Purpose or Case Report: Epiphyseal separation of the distal femur is a rare entity. It usually occurs as a complication of a traumatic vaginal or cesarean delivery, particularly in breech position. It is considered a Salter-Harris type 1 fracture, where the epiphysis separates from the metaphysis resulting in physeal widening and frequently displacement of the epiphysis in relationship to the metaphysis. Only a few cases are reported in the literature.

We report a case of distal epiphyseal separation of the distal femur in a 5-day-old patient who presented to the emergency department with swollen right foot. He was born at term in breech presentation via C-section delivery. At the presentation the physical examination revealed edema from the thigh to foot, decreased range of movement of the right hip, and he cried with palpation of the right tight.

Lateral radiographs of the right lower limb showed metaphyseal irregularity involving the distal femur simulating a corner fracture, and posterior displacement of the distal femoral epiphysis relative to the metaphysis. Subsequent ultrasound of the right knee demonstrated posterior displacement of the distal femoral epiphysis with a 5 mm gap and significant soft tissue swelling, confirming epiphyseal separation of the distal femoral epiphysis.

As the distal femoral epiphyseal secondary ossification center appears at approximately 38th week of gestational age, the radiograph is commonly the first imaging method to depict distal femoral epiphyseal separation. However, the radiographic diagnosis is challenging, and it can mimic a corner fracture leading to the suspicion of non-accidental injury. Therefore, ultrasound is an excellent complementary tool given its lack of radiation, no need for sedation, short examination time and high ability to assess the musculoskeletal structures with direct evaluation of the surface of osseous elements, cartilage and soft-tissue structures.

Although this condition is uncommon, awareness of its imaging features among pediatric radiologists is crucial to avoid misdiagnosis and unnecessary additional imaging.
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