AM Bundle Function

For Medical Students

The Principle

Two ropes attached at slightly different points change length differently when the joint moves. One can tighten while the other slackens.

Which one tightens depends on where each is attached. A rope attached further from the axis of movement changes length more.

So describing a bundle means describing where it is attached and what happens to its length as the knee bends.

At the Knee

The anteromedial bundle is named for its tibial attachment, which sits anteriorly and medially in the footprint. 2.7.8 covers the anatomy.

Its femoral origin is the more proximal and anterior of the two, high in the notch when the knee is extended.

Conventional teaching has it tightening as the knee flexes, with maximal tension around 60 to 90 degrees.

That makes it the bundle resisting anterior translation in flexion, which is the position most clinical and instrumented testing uses. 3.7.8 covers anterior stability.

What Changes It

Flexion angle, which is what it responds to.

Rotation, which alters its length as well.

Partial injury affecting this bundle, which leaves laxity in flexion more than in extension.

Graft placement in a reconstruction, which decides whether this behaviour is reproduced.

Clinical Relevance

A knee with an isolated anteromedial bundle injury is lax in flexion and tighter near extension, which is a recognisable pattern. Section 4 covers the tests.

Testing at 90 degrees loads this bundle, so a normal finding there does not exclude injury to the other one.

Reconstruction usually reproduces this bundle's behaviour more closely than the other's, which is a consequence of tunnel position. 3.7.20 covers the construct.

Knowing which bundle a test loads makes the findings interpretable rather than memorised.

MRCS Level

The Principle

Length change through the range is the quantity that defines a bundle, and it follows from the geometry of its two attachments relative to the moving axis. 3.3.7 covers the instant centre.

The anteromedial bundle's femoral origin sits closer to the flexion axis's path, so its length changes less dramatically than the posterolateral bundle's.

That relative isometry is why it is the bundle a single graft most often mimics, and why it is the one most testing samples.

At the Knee

Conventional teaching describes the anteromedial bundle as relatively isometric compared with the posterolateral, tightening progressively into flexion and reaching maximal tension in the mid to deep flexion range.

At 90 degrees it carries most of the anterior restraint, which is why the anterior drawer at that angle is a test of this bundle.

In rotation it contributes less than the posterolateral bundle near extension and more in flexion. 3.7.7 covers rotation.

Its fibres are longer, which affects the strain for a given displacement: a longer fibre strains less for the same elongation.

What Changes It

Flexion angle, as the dominant variable.

Tibial and femoral attachment position in a reconstruction, which sets the pattern.

Rotation and combined loading.

Partial rupture, commonly involving this bundle in a knee that still has an endpoint on testing.

How It Is Measured

Cadaveric fibre length and strain measurement through the range, with the bundle selectively isolated.

In situ force measurement after selective sectioning of the other bundle.

Magnetic resonance imaging at multiple angles describes the geometry without load.

Selective bundle studies are technically demanding and the figures vary between them accordingly.

Clinical Relevance

Read a test at 90 degrees as sampling this bundle rather than the whole ligament.

Expect a partial tear with an endpoint to involve this bundle commonly.

Account for fibre length when comparing strain figures between bundles.

Place a single graft knowing it approximates this bundle's behaviour most closely.

FRCS Level

At the Knee

Treat the anteromedial bundle as the behaviour a single bundle reconstruction most nearly reproduces, and treat that as a partial restoration rather than a success.

A construct that reproduces this bundle restores anterior translation at the angles clinical testing samples, which is why a reconstruction can measure well and leave rotational symptoms. 3.7.7 covers rotation.

Conventional teaching holds that a transtibial femoral tunnel tends to land in a more anteromedial position than an anatomic one, which is a technical cause of that pattern. 3.7.20 covers the construct.

What Changes It

Femoral tunnel position, which decides which bundle's behaviour is reproduced.

Drilling technique, which influences where the tunnel can land.

Graft tensioning angle, which sets where the construct is tightest.

Notch geometry, which constrains the available positions.

How It Is Measured

Postoperative computed tomography locates the tunnel against footprint maps and is how a non anatomic position is identified.

Robotic testing shows which degrees of freedom a given tunnel position restores.

Instrumented pivot shift measures the rotational result the position predicts.

Arthrometry measures the translation this bundle controls, which is the test most likely to look normal.

Clinical Relevance

Verify femoral tunnel position when a reconstruction measures well and feels unstable.

Expect a more anteromedial tunnel to restore translation and leave rotation.

Choose the tensioning angle deliberately rather than by habit.

Treat a normal arthrometer reading as evidence about this bundle only.