Ligaments

For Medical Students

What It Shows

The knee's ligaments appear as dark bands, because they are dense organised collagen with little free water. 5.2.12 covers the magnetic resonance basic principles.

The cruciates run obliquely within the joint and the collaterals run close to the coronal plane outside it. 5.7.1 covers the normal cruciate.

It matters because an obliquely running structure is cut obliquely by every orthogonal plane, so plane and position decide whether it can be assessed. 5.5.1 covers the standard protocol.

How It Is Acquired

Assess the cruciates on the sagittal images with the knee in slight external rotation, which brings the anterior cruciate into plane. 5.7.1 covers the normal cruciate.

Confirm on the coronal and axial planes, because the sagittal plane alone can make a normal ligament disappear. 5.5.1 covers the standard protocol.

Assess the collaterals on the coronal images along their length. 5.9.1 covers the medial collateral anatomy.

Assess the posterolateral structures deliberately, since they are oblique and small. 5.10.3 covers posterolateral corner injuries.

Know the normal striated appearance of the anterior cruciate, which is bundle anatomy. 5.7.1 covers the normal cruciate.

What Normal Looks Like

Dark continuous bands following their expected courses. 5.7.1 covers the normal cruciate.

A striated anterior cruciate with fine bright lines between its bundles, which is normal. 5.7.1 covers the normal cruciate.

A uniformly dark posterior cruciate with a smooth curve. 5.8.1 covers the normal posterior cruciate.

Collaterals as thin dark bands with the expected layers medially. 5.9.1 covers the medial collateral anatomy.

MRCS Level

How It Is Acquired

In a viva the expected answer names the plane and the position for each ligament rather than describing appearances.

The expected points are the sagittal cruciate assessment with rotation, the confirmation planes, the coronal collaterals, the posterolateral structures, and the normal striations.

Say that the sagittal plane alone can make a normal cruciate disappear. 5.7.1 covers the normal cruciate.

Say that the striations are normal anatomy. 5.7.2 covers partial cruciate tears.

Say that the posterolateral structures need deliberate assessment. 5.10.3 covers posterolateral corner injuries.

What Normal Looks Like

The reasoning expected is that an obliquely running ligament crossing a slice plane is partially volume averaged with the fat and fluid around it, so it can appear interrupted without being torn. 5.2.5 covers magnification.

Knee position changes the obliquity, which is why the external rotation is a protocol requirement and not a convenience. 5.5.1 covers the standard protocol.

The anterior cruciate is two bundles in a common sheath, so the interfascicular tissue between them carries signal and produces striations in a normal ligament. 2.7.8 covers the anteromedial bundle.

The posterior cruciate is a thicker more uniform structure, which is why it is read more easily and why its buckling is used as a translation sign. 5.8.1 covers the normal posterior cruciate.

How Good It Is

Magnetic resonance imaging reads the normal ligaments reliably when the plane and position are right.

Its best contribution is the continuous dark band following a normal course, which is a strong normal. 5.7.1 covers the normal cruciate.

It is limited for the obliquely running small structures of the posterolateral corner, and for partial tears of the cruciate. 5.7.2 covers partial cruciate tears.

No figures appear for accuracy on this page, under decision 028, and no rotation angles or thickness values appear either: those are local protocol quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level a confident normal ligament is worth as much as a confident abnormal one.

A continuous dark cruciate on three planes in a stable knee is a secure normal. 5.7.1 covers the normal cruciate.

A collateral assessed along its length with its layers separated gives a grade that informs management. 5.9.3 covers deep medial collateral injuries.

The posterolateral corner needs deliberate assessment and still performs inconsistently, which bounds what a negative means. 5.10.3 covers posterolateral corner injuries.

A ligament reported abnormal in a stable knee should be checked against the protocol and the position before it is believed. 5.1.2 covers clinical examination versus imaging.

Where It Misleads

The commonest error is reading normal cruciate striation as a partial tear. 5.7.2 covers partial cruciate tears.

An obliquely cut normal ligament is reported as discontinuous. 5.2.5 covers magnification.

The knee was positioned without rotation and the cruciate is out of plane. 5.5.1 covers the standard protocol.

The posterolateral structures are not assessed and the corner is reported intact. 5.10.3 covers posterolateral corner injuries.

A single plane is accepted as sufficient. 5.5.1 covers the standard protocol.

Fellowship Level

Where It Misleads

At fellowship level the ligaments show most clearly that a magnetic resonance study measures continuity and the clinical question is restraint.

A continuous ligament can be functionally lax and a dark band says nothing about stiffness, which is the conclusion every graft, repair and healing page in this section reaches. 5.7.8 covers cruciate healing.

Normal ligament signal has no agreed definition, so the boundary between normal striation and a partial tear is set by the reader, which makes the least reproducible call in the section a definitional problem rather than a technical one. 5.7.2 covers partial cruciate tears.

Knee position in the scanner is not recorded, so a ligament's apparent tension and line cannot be corrected for by a reader. 5.5.1 covers the standard protocol.

Three dimensional isotropic acquisition removes the oblique plane problem by allowing reformatting along any structure, which is the clearest technical gain available here. 5.2.18 covers gradient echo imaging.

The useful report describes continuity, line and position as three separate observations and does not infer function from them. 5.20.3 covers reporting cruciate injuries.