Os Trigonum Syndrome

Os Trigonum Syndrome

What is the “Os Trigonum”

The os trigonum is an additional bone at the back of the ankle bone called the talus. It is joined to the talus bone via a fibrous connection. It is a type of “accessory” bone.

It is present between 3 to 15% of the population. It is often bilateral.

In some patients, the additional bone is joined to the ankle talus bone and this is called a “Steida’s” process.

Is “Os Trigonum” Harmful?

Os Trigonum is an anatomical variant with an incidence between 3 to 15%.

Many people with this anatomical variant in their ankles will live their entire life without knowing this.

Os Trigonum Syndrome

In some patients, the “os trigonum” can cause pain at the back of the ankle. It usually occurs after some form of ankle injury such as an ankle sprain.

It can also cause pain in people who have to tip-toe repeatedly e.g. ballet dancers.

Investigations for Os Trigonum Syndrome

X-rays of the ankle can sometimes show the os trigonum:

Os Trigonum

A Bone Scan can prove that the cause of the pain is the Os Trigonum:

It will light up “bright” as a dark spot on bone scans.

An MRI scan can also be useful for diagnosis:

What are the Possible Treatments?

  1. Rest the ankle. Use crutches, CAM walkers or rocker bottom shoes.
  2. Oral anti-inflammatory medications – do check for drug allergies.
  3. Surgery to remove the accessory os trigonum if conservative treatments failed.

Surgical Options

2 options –

  • Open surgery to remove the os trigonum. This involves a larger scar.
  • Arthroscopic surgery to remove the os trigonum. This is much less invasive. It involves 2 small keyhole scars at the back of the ankle.

I usually offer arthroscopic excision of Os Trigonum for my patients.

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