Combined PLC Injuries

For Medical Students

What It Tests

An isolated posterolateral corner injury is uncommon. The corner is usually injured with a cruciate, and which cruciate decides almost everything that follows.

Assessing a combined injury means establishing the corner, establishing the cruciates, and reading the two together rather than listing them. 4.13.4 covers the corner assessment.

The reason the pattern matters is that a cruciate reconstruction into an uncontrolled corner stretches, so the corner determines whether the cruciate operation will hold. 3.10.8 covers reconstruction biomechanics.

How It Is Performed

Examine the common peroneal nerve and the circulation first in any knee with corner signs, because a combined injury raises the possibility of a dislocation. 4.11.7 covers that sequence.

Perform the dial test at thirty and ninety degrees. 4.13.5 covers it.

Perform the posterior drawer and look for a posterior sag. 4.11.2 covers the drawer.

Perform the Lachman test and the pivot shift. 4.10.4 covers the Lachman test.

Perform varus stress at both angles and palpate the collateral ligament. 4.13.1 covers varus stress.

Look at the standing alignment and the gait, because a varus thrust changes the operation whatever the ligament findings are. 4.11.5 covers dynamic assessment.

What a Positive Means

An increased dial difference at thirty degrees only, with normal cruciates, is an isolated corner injury.

An increased dial difference at both angles, with a posterior sag and an increased posterior drawer, is a corner and posterior cruciate injury. That is the commonest combination and the one most often treated incompletely. 3.10.9 covers it.

An increased dial difference at thirty degrees with a positive Lachman and a large pivot shift is a corner and anterior cruciate injury, where the corner makes the rotational instability worse than the cruciate injury alone would. 4.14.1 covers the pivot shift.

All of the above with gross laxity in several planes is a multiligament knee, and the assessment and the plan change accordingly. 4.11.7 covers it.

MRCS Level

How It Is Performed

In a viva this is a pattern recognition station, and the answer is the pattern rather than the list of positive tests.

The expected points are nerve and circulation first, the dial test at two angles, both cruciates, varus at two angles, the collateral palpation, and the standing alignment.

Say that you would state the pattern rather than the tests, because the pattern is what the operation is planned from. 4.1.8 covers the record.

Say that you would examine the other knee fully, since every corner finding is a comparison. 4.1.2 covers the principle.

Say that examination under anaesthesia is part of the assessment in the acute high energy knee rather than an addition. 4.14.6 covers it.

Say that stress radiographs in varus and posteriorly give the grades the examination estimates.

What a Positive Means

The reasoning expected is that the corner and each cruciate resist overlapping but different movements, so the combination produces a laxity pattern rather than a sum of findings. 3.10.6 covers the corner.

The corner and the posterior cruciate both resist posterior translation and external rotation, which is why their combined loss gives a dial difference that grows between thirty and ninety degrees. 3.10.7 covers the mechanics.

The corner and the anterior cruciate both resist internal rotation of the tibia, which is why their combined loss gives a larger pivot shift than either alone. 3.11.4 covers pivot shift control.

A combined injury also removes the secondary restraint each structure provides to the other, so the laxity is greater than either injury predicts, and that is the mechanical argument for addressing both. 3.10.8 covers the biomechanics.

How Good It Is

The examination is good at establishing the pattern and poor at grading each component, and the pattern is what the operation needs.

The dial test at two angles carries the separation of corner from posterior cruciate, and it does it reliably. 4.13.5 covers it.

The pivot shift carries the corner and anterior cruciate combination and does it less reliably, because the pivot shift is hard to grade awake. 4.14.1 covers it.

In the acute high energy knee the whole assessment is unreliable, and examination under anaesthesia is what the plan is actually built on. 4.14.6 covers it.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the pattern decides what is reconstructed, in what order, and whether it is staged, and those are the decisions that determine the result.

A corner and posterior cruciate injury is reconstructed in both, and the corner is never left, because leaving it is the commonest cause of a stretched posterior cruciate graft. 3.10.9 covers the combination.

A corner and anterior cruciate injury is reconstructed in both where the corner laxity is clear, and the decision is harder where the corner finding is borderline, since an unnecessary corner reconstruction adds stiffness. 4.13.4 covers that assessment.

A varus thrust changes the operation to include correction, and in a chronic knee the osteotomy may precede or replace the ligament work.

Timing matters. Acute corner repair has a window that a delayed reconstruction does not, so the assessment has to happen early and under anaesthesia if it cannot happen awake. 4.14.6 covers it.

Where It Misleads

The commonest error is reconstructing the cruciate and leaving the corner.

The second is performing the dial test at one angle, which removes the separation the pattern depends on.

The third is listing positive tests in a letter without stating the pattern, which leaves the next surgeon to reconstruct the reasoning.

The fourth is treating a borderline corner finding as definite, which produces a stiff lateral side the patient did not need.

The fifth is planning without the alignment, in a knee where the limb loads the reconstruction with every step.

Fellowship Level

Where It Misleads

In revision practice the combined injury arrives as a failed single reconstruction, and the examination's job is to establish what was not treated.

The commonest finding is a stretched posterior cruciate graft with an increased dial difference at both angles, which is the pattern that was present before the first operation and was not read. 4.13.5 covers the test.

A failed anterior cruciate reconstruction with an increased dial difference at thirty degrees and a persisting pivot shift has an unaddressed corner rather than a technically poor graft, and the revision addresses both. 4.10.12 covers revision assessment.

A knee with both reconstructions intact and a varus thrust has an alignment problem that will fail them both, and the osteotomy is the operation rather than a revision of either. 4.11.5 covers the dynamic examination.

An over constrained corner reconstruction in a combined knee presents as lateral pain and loss of internal rotation, and it is the complication that follows treating a borderline finding as definite. 4.2.4 covers stiffness.

A common peroneal nerve palsy that has not recovered limits what any of this achieves, and in a knee with several reconstructions it is often the main determinant of the patient's function. 4.23.6 covers complex injuries.