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

Abstract

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.

Keywords: ACL; General sports trauma; Imaging; Knee; Radiology.

Conflict of interest statement

Conflict of interest

All authors declare that they have no conflict of interest.

Ethical aproval

This retrospective study did not have to be reviewed by a medical ethics board according to the Dutch Law on Medical Research with Humans (WMO).

Figures

Fig. 1

ACL-deficient knee with a deep lateral femoral notch sign. The depth was measured by drawing a line over the femoral condyle (long line) and a small line perpendicular to this line and the deepest point of the depression (black arrow)

Fig. 2

ACL-deficient knee illustrating the measured location for the lateral femoral notch sign (LFNS). The white line is the Blumensaat line. The black double-headed arrow (tangent line) is the distance between the Blumensaat line and the posterior beginning of the LFNS

Fig. 3

The left side shows the median lateral femoral notch sign depth in patients with lateral meniscal injury (n = 38), with a median depth of 0.8 mm; the right side shows the median lateral femoral notch sign depth in patients without lateral meniscal injury (n = 80), with a median depth of 0.5 mm (p = 0.040). The vertical lines show the 95% CI