The Sinus Tarsi is a small canal between the heel and ankle bones. It contains ligaments, blood vessels and joint capsules running through the canal. Sinus Tarsi Syndrome is characterized with persistent pain on the outside of the ankle due to an injury or swelling in the Sinus Tarsi. It occurs following an ankle sprain, if osteoarthritis is present in the joint or repetitive trauma of the soft tissue in the Sinus Tarsi due to poor biomechanics when the foot over pronates (flattens).