Sinus tarsi syndrome is something of a paradox. It can be caused by an excessively inverted foot and sinus tarsi syndrome can also be caused by an excessively everted foot.
Sinus tarsi syndrome caused by an excessively inverted foot is the ankle sprain. During an ankle sprain the structures in the sinus tarsi can get stretched. They often get missed as the initial diagnosis of an ankle sprain due to the pain and swelling. It is only later that sinus tarsi syndrome gets diagnosed as a chronic problem following the ankle sprain. The pain is usually felt on the outside of the ankle joint. Sinus tarsi syndrome following ankle trauma usually responds really well to physical therapy and home exercise programs. Sometime a steroid injection into the sinus tarsi is helpful.
The other cause of sinus tarsi syndrome is an excessively pronated foot. This foot forces the subtalar joint to the end range of pronation or eversion and compresses the lateral side of the sinus tarsi. This produces pain on the outside of the ankle joint. This type of sinus tarsi syndrome responds really well to foot orthoses designed to keep the subtalar joint from pronating to its end range of motion.