In skeletally immature children, it can often be difficult to differentiate occult Salter-Harris I fibula fractures from ankle sprains based on physical examination, and often, initial radiographs in both conditions are only notable for soft tissue swelling. The likelihood of a child having subsequent plain radiographic evidence of a fracture in this setting and the likelihood of subsequent fracture displacement have not been previously reported. The purpose of our study was to determine the incidence of occult fracture in these patients and the risk of fracture displacement. We performed a retrospective review of all children seen for acute ankle injuries over a 14-month period at a large tertiary care children's hospital. To be included in the study, patients needed to have acute ankle trauma, an open distal fibula physis, normal radiographs, and localized distal fibular tenderness on examination. Thirty-seven consecutive children met the inclusion criteria, with 1 child having 2 isolated injuries, one of each ankle, 4 months apart. All patients were initially placed in a short leg walking cast and allowed to weight bear as tolerated. Anteroposterior, mortise, and lateral radiographs of the ankle taken 3 weeks after injury were evaluated for periosteal new bone formation and/or fracture displacement. By 3 weeks after injury, there was periosteal new bone formation about the distal fibula in 7 (18%) of 38 ankles. No fractures were displaced during treatment, and no radiographs had greater than 1 mm of new bone formation visible. This is the first investigation reporting the frequency of plain radiographic evidence of occult distal fibula fractures in children. This study finds that 7 (18%) of 38 acute ankle injuries in children presenting with distal fibula tenderness and normal radiographs show evidence of periosteal new bone formation on follow-up radiographs, implying the presence of an occult fracture. There was no evidence of fracture displacement during treatment, and no fracture demonstrated greater than 1 mm of periosteal new bone.
