The sinus tarsi is a small, complex anatomical space located in the hindfoot. This structure is positioned where the ankle meets the foot and plays a role in foot mechanics. It is integral to the subtalar joint, which is responsible for the side-to-side tilting motion of the foot. Understanding the sinus tarsi provides insight into a common source of persistent pain in the lower leg.
Precise Anatomical Location
The sinus tarsi is a cylindrical cavity positioned on the lateral aspect of the foot, just in front of the ankle bone. It is a space formed by the contours of two major bones: the talus above and the calcaneus (heel bone) below. This bony space is situated between the anterior and posterior sections of the subtalar joint.
The structure is roughly cone-shaped, with the wider opening facing outward and forward. This opening is often visible as a slight indentation just below the prominent bony knob of the outer ankle (the lateral malleolus). The sinus then narrows into a smaller, medial channel known as the tarsal canal, which extends deeper into the foot.
What Occupies This Space
The space within the sinus tarsi is filled with soft tissues that provide mechanical stability and sensory feedback. The stabilizing structures are a network of ligaments, notably the cervical ligament and the interosseous talocalcaneal ligament. The cervical ligament limits the inward rotation (inversion) of the hindfoot, while the interosseous talocalcaneal ligament maintains the close opposition of the talus and calcaneus.
The sinus also contains a pad of fatty tissue that acts as cushioning. This area is richly supplied with nerve endings and mechanoreceptors, which are specialized sensory receptors. These receptors are proprioceptors, constantly relaying information about the foot’s position, movement, and balance back to the brain. This function is why the sinus tarsi is sometimes called the “eye of the foot,” as it aids stability on uneven terrain.
Understanding Sinus Tarsi Syndrome
Sinus Tarsi Syndrome (STS) is a condition characterized by chronic pain and tenderness in this anatomical space. It is a common cause of persistent lateral ankle pain, often developing after an inversion ankle sprain. The trauma from the sprain can damage the ligaments, leading to inflammation or scar tissue formation within the sinus tarsi.
The defining symptom of STS is a deep, aching pain located on the outer side of the ankle and foot. This pain is often worse during weight-bearing activities. Patients report discomfort when walking, standing for long periods, or navigating uneven surfaces. This difficulty on uneven ground is related to the proprioceptive deficit caused by inflammation affecting the nerve endings inside the sinus.
Diagnosis begins with a physical examination that reveals tenderness when pressure is applied directly over the sinus tarsi. Imaging tests, such as magnetic resonance imaging (MRI), are used to confirm the diagnosis by visualizing inflammation, scar tissue, or ligament tears. Injecting a local anesthetic directly into the sinus is a diagnostic tool; temporary pain relief strongly supports STS. Treatment usually starts with conservative measures, including rest, anti-inflammatory medications, physical therapy, and orthotic shoe inserts to correct excessive foot pronation.