Bundle Function

For Medical Students

The Principle

A single rope between two points is tight at one length and slack at others. A joint that moves through a large range cannot be controlled by one length of rope.

Splitting the rope into parts that run slightly differently solves it. As the joint moves, one part tightens while another slackens.

That is reciprocal tightening, and it is how a ligament stays effective across a range.

At the Knee

The anterior cruciate is conventionally described as two functional bundles, named for their tibial attachments: anteromedial and posterolateral. 2.7.8 and 2.7.9 cover them.

They are not separate ligaments. They are regions of one structure whose fibres run differently enough to behave differently.

Conventional teaching has the anteromedial bundle tightening in flexion and the posterolateral bundle tightening near extension. 3.7.5 and 3.7.6 cover each.

The femoral attachment is what makes this work. The two bundles' femoral origins sit in different places, so flexing the knee changes their relative lengths.

What Changes It

Flexion angle, which is what the whole arrangement responds to.

Rotation, which stretches the bundles unequally.

Partial injury, which can involve one bundle more than the other.

Tunnel position in a reconstruction, which determines how a graft behaves through range.

Clinical Relevance

A partial tear can leave a knee tight at one flexion angle and lax at another, which is why testing at more than one angle matters. Section 4 covers the tests.

A graft placed in one position cannot reproduce both bundles exactly, which is the mechanical basis of the single and double bundle discussion. 3.7.20 covers the construct.

Tunnel position matters because it sets how the graft's length changes through the range.

Describing the ligament as one rope makes reconstruction look simpler than it is.

MRCS Level

The Principle

The useful concept is isometry. A fibre is isometric if its length does not change through the range, and no fibre in the anterior cruciate is fully isometric.

Fibres therefore have a length change pattern, and the bundle names group fibres with similar patterns. The grouping is a simplification of a continuum.

That is why conventional teaching describes the two bundle model as functionally useful and anatomically approximate.

At the Knee

The femoral footprint is the critical geometry. Fibres originating more proximally and posteriorly in the notch behave differently from those more distally and anteriorly. 2.7.6 covers the footprint.

Conventional teaching gives the anteromedial bundle maximal tension at around 60 to 90 degrees of flexion and the posterolateral bundle maximal tension near full extension.

In rotation the bundles load unequally, with the posterolateral bundle contributing more to rotational restraint near extension. 3.7.7 covers rotation.

The reciprocal behaviour means the whole ligament is never uniformly tense, so a single tension figure for the ligament is not meaningful.

What Changes It

Flexion angle, as the primary variable.

Rotation and coronal load, which distribute between bundles unequally.

Partial rupture, which commonly involves one bundle region predominantly.

Femoral tunnel position in a graft, which is the single largest determinant of length change behaviour.

How It Is Measured

Cadaveric fibre length measurement through the range, with markers or with imaging, maps the length change patterns.

In situ force measurement per bundle after selective sectioning apportions the load.

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

The two bundle description predates the measurements and has been refined rather than replaced by them.

Clinical Relevance

Treat the two bundle model as functionally useful and anatomically approximate.

Expect a partial tear to produce angle specific findings.

Place a femoral tunnel knowing it is the main determinant of length change behaviour.

Avoid quoting a single tension figure for the whole ligament.

FRCS Level

At the Knee

Treat the bundle model as the mechanical content of the tunnel position debate. The question is which length change pattern a graft should reproduce, and a single graft can reproduce one.

Conventional teaching holds that double bundle reconstruction restores rotational control measurably better in cadaveric testing and has not shown a consistent clinical advantage, which is a real tension between two levels of evidence.

Anatomic single bundle reconstruction is the common resolution: place the graft in the native footprint centre so its behaviour approximates the whole ligament rather than either bundle. 3.7.20 covers the construct.

What Changes It

Femoral tunnel position within the footprint, which sets the length change pattern.

Graft diameter, which determines how much of the footprint is spanned.

Tensioning angle, which sets where the graft is tightest.

Notch morphology, which constrains where a tunnel can go.

How It Is Measured

Robotic testing compares single and double bundle constructs against the intact knee in all six degrees of freedom.

Randomised clinical trials compare them on laxity and outcome, and have not shown a consistent difference.

Tunnel position is measured postoperatively on computed tomography against footprint maps.

The gap between the cadaveric and the clinical result is the honest state of this question.

Clinical Relevance

Place the graft in the native footprint centre rather than aiming at a bundle.

State the gap between cadaveric and clinical evidence when the double bundle question arises.

Choose graft diameter with footprint coverage in mind.

Verify tunnel position on imaging when a reconstruction underperforms.