The MR dimple sign in irreducible posterolateral knee dislocations.

While posterolateral knee dislocation is a rare entity, its identification has important implications. The invagination of skin seen in posterolateral knee dislocations is referred to as the "dimple sign" on physical exam. We report a case where the "dimple sign" was also demonstrated on magnetic resonance imaging. Failure to recognize this finding on imaging will render the knee irreducible, as a result of interposed tissues, until discovered by the orthopedic surgeon intraoperatively. The incarcerated medial capsule and medial collateral ligament could also be misinterpreted as torn meniscus, as occurred in our case.