PM Bundle

For Medical Students

Structure and Location

The posteromedial bundle is the smaller part of the posterior cruciate ligament. Its name describes both ends: posterior on the femoral footprint, medial on the tibial one.

On the medial wall of the notch it occupies the back of the crescent shaped footprint, further from the articular margin and lower down the wall than the anterolateral bundle.

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

Conventional teaching gives it roughly one third of the ligament's cross sectional area, with a mid substance width of about 6 mm.

Like the anterolateral bundle it is a region of a continuous ligament rather than a separate structure.

Relations

The anterolateral bundle lies in front of and above it on the femur and lateral to it on the tibia.

The posterior meniscofemoral ligament runs immediately behind it to the medial condyle. The two are often indistinguishable in the notch, and the only reliable way to tell them apart is to follow each to its attachment.

Behind both lies the posterior capsule, then the popliteal vessels.

Function

The bundle tightens as the knee extends. Near full extension it takes up the restraint that the anterolateral bundle has let go.

It slackens with flexion, so by 90 degrees it contributes little to resisting posterior translation.

The two bundles therefore cover the range between them. 3.8.5 covers the mechanics.

Clinical Relevance

A knee with an intact posteromedial bundle and a torn anterolateral one can feel sound in extension and lax at 90 degrees. The reverse pattern, lax near extension and sound in flexion, points away from an isolated ligament injury and towards the capsule and corners.

Because the bundle is small, its individual contribution is easy to underestimate. The part of the range it covers is the part a patient uses walking on the flat.

It is the part of the ligament a single bundle reconstruction does not reproduce.

MRCS Level

Structure and Location

Locate the femoral attachment by working backwards and distally from the anterolateral footprint along the wall. Conventional teaching places the centres of the two femoral footprints about 10 mm apart.

On the tibia the two attachments are closer, with their centres about 5 mm apart within a fossa only a little over 1 cm across.

Conventional teaching gives the bundle a stiffness of the order of 60 newtons per millimetre and an ultimate tensile load of the order of 400 newtons, roughly half and a third of the anterolateral figures.

Its fibres are shorter than the anterolateral bundle's, which is part of why its length change through range is the reciprocal of the larger bundle's rather than the same.

Relations

The posterior meniscofemoral ligament is the structure most often mistaken for it, and a large one can be as thick as the bundle itself.

The anterolateral bundle lies anterior and proximal on the femur. There is no plane between them, so a description of either is a description of a region.

The posteromedial corner lies beyond the capsule on the same side, and the two share the restraint they provide near extension. 2.9.4 covers the corner.

Blood Supply and Innervation

The middle genicular artery supplies this bundle more directly than the anterolateral one, because the vessel enters from behind and the bundle is the posterior part of the ligament.

Periligamentous vessels in the synovial sheath run along it and are the supply that survives when the ligament is partly torn.

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

Function

Through extension the bundle's share of the posterior restraint rises. Conventional teaching puts it at a minority throughout, with its largest share in the last 20 degrees of extension.

Its contribution near extension overlaps with the capsule and the posteromedial corner, so an isolated loss is hard to detect by examination.

It contributes to resisting external rotation in the extended knee, which is where its attachment geometry gives it the longest lever. 3.8.5 carries the detail.

Clinical Relevance

Interpret laxity near extension with the corners in mind. The anatomy means this bundle and the posteromedial corner are tested together and cannot be separated clinically.

Expect a knee reconstructed with a single anterolateral bundle graft to retain some laxity near extension, because the structure that restrains there has not been reproduced. 7.34 carries the double bundle discussion.

Follow a band to its attachment before naming it. In the notch the posterior meniscofemoral ligament and this bundle look alike.

FRCS Level

Relations

At the femoral end this bundle's footprint is the posterior and distal part of the crescent, which puts it further from cartilage and closer to the back of the wall. The constraint there is the posterior cortex rather than the articular margin.

At the tibial end the two footprint centres are close enough that separating them is a matter of millimetres within one fossa.

The posterior meniscofemoral ligament occupies the space behind it. Preserving that ligament preserves restraint, and it is also the structure most likely to be taken for the bundle during exposure.

Function

Treat the bundle as the extension half of the pair. It is the part of the ligament that restrains the knee in the position most used in gait on the flat.

Its small cross sectional area means the ligament's measured properties are dominated by the other bundle, so figures quoted for the whole ligament understate what is lost when this bundle alone fails.

Reciprocal loading is the reason a double bundle construct is argued for, and the argument is anatomical before it is clinical. 3.8.3 and 7.34 carry it.

Clinical Relevance

Consider the posteromedial corner alongside this bundle in any knee with laxity near extension. The anatomy pairs them.

Accept that an isolated injury to this bundle is a diagnosis made on imaging rather than on examination, because the restraint it provides is shared.

Preserve the posterior meniscofemoral ligament where it is intact. It restrains in the same part of the range.

Where restraint near extension is the problem, a construct reproducing only the anterolateral bundle addresses the wrong part of the range.