Dashboard Injury

For Medical Students

What It Tests

A dashboard injury is a blow to the front of the flexed tibia that drives it backwards relative to the femur. The name comes from a car collision but the mechanism occurs in falls onto a flexed knee and in sport.

Asking about it tests whether the posterior cruciate was loaded, because it is the structure that resists exactly this.

It also screens for injuries elsewhere, since the energy involved is frequently high and the knee is rarely the only thing hurt. 3.8.1 covers the ligament's function.

How It Is Performed

Ask whether the knee was bent and whether something struck the front of the shin rather than the knee itself.

Ask about the energy: a road traffic collision, a fall from height, or a sporting impact.

Ask about the hip and the femur, because a posteriorly directed force on a flexed knee also loads them and a hip dislocation or femoral fracture takes priority.

Ask about the foot and ankle, which are injured in the same events and are frequently missed.

Ask about numbness, tingling or weakness, and whether the knee was ever obviously out of shape.

What a Positive Means

A dashboard mechanism with posterior sag on examination is a posterior cruciate injury. 4.11.1 covers the sag sign.

The same mechanism with external rotation instability raises the posterolateral corner as well, which is the combination that matters most. 4.11.7 covers combined assessment.

A dashboard injury with an inability to weight bear raises a tibial plateau or patellar fracture.

A high energy dashboard injury with gross multidirectional laxity is a knee dislocation and the vascular assessment comes first.

An isolated posterior cruciate injury with no other findings is possible and is less common than the combination.

MRCS Level

What a Positive Means

The expected reasoning is that the posterior cruciate is the primary restraint to posterior translation and does most of that work in flexion, which is exactly the position of this mechanism. 3.8.2 covers posterior stability.

So a flexed knee struck from the front loads the posterior cruciate maximally, which is why the mechanism is so characteristic.

The posterolateral structures resist external rotation and are loaded when the force has a rotational component, which is common in real collisions.

The dial test at two flexion angles is what separates the isolated from the combined injury, and the mechanism is what prompts it. 4.13.5 covers the test.

A posterior cruciate injury is under-diagnosed acutely because posterior sag is subtle in a swollen knee and the patient can often walk.

How Good It Is

The dashboard mechanism is one of the most specific histories in the knee, because very little else produces it.

When the mechanism is clear it should direct the examination towards posterior translation and rotation specifically, and that direction is its whole value.

It performs poorly for grading, because the acute examination under-reports posterior laxity for the same reasons it under-reports anterior laxity.

A negative answer excludes little, because posterior cruciate injuries also occur with hyperextension and with rotational mechanisms. 4.2.19 covers hyperextension.

FRCS Level

How Good It Is

At decision level the dashboard history is mainly a prompt to assess breadth, because the isolated and combined injuries are managed differently and the combined one is easy to miss.

An isolated posterior cruciate injury is frequently managed without surgery, and 3.17.2 sets out why that balance is genuinely close.

A combined posterior cruciate and posterolateral injury is a different proposition and much more often reconstructed. 4.13.8 covers combined lateral injuries.

The history also establishes whether this was a polytrauma event, which changes the timing of everything: a knee ligament injury in a patient with other injuries is assessed early and reconstructed late.

Where It Misleads

The commonest error is diagnosing an isolated posterior cruciate injury because the posterior drawer was positive and nothing else was tested.

The second is accepting an acute examination as definitive. A posterior cruciate injury assessed at two to six weeks gives a different and more useful picture. 4.1.5 covers the timing problem.

The third is missing the associated injury outside the knee, particularly the ankle and the hip, in a patient whose knee is the loudest complaint.

And a dashboard injury in an elderly patient raises a fracture at lower energy, so the radiograph threshold is lower.

Fellowship Level

Where It Misleads

In referred practice the dashboard injury appears as a chronic posterior cruciate deficiency, often years later, with a gait pattern the patient has adapted to. 4.11.6 covers the chronic presentation.

The history that matters then is the original mechanism and whether the posterolateral corner was ever assessed, because a chronic combined injury is the common reason a posterior cruciate reconstruction fails. 4.13.4 covers the assessment.

A varus thrust during walking in a chronic posterior cruciate deficient knee indicates a lateral side component and changes the operation from a reconstruction to a reconstruction plus a realignment. 4.4.4 covers the gait pattern.

The second question is what the patient actually cannot do, since many chronically deficient knees function well and the mechanical finding is not the indication. 3.17.2 covers the balance.

No figure here supports these claims, under decision 023.