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.
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.
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.
When the presence of the unfused bone is combined with an ankle injury, Os Trigonum Syndrome can develop. It is usually caused by either:
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.
The most common symptoms of Os Trigonum syndrome are:
The condition usually affects one foot, but in approximately one third of cases, both feet are affected.
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.
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:
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.
Page Last Updated: 2019-06-18
Next Review Due: 2021-06-18