Combined PCL and PLC Injury

For Medical Students

The Principle

Two injuries together are worse than the sum when each structure was also a secondary restraint for the other's direction.

If neither structure heals well, the combination does not resolve with time the way a medial injury does.

So the combination is both mechanically worse and clinically less forgiving.

At the Knee

The posterior cruciate and the posterolateral corner both resist posterior translation and external rotation, in different proportions at different angles. 3.8.2 and 3.10.2 cover them.

Losing both produces a larger posterior and rotational laxity than either alone, which is the apportionment showing itself. 3.8.1 covers the cruciate's functions.

Neither heals reliably. The cruciate heals partly and at length, and the corner heals badly. 3.10.6 covers the corner.

Conventional teaching identifies an unaddressed corner injury as the commonest cause of posterior cruciate graft failure. 3.8.8 covers the construct.

What Changes It

Whether the corner is identified and reconstructed.

Alignment, since varus loads both. 3.13 covers alignment.

Chronicity, since the structures stretch.

Peroneal nerve status, which can accompany the corner injury. 2.10.6 covers the nerve.

Clinical Relevance

Examining for the corner in every posterior cruciate injury is the practical consequence, and the dial test is how. 3.10.7 covers the test.

Reconstructing the cruciate without the corner reproduces the failure, which is why both are addressed. Section 6 covers management.

A varus limb with corner laxity needs the alignment corrected as well, which makes this a multi procedure problem.

6.24 covers the clinical condition.

MRCS Level

The Principle

The mechanical statement is shared restraint in two directions, so the combined deficit exceeds the sum. 3.7.3 covers secondary restraints as a general idea.

The clinical statement is that neither structure heals, which removes the option of waiting that the medial side offers. 3.9.10 covers that contrast.

Together they make this the combination where reconstruction of both is the usual answer rather than a selected one.

At the Knee

Sectioning studies show that dividing the corner in a posterior cruciate deficient knee increases posterior translation and external rotation more than dividing it in an intact knee.

The dial test at 30 and 90 degrees is the clinical separation: worse at 90 implicates the cruciate and a finding at 30 implicates the corner. 3.10.7 covers the mechanics.

Varus thrust and hyperextension in gait are the chronic findings, and 3.14 covers gait in detail.

Alignment interacts with both. A varus limb loads the corner in stance and increases the posterior and rotational demand. 3.13 covers alignment.

What Changes It

Whether both structures are reconstructed.

Alignment and whether it is corrected.

Chronicity and the degree of stretching.

Peroneal nerve function.

How It Is Measured

Stress radiography measures the posterior component reproducibly.

Dial test at two angles measures the rotational component without an instrument.

Long leg radiography measures the alignment.

Robotic testing apportions the restraint in the combined state, which is the relevant experiment.

Clinical Relevance

Perform a dial test at both angles in every posterior cruciate injury.

Reconstruct both structures rather than the cruciate alone.

Measure alignment, since varus loads both.

Document peroneal nerve function.

FRCS Level

At the Knee

Treat this as the combination where the environment, the structures and the alignment all have to be addressed, and where a single sitting may not be possible. 3.10.8 covers the corner construct.

Conventional teaching reconstructs both the cruciate and the corner, and corrects varus alignment where present, because reconstructing one leaves the loads that defeated it.

Tunnel convergence is the geometric constraint that decides the technique: a posterior cruciate tibial tunnel and the corner's tunnels compete, which is an argument for an inlay. 3.8.10 covers the inlay.

What Changes It

Alignment and whether an osteotomy is included.

Tunnel trajectories and convergence.

Graft availability for three or four limbs.

Peroneal nerve status, which may limit the dissection.

How It Is Measured

Computed tomography plans the tunnels and assesses convergence.

Stress radiography and the dial test measure the two components before and after.

Long leg radiography measures and plans the alignment correction.

Multiligament series report outcomes in small heterogeneous groups, which limits what can be concluded.

Clinical Relevance

Reconstruct both structures and correct varus where present.

Plan tunnels to avoid convergence, considering an inlay for the cruciate.

Measure both components before and after, by their own methods.

Counsel that outcomes in this combination are worse than in isolated injury.