PM Bundle Mechanics

For Medical Students

The Principle

A smaller part of a structure can still matter, if it works at an angle the larger part does not cover.

Covering the range is the point of having two parts. Each handles the part of the arc where the other is slack.

Whether the smaller part is worth replacing surgically is a separate question from whether it does something.

At the Knee

The posteromedial bundle is named for its femoral attachment, which sits posteriorly and medially within the footprint. 2.8.3 covers the anatomy.

It is the smaller of the two. Conventional teaching gives it roughly half the cross sectional area of the anterolateral bundle.

It tightens as the knee extends, reaching maximal tension near full extension, which is the reciprocal of the other bundle. 3.8.3 covers the pair.

Near extension the posterior capsule and the posterolateral corner also contribute, so this bundle is one of several restraints rather than the dominant one.

What Changes It

Flexion angle, which it responds to in the opposite direction from the other bundle.

Rotation, which alters its length.

Capsular and corner integrity, which share the restraint in its range.

Whether a reconstruction includes a second bundle. 3.8.9 covers double bundle.

Clinical Relevance

Laxity near extension in a posterior cruciate injury involves this bundle and the capsule together, which is why isolated findings there are uncommon. Section 4 covers the tests.

A single bundle reconstruction leaves this bundle's range less well controlled, which is the mechanical argument for adding a second. 3.8.8 covers the construct.

Its small size means the argument is about a modest gain rather than a large one.

Knowing it is tight near extension explains why graft tensioning angle matters.

MRCS Level

The Principle

The mechanical question is whether this bundle's range is already covered. Near extension the posterior capsule is taut and the corner contributes, so the bundle's marginal contribution is smaller than its size alone suggests.

That is the honest form of the double bundle argument: the gain is real, measurable in a rig, and small in a knee with an intact capsule.

Where the capsule and corner are also injured the argument changes, because then nothing else covers the range.

At the Knee

Conventional teaching places maximal posteromedial bundle tension near full extension and describes it as slackening through flexion.

In situ force measurement gives it a small share of the posterior restraint at 90 degrees, where the drawer is performed.

Its share near extension is larger in relative terms and the absolute posterior loads there are smaller.

Adding a second graft bundle to reproduce it changes the construct's behaviour near extension measurably in cadaveric testing. 3.8.9 covers that.

What Changes It

Flexion angle, in the opposite direction from the other bundle.

Capsular and corner integrity, which determine the marginal contribution.

Tensioning angle of any graft reproducing it.

Combined injury patterns, which change what else is covering the range.

How It Is Measured

Selective sectioning with in situ force measurement gives its share by angle.

Cadaveric double bundle construct testing measures the gain from reproducing it.

Clinical trials report outcomes that have not separated the bundles' contributions.

The capsule's contribution is hard to isolate, which limits precision here.

Clinical Relevance

Expect this bundle's marginal contribution to be smaller than its size suggests.

Reconsider that where the capsule and corner are also injured.

Read the double bundle gain as real, measurable and small in an otherwise intact knee.

Account for the capsule when interpreting near extension findings.

FRCS Level

At the Knee

Treat this bundle as the content of the double bundle question and treat that question as settled in the same unsatisfying way as its anterior counterpart: a cadaveric advantage without a demonstrated clinical one. 3.8.9 covers it.

Where it becomes practically relevant is the combined injury. In a knee with a deficient corner and capsule, the near extension range has no other restraint, and a construct that covers it has a mechanical case.

Conventional teaching reserves double bundle reconstruction for selected cases rather than as a default, which is consistent with that reading. 2.10.4 covers the corner.

What Changes It

Corner and capsular status, which decide whether anything else covers the range.

Graft availability, since a second bundle needs more tissue.

Tunnel space on the femur, which constrains two tunnels.

Operative time and complexity against the expected gain.

How It Is Measured

Robotic testing compares single and double bundle constructs against the intact knee at several angles.

Clinical series report comparable outcomes in small numbers.

Stress radiography measures the result reproducibly.

No trial has isolated the benefit in the combined injury subgroup, where the mechanical case is strongest.

Clinical Relevance

Consider double bundle in a combined injury where the corner and capsule are deficient.

Treat it as selected rather than default, on the current evidence.

Account for graft quantity and femoral tunnel space before planning two bundles.

State that the subgroup with the strongest mechanical case has no trial evidence.