Drop sign and radial head subluxation in lateral collateral ligament complex injury: computed tomography scan study...

Posterolateral elbow instability can be difficult to diagnose. Drop sign and radial head subluxation are frequently reported after lateral collateral ligament complex (LCLc) injuries; however, their physiological ranges and diagnostic utility are not well established. This study aimed to define the physiological range of these markers using computed tomography (CT) and correlate changes in these markers with progressive LCLc injury.

Nine adult cadaveric elbows were mounted on a setting that allows controlled posterolateral distraction at 90° flexion. CT scans were performed before and after distraction to evaluate drop sign and radial head subluxation. The drop sign was measured as the distance between the Trochlear Sulcus and Olecranon (TSO). LCLc injuries were sequentially induced, including annular ligament division and progressive soft tissue lesions, up to complete LCLc disruption.

The physiological TSO was 3.3 mm in normal elbow without distraction and increased to 3.8 mm under distraction (P = .02). TSO greater than 4.3 mm was defined as a positive drop sign, and an average TSO of 4.95 mm was observed with a lesion of the capsule and ulnar band of the LCL. The drop sign progressively increased after each stage of LCLc injury, reaching 9.7 mm (95% confidence interval: 8.8-10.5) after complete LCLc disruption. Radial head subluxation also rises from 2.1 mm (95% confidence interval: 1.3-2.8) under distraction in normal elbow to 14.4 mm following complete LCLc lesion (P < .01).

CT imaging of the drop sign and radial head subluxation correlates with the severity of LCLc injury. These markers provide valuable diagnostic information for posterolateral elbow instability.