The "sliver sign": a specific radiographic sign of acute lateral patellar dislocation.

OBJECTIVE: The objective of this work is to assess the prevalence of the sliver sign, defined as an intraarticular linear or curvilinear ossific density, in association with knee effusion in patients with acute knee trauma, as a predictor of recent lateral patellar dislocation (LPD).
MATERIALS AND METHODS: A retrospective radiology database search for the term 'patellar dislocation' on MRI knee exams performed at our institution over a 7-year period identified 216 studies. Of these, 142 exams met true positive gold standard diagnostic criteria for LPD. Imaging findings of both the retrospectively identified radiographs and subsequent MRI were recorded. Accuracy of radiographic interpretation was also analyzed.
RESULTS: After review by an experienced musculoskeletal radiologist, 27 patients (19%) with LPD had knee radiographs demonstrating intraarticular osseous fragments. The majority of these patients had fragments (22/27, 81%) that were linear or curvilinear in configuration. A smaller subset of patients had fragments (5/27, 19%) that were rounded or oblong, felt to most likely represent sequelae of chronic dislocation. The fragments were identified on the patellar view only in eight of 27 patients (30%). These fragments were often initially misinterpreted, with the diagnosis explicitly stated in the report in 2/27 cases (7%). More often, the fractures were described vaguely (13/27, 48%), interpreted incorrectly (6/27, 22%), or missed (6/27, 22%). All 27 patients had moderate or large knee joint effusions.
CONCLUSIONS: In the setting of acute knee trauma, knee radiographs demonstrating a joint effusion and an intraarticular sliver-like osseous fragment correlate with recent lateral patellar dislocation. The routine trauma knee radiographic series does not include a patellar view but probably should, especially in young patients.