Radiology of postnatal skeletal development. VIII. Distal tibia and fibula.

Initially the distal tibial physis is a relatively transverse structure. As the epiphysis matures, undulations develop within the physis and lappet formation occurs peripherally. Within the first two years a significant physeal undulation develops anteriorly above the medial malleolus. This undulation must not be misinterpreted as premature epiphyseodesis following distal tibial fracture. Secondary ossification in the distal tibia begins centrally and initially expands to fill the area over the tibial plafond. At the lateral side of the tibial epiphysis the ossification center may be wedge-shaped. The medial margin adjacent to the medial malleolus is often irregular and may show small peripheral foci of ossification. By seven to eight years, the secondary center extends into the medial malleolus, with complete distal extension often not occurring until adolescence (although usually complete by ten to eleven years). The malleolar tip may exhibit an accessory ossification center. However, this center also may be a traumatic avulsion in the symptomatic patient. Physiologic epiphyseodesis begins over the medial malleolus and subsequently extends laterally. This pattern of closure appears to predispose to fracture of the lateral portion of the distal tibial epiphysis (fracture of Tillaux), as well as to triplane fractures. The articular surface curves onto the lateral side of the distal tibia to form an articulation with the lateral malleolus (distal tibiofibular joint). A similar extension occurs along the medial side of the fibula. These surfaces extend proximally as a recess to the level of the distal tibial physis, at which point the syndesmosis begins. The initially transverse distal fibular physis becomes a convoluted structure, with extensive peripheral lappet formation. Within these regions of physeal overlap there may be small areas of accessory ossification (both medially and laterally) that should not be misinterpreted as fractures. This overlapping also minimizes specific physeal separation and displacement (especially when compared to the incidence of distal tibial physeal injuries). Stress views may be necessary to show such an undisplaced fracture. The fibular physis normally is level with the tibial articular surface or distal extent of the tibial ossification center, especially after the second year of life (however, it may be more proximal in infants). As in the medial malleolus, there may be accessory ossification at the tip of the fibula. While this usually is a normal variant of secondary ossification, occasionally it also may result from trauma. Extensive porosity of the distal fibular metaphysis predisposes to buckling or torus injuries that may have severe, multiangular deformation.