingement.

The talus bone has several parts to it, which join together during development. The os trigonum, at the back, may fail to join up with the other parts and remain separate. Usually, this is not a problem as the os is tightly bound by fibrous tissue to the rest of the talus. If the join is damaged by an injury, the os trigonum can become mobile and then painful.

ballet dancer en pointe cause of posterior impingement

Even if the os trigonum is not loose, posterior impingement can occur. This is because the back of the talus bone can be trapped between the heel bone and the ankle.

This was thought to be limited to ballet dancers in the en pointe position, where the ankle if pointed downwards in quite an extreme way. However, we now know that it is far more common, and can occur in basketball players, netball players, tennis players, and footballers – who may be injured by the position of the ankle in a sliding tackle.

MRI of os trigonum

Another cause for pain at the back of the ankle is tendonitis of the Flexor Hallucis Longus tendon, which runs next to the Os Trigonum. If this is irritated, then similar pain can be caused at the back of the ankle.

The indications that there is a problem can be seen on an MRI scan. There may be fluid around the tendon, damage in the join between the Os Trigonum and the rest of the talus, or bruising in the bone.

If you have pain at the back of your ankle, it is best to see an Orthopaedic Surgeon who specialises in problems of the foot and ankle. You will need to have an xray as well as an MRI scan to help make the diagnosis.

What Does Surgery Involve?

This is a keyhole procedure. Two small cuts are made at the back of the ankle, either side of the Achilles Tendon. A camera on a thin metal tube is passed inside. An instrument is passed through the other cut to allow the procedure to be performed. The individual procedure is determined by the problem, but if there is a loose Os Trigonum, this is removed. The FHL tendon is cleared of adherent tissue and debris.

Surgery takes place under general anaesthesia, but local anaesthetic injections are given to help relieve pain after the operation. Stitches are placed in the skin, as well as absorbant dressings.

You can see a video of a keyhole procedure below.

Risks of Surgery

Bleeding, infection, poor bone healing, poor skin healing, injury to tendons, injury to nerves, recurrence, and a need for further surgery.

There is a small risk of blood clots in the legs or lungs (DVT and PE), and there are also risks from anaesthesia – the process of being put to sleep for your operation.

Risks of Anaesthesia

The injection behind your knee is given using an ultrasound machine to guide the needle. There is a less than 1% chance of injury to the nerve. General anaesthetic also carries risks. These risks are proportional to your general health. You will need to be assessed for your fitness for surgery and an Anaesthetist will be able to advise you on your individual risk.

postoperative shoe foot surgery

What Happens After Surgery?


You will have had injections to numb your foot so that you are not in pain after the operation. This injection will wear off after 18-24 hours, so you must take regular painkillers so that you are not in severe pain when the injection wears off. Ibuprofen or Diclofenac can be used for a month after surgery to help with pain.

The pain will settle over a few weeks. You may see blood stains on the dressings. This is normal and not a cause for alarm. If blood is dripping from the dressings however, return to hospital.


The ankle will be swollen as well as sore. You should keep the foot elevated as much as possible for the first two weeks. Keep walking down to a minimum – going to the toilet or for meals. Letting the leg hang down will cause the foot to become more swollen.

If you are resting on your sofa, keep your foot elevated on the back rest or arm rest. If you are resting in bed, then place a few pillows under the foot to keep it elevated.


You will not need a plaster cast or a plastic boot. You will have thick dressings which will stay in place for two weeks You can fully weight bear through the operated ankle, although this may be sore to begin with. The shoe is for your comfort when walking, and when the stitches have been removed, you can return to normal shoes.


Although any loose bone has been removed, you will initially not notice any change in your symptoms due to the operation pain. Only when the operation pain has settled after six weeks will you be aware if the original pain has been resolved. After the stitches have been removed, and the pain is settling, you will need physiotherapy to stretch the  ankle.


You will have a sore ankle for at least a few weeks, and in some cases a few months. I recommend that you have at least two weeks off work with a possibility of requiring another four weeks if you are very sore from having bone shaved off the front of the ankle joint.

Some patients have recurrent pain around six weeks after surgery. This is due to inflammation and scarring around the ankle and may need an injection into the ankle to settle it down. Remember that not everyone is the same, and some people take a longer time to recover from their surgery. Please request a sick note before you leave hospital, if you need one.

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