Lateral capsular sign | Radiology Signs

Answer:

Small avulsion fragment from the lateral aspect of the lateral tibial plateau, just distal to the joint line, seen on anteroposterior knee radiograph, representing avulsion of the lateral capsular ligament (and/or anterolateral ligament) insertion. This is synonymous with the Segond fracture and is a highly specific indirect marker of severe internal derangement of the knee, most notably anterior cruciate ligament (ACL) tear, often with associated lateral meniscal injury.

Why is it called so?

Named for its location as a small fleck of bone adjacent to the lateral aspect of the lateral tibial plateau, indicating avulsion from the lateral knee joint capsule — hence the term Lateral capsular sign, coined by Woods et al. in 1979. The underlying fracture itself was first described in 1879 by French surgeon Paul Segond in cadaveric experiments, and is eponymously known as the Segond fracture.

Pathophysiology

High-energy rotational or valgus trauma with internal rotation and anterior tibial subluxation (pivot-shift mechanism) places forceful tension on the mid-third lateral capsular ligament/anterolateral ligament at its tibial insertion, avulsing a thin curvilinear bone fragment from the lateral tibial plateau. This occurs during the same injury mechanism that disrupts the ACL, which is why the two findings are so strongly associated.

Alternative names: Segond fracture (the same entity — the “lateral capsular sign” is the descriptive radiographic term, “Segond fracture” the eponymous term)

Other associated named signs: Reverse Segond fracture (avulsion of the deep medial collateral ligament from the medial tibial plateau, associated with PCL and medial meniscal injury), lateral femoral notch sign (deep sulcus on the lateral femoral condyle, another indirect radiographic marker of ACL tear)

References

1. Woods GW, Stanley RF, Tullos HS. Lateral capsular sign: x-ray clue to a significant knee instability. Am J Sports Med. 1979;7(1):27-33.
2. 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(6):1163-1167.