Avulsion Injuries

For Medical Students

What It Shows

An avulsion fracture is a fragment of bone pulled away at the attachment of a ligament or tendon. 5.2.2 covers the image formation.

The fragment's position names the structure, because each attachment sits at a known site. 2.7.7 covers the cruciate tibial footprint.

It matters because a small bony fragment on a radiograph is the visible sign of a soft tissue injury that is otherwise invisible on that film. 5.3.15 covers Segond fractures.

How It Is Acquired

Look at the known attachment sites rather than scanning the whole film, since each avulsion has a location. 2.11.4 covers the Segond lesion.

Assess the fragment's size and displacement, which decide whether it can be fixed directly. 5.16.1 covers the fracture assessment.

Obtain a tunnel view where a cruciate avulsion is suspected, because the intercondylar region is foreshortened on the standard views. 5.3.4 covers the tunnel view.

Consider the skeletal age, since an avulsion in a child may involve the physis. 5.15.2 covers the growth plates.

Proceed to a magnetic resonance study to assess the attached structure, which the radiograph does not show. 5.7.4 covers the acute cruciate injury.

What Normal Looks Like

Smooth corticated outlines at the known attachment sites with no separate ossific fragment.

An intact intercondylar eminence on the tunnel view. 5.3.4 covers the tunnel view.

Normal physes in a skeletally immature knee, with no widening. 5.15.2 covers the growth plates.

No ossific density in the soft tissues adjacent to an attachment. 5.3.15 covers Segond fractures.

What Abnormal Looks Like

A bony fragment at the tibial cruciate attachment, raising a cruciate avulsion. 2.7.7 covers the cruciate tibial footprint.

A fragment at the lateral tibial margin, which is a Segond fracture. 5.3.15 covers Segond fractures.

A fragment at the fibular head, raising a posterolateral corner injury. 5.10.3 covers posterolateral corner injuries.

A fragment at the medial femoral epicondyle in a child, raising a collateral avulsion. 5.9.1 covers the medial collateral anatomy.

MRCS Level

How It Is Acquired

In a viva this is the page behind the answer that a small fragment is a soft tissue diagnosis.

The expected points are the known sites, the fragment size and displacement, the tunnel view, the skeletal age, and the magnetic resonance study.

Say that the fragment names the injured structure. 2.11.4 covers the Segond lesion.

Say that a bony avulsion can be fixed directly where a mid substance tear cannot. 5.7.3 covers complete cruciate tears.

Say that in a child the equivalent injury is often bony where in an adult it is ligamentous. 5.15.5 covers paediatric cruciate injury.

What Abnormal Looks Like

The reasoning expected is that a ligament and its bony attachment form a chain, so the injury happens at whichever link is weakest, and in a growing skeleton that is the bone. 2.7.1 covers the cruciate gross anatomy.

That is why the same mechanism gives a bony avulsion in a child and a mid substance tear in an adult. 5.15.5 covers paediatric cruciate injury.

A bony fragment is fixable with the ligament intact, which is a better situation than the ligamentous equivalent and a reason to look for it. 5.7.3 covers complete cruciate tears.

A small fragment can also indicate a complete disruption of a structure whose remaining substance is torn, so the fragment's size does not grade the injury. 5.10.3 covers posterolateral corner injuries.

How Good It Is

A radiograph detects an avulsion fragment well and says nothing about the structure attached to it.

Its best contribution is directing attention to a specific soft tissue structure, which is a high value finding on a cheap film. 5.3.15 covers Segond fractures.

It is limited by not showing the ligament, so the fragment's presence and the structure's integrity are two separate questions. 5.7.4 covers the acute cruciate injury.

No figures appear for accuracy on this page, under decision 028, and no fragment size thresholds appear either: the thresholds for direct fixation are published quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level an avulsion fragment is one of the most useful findings a plain film gives, because it redirects the whole assessment.

A tibial cruciate avulsion in a child is a repair with an intact ligament, which is a different operation from a reconstruction. 5.15.5 covers paediatric cruciate injury.

A fibular head fragment makes a posterolateral corner injury likely, and that changes the timing and the operation. 5.10.3 covers posterolateral corner injuries.

A Segond fracture indicates anterolateral injury and an associated cruciate tear. 5.3.15 covers Segond fractures.

In every case the fragment is the prompt and the magnetic resonance study is the assessment. 5.7.4 covers the acute cruciate injury.

Where It Misleads

The commonest error is reporting the fragment and not naming the structure it came from. 5.20.1 covers structured reporting.

A fragment is treated as the whole injury and the ligament is not assessed. 5.7.4 covers the acute cruciate injury.

An accessory ossicle or a fabella is read as an avulsion. 5.15.7 covers developmental variants.

A small fragment is taken to indicate a minor injury. 5.10.3 covers posterolateral corner injuries.

The intercondylar region is assessed without a tunnel view. 5.3.4 covers the tunnel view.

Fellowship Level

Where It Misleads

At fellowship level avulsion fragments are the best demonstration in this chapter that a radiograph's value lies in what it implies rather than in what it shows.

The fragment is a few millimetres of bone and its diagnostic weight comes entirely from anatomical knowledge of what attaches there, which means the finding is only as good as the reader's anatomy. 2.11.4 covers the Segond lesion.

Several of these fragments are reported descriptively, as a cortical irregularity or a small ossific density, which loses the implication entirely. 5.20.8 covers common reporting errors.

The paediatric versions interact with the physis, so the same fragment carries a growth consequence as well as a ligamentous one. 5.15.2 covers the growth plates.

Magnetic resonance imaging detects the soft tissue injury directly and the small bony fragment less well than a radiograph, which makes the two genuinely complementary rather than one superseding the other. 5.1.3 covers choosing the correct investigation.

The practical conclusion is that these films should be read by attachment site rather than by scanning, and the report should name the structure implicated. 5.20.1 covers structured reporting.