This refers to the injury of the ligaments that join the talus bone to the calcaneus bone, the two large bones in the back of the foot. Between the two bones is a tunnel which contains the ligaments called the sinus tarsi. The ligaments are damaged in a significant injury to the foot such as an inversion injury. They can also become injured and inflamed as part of an overuse injury, particularly associated with abnormal foot biomechanics.
This injury is often associated with an injury to the lateral ligament complex of the ankle, and it is easy to overlook the sinus tarsi injury and concentrate on the ankle. The ankle ligaments may heal up quite well but you will be left with some pain on the outside of the heel in front of the lateral malleolus. This is often worst in the morning and settles as the foot gets warmed up.
If you think you have this condition it is well worth getting it checked out by Mr Willis-Owen. In the mean time you should avoid any aggravating activities which may prolong the problem. It generally won't get better on its own. Anti-inflammatories and ice are helpful after exercise.
Treatment involves first confirming the diagnosis then establishing the cause. An MRI scan can be useful to exclude other problems and confirm the diagnosis. Treatment options may include specific physiotherapy, taping, orthotics, and injections. Once you are recovered it is important to address any biomechanical problems or other underlying factors in order to prevent recurrence.