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(Radiology. 2001;219:800-801.)
© RSNA, 2001

Signs in Imaging

The Lateral Femoral Notch Sign1

Duke G. Pao, MD

1 From the Department of Radiology, University of Virginia Medical System, Box 170, Charlottesville, VA 22908. Received April 9, 1999; revision requested May 21; revision received October 11; accepted October 21. Address correspondence to the author (e-mail: dgp2p@virginia.edu).

Index terms: Knee, injuries, 452.419, 4526.4857 • Knee, ligaments, menisci, and cartilage, 4526.4857 • Knee, radiography, 4526.11, 458.11 • Signs in Imaging


   APPEARANCE
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 APPEARANCE
EXPLANATION
 DISCUSSION
 REFERENCES
 
The lateral femoral notch sign is a finding that can be seen on the lateral radiograph of the knee. This sign is characterized by an abnormally deep depression of the lateral condylopatellar sulcus (Fig 1).



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  		Figure 1. Lateral femoral notch sign. Lateral radiograph of the knee shows an abnormally deep depression with sharp angulation of the lateral condylopatellar sulcus (straight arrow). Associated large joint effusion (curved arrows) is also present.

 

   EXPLANATION
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 EXPLANATION
DISCUSSION
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The lateral condylopatellar sulcus, also known as the lateral femoral notch, normally forms a shallow groove in the middle of the lateral femoral condyle. It represents the junction zone on the lateral femoral condyle where the tibiofemoral and patellofemoral radii of curvature meet. On the lateral radiograph of the knee, it is aligned more parallel to the central ray of the x-ray beam than the anteriorly located medial condylopatellar sulcus. This results in the lateral condylopatellar sulcus projecting farther posteriorly and appearing more conspicuous than the medial sulcus (1). This appearance of the lateral sulcus also facilitates distinction between the lateral femoral condyle and the overlapping medial femoral condyle on the lateral projection.

An abnormally deep lateral condylopatellar sulcus has been attributed to an impacted osteochondral fracture (2). A localized chondral or transchondral abnormality overlying the lateral condylopatellar sulcus has been observed with a torn anterior cruciate ligament (ACL) during surgical reconstruction and magnetic resonance (MR) imaging of the knee (2–4).

The most common mechanism of an ACL tear is rotation and valgus stress (4). Disruption of the ACL with valgus stress causes the posterior aspect of the lateral tibial plateau and the middle to anterior portion of the lateral femoral condyle to forcefully impact against one another. This causes a pattern of injuries known as "kissing contusions," which are usually radiographically occult injuries to the cartilage and bone demonstrated as bone contusions at MR imaging (Fig 2).



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  		Figure 2. Sagittal T1-weighted spin-echo MR image (600/12 [repetition time msec/echo time msec]) obtained in the same patient as in Figure 1 demonstrates a deep depression of the middle portion of the lateral femoral condyle (arrowhead) and surrounding bone marrow edema (straight solid arrows). There is associated bone marrow edema of the posterior aspect of the lateral tibial plateau (open arrows). A large joint effusion (curved arrows) is present. Other MR images (not shown) demonstrated an ACL tear.

 

   DISCUSSION
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 APPEARANCE
 EXPLANATION
 DISCUSSION
REFERENCES
 
The lateral femoral notch sign has been described (2,3) as an indirect sign of a torn ACL. The depth of the lateral condylopatellar sulcus can be measured on the lateral radiograph by drawing a tangent line across the sulcus on the articular surface of the lateral femoral condyle. The depth of the sulcus is then measured perpendicular to this line at its deepest point.

Using this method of measuring the sulcus, Warren et al (3) compared the depth of the sulcus in patients with and those without ACL tears. They found that only one (2%) of 47 patients with clinically intact ACLs had a sulcus greater than 1.0 mm in depth. In contrast, two (4%) of 52 patients with acute ACL tears and 13 (13%) of 101 patients with chronic ACL tears had a sulcus greater than or equal to 1.5 mm in depth. The authors concluded that a lateral femoral notch greater than 2.0 mm in depth was highly suggestive of an ACL injury but that the lateral femoral notch sign was an inconstant finding in ACL–deficient knees.

Using the same method of measuring the sulcus, Cobby et al (2) found a clinically substantial difference in the mean depth of the sulcus between patients with and those without ACL tears. In the 62 patients with clinically and/or arthroscopically confirmed normal ACLs, the mean depth was 0.45 mm, with a range of 0–1.2 mm. In 41 patients with confirmed ACL tears, the mean depth was 0.89 mm, with a range of 0–5.0 mm. Five (12%) of the patients with ACL tears had a sulcus larger than 1.5 mm in depth, and one (2%) of these five patients had a 5-mm-depressed angulated fracture at the sulcus. They concluded that a sulcus deeper than 1.5 mm, which was equivalent to 3 SDs larger than the mean, was a reliable indirect conventional radiographic sign of a torn ACL.

There are few findings suggestive of an ACL tear on conventional radiographs of the knee. Five characteristic fractures on radiographs that have a high association with a torn ACL are the following: (a) avulsion fracture of the lateral tibial rim (Segond fracture), (b) avulsion fracture of the tibial spine, (c) avulsion fracture of the posteromedial tibial plateau, (d) posterior fracture of the lateral tibial plateau, and (e) the lateral femoral notch sign (2,5–7). The lateral femoral notch sign and posterior fracture of the lateral tibial plateau are infrequently seen radiographic signs corresponding to the kissing contusions seen at MR imaging.

An impaction fracture of the lateral femoral condyle with sufficient depression could be apparent on the lateral radiograph of the knee as the lateral femoral notch sign. However, a limitation of this sign remains the difficulty in distinguishing a prominent but normal lateral condylopatellar sulcus and a shallow impacted fracture at the sulcus. Although the lateral femoral notch sign is a highly infrequent finding in patients with ACL tears, an abnormally deep lateral condylopatellar sulcus is highly suggestive of a torn ACL. It is important to carefully search for the lateral femoral notch sign and to recognize its importance so that further evaluation with MR imaging can be performed to confirm the presence of an associated ACL tear.


   FOOTNOTES
 
Abbreviation: ACL = anterior cruciate ligament

A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.


   REFERENCES
TOP
 APPEARANCE
 EXPLANATION
 DISCUSSION
 REFERENCES
 

  1. Weissman BNW, Sledge CB. Orthopedic radiology Philadelphia, Pa: Saunders, 1986; 502, 511.
  2. Cobby MJ, Schweitzer ME, Resnick D. The deep lateral femoral notch: an indirect sign of a torn anterior cruciate ligament. Radiology 1992; 184:855-858.[Abstract/Free Full Text]
  3. Warren RF, Kaplan N, Bach BR. The lateral notch sign of anterior cruciate ligament insufficiency. Am J Knee Surg 1988; 1:119-124.
  4. Kaplan PA, Walker CW, Kilcoyne RF, Brown DE, Tusek D, Dussault RG. Occult fracture patterns of the knee associated with anterior cruciate ligament tears: assessment with MR imaging. Radiology 1992; 183:835-838.[Abstract/Free Full Text]
  5. Goldman AB, Pavlov H, Rubenstein D. The Segond fracture of the proximal tibia: a small avulsion that reflects major ligamentous damage. AJR Am J Roentgenol 1988; 151:1163-1167.[Abstract/Free Full Text]
  6. Stallenberg B, Gevenois PA, Sintzoff SA, Matos C, Andrianne Y, Struyven J. Fracture of the posterior aspect of the lateral tibial plateau: radiographic sign of anterior cruciate ligament tear. Radiology 1993; 187:821-825.[Abstract/Free Full Text]
  7. Chan KK, Resnick D, Goodwin D, Seeger LL. Posteromedial tibial plateau injury including avulsion fracture of the semimembranous tendon insertion site: ancillary sign of anterior cruciate ligament tear at MR imaging. Radiology 1999; 211:754-758.[Abstract/Free Full Text]



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