The ankle joint presents an interesting dichotomy of strength and weakness. Whereas the ankle joint supports your body weight while managing sharp twists and turns and also prevent falls, it simultaneously can be weak and highly susceptible to sprains, strains, and even fractures from the simplest of movements, such as stepping off a curb.

The healing process seems simple enough with what is known as the RICE principle (Rest, Ice, Compression & Elevation), and of course there is also often simultaneous use of an ankle brace or tape while performing activity. However, not all ankle trauma follows this roadmap to recovery and healing and when an ankle continues to demonstrate pain for longer than the typical expected healing time period, misdiagnosis may be worth consideration. In particular, as I describe here, perhaps it may be a fracture of the os trigonum.

What is the Os Trigonum?

The os trigonum is an elongation of the talus bone, one of the key bones in the ankle joint. The prevalence of os trigonum in humans has varied widely in the literature, but it is more common than once thought. A 2019 study of computed tomography (CT) scans of the ankle in 586 Chinese patients found an incidence of 27%.¹ In a 2018 study of CT scanning of 1256 ankles (628 patients with 665 symptomatic ankles), os trigonum was found in 30.3% of the entire cohort. Furthermore, nearly 24% of unaffected ankles were marked by the condition, making it far more common than previously reported. The first appearance of an os trigonum is commonly at approximately 9 years of age, but it has been seen in children as young as 7 months. In rare situations in older adults, the bone is fused to other ankle bones.

Repetitive Microtrauma Leads to Chronic Pain

Because fracture of the os trigonum is rare, the condition can be misdiagnosed, which can lead to chronic pain, instability, diminished function in the ankle, and other injuries. A patient with an os trigonum fracture typically has chronic ankle pain or chronic ankle pain that does not completely resolve. What is commonly referred to as an “inversion sprain” is the prime mechanism for acute fracture of the os trigonum, accounting for nearly 85% of cases. Again, these are often misdiagnosed. Once the os trigonum escalates into a chronic problem, consistent ankle pain does not resolve with normal treatment and surgery might be required.

How to Identify an Os Trigonum Fracture

On physical evaluation, an individual may report increased pain when the injured area of the ankle is touched (palpated). In addition, pain can be present when the ankle is passively moved in different directions. Because of the relationship of the certain foot tendons to the os trigonum area, a patient can also experience pain with active or passive movements of big toe. A patient might have cracking and popping (crepitus), redness or a combination of these findings. There may also be noticeable weakness when placing weight on the side of the injury.

On imaging, a positive fracture is marked by a clear separation and a rough, jagged edge in the region of the os trigonum. Smooth areas on the radiographs are a sign that there is no fracture. In some patients, a nuclear bone scan can determine the integrity of the bony components and also verify the diagnosis.

Surgical Intervention and Rehabilitation

When a fracture of an os trigonum is caught early, a short-leg cast might help. If the condition becomes chronic however, surgical intervention might be the only avenue to a full recovery. Factors that lead to surgical intervention include:

• Severe pain
• Severe decrease in function
• Severe Swelling
• Inability to perform a particular activity that one wishes to (ex. Sport)

The common surgical technique for this condition involves taking a lateral (from the outside) approach to the ankle. An incision is made, the os trigonum is identified and removed by the surgeon with additional exploration to rule out any other further problems in the area. Then, postsurgical images are taken, and the patient is sent for rehabilitation. The physical therapy protocol consists of evaluating and treating the patient for 4 to 6 weeks, addressing swelling issues with walking, overall function, normalizing strength, restoring range of motion and proprioception, all while managing pain.

Hopefully this information is helpful in addressing those challenging cases where things are just not lining up after ankle injury.

References:

1. Russell JA, Kruse DW, Koutedakis YI, McEwan IM, Wyon MA. Pathoanatomy of posterior ankle impingement in ballet dancers. ClinAnat. 2010;23(6):613-621.