Abstract
An 11-year-old girl presented with pain in the sinus tarsi and a foot deformity after spraining her left foot. She had flat foot due to spasticity of the peroneal tendon, which was difficult to correct manually. An imaging study revealed an accessory anterolateral talar facet (AALTF) and bone marrow oedema at the lateral aspect of the posterior talocalcaneal joint. We diagnosed peroneal spastic flat foot with possible involvement of sinus tarsi syndrome and AALTF. When conservative therapy failed, we applied a cast with the foot in full plantar flexion and an inverted position. After 4 weeks, the patient had a normal range of motion in the foot, the pain had resolved completely and the peroneal spasticity had disappeared. Orthopaedic treatment with a cast may be worth attempting before surgery in such cases.