The sinus tarsi is a small, tunnel-like space located on the outer side of the foot, situated where the ankle meets the heel. This area contains a complex network of tissues integral to foot mechanics and stability. When this passage becomes inflamed or injured, it can cause persistent pain, leading to a condition known as Sinus Tarsi Syndrome. Understanding the structure and purpose of this region is the first step in addressing the chronic discomfort it can cause.
Anatomy and Location
The sinus tarsi is an irregularly shaped, cone-like cavity found on the outside of the foot, positioned just below the prominent ankle bone. This space is structurally defined by the concave surfaces of the two largest bones in the hindfoot: the talus, which sits superiorly, and the calcaneus (heel bone), which sits inferiorly. Together, these two bones form the subtalar joint, and the sinus tarsi is the opening that separates the joint’s anterior and posterior sections.
The cavity narrows as it extends inward, connecting to a medial passage called the tarsal canal. This confined area is filled with a dense collection of soft tissues, including a protective fat pad that acts as a shock absorber.
Within this fat pad are several strong ligaments, including the interosseous talocalcaneal ligament and the cervical ligament, which act like internal tension wires tethering the talus and calcaneus together. The sinus tarsi also hosts a rich supply of small blood vessels and nerve endings, making it a highly sensitive and responsive area within the foot.
Primary Function and Role in Stability
The contents of the sinus tarsi have a specialized function beyond simple structural support. The space is a key sensory area, heavily populated with specialized nerve endings called mechanoreceptors. These receptors constantly monitor the stretching and compression of surrounding tissues, sending information to the brain about the foot’s position and movement.
This constant feedback loop is known as proprioception, which is fundamental for balance and coordinated movement. This sensory input allows for rapid, unconscious adjustments of the foot to maintain stability and prevent falls, particularly when walking on uneven terrain.
The ligaments housed in the sinus tarsi are the primary stabilizers of the subtalar joint, which controls the side-to-side motion of the foot. Specifically, the cervical ligament limits the inward rolling motion (inversion), while the interosseous talocalcaneal ligament helps restrict outward rolling (eversion). This dual action ensures that the joint remains controlled and prevents excessive motion that could otherwise damage the joint surfaces.
Understanding Sinus Tarsi Syndrome
Sinus Tarsi Syndrome (STS) is a condition characterized by pain and inflammation affecting the soft tissues inside this anatomical space. The primary symptom is a deep, localized ache felt on the outside front of the ankle, directly over the sinus tarsi opening. This pain is often persistent, sometimes described as a dull throb.
A defining characteristic of STS is that the pain is often significantly aggravated by activities that challenge the subtalar joint. These include walking or running on uneven surfaces, pivoting, or standing for long periods. Patients frequently report a subjective feeling of instability or a “giving way” sensation in the hindfoot, as the damaged sensory receptors and ligaments fail to adequately stabilize the joint.
Tenderness is highly localized; firm pressure applied directly into the sinus tarsi area will reproduce the deep pain. Diagnosis is often confirmed by the temporary relief of symptoms following a local anesthetic injection directly into the sinus tarsi. If the pain is caused by inflamed structures, numbing them should stop the pain.
Common Causes of Injury and Pain
The development of Sinus Tarsi Syndrome is traced back to two main categories of mechanical stress. The most frequent cause is trauma, specifically an inversion ankle sprain where the foot rolls inward. When the ankle rolls in sharply, it can stretch or partially tear the ligaments inside the sinus tarsi, such as the cervical and interosseous ligaments.
Even after the initial ankle sprain heals, residual inflammation, scar tissue, or chronic laxity in the damaged ligaments can persist within the confined space. This chronic irritation and instability within the subtalar joint can then lead to the ongoing symptoms of STS.
The second major cause relates to long-term biomechanical factors, particularly excessive pronation, or “flat feet”. In a foot with excessive pronation, the arch collapses and the foot rolls inward too much during walking or running. This rolling motion can cause the talus and calcaneus bones to repeatedly compress or impinge the soft tissues and nerves within the sinus tarsi, leading to chronic inflammation and pain.