The distal femoral metaphysis is a 9 cm transitional zone from the thick cortical region of the shaft to the broad condylar region. There is a thin cortex and a wide cancellous area. There are numerous muscle and ligamentous attachments, which cause deforming forces. Gastrocnemius rotates the distal fragment posteriorly, while the strong adductors cause varus angulation. Intercondylar fractures are splayed open by discordant muscle action.
In most cases, axial loading with a varus or valgus force leads to a distal femoral fracture. In the young this is usually a high-energy injury, typically an RTA, with considerable comminution, displacement and soft tissue trauma. In the elderly with osteoporotic bone, a fall on a flexed knee is the most common mechanism of injury. A long spiral supracondylar fracture is the result, sometimes complicated by a femoral prosthesis above or below the fracture. Rarely, a fracture occurs without obvious trauma in the immobile, as a result of severe osteoporosis.
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