The lateral femoral notch sign as a diagnostic tool for acute ACL injuries

Overview

The lateral femoral notch sign (LFNS) on conventional radiographs shows promise as a reliable indicator for acute anterior cruciate ligament (ACL) injuries. This study evaluated the depth and location of the LFNS, demonstrating its potential utility in diagnosing ACL ruptures in the acute setting with good inter- and intra-observer reliability.

Background

Diagnosing ACL injuries acutely is challenging due to swelling and pain that reduce clinical exam reliability, and MRI is not always immediately available. The LFNS, a depression in the lateral femoral condyle, has been recognized in chronic ACL deficiency but its prevalence and diagnostic value in acute injuries remain unclear. Accurate and simple radiographic markers like the LFNS could aid early diagnosis and prevent treatment delays that risk further knee damage. This study aimed to validate the LFNS depth measurement and assess its reliability as a diagnostic tool in acute ACL injury.

Data Highlights

LFNS location relative to Blumensaat line was categorized as 0 mm, 1–10 mm, and > 10 mm to differentiate normal and pathological sites.

Key Findings

• The LFNS depth measurement on lateral knee radiographs is a simple and reproducible method with good inter- and intra-observer reliability.
• A depth of ≥ 1.5 mm in the lateral femoral condyle is considered pathological and associated with acute ACL injury.
• The location of the LFNS relative to the Blumensaat line further refines diagnostic accuracy, with pathological locations extending beyond 10 mm posteriorly.
• Combining LFNS depth and location measurements improves prediction of ACL injury compared to either parameter alone.
• The presence of a deep LFNS correlates with lateral meniscal injury, indicating associated intra-articular damage.
• Blinded assessment by multiple specialists confirmed the robustness of LFNS as a diagnostic sign in the acute setting.

Clinical Implications

The LFNS can be used as an accessible radiographic marker to support early diagnosis of acute ACL ruptures when MRI is unavailable. Recognizing a pathological LFNS depth and location on routine lateral knee radiographs may prompt timely referral for further evaluation and management, potentially reducing delays that worsen joint outcomes. Incorporating LFNS assessment into acute knee injury protocols could enhance diagnostic confidence in emergency and outpatient settings.

Conclusion

The lateral femoral notch sign is a valid and reliable radiographic indicator for acute ACL injury. Its measurement is simple, reproducible, and can aid early diagnosis, improving patient care when advanced imaging is not immediately accessible.

References

1. Original Study (2013) -- The lateral femoral notch sign: an effective diagnostic indicator for acute anterior cruciate ligament injuries