MRI Anatomy

For Medical Students

Structure and Location

On a sagittal image the normal posterior cruciate is a thick band of uniformly low signal, running from the back of the tibia up and forwards to the medial femoral condyle. It is the easiest ligament in the knee to find.

It is darker and thicker than the anterior cruciate, and conventional teaching gives its anteroposterior thickness as about 4 to 6 mm on a sagittal image.

Its shape is curved rather than straight, convex forwards, and the curve is most obvious with the knee extended in the scanner. The ligament straightens as the knee flexes.

The tibial attachment is seen below the joint line on the posterior cortex, which is further down than students expect.

The two bundles are not separable on a standard image. What is seen is one band.

Relations

The anterior cruciate crosses in front of it, and on a mid sagittal slice both are visible with the posterior cruciate the clearer of the two.

The meniscofemoral ligaments appear as small round or oval low signal structures immediately in front of and behind it on sagittal slices, and as bands running to the lateral meniscus on coronal slices.

The posterior capsule lies behind it, and behind that the popliteal vessels, which are bright or dark depending on the sequence.

Function

A normal image establishes the ligament's course, its continuity and its thickness, and those three together are what a report describes.

The curve is a function of position rather than of the ligament, so a straighter ligament in a flexed knee is a normal finding.

Pattern recognition starts from the normal. 5.8.1 is the imaging academy's page on the normal ligament and 5.8 covers the rest.

Clinical Relevance

A round low signal structure beside the ligament is usually a meniscofemoral ligament rather than a loose body. Following it to the lateral meniscus on the next slices settles it.

A large posterior meniscofemoral ligament lying against the main band gives a double posterior cruciate appearance. That is a normal variant.

Thickness is read against the opposite knee where one is available, because the normal range is wide.

Tear patterns belong to 5.8.2 and 5.8.3 and are deliberately not described here.

MRCS Level

Structure and Location

Use the sagittal proton density or T2 weighted sequence to assess it. The ligament's low signal against bright fluid and fat is what makes it legible.

Conventional teaching describes the normal ligament as uniformly low in signal throughout, with no internal bright line and no change in calibre along its length.

The femoral footprint is seen on coronal and sagittal images as a broad attachment to the medial wall of the notch. Its anterior border sits close to the articular cartilage, which is visible on the same slice.

On axial images the ligament appears as an oval low signal structure in the notch, and the axial plane is the one that shows its width best.

Relations

The anterior meniscofemoral ligament is seen in front of the main band and the posterior one behind it. Either can be absent, and conventional teaching finds at least one present in the large majority of knees.

The oblique popliteal ligament and the posterior capsule lie behind, and the plane between them and the ligament is where fluid collects.

The posterior horn of the medial meniscus lies below and medial to the tibial attachment, close enough that the two are described together on a posterior slice.

Blood Supply and Innervation

The middle genicular artery is not reliably seen on a standard knee protocol. Its course through the posterior capsule can sometimes be inferred from the fat plane it crosses.

Vascular anatomy is therefore a matter of knowing rather than of seeing, which is worth stating in a report only when the posterior capsule itself is abnormal.

Nerve supply is not visible at this resolution.

Function

A normal study answers three questions: is the ligament continuous, is it uniform in signal, and is its calibre even. 5.8 covers what the answers mean when they are no.

The curve is a positional finding. Reading it as pathology is the most common error made with this ligament.

Where the knee is scanned in a few degrees of flexion, as most are, the ligament sits between its extended and flexed shapes.

Clinical Relevance

Confirm a round low signal structure by following it across slices before calling it a loose body. The meniscofemoral ligaments account for most of them.

Compare thickness with the contralateral knee where a study is available, rather than against a single quoted figure.

Read the posterior capsule on the same slices. A posterior cruciate injury with an injured capsule is a different problem from one without, and the capsule is visible.

Leave tear description to 5.8.2 and 5.8.3. This page is the reference for normal.

FRCS Level

Relations

The anatomy that matters on a preoperative study is posterior. The capsule, the fat plane and the vessels behind the tibial attachment are all visible, and their state is what changes a plan.

The femoral footprint's relationship to the articular margin is measurable on a coronal slice, and that measurement is the one a tunnel plan rests on.

The meniscofemoral ligaments are visible and their presence is part of what the knee still has. An intact posterior meniscofemoral ligament is restraint.

Function

Use the normal study as the reference for the contralateral comparison rather than as a set of absolute values. The published ranges are wide enough that a single figure decides little.

Read continuity, uniformity and calibre as three separate observations. A ligament can be continuous and abnormal in calibre, and the combination is what distinguishes a healed injury from an uninjured ligament.

Position in the scanner changes shape and not substance, so a report that comments on curvature without stating the knee's position has not said anything.

Clinical Relevance

Assess the posterior capsule and the fat plane on the same study as the ligament. Their state governs the approach, and 2.8.6 covers why.

Measure the distance from the femoral footprint to the articular margin where a tunnel is planned, rather than estimating it at operation.

Document the presence of the meniscofemoral ligaments. They are restraint that a deficient knee has not lost.

Avoid reading a curved ligament in an extended knee as a healed injury. The curve is normal and the position explains it.