Outer foot pain can have many causes, but one common cause is sinus tarsitis.  The sinus tarsi is a trumpet-shaped passage way located deep in the foot just below the ankle. It passes into the subtalar joint, which is the joint that is located just under the ankle, and allows the foot to roll inward and outward. The sinus tarsi is essentially the opening into this joint on the outer side of the foot, and is located in front of the outer ankle bone and slightly inward toward the top of the foot. There are several structures that run through this passage way, including a plug of fatty-fibrous tissue, as well as a small nerve. Inflammation in this space can create a vague pain in the outer side of the foot that can be difficult to diagnose.

The sinus tarsi can become inflamed in several ways. An acute injury, like rolling the foot inward during an ankle sprain, can damage this tissue. This site can also become inflamed when the foot rolls outward during the typical movement of someone with flat feet. Over time, this constant flattening of the foot can irritate the tissue in the sinus tarsi, and inflammation and pain can develop.

The symptoms of sinus tarsitis are often vague, and consist of pain felt in a broad pattern on the side of the foot near the ankle bone. This pain can be a dull ache, or it can be sharp and stinging. It usually feels better at rest, and worse with increased activity. More stable shoes improve the pain.

Sinus tarsitis is diagnosed primarily by physical exam. There are no specific tests for this condition, although an MRI can demonstrate inflammation in this space. It is often missed by those not well experienced in treating foot and ankle conditions, and is often mistaken for an ankle sprain or tendonitis.

Treatment of sinus tarsitis involves stabilizing the joint with a brace, as well as anti-inflammatory medication, icing, and often a corticosteroid injection. For those with flat feet, the use of prescription orthotics can limit the potential for this condition to redevelop after it is treated initially. In rare cases, surgical cleaning of the contents of this space may be needed if one does not respond to nonsurgical treatment.