summary Iselin's Disease is a common acquired condition cause by traction apophysitis of the the peroneus brevis tendon at the tuberosity of the fifth metatarsal in children and presents with lateral foot pain. Diagnosis is made clinically with pain over the 5th metartarsal base with radiographs showing an enlarged apophysis with disordered ossification and widened chondro-osseous junction. Treatment is usually nonoperative with activity modifications, stretching, and a short period of cast immobilization as the condition typically resolves over time. Epidemiology Demographics most often seen in physically active boys and girls between the ages of 8 and 13 years of age Risk factors common in soccer players, basketball players, gymnasts, dancers, and other sports with repetitive inversion stresses Etiology Pathophysiology due to repetitive traction of peroneus brevis tendon at the site of its attachment Anatomy Osteology fifth metatarsal apophysis Muscles peroneus brevis tendon insertion Presentation History often no history of trauma Symptoms pain in lateral foot worse with activity and improves with rest Physical exam inspection swelling and erythema palpation tenderness over base of 5th metatarsal at the insertion of peroneus brevis provocative test pain with resisted eversion and extreme plantar flexion Imaging Radiographs may show an enlarged apophysis with disordered ossification and widened chondro-osseous junction Differential Normal apophysis longitudinally oriented apophysis without fragmentation Base of 5th metatarsal fractures transverse orientation of fracture fragment differentiates it from nearby vertically oriented apophysis may be difficult to differentiate in adolescents Os vesalianum intratendinous accessory bone within the substance of the peroneus brevis often incidental but can be associated with pain Treatment Nonoperative stretching, rest, activity modification, icing indications standard of care as conditions resolves with time cast immobilization indications pain despite attempts at other non-surgical management Operative surgical excision indications pain despite immobilization and other non-surgical management nonunion Complications Nonunion rarely occurs