Segond Lesion Mechanics

For Medical Students

The Principle

When a soft tissue is pulled harder than its bony attachment can hold, the bone fails instead of the tissue. That is an avulsion.

An avulsion is therefore evidence about two things: that a structure was attached at that spot, and that it was loaded in the direction the fragment moved.

A small fragment can mean a large injury, because the fragment's size says nothing about the force or the structure.

At the Knee

The Segond fracture is an avulsion of a thin flake of bone from the lateral tibial margin, just proximal and posterior to Gerdy's tubercle. 2.11.4 covers the lesion.

It is produced by internal rotation and varus loading, which is the loading that stresses the anterolateral structures. 3.11.5 covers internal rotation control.

Conventional teaching treats it as nearly pathognomonic of anterior cruciate rupture, because the same mechanism injures both. 3.7.14 covers the mechanism.

It is indirect evidence that something is attached at that point on the tibia, which is part of the anterolateral ligament's anatomical case. 3.11.3 covers that structure.

What Changes It

The loading direction and magnitude at injury.

Bone quality, which decides whether the bone or the soft tissue fails.

Whether the structure attached there was intact beforehand.

Age, since a younger bone is more likely to avulse.

Clinical Relevance

Seeing it on a plain radiograph should prompt a search for a cruciate injury rather than treatment of the fragment. Section 5 covers imaging.

It indicates anterolateral injury, which bears on whether a lateral procedure is added. 3.11.7 covers the tenodesis.

The fragment itself rarely needs fixing, which is why the finding matters more than the fracture. Section 6 covers management.

6.100 covers the clinical condition.

MRCS Level

The Principle

An avulsion fracture localises a structure's attachment more reliably than dissection does, because it is produced in vivo by the structure's own loading.

That makes the Segond lesion a piece of mechanical evidence in the anterolateral anatomy argument rather than only a radiographic sign. 3.11.3 covers the argument.

What it cannot do is name the structure, because several anterolateral structures attach in that region. 2.11.5 covers the complex.

At the Knee

The fragment arises from the lateral tibial rim, conventionally described as 2 to 3 centimetres posterior to Gerdy's tubercle, which corresponds to the described anterolateral ligament tibial attachment region.

Conventional teaching reports a high association with anterior cruciate rupture, and also with lateral meniscal and anterolateral complex injury. 6.4 covers associated injuries.

It is best seen on an anteroposterior radiograph and is easily missed, which is why the finding is specifically looked for. 5.10 covers imaging.

Its presence is associated with a higher grade pivot shift in reported series, which is consistent with anterolateral involvement. 3.11.4 covers pivot shift control.

What Changes It

Loading direction and bone quality at injury.

Age, since the pattern differs between skeletally immature and mature bone.

Whether the fragment is displaced.

Concurrent injuries, which are common.

How It Is Measured

Plain anteroposterior radiography identifies it, and computed tomography localises it precisely.

Magnetic resonance imaging demonstrates the associated soft tissue injury. 5.10 covers imaging.

Cadaveric loading studies reproduce the fracture and establish the mechanism.

Correlation studies relate its presence to pivot shift grade and to associated injuries.

Clinical Relevance

Look for it specifically on the anteroposterior radiograph, since it is easily missed.

Read it as indicating anterolateral and cruciate injury rather than as a fracture to treat.

Expect a higher grade pivot shift where it is present.

Use it as evidence of an attachment region rather than of a named structure.

FRCS Level

At the Knee

Treat the lesion as an indication to consider a lateral procedure, on the ground that it is direct evidence of anterolateral injury in a knee where that is otherwise inferred. 3.11.7 covers the tenodesis.

Conventional teaching does not fix the fragment, because it is small, extra-articular and the attached tissue heals to the tibia. The exception is a large displaced fragment.

Its value is diagnostic and prognostic rather than therapeutic, which is an unusual position for a fracture and worth stating as such.

What Changes It

Fragment size and displacement.

Whether a lateral procedure is being added anyway.

Associated injuries, which drive the operation.

Pivot shift grade under anaesthetic.

How It Is Measured

Plain radiography and computed tomography characterise the fragment.

Examination under anaesthetic establishes the rotatory laxity it predicts.

Cohort studies relate its presence to outcome and to associated injury.

No evidence establishes that fixing the fragment changes the result.

Clinical Relevance

Consider a lateral procedure where the lesion is present.

Leave the fragment unfixed unless it is large and displaced.

Grade the pivot shift under anaesthetic, which the lesion predicts will be higher.

Describe its value as diagnostic and prognostic rather than therapeutic.