The lateral femoral notch sign: a reliable diagnostic measurement in acute anterior cruciate ligament injury

PURPOSE: To describe the validity and inter- and intra-observer reliability of the lateral femoral notch sign (LFNS) as measured on conventional radiographs for diagnosing acute anterior cruciate ligament (ACL) injury. METHODS: Patients (≤ 45 years) with a traumatic knee injury who underwent knee arthroscopy and had preoperative radiographs were retrospectively screened for this case–control study. Included patients were assigned to the ACL injury group (n = 65) or the control group (n = 53) based on the arthroscopic findings. All radiographs were evaluated for the presence, depth and location of the LFNS by four physicians who were blind to the conditions. To calculate intra-observer reliability, each observer re-assessed 25% of the radiographs at a 4-week interval. RESULTS: The depth of the LFNS was significantly greater in ACL-injured patients than in controls [median 0.8 mm (0–3.1 mm) versus 0.0 mm (0–1.4 mm), respectively; p = 0.008]. The inter- and intra-observer reliabilities of the LFNS depth were 0.93 and 0.96, respectively. Secondary knee pathology (i.e., lateral meniscal injury) in ACL-injured patients was correlated with a deeper LFNS [median 1.1 mm (0–2.6 mm) versus 0.6 mm (0–3.1 mm), p = 0.012]. Using a cut-off value of 1 mm for the LFNS depth, a positive predictive value of 96% was found. CONCLUSION: This was the first study to investigate the inter- and intra-observer agreement of the depth and location of the LFNS. The depth of the LFNS had a very high predictive value for ACL-injured patients and could be used in the emergency department without any additional cost. A depth of > 1.0 mm was a good predictor for ACL injury. Measuring the depth of the LFNS is a simple and clinically relevant tool for diagnosing ACL injury in the acute setting and should be used by clinicians in patients with acute knee trauma. LEVEL OF EVIDENCE: Diagnostic study, level II.