MRI Anatomy

For Medical Students

Structure and Location

A normal anterior cruciate ligament is seen best on sagittal images. It appears as a band of low signal running from the anterior tibia upwards and backwards to the lateral femoral condyle.

It is not uniformly dark like a meniscus. The fibres are separated by connective tissue and the ligament has a striated appearance, with thin lines of higher signal between the fibre bundles. That striation is normal.

On a sagittal image the normal ligament runs roughly parallel to the roof of the notch, Blumensaat's line. Loss of that parallel orientation is the most useful single sign of abnormality.

On coronal images it is seen crossing the notch, and on axial images as an oval in the notch alongside the posterior cruciate.

Relations

The posterior cruciate lies behind and medial, and is uniformly dark and curved. The two are easily told apart: the anterior cruciate is straight and striated, the posterior is curved and dark.

The notch roof lies above, and the relationship between the two is the orientation reference.

The anterior horns of the menisci and the intermeniscal ligament lie in front of its tibial attachment.

The fat pad lies anteriorly and the posterior capsule behind.

Function

The imaging questions are whether the ligament is intact, whether it is in the right orientation, and whether its fibres are continuous.

Sagittal images answer orientation and continuity. Coronal and axial images confirm and help when the sagittal plane is ambiguous.

Clinical Relevance

Striation is normal and is not a partial tear. That is the commonest overcall on this ligament.

Tear patterns, the secondary signs and the accuracy of each belong to 5.7.2 to 5.7.5. This page stops at the normal appearance, which is what the architecture assigns it.

MRCS Level

Structure and Location

Describe the appearance plane by plane. Sagittal: a straight band of intermediate to low signal with internal striation, running from the anterior intercondylar area to the lateral wall, roughly parallel to the notch roof. Coronal: the ligament crossing the notch, with its femoral attachment on the lateral wall visible on the posterior slices. Axial: an oval structure in the notch, anterolateral to the posterior cruciate.

Signal is not uniform. The ligament is brighter than the posterior cruciate on most sequences because of the connective tissue between its fascicles, and brighter near its tibial attachment where the fibres fan.

Oblique sagittal and oblique coronal planes aligned to the ligament improve visualisation, because a true sagittal plane cuts it obliquely. Whether they are acquired is a protocol decision.

The two bundles are not reliably separable on routine imaging.

Relations

Blumensaat's line is the orientation reference and the most useful landmark on the sagittal view.

The posterior cruciate is the comparator. Any uncertainty about which cruciate is being looked at is resolved by its curve and its uniform low signal.

The intermeniscal ligament and the anterior meniscal horns lie anterior to the tibial attachment and can produce confusing signal at that level.

The lateral meniscal anterior root attaches in the same region and overlaps the footprint, which 2.6.4 covers.

Blood Supply and Innervation

Vascularity is not assessable on routine sequences. The synovial sheath is not resolved as a separate structure.

Contrast enhancement around the ligament reflects synovial rather than ligamentous perfusion.

Signal change within the ligament reflects water content and fibre disruption rather than perfusion, which is why increased signal is interpreted structurally.

Function

Imaging establishes continuity, orientation and attachment. It does not establish function, and a ligament that looks continuous can be incompetent.

That limit is why examination and imaging are read together rather than one confirming the other, and it is more true of this ligament than of the menisci.

After reconstruction the graft is assessed on the same criteria: orientation relative to the notch roof, continuity, and tunnel position.

Clinical Relevance

Do not call a partial tear on striation alone. The normal ligament has internal signal, and the criteria for partial tear are in 5.7.

Assess orientation first. A ligament that has lost its parallel relationship to the notch roof is abnormal even where its fibres look continuous.

After reconstruction, assess graft orientation and tunnel position rather than graft signal, because graft signal changes with remodelling and is a poor guide to competence.

Oblique planes improve assessment and are not universally acquired, which is worth knowing when a report is equivocal.

No measurement appears on this page. Every angle, ratio and signal criterion belongs to Section 5.

FRCS Level

Relations

Before revision the relations to establish on imaging are tunnel position, tunnel diameter, tunnel convergence and bone quality. Computed tomography answers the first three better than magnetic resonance does.

The relation between the existing femoral tunnel and the native footprint on the lateral wall is the single most useful preoperative measurement in revision.

Function

Imaging decides whether a reconstruction has failed technically or biologically. An anterior or vertical tunnel with a continuous graft is a technical failure; an anatomical tunnel with a discontinuous graft is not.

That distinction changes the revision plan, and it is made before the operation on images rather than at arthroscopy.

Clinical Relevance

Obtain computed tomography before revision. Tunnel geometry and bone quality are what stage the procedure and magnetic resonance underestimates both.

Assess graft orientation on the sagittal plane against the notch roof, exactly as for a native ligament.

Treat graft signal with caution in the first year. Remodelling produces increased signal in a competent graft and the finding does not mean failure.

In the paediatric knee, assess the physes as well as the ligament, and Section 10 owns the consequences.