Bundle Function

For Medical Students

The Principle

Two parts of a ligament attached at slightly different points tighten at different joint angles. One can be tight while the other is slack.

Which one tightens in flexion depends on the geometry of the attachments, and it is not the same for every ligament.

So the arrangement has to be learned per ligament rather than assumed from another one.

At the Knee

The posterior cruciate is described as two functional bundles, named for their femoral attachments: anterolateral and posteromedial. 2.8.2 and 2.8.3 cover them.

The anterolateral bundle is the larger. Conventional teaching gives it roughly twice the cross sectional area of the posteromedial.

Conventional teaching has the anterolateral bundle tightening in flexion and the posteromedial bundle tightening near extension.

That is the reverse of the anterior cruciate's arrangement, where the larger bundle is tight in flexion for a different reason. 3.7.4 covers that chapter's bundles.

What Changes It

Flexion angle, which is what the arrangement responds to.

Rotation, which alters the bundles' lengths unequally.

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

Graft position in a reconstruction, which decides what is reproduced.

Clinical Relevance

Posterior loads are largest in flexion, and the larger bundle is the one tight in flexion. The ligament is built for where the work is.

A single bundle reconstruction usually targets the anterolateral bundle, on the same reasoning. 3.8.8 covers the construct.

Testing at 90 degrees samples the anterolateral bundle, which is the one that matters most. Section 4 covers the tests.

Remembering that the arrangement is reversed from the anterior cruciate's prevents a common error.

MRCS Level

The Principle

The functional asymmetry here is larger than in the anterior cruciate. One bundle is roughly twice the size of the other and carries most of the load at the angle that matters.

That asymmetry is why single bundle reconstruction of this ligament is more defensible than it is for the anterior cruciate: reproducing the dominant bundle reproduces most of the function.

Length change patterns are reciprocal, so a single tension figure for the ligament is as meaningless here as it is anteriorly. 3.7.4 covers that point.

At the Knee

Conventional teaching gives the anterolateral bundle maximal tension in mid to deep flexion, commonly placed beyond 70 or 80 degrees, and the posteromedial bundle maximal tension near full extension.

The anterolateral bundle is therefore the restraint tested by a posterior drawer at 90 degrees. 3.8.6 covers the drawer.

The posteromedial bundle's contribution near extension is small relative to the capsule and the corner in that position.

The meniscofemoral ligaments run alongside the bundles and contribute to posterior restraint, which complicates sectioning studies. 2.6.3 covers them.

What Changes It

Flexion angle, which is the dominant variable.

Rotation and combined loading.

Partial rupture, which commonly spares one bundle.

Femoral tunnel position in a reconstruction, which determines the length change pattern.

How It Is Measured

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

Cadaveric length change measurement maps the reciprocal patterns.

Magnetic resonance imaging at several flexion angles describes the geometry without load. 5.8 covers imaging.

The meniscofemoral ligaments have to be accounted for or the bundle figures are inflated.

Clinical Relevance

Test at 90 degrees, which samples the dominant bundle.

Treat single bundle reconstruction as reproducing most of the function here.

Expect a partial rupture to spare one bundle and give angle specific findings.

Account for the meniscofemoral ligaments when reading apportionment figures.

FRCS Level

At the Knee

Treat the size asymmetry as the argument that settles the single against double bundle question differently from the anterior cruciate. Here the dominant bundle really is dominant.

Conventional teaching reports that double bundle reconstruction restores posterior translation closer to normal in cadaveric testing, with clinical equivalence, which is the same shape of evidence as anteriorly and with a smaller mechanical gap to close. 3.8.9 covers it.

Femoral tunnel position within the anterolateral footprint is the variable that matters most in a single bundle reconstruction. 3.8.8 covers the construct.

What Changes It

Femoral tunnel position within the footprint.

Graft tensioning angle, which here is in flexion rather than extension.

Whether a second bundle is added.

Corner integrity, which determines whether the cruciate graft is working alone.

How It Is Measured

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

Clinical trials report comparable outcomes, with small numbers relative to the anterior cruciate literature.

Postoperative computed tomography locates the tunnels against footprint maps.

Stress radiography measures the clinical result and is the reproducible outcome measure here.

Clinical Relevance

Place the femoral tunnel within the anterolateral footprint in a single bundle reconstruction.

Tension in flexion, since that is where this ligament's dominant bundle works.

Treat the cadaveric advantage of double bundle as real and the clinical advantage as unproven.

Measure the result with stress radiography rather than by hand.