The sinus tarsi is a canal running from an opening anterior and inferior to the lateral malleolus to a point just behind the medial malleolus. It forms part of the subtalar joint and contains some blood vessels, fat, subtalar ligaments.
Injury to it is either chronic overuse due to poor biomechanics (over pronation etc) or as the result of an acute ankle sprain.
Pain usually anterior to the lateral malleolus. It is often worse in the morning and may diminish with exercise. Can also have ankle and foot stiffness and occasionaly of weakness.
Treatment is usually rest, ice etc but in particular mobilisation of the subtalar joint. Corticosteriod injection can help but the biomechanics need to be addressed to prevent recurrance.
Any help??
Sounds like you've been doing your research. Have you considered a peroneal tendinopathy or a stress fracture of your talus??