The drop sign, a radiographic warning sign of elbow instability.

Persistent instability or redislocation is uncommon but of significant concern in treating elbow dislocations. Finding an objective, static radiographic sign that might correlate with the presence of instability was the purpose of this study. Pre- and post-reduction radiographs of 10 consecutive simple and complete adult elbow dislocations were reviewed and compared with radiographs of 20 consecutive adult elbows without any trauma history. A statistically significant measured increase in static ulnohumeral distance was noted on the routine unstressed post-reduction lateral radiographs of patients sustaining dislocation. We have termed this increased distance the drop sign. It differs from the radiographic ulnohumeral separation noted during O'Driscoll's test for posterolateral rotary instability, which is present only with axial compression. The drop sign becomes concerning only if persistent after the first reduction radiograph and may be a warning sign of the presence of instability.