Physeal injuries are common in the pediatric population and are usually classified by the Salter Harris system. One of the complications of physeal fractures is trapped periosteum. This can render a fracture as irreducible which may result in premature physeal closure. Trapped periosteum is important to recognize on imaging because it requires open reduction to prevent growth disturbances.
On radiographs and CT with mulitplanar reconstruction, peristent physeal widening > 3mm after closed reduction is suggestive of trapped periosteum. The MRI findings of trapped periosteum have not been adequately described in the literature. A few reports seem to concur that on proton density weighted fat-suppressed images trapped periosteum appears as a low signal band insinuating at the physis.
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Whan A, Breidahl W, Janes G. MRI of trapped periosteum in a proximal tibial physeal injury of a pediatric patient. AJR Am J Roentgenol 2003;181:1397-9.