Posterolateral Bundle

For Medical Students

Structure and Location

The posterolateral bundle is the group of fibres attaching posterolaterally on the tibial footprint and distally on the femoral wall.

It is the shorter of the two bundles and the more obliquely oriented. At the femur it lies distal to the bifurcate ridge, closer to the articular margin.

Its femoral attachment moves a long way as the knee flexes, because a point low on the wall travels when the condyle rotates. That is the anatomical reason the bundle's behaviour differs from the anteromedial.

As with the anteromedial bundle, the division is a convention with no septum between them.

Relations

At the femur: distal to the bifurcate ridge, nearest the posterior articular margin of the lateral condyle.

At the tibia: posterolateral within the footprint, closest to the anterior root of the lateral meniscus.

Through its course: posterior and lateral to the anteromedial fibres, with the two twisting around each other in flexion.

The posterior wall of the femoral tunnel region lies immediately behind its femoral attachment, which is why a posteriorly placed tunnel risks a wall blowout.

Function

Orientation only, because bundle function through range is owned by 3.7.6. The posterolateral fibres are more obliquely oriented and their length changes more across the range than the anteromedial fibres do.

They are the fibres associated with rotational control, which is why a pivot shift rather than a Lachman test is the sign of their loss.

The reciprocal relationship between the two bundles through the range belongs to 3.7.5 and 3.7.6.

Clinical Relevance

Isolated posterolateral bundle injury presents with a positive pivot shift and a relatively normal Lachman test, which is the mirror of the anteromedial pattern.

A vertically placed graft reproduces anteromedial behaviour and not posterolateral, which is why such a graft controls translation and not rotation.

Its femoral attachment is the harder of the two to reach, being distal and posterior on the wall, and that is a practical consequence rather than a theoretical one.

MRCS Level

Structure and Location

Define it by its attachments. Tibially: the posterolateral part of the footprint, adjacent to the anterior root of the lateral meniscus. Femorally: distal to the bifurcate ridge on the lateral wall, close to the posterior articular margin.

It is conventionally described as the shorter bundle with the smaller cross sectional area, and its fibres are more obliquely oriented in the coronal plane than the anteromedial fibres.

Because its femoral attachment is distal and posterior, it is the part of the footprint closest to the posterior wall and to the articular cartilage, and therefore the part a tunnel can most easily compromise.

It is the same tissue as the anteromedial bundle. The distinction is positional.

Relations

The posterior wall of the lateral condyle, immediately behind. Wall thickness here is what a surgeon assesses before drilling, and it is thinner than it looks.

The articular margin of the lateral condyle, immediately distal.

The anterior root of the lateral meniscus at the tibial end, which the footprint overlaps.

The anteromedial fibres anteriorly and proximally, sharing a synovial sheath.

Blood Supply and Innervation

Shared with the whole ligament through the sheath. There is no bundle specific supply.

Its femoral attachment sits in the better vascularised part of the footprint region, which is relevant to proximal tear healing.

Mechanoreceptor distribution is by insertion rather than by bundle, and the femoral insertion region is often described as the richer.

Function

Orientation only. The posterolateral fibres are more obliquely oriented and their attachment geometry means their length changes substantially across the range.

That greater length change is the anatomical basis of their association with rotational control and of the reciprocal behaviour of the two bundles.

3.7.6 owns the quantitative account, which the architecture excludes from here by name.

Clinical Relevance

The pivot shift is the clinical sign of posterolateral bundle deficiency, and a knee with a normal Lachman and an abnormal pivot shift is the pattern to recognise.

A graft that fails to reproduce this bundle controls translation and leaves rotation, which is the commonest complaint after a vertically placed reconstruction.

Reaching its femoral attachment requires a trajectory that clears the articular margin and keeps the posterior wall intact. That is a geometric constraint and the reason portal and drilling choices are argued about. 7.20 owns them.

No measurement appears on this page, and bundle function through range is excluded from it by the architecture.

FRCS Level

Relations

The posterior wall is the structure that limits how faithfully this bundle's attachment can be reproduced. A tunnel on the anatomical position with a compromised wall is not a better reconstruction than a slightly anterior tunnel in intact bone.

That trade is made at the time, from what the wall looks like, and it is anatomical.

Function

Rotational control is the clinical property this bundle is held to supply, and it is the property single bundle reconstruction most often fails to restore.

Whether that failure is best addressed by a second bundle, a more anatomical single bundle, or an added lateral procedure is the central argument in cruciate surgery. The anatomy here says only that a vertical graft cannot reproduce an oblique bundle.

Clinical Relevance

Examine the pivot shift and record it, before and after. It is the sign that tracks this bundle and the one a stability examination based on translation alone misses.

Assess the posterior wall before drilling and accept a compromise in position rather than a blowout.

In revision for residual rotational laxity, consider the lateral structures and the lateral meniscal root as well as the tunnel position. 2.6.4 and 2.11 cover them.

Nothing quantitative appears here, by the architecture's instruction rather than by the figure policy.