Lateral femoral notch sign (knee)

The (deep) lateral femoral notch sign describes a depression on the lateral femoral condyle at the terminal sulcus, a junction between the weight-bearing tibial articular surface and the patellar articular surface of the femoral condyle. It is occasionally referred to as a deep sulcus sign, not to be confused with the deep sulcus sign in pneumothorax on supine chest radiograph.

Pathology

The likely mechanism is a hyperextension or impaction injury with a collision of the femoral condyle and the posterior tibial plateau during the rotational movement responsible for injuring the ACL, most commonly the pivot-shift. 

Radiographic appearance

Lateral femoral notch sign is usually first appreciated on the lateral radiograph and is suggestive of an osteochondral fracture ^1,2. The depth of the lateral femoral notch sign has been shown to correlate with anterior cruciate ligament (ACL) tear ². 

A normal sulcus is located within 10 mm of Blumensaat's line on lateral projection ³.

Plain radiograph

On a lateral projection, a line is drawn from the normal articular surface of the lateral femoral condyle. The depth of the abnormal depression/notch can then be measured. A study compared the depth of the lateral femoral notch in patients with proven ACL tear and healthy individuals. A notch depth of 1.5 mm is shown as a useful additional sign of a torn ACL ².

Internal derangement should also be suspected if the notch is irregular.