Sinus tarsi syndrome is a problem that occurs between two bones in the rearfoot (talus and calcaneus) and the pain is usually felt on the outside of the ankle joint. It can be a chronic problem following an ankle sprain or it can be from a very flat pronated foot.
If the sinus tarsi syndrome follows and ankle sprain, then my advice is:
1. Make sure that the rehabilitation from the original ankle sprain is adequate
2. Get some physical therapy done to mobilize the rearfoot.
3. Get some advice on exercises to carry out at home
4. If that does not work, consider a steroid injection
If the sinus tarsi syndrome is due to a very pronated foot, then my advice is:
1. You need foot orthotics that are designed to move the subtalar joint (the joint across the sinus tarsi) away from its end range of motion. Just supporting the arch is not enough
2. A good motion controlling running shoe is really important
3. If that does not work, then consider a steroid injection