AL Bundle

For Medical Students

Structure and Location

The anterolateral bundle is the larger part of the posterior cruciate ligament. Its name describes both of its ends: anterior on the femoral footprint, lateral on the tibial one.

On the medial wall of the notch it occupies the front of the crescent shaped footprint, nearer the articular margin of the medial condyle and nearer the roof.

On the tibia it occupies the lateral part of the posterior intercondylar fossa, closer to the lateral plateau.

Conventional teaching gives it roughly two thirds of the ligament's cross sectional area, with a mid substance width of about 10 mm.

It is not a separate structure. The bundle is a region of a continuous ligament, named because its fibres behave together and differently from the rest.

Relations

The posteromedial bundle lies behind and below it at both ends. The two share a fibre continuum rather than a plane of separation.

The anterior meniscofemoral ligament runs immediately in front of it on its way to the medial condyle, and in the notch the two can look like one band.

The anterior cruciate crosses in front of it, and the roof of the notch lies above.

Function

The bundle tightens as the knee flexes. At 90 degrees it is the main restraint to the tibia moving backwards.

It slackens towards extension, where the posteromedial bundle and the posterior capsule take up the work.

That reciprocal pattern is the reason the posterior drawer is tested in flexion. 3.8.4 covers the mechanics through range.

Clinical Relevance

A posterior drawer at 90 degrees is substantially a test of this bundle. A knee with an intact posteromedial bundle and a torn anterolateral one can therefore feel stable in extension and lax in flexion.

Because it is the larger bundle, it is the one a single bundle reconstruction aims to reproduce.

Its femoral attachment sits close to the articular cartilage of the medial condyle, which limits how far forwards a tunnel can be placed.

MRCS Level

Structure and Location

Locate the femoral attachment by the articular margin rather than by the notch roof. Conventional teaching places the anterior border of the bundle's footprint within about 3 mm of the cartilage edge of the medial condyle.

Its tibial attachment sits laterally within the fossa, and at arthroscopy the lateral edge of the fossa is a more reliable landmark than the fibres themselves.

Conventional teaching gives the bundle a stiffness of the order of 120 newtons per millimetre and an ultimate tensile load of the order of 1,100 newtons, both larger than the posteromedial bundle by a factor of about two.

Its fibres are longer at the posterior edge of the femoral footprint than at the anterior edge, so the bundle fans rather than running as parallel strands.

Relations

The anterior meniscofemoral ligament is the structure most often mistaken for it. The distinguishing feature is the attachment: the meniscofemoral ligament runs to the lateral meniscus, the bundle to the tibia.

The posteromedial bundle lies posterior and distal on the femur and medial on the tibia. There is no sharp border between them.

The posterior capsule lies behind the tibial end, and the popliteal vessels behind that.

Blood Supply and Innervation

The middle genicular artery supplies the bundle from behind, with periligamentous vessels running forwards in the synovial sheath.

The anterior part of the femoral footprint is the least well supplied region of the ligament, because it is furthest from the vessel's point of entry.

Nerve supply is shared with the rest of the ligament, from the posterior articular branches of the tibial nerve.

Function

Through flexion the bundle's share of the posterior restraint rises. Conventional teaching puts its contribution at the large majority by 90 degrees and a minority near full extension.

The reciprocal behaviour of the two bundles means the ligament as a whole restrains posterior translation across the range while neither bundle does so alone.

Its contribution to resisting external rotation is smaller than its contribution to resisting translation, and it is the corners rather than this bundle that govern rotation. 2.10.4 covers the posterolateral corner.

Clinical Relevance

Read a posterior drawer at 90 degrees as a test of this bundle and the posterior drawer near extension as a test of the capsule and corners. The pair of findings localises the injury.

Where a graft reproduces only this bundle, expect restraint to be restored in flexion and less well near extension. That is an anatomical prediction rather than a surgical opinion, and 7.33 carries the operative discussion.

Protect the articular margin of the medial condyle when working at the anterior edge of the footprint. The cartilage is within a few millimetres.

FRCS Level

Relations

The constraint that matters at the femoral end is cartilage, not bone. The footprint abuts the articular margin, so the anterior limit of a tunnel is set by the cartilage edge rather than by the shape of the notch.

At the tibial end the constraint is vascular. The bundle's attachment is lateral within a fossa whose posterior wall is thin, and the artery lies behind it.

The anterior meniscofemoral ligament shares the space in front of the bundle. In a deficient knee it is intact tissue occupying part of the field.

Function

Treat the bundle as the flexion restraint and the posteromedial bundle with the capsule as the extension restraint. Reconstructing one reproduces one half of a reciprocal pair.

The bundle's length changes little through the mid range and more at the extremes, so graft behaviour depends on the flexion angle at which it is fixed. 3.8.4 carries the mechanics and 7.33 the technique.

Its larger cross sectional area explains why it dominates the ligament's measured properties: figures quoted for the whole ligament are close to figures for this bundle alone.

Clinical Relevance

Identify the cartilage margin before the footprint when working on the medial wall. The margin is the fixed landmark and the footprint is read from it.

Consider the flexion angle at which restraint is most needed when planning which part of the footprint to reproduce. The anatomy makes that a choice rather than a default.

Where the anterior meniscofemoral ligament is intact, preserve it. It carries a share of the posterior restraint that a deficient knee has not lost.

Expect a single bundle graft to behave like this bundle, which means well in flexion and less well near extension.