Diagnosis: Segond fracture with complete tears of the anterior cruciate and medial collateral ligaments

Frontal knee radiograph (A) demonstrates an avulsion fracture of the lateral tibial rim (Segond fracture; yellow arrow) and a fracture of the tibial eminence (red arrow). Cross-table lateral radiograph of the knee (B) shows a fat–fluid level, or lipohemarthrosis (blue arrow), indicating concomitant intra-articular fracture with leakage of marrow fat into the joint capsule. Coronal proton density fat-suppressed (C) and sagittal proton density (D) MR images show complete tears of the femoral attachment of the medial collateral ligament (green arrow) and anterior cruciate ligament (white arrow). The Segond fracture (yellow arrow) and avulsed tibial spine (red arrow) are also seen on the coronal image. There is focal bone narrow edema of the trochlea (black arrow).

Discussion

A fracture of the non-articular surface of the lateral tibial rim is called a Segond fracture.

The site of a Segond fracture is posterior and just proximal to Gerdy’s tubercle, which is the tibial attachment of the iliotibial band.
Opinion varies about the exact anatomic components involved in this injury. Paul Segond, who identified the injury in 1879 before the advent of x-rays, believed it was the result of avulsion of the central portion of the lateral capsular ligament. Since then, injury to the posterior fibers of the iliotibial band and the anterior oblique band of the fibular collateral ligament has been postulated, although the exact structures involved remain controversial.

Segond fractures may be inconspicuous, but are essential to recognize, as the fracture is associated with extensive ligamentous injury and lateral knee instability. If the fracture is not initially identified and appropriately treated, chronic instability can occur.
Radiographs are the initial imaging modality for detecting a Segond fracture. Once it has been identified by plain film, subsequent MRI is essential to evaluate for associated injuries, including:
- anterior cruciate ligament (ACL) tears (most common, in 75–100% of cases)
- medial or lateral meniscal tears (in 66–75% of cases)
- other avulsion injuries including avulsion of the fibular head, tibial spine, and Gerdy’s tubercle
- posterolateral corner injuries

Clinical synopsis

The patient was discharged from the Emergency Department with a hinged knee brace and instructions for immobilization. He did well with conservative treatment.

Self-assessment

What is the “lateral capsular” sign?	This term describes the Segond tibial rim avulsion fracture fragment.
Is MR an optimal imaging modality for detection of a Segond fracture?	No. In most patients, the fracture fragment may not be apparent on MRI, even when it is seen on radiography. If there is lateral tibial plateau edema on MR, a Segond fracture should be suspected – the ligaments and menisci should be closely scrutinized. Often, Segond fracture is the only finding on radiographs in patients with ACL tear.
How do Segond and arcuate complex avulsion fractures differ?	Segond fractures involve the lateral tibial rim, while arcuate complex avulsion fractures involve the proximal fibula. On plain AP radiographs, Segond fracture fragments typically project parallel to the tibial shaft, while arcuate complex avulsion fracture fragments project perpendicular to the long axis of the tibia and fibula.