Sinus Tarsi Syndrome (STS) is a localized, painful condition affecting the outer front portion of the ankle and foot, arising from inflammation or injury within a small anatomical tunnel called the sinus tarsi. STS is frequently confused with a standard ankle sprain, leading to delayed or incorrect treatment for patients experiencing chronic lateral ankle pain. The chronic nature of the pain and subjective feeling of instability often distinguish STS from other forms of ankle discomfort.
The Sinus Tarsi Explained
The sinus tarsi is a small, cone-shaped channel located on the outer side of the foot. This space is situated between the talus (ankle bone) and the calcaneus (heel bone). The sinus tarsi is part of the subtalar joint, which controls the side-to-side movement of the foot.
The structures housed within this bony passage become irritated in Sinus Tarsi Syndrome. It contains a complex of ligaments, including the cervical and interosseous talocalcaneal ligaments, a protective fat pad, blood vessels, and nerve endings. These ligaments provide stability to the subtalar joint and limit excessive foot movement. Inflammation or damage to these soft tissues causes the syndrome’s characteristic pain.
Recognizing the Symptoms
The primary sign of Sinus Tarsi Syndrome is a deep, aching pain located on the outer side of the ankle. This pain is specifically felt when pressure is applied to the depression in front of the lateral malleolus. The discomfort is often exacerbated by weight-bearing activities, such as standing for extended periods or walking.
A distinguishing symptom is the worsening of pain when walking on uneven ground, such as gravel or a sloped surface. This happens because these surfaces force the foot into extreme inversion and eversion, stressing the inflamed tissues. Many patients also report a subjective feeling of instability or a sense that the ankle is “giving way.” This feeling stems from damage to the ligaments and mechanoreceptors, the sensory nerves that aid in balance and joint position awareness.
Common Causes and Risk Factors
The most common underlying cause for Sinus Tarsi Syndrome is a past traumatic event, particularly a severe or recurrent inversion ankle sprain. An inversion sprain causes the foot to roll inward, overstretching and damaging the ligaments within the sinus tarsi. This injury can lead to instability in the subtalar joint, causing excessive movement that chronically irritates the surrounding tissues.
Biomechanical abnormalities also contribute to the syndrome’s development. Excessive foot pronation (flat feet) is a major risk factor, as this flattening of the arch increases pressure on the sinus tarsi area during activity. People who participate in sports involving frequent quick turns, pivoting, or running on uneven surfaces, like trail runners, are also more susceptible due to repetitive strain.
Confirming the Diagnosis and Treatment Options
Diagnosis typically begins with a detailed clinical examination, where a healthcare provider palpates the area in front of the outer ankle bone to check for localized tenderness. Imaging studies rule out other conditions causing similar pain, such as fractures or arthritis. X-rays assess bony structures, while an MRI scan visualizes soft tissues, including the ligaments and fat pad within the sinus tarsi.
The gold standard for confirming the diagnosis is the use of a diagnostic injection. This involves injecting a local anesthetic, such as lidocaine, directly into the sinus tarsi space. If the patient experiences a significant, temporary reduction of pain immediately following the injection, it strongly suggests the sinus tarsi is the source of the discomfort.
Treatment is overwhelmingly conservative, focusing on reducing inflammation and pain. Resting the ankle and avoiding activities that aggravate symptoms is the primary initial step. Nonsteroidal anti-inflammatory drugs (NSAIDs) can be used to manage pain and swelling.
Physical therapy is a fundamental component of rehabilitation, focusing on strengthening the muscles around the ankle and improving proprioception (the body’s sense of joint position). Therapists often prescribe exercises like the “ankle alphabet” to restore range of motion and stability. Custom orthotics or supportive bracing may also be prescribed to control excessive foot pronation and reduce strain on the subtalar joint.
If conservative measures do not provide adequate relief, a corticosteroid injection, sometimes performed under ultrasound guidance, may be used to deliver an anti-inflammatory agent directly to the affected area. Surgical intervention, such as sinus tarsi arthroscopy to remove inflamed tissue or repair damaged ligaments, is considered a last resort. Surgery is reserved for chronic cases that have not responded to a comprehensive course of non-surgical treatment.