Deep Lateral Femoral Notch Sign

1. High-Yield Overview & Pathognomonic Essence

• Morphological Hallmark: An abnormal focal depression, concavity, or deepening of the lateral femoral sulcus (sulcus terminalis) on the lateral femoral condyle, defined quantitatively on lateral plain radiography as a depth exceeding 1.5 mm (Cobby threshold) to 2.0 mm (Jones threshold) measured perpendicular to the condylar tangent line, or identified on sagittal MRI as a distinct osteochondral impaction step-off.
• Pathognomonic Significance: An established indirect, pathognomonic secondary sign of an acute complete anterior cruciate ligament (ACL) tear. The notch represents an impaction fracture of the lateral femoral condyle colliding against the posterior margin of the lateral tibial plateau during violent anterior subluxation in a noncontact pivot-shift mechanism, serving as the knee’s direct biomechanical analogue to the Hill-Sachs lesion of the shoulder.
• Critical Clinical Implication: Presence of the deep notch sign confirms high-energy rotatory knee trauma with near 100% specificity and positive predictive value. Beyond confirming an ACL tear, it strongly correlates with high-grade anterolateral rotatory instability (ALRI), high rates of concomitant posterior root tears of the lateral meniscus (up to 48%), anterolateral ligament (ALL) disruption (92% specificity), and a steeper posterior tibial slope, signaling the need for meticulous meniscal root repair and consideration of lateral extra-articular tenodesis (LET).

2. Diagnostic Sign Summary Matrix

3. Clinical Case Imaging & Standardized Figures

Figure 1 demonstrates the classic radiographic appearance of the deep lateral femoral notch sign on a non-weight-bearing lateral projection of the knee in an acute post-traumatic presentation. A pronounced, sharply demarcated focal indentation is visible along the articular surface of the lateral femoral condyle at the junction between the distal weightbearing femoral condyle and the anterior patellofemoral trochlea. Under standard radiographic measurement technique, a tangent line drawn across the adjacent articular contours reveals that the perpendicular depth of the sulcus exceeds 2.0 mm, fulfilling the validated threshold for an acute impaction fracture. The subtle underlying zone of subchondral radiodensity immediately deep to the notch represents microtrabecular bone compaction resulting from the violent collision between the lateral femoral condyle and the posterior rim of the lateral tibial plateau during anterior tibial subluxation.

4. Pathophysiology & Biomechanical Mechanism

The deep lateral femoral notch sign is a permanent structural footprint of violent knee kinematics occurring during a noncontact pivot-shift injury. The inciting trauma typically involves an axial compressive load delivered to a knee positioned in slight flexion (approximately 15 to 30 degrees), accompanied by acute dynamic valgus loading and internal tibial rotation relative to the femur (or external femoral rotation on a fixed foot). Under these combined loads, the anterior cruciate ligament experiences peak tensile strain and catastrophic structural failure.

Upon complete disruption of the primary restraint to anterior tibial translation, the lateral compartment exhibits an explosive anterior subluxation of the lateral tibial plateau relative to the lateral femoral condyle. As the posterior rim of the lateral tibial plateau translates anteriorly beneath the femoral condyle, the hard, wedge-shaped cortical margin of the posterior tibia forcefully collides with the midportion of the lateral femoral condyle. The anatomical site of impact corresponds precisely to the sulcus terminalis, the anatomical transitional groove that separates the anterior patellar articular surface from the distal weightbearing tibiofemoral articular surface.

Because the contact area is exceedingly small and the cortical bone of the posterior tibial margin is structurally denser than the softer cancellous trabeculae of the femoral condyle, the collision exceeds the compressive yield strength of the femoral subchondral bone. This produces an acute osteochondral impaction fracture characterized by microscopic trabecular microfractures, trabecular compaction, and focal plastic depression of the overlying articular cartilage. This biomechanical mechanism represents the direct lower extremity analogue of the Hill-Sachs lesion of the humeral head, which occurs when the posterior humeral head impacts the rigid anterior glenoid rim during anterior glenohumeral dislocation.

Simultaneously, the violent rotatory deceleration generates reciprocal energy dissipation across the joint, establishing the characteristic “kissing contusion” pattern on magnetic resonance imaging between the terminal sulcus of the lateral femoral condyle and the posterolateral tibial plateau. The extreme lateral compartment translation places catastrophic tension on the secondary anterolateral restraints, including the anterolateral ligament (ALL) and the deep capsulo-osseous layer of the iliotibial band, while simultaneously subjecting the posterior horn and root of the lateral meniscus to severe shear and entrapment forces between the colliding articular surfaces.

5. Modality-Specific Imaging Findings

Plain Radiography

Plain radiography serves as the primary screening examination in acute knee trauma and provides the original diagnostic description of the sign. Key radiographic considerations include:

• Projection Geometry: A true lateral projection of the knee is mandatory. The posterior margins of the medial and lateral femoral condyles must be strictly superimposed (divergence less than 5 mm, ideally 0 to 2 mm). Rotational deviation introduces projectional artifact that can either obscure a true depression or falsely create an apparent notch.
• Measurement Technique: On the true lateral view, identify the groove of the sulcus terminalis on the lateral femoral condyle. Construct a baseline tangent line connecting the normal articular surfaces immediately anterior and posterior to the notch. Measure the depth of the notch along a line perpendicular to this tangent at the point of maximal condylar depression.
• Diagnostic Thresholds: A normal anatomical sulcus terminalis exhibits a gentle concavity with a depth rarely exceeding 0.8 mm to 1.0 mm (mean 0.5 to 0.7 mm). Cobby et al. (1992) established that a depth exceeding 1.5 mm is abnormal and highly specific for ACL rupture. Jones et al. (1993) validated that a threshold depth exceeding 2.0 mm achieves virtually 100% specificity and positive predictive value for acute complete ACL tear.
• Associated Plain Film Signs: Subchondral trabecular condensation manifested as a faint curvilinear radiodense line beneath the depressed cortex; a concomitant Segond fracture (avulsion fracture of the anterolateral tibial margin); lipohemarthrosis visible in the suprapatellar pouch on cross-table lateral views; and subtle avulsion of the fibular styloid (arcuate complex sign).

Computed Tomography (CT)

Computed tomography provides sub-millimeter isotropic resolution that eliminates projectional superimposition and accurately characterizes the three-dimensional morphology of the impaction injury:

• Multiplanar Mapping: Thin-slice sagittal reformations (0.5 mm to 1.0 mm) allow direct measurement of the cortical defect depth and width without the geometric distortions inherent to plain radiography.
• Subchondral Trabecular Compaction: Visualizes the extent of subchondral comminution, microtrabecular impaction, and underlying bone densification beneath the articular step-off.
• Articular Fragment Evaluation: Detects small cortical flakes or displaced osteochondral fragments within the lateral gutter, intercondylar notch, or posterior capsular recesses that may escape radiographic detection.
• Associated Fractures: Accurately delineates subtle posterolateral tibial plateau impaction fractures, Segond fractures, and fibular head avulsions, which Korthaus et al. (2022) identified as frequently coexisting with high-energy lateral condylar impactions.

Magnetic Resonance Imaging & Ultrasound

Magnetic resonance imaging is the gold standard modality for definitive evaluation of both the osteochondral depression and associated ligamentous and meniscal disruptions:

• Sagittal Sequences & Bone Marrow Edema: Proton density (PD) and T2-weighted fat-suppressed (FS) sequences demonstrate marked hyperintensity within the trabecular marrow of the lateral femoral condyle immediately subjacent to the notch, paired with a reciprocal contusion along the posterolateral tibial plateau. On T1-weighted sequences, this compaction manifests as a focal low-signal band reflecting impacted trabeculae and microvascular disruption.
• Articular Cartilage Evaluation: Dedicated cartilage sequences (3D PD-FS or T2 mapping) demonstrate focal articular cartilage fissuring, delamination, or full-thickness chondral loss over the notch. Behzadi et al. (2018) demonstrated that patients with a deep lateral femoral notch sign exhibit significantly elevated T2 relaxation times in the anterolateral condylar cartilage even following successful ACL reconstruction, indicating persistent microstructural cartilage damage.
• Ligamentous & Meniscal Trajectory: MRI definitively confirms complete ACL rupture (discontinuity, fluid gap, abnormal sagittal orientation angle less than 45 degrees). Furthermore, MRI identifies critical associated injuries, specifically lateral meniscus posterior root tears (Berthold et al. 2020), anterolateral ligament (ALL) tears (Dimitriou et al. 2021), and posterolateral corner strains (Korthaus et al. 2022).
• Ultrasonography: High acoustic impedance and cortical reflection prevent direct sonographic evaluation of the intra-articular notch. However, dynamic high-frequency ultrasound provides valuable assessment of the extracapsular soft tissues, revealing hematoma along the anterolateral capsule, disruption of the proximal or distal ALL attachments, and dynamic meniscal extrusion under manual rotatory stress.

6. Diagnostic Utility, Sensitivity & Specificity

The diagnostic performance of the deep lateral femoral notch sign is characterized by exceptional specificity and high positive predictive value, balanced by relatively low sensitivity across standard imaging modalities. In the seminal radiological cohort published by Cobby et al. (1992), a lateral condylar depression exceeding 1.5 mm on plain radiographs demonstrated a specificity of 100% and a positive predictive value of 100% for an acute ACL rupture. Jones et al. (1993) corroborated these findings using a 2.0 mm threshold, confirming near 100% specificity in patients presenting with acute post-traumatic hemarthrosis.

Despite its outstanding specificity, the sensitivity of the sign on plain radiographs is modest, ranging between 10% and 25%. This low sensitivity reflects the fact that only a fraction of ACL injury mechanisms involve the extreme axial loading and subluxation velocity required to produce permanent cortical plastic deformation against the tibial margin. On magnetic resonance imaging, sensitivity increases to approximately 30% to 40% as subchondral bone marrow edema and minor contour irregularities are recognized earlier. Consequently, the negative predictive value of the sign is moderate (60% to 75%); the absence of a deep lateral femoral notch sign can never be used to exclude an ACL rupture.

Technical factors strongly influence visualization on plain radiography. Rotational deviation of more than 5 degrees from a true lateral projection obscures the lateral condylar profile behind the medial femoral condyle or creates an apparent false notch. True lateral projection is confirmed by superimposition of the posterior condylar outlines within 2 mm to 3 mm. When strict lateral alignment is achieved, identification of a notch depth greater than 2.0 mm serves as an absolute surrogate marker for complete ACL disruption.

7. Differential Diagnosis & Radiographic Mimics

8. Clinical Significance & Management Implications

The presence of a deep lateral femoral notch sign extends far beyond establishing an indirect diagnosis of an ACL tear; it identifies a distinct subpopulation of knee injuries characterized by extreme rotatory energy transfer, multi-structure disruption, and high risks of residual instability:

• Surrogate Marker of Severe Rotatory Energy & High-Grade ALRI: A deep notch indicates that the knee experienced massive anterior translation and rotatory displacement during the pivot-shift event. Clinically, these patients demonstrate high-grade anterolateral rotatory instability (Grade II or Grade III pivot-shift) during physical examination under anesthesia.
• Mandatory Evaluation of the Lateral Meniscus Posterior Root: In a landmark comparative cohort, Berthold et al. (2020) demonstrated that the deep lateral femoral notch sign was present in 48% of patients with combined ACL and lateral meniscus posterior root tears (LMPRT), compared to only 15% of patients with isolated ACL tears (p < 0.001). During diagnostic arthroscopy, the surgeon must not conclude inspection with an anterior probe examination; a 70-degree arthroscope and posterior trans-notch probing are mandatory to identify and repair hidden root tears. Anatomical transosseous pullout repair or suture anchor fixation restores meniscal hoop stress and protects the lateral compartment against accelerated chondrolysis.
• Indication for Lateral Extra-Articular Tenodesis (LET): Dimitriou et al. (2021) demonstrated that a lateral femoral notch depth exceeding 2.0 mm has a 92% specificity for concomitant anterolateral ligament (ALL) rupture. Combined ACL reconstruction and lateral extra-articular tenodesis (such as a modified Lemaire procedure) is strongly indicated in patients exhibiting the deep notch sign, as isolated intra-articular ACL reconstruction often fails to control residual rotatory laxity, predisposing the graft to premature elongation or rupture.
• Influence of Increased Posterior Tibial Slope: Braz et al. (2024) demonstrated that ACL-injured patients exhibiting a deep lateral femoral notch sign possess a significantly steeper lateral posterior tibial slope (mean increase of 2.7 degrees) and a 19% higher frequency of lateral meniscal tears. A steep tibial slope amplifies anterior tibial shear forces during axial loading, predisposing to violent pivot-shift subluxation and increased ACL graft strain. In revision settings or high-grade instability, slope-reducing deflexion osteotomy should be considered.
• Chondral Restoration Considerations: Extensive osteochondral depressions involving more than 3 mm to 4 mm of depth or extending into the functional weightbearing arc of the condyle warrant formal cartilage preservation. While many shallow lesions remodel spontaneously, symptomatic deep depressions may require subchondral elevation with bone grafting, autologous osteochondral transfer (OATS), or focal osteochondral allograft transplantation to prevent focal articular degeneration.

9. Master Class Pro-Tip

Master Class Pro-Tip: When a lateral knee radiograph demonstrates a deep lateral femoral notch exceeding 2.0 mm in an acute hemarthrosis, approach the case as a high-energy multi-structure rotatory injury rather than an isolated ACL rupture. The presence of this impaction signature carries a near 50% probability of an occult posterior root tear of the lateral meniscus and over 90% specificity for concomitant anterolateral ligament (ALL) disruption. During diagnostic arthroscopy, perform a 70-degree arthroscope inspection of the lateral meniscal root beneath the posterior cruciate ligament, and maintain a low threshold to augment primary ACL reconstruction with a modified Lemaire lateral extra-articular tenodesis (LET) to protect the intra-articular graft against persistent rotatory torque and early failure.

Master Class Radiological Pearl: Deep Lateral Femoral Notch Sign

10. Evidence-Based Synthesis

11. References

1. Cobby MJ, Schweitzer ME, Resnick D. The deep lateral femoral notch: an indirect sign of a torn anterior cruciate ligament. Radiology. 1992;184(3):855-858. DOI [Level IV: Diagnostic Cohort Study]
2. Jones AR, Finlay DB, Learmonth DJ. A deep lateral femoral notch as a sign of acutely torn anterior cruciate ligament. Injury. 1993;24(9):601-602. DOI [Level IV: Diagnostic Validation Series]
3. Berthold DP, Muench LN, Herbst E, et al. High prevalence of a deep lateral femoral notch sign in patients with anterior cruciate ligament (ACL) and concomitant posterior root tears of the lateral meniscus. Knee Surg Sports Traumatol Arthrosc. 2020;28(10):3126-3133. DOI [Level IV: Comparative Cohort Study]
4. Dimitriou D, Reimond M, Foesel A, et al. The deep lateral femoral notch sign: a reliable diagnostic tool in identifying a concomitant anterior cruciate and anterolateral ligament injury. Knee Surg Sports Traumatol Arthrosc. 2021;29(8):2536-2544. DOI [Level IV: Diagnostic Cohort Study]
5. Braz JNPDSV, Alves LFTG, Ferreira FAL, et al. The presence of a deep lateral femoral notch sign in ACL-injured patients is associated with a 2.7° steeper posterior tibial slope and a 19% higher frequency of lateral meniscal injuries. Knee Surg Sports Traumatol Arthrosc. 2024;32(10):2657-2665. DOI [Level IV: Retrospective Comparative Study]
6. Korthaus A, Warncke ML, Pagenstert G, et al. Lateral femoral notch sign and posterolateral tibial plateau fractures and their associated injuries in the setting of an anterior cruciate ligament rupture. Arch Orthop Trauma Surg. 2022;142(8):1985-1993. DOI [Level IV: Retrospective Cohort Study]