Contact Injury

For Medical Students

What It Tests

A contact injury is one where something struck the knee or the leg, so the forces came from outside and have a direction.

Asking about it tests which way the knee was pushed, and that points at the structures on the opposite side, since a ligament fails in tension.

Contact injuries are more likely than non-contact ones to involve the collaterals and more likely to produce combined injuries, because an external force is not limited by what the patient can generate.

How It Is Performed

Ask where the blow landed: the outside of the knee, the inside, the front, or the lower leg.

Ask which way the knee bent or was pushed as a result.

Ask whether the foot was planted at the time, because a planted foot converts a blow into a much larger joint force.

Ask whether they could weight bear immediately and whether the knee swelled, and how fast.

Ask about the knee feeling like it came apart or went out and back in, which raises a dislocation. 4.23.6 covers multiligament assessment.

What a Positive Means

A blow to the outside of the knee forces valgus and loads the medial structures. 4.12.1 covers valgus stress testing.

A blow to the inside forces varus and loads the lateral structures, which is less common and more often associated with serious combined injury. 4.13.1 covers varus stress testing.

A blow to the front of the flexed knee drives the tibia backwards and loads the posterior cruciate. 4.2.21 covers the dashboard mechanism.

A high energy contact injury with gross instability in several directions is a knee dislocation until proven otherwise, and the vascular assessment comes first.

An inability to weight bear after a direct blow raises a fracture rather than a ligament injury.

MRCS Level

What a Positive Means

The reasoning expected is that a ligament fails in tension, so the injured structure is opposite the direction of the blow.

A valgus force at full extension loads the superficial medial collateral and, with enough energy, the cruciate as well. 4.12.2 covers assessment at full extension.

A varus force loads the lateral collateral and the posterolateral structures, and an isolated lateral collateral injury is uncommon. 4.13.8 covers combined lateral injuries.

A posteriorly directed force on a flexed knee loads the posterior cruciate and, with more energy, the posterolateral corner.

Any high energy contact injury should prompt assessment for a combined injury rather than a single structure, which is the main difference from the non-contact mechanism.

How Good It Is

Contact mechanisms are well recalled for the fact of the blow and poorly recalled for its direction, which is the same pattern seen with giving way.

Witnesses, team mates and video are more reliable than the patient for direction, and asking whether anybody saw it is a legitimate history question.

The energy of the injury is reliably recalled and is the most useful single feature, because it predicts combined injury better than the direction does.

The examination outperforms the history for which structure, so the history's job is to establish the energy and to prompt a complete assessment.

FRCS Level

How Good It Is

At decision level the energy of a contact injury determines the breadth of the assessment and the urgency.

A low energy valgus injury in a sportsperson is a medial structure injury that usually heals, and the decision is about bracing and rehabilitation rather than surgery. 4.12.1 covers the assessment.

A high energy injury is a multiligament problem until excluded, and the sequence is vascular, then neurological, then ligamentous. The history establishes the energy and therefore the sequence.

The combination most often missed is a medial injury with a cruciate injury, where the cruciate is diagnosed and the medial component is assumed to be minor. 4.9.10 and 4.12.7 cover related separations.

Where It Misleads

The commonest error is anchoring on the structure the blow suggests and performing a targeted examination.

The second is accepting the patient's direction, which is unreliable, and then examining only the structures it implies.

The third is under-estimating energy from the sport rather than the event. A low level collision can carry high energy and a professional collision can carry little.

And a contact injury in a child or adolescent raises a physeal injury that may present as ligamentous instability, because the growth plate is weaker than the ligament. 4.19.1 covers it.

Fellowship Level

Where It Misleads

In referred practice the contact injury that reaches a fellowship clinic is usually a multiligament knee or a missed component of one.

The history of the original event is the most useful record available, because the examination at presentation was performed on a painful, swollen, guarding knee and under-reported the laxity. 4.1.5 covers the timing problem.

The energy, the direction and whether the knee reduced spontaneously are the three features worth reconstructing, and the third is frequently never asked.

A spontaneously reduced dislocation can present as a ligamentous injury with a normal radiograph, and the vascular history then becomes the critical part.

A contact injury with a peroneal nerve deficit, however mild, indicates a lateral side injury of higher energy than the examination may suggest. 4.13.8 covers combined lateral injuries.

No figure here supports these claims, under decision 023.