Frog-Leg Test Maneuver for the Diagnosis of Injuries to the Posterolateral Corner of the Knee: A Diagnostic Accuracy Study

Objective: To determine the sensitivity and specificity of a new clinical test for the diagnosis of injuries to the posterolateral corner of the knee by using magnetic resonance imaging (MRI) as the reference standard. Design: Diagnostic accuracy study. Setting: A tertiary care teaching hospital. Participants: Twelve subjects with chronic instability of the knee and posterolateral corner injury diagnosed by intraoperative findings and 9 subjects without posterolateral corner injury (used as controls). Interventions: Injured and uninjured knees were assessed blindly by 2 examiners using a new clinical diagnostic test (the frog-leg test) and the classic varus stress test. Main Outcome Measures: Injuries to the posterolateral corner of the knee identified by the 2 clinical tests. Findings were compared and examined for reproducibility, and kappa statistic was used to assess interobserver agreement. Test results were compared with those of MRI and intraoperative findings to determine diagnostic accuracy. Results: The frog-leg test showed high interobserver agreement (kappa, 0.86), with a high rate (83%-100%) of agreement with the intraoperative diagnosis. The frog-leg test had high sensitivity (91.7%) and specificity (94.5%) for detecting posterolateral corner injuries. The sensitivity of the varus stress test increased from 83.3% to 90.0% when combined with the frog-leg test. Conclusions: Examiners were able to identify posterolateral corner injuries and differentiate injured from uninjured knees using the frog-leg test, which could potentially be used as an ancillary tool to the varus stress test in diagnosing injuries to the posterolateral corner of the knee. Larger studies are needed to confirm our findings.

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