PL Bundle Function

For Medical Students

The Principle

A rope attached further from the axis of a hinge changes length more as the hinge moves. Its behaviour is therefore more dramatic across the range.

A shorter rope also strains more for the same stretch, because strain is the stretch divided by the original length.

So a short rope attached far from the axis is tight over a narrow part of the range and highly strained when it is.

At the Knee

The posterolateral bundle is named for its tibial attachment, which sits posteriorly and laterally in the footprint. 2.7.9 covers the anatomy.

Its femoral origin is the more distal and posterior of the two, which puts it further from the flexion axis in extension.

Conventional teaching has it tightening as the knee extends, with maximal tension near full extension.

It is therefore the bundle controlling the knee in the position where pivoting injuries occur, which is near extension with the knee rotating. 3.7.14 covers mechanisms.

What Changes It

Flexion angle, which it responds to more sharply than the other bundle.

Rotation, where it contributes most near extension. 3.7.7 covers rotation.

Partial injury affecting this bundle, which leaves rotational laxity with a near normal test in flexion.

Tunnel position in a reconstruction, since reproducing this behaviour is the harder half.

Clinical Relevance

A knee lax on rotational testing and near normal on translation testing in flexion fits an injury affecting this bundle. Section 4 covers the tests.

Testing at 90 degrees samples the other bundle, so this one can be injured and missed.

The pivot shift tests this bundle's territory, which is why it is the more informative test in a pivoting athlete. 3.7.9 covers the pivot shift.

This is the bundle a single graft reproduces less well, which matters for the symptom patients complain of.

MRCS Level

The Principle

This bundle's length change through the range is larger than the anteromedial bundle's, so it is further from isometric and more sensitive to attachment position.

Sensitivity to position cuts both ways. It is harder to reproduce in a reconstruction and more informative about function when it is intact.

Strain rather than force is the quantity to compare between bundles, because the bundles differ in length and cross sectional area. 3.7.10 covers strain.

At the Knee

Conventional teaching gives the posterolateral bundle maximal tension at or near full extension, slackening progressively with flexion, which is the reciprocal of the anteromedial pattern.

In rotation it carries a larger share of the restraint near extension, which is why rotational laxity is assessed near extension. 3.7.7 covers rotation.

Its contribution is why an anterior drawer at 90 degrees can be near normal in a knee with a demonstrable pivot shift.

It is shorter and its fibres are more obliquely oriented, so for a given tibial displacement it strains more than the anteromedial bundle.

What Changes It

Flexion angle, more sharply than for the other bundle.

Rotation and combined loading, where its share is highest near extension.

Attachment position, to which it is more sensitive.

Partial rupture, where its isolated injury produces rotational rather than translational findings.

How It Is Measured

Selective sectioning with in situ force measurement apportions the restraint between bundles at each angle.

Cadaveric strain measurement with the bundle isolated maps its length change pattern.

Instrumented pivot shift measures the clinical consequence of its loss, without an agreed threshold. 3.7.9 covers the test.

Isolated posterolateral bundle injury is uncommon, so much of the evidence is from sectioning rather than from patients.

Clinical Relevance

Assess rotation near extension, since that is where this bundle's share is highest.

Expect a near normal drawer at 90 degrees in a knee with a positive pivot shift.

Compare bundles by strain rather than by force, given their different dimensions.

Treat isolated injury to this bundle as uncommon and the sectioning evidence as the main source.

FRCS Level

At the Knee

Treat this bundle's behaviour as the half a reconstruction most often fails to restore, and treat residual rotational laxity as the expected sign of it.

That is the mechanical content of the anterolateral augmentation argument. If the graft does not restore the near extension rotational control, something else can. 3.11 covers the constructs.

Conventional teaching holds that a more anatomic, lower and more posterior femoral tunnel restores rotational control better than a high anteromedial one, which is this bundle's territory being reproduced.

What Changes It

Femoral tunnel position, to which this bundle's behaviour is most sensitive.

Whether a lateral augmentation was added.

Posterior tibial slope, which increases the demand this control has to meet. 3.13 covers alignment.

Graft diameter and footprint coverage.

How It Is Measured

Robotic testing in rotation at low flexion angles is the measurement that distinguishes constructs here.

Instrumented pivot shift measures the clinical result with no agreed threshold.

Postoperative computed tomography locates the tunnel, which is the actionable variable.

Reinjury and return to pivoting sport rates measure the consequence.

Clinical Relevance

Place the femoral tunnel to reproduce near extension rotational control.

Consider lateral augmentation where rotational laxity is high or the knee is a revision.

Measure slope, which raises the demand on this control.

Expect residual rotational laxity to be the sign that this behaviour was not restored.