Cartilage

For Medical Students

What It Shows

Articular cartilage covers the femoral condyles, the tibial plateaux and the patella, and magnetic resonance imaging is the only modality that images it. 5.11.1 covers normal cartilage.

It appears as a layer of intermediate to high signal over the dark subchondral plate. 5.5.6 covers the bones.

It matters because cartilage is what the compartment question is about, and the routine knee protocol was built for menisci and ligaments. 5.5.1 covers the standard protocol.

How It Is Acquired

Use a sequence with good contrast between cartilage and joint fluid, which the gradient echo family provides. 5.2.18 covers gradient echo imaging.

Use thin slices, because a thin layer is averaged with what is beside it. 5.2.5 covers magnification.

Assess the surface separately from the substance, since a surface defect and an internal change are different findings. 5.11.3 covers cartilage fissuring.

Assess the subchondral plate and marrow alongside, because the two change together. 5.11.7 covers bone marrow lesions.

Know that the patellar and trochlear cartilage needs the axial plane. 5.12.6 covers patellofemoral cartilage injury.

What Normal Looks Like

A smooth layer of uniform thickness with a sharp surface against joint fluid. 5.11.1 covers normal cartilage.

A normal signal variation through the layer's depth, which reflects its zonal structure and is not damage. 5.11.1 covers normal cartilage.

Thicker cartilage on the patella and the trochlea than on the condyles. 5.12.6 covers patellofemoral cartilage injury.

An intact dark subchondral plate beneath it. 5.5.6 covers the bones.

MRCS Level

How It Is Acquired

In a viva this is the tissue to name when asked what the routine protocol understates.

The expected points are the cartilage sensitive sequence, the thin slices, the surface against the substance, the subchondral plate, and the axial plane.

Say that a cartilage question needs a cartilage sequence requested by name. 5.5.1 covers the standard protocol.

Say that normal zonal signal variation is not damage. 5.11.1 covers normal cartilage.

Say that cartilage and subchondral marrow change together. 5.11.7 covers bone marrow lesions.

What Normal Looks Like

The reasoning expected is that cartilage is a thin layer with an organised zonal structure, so its water content and collagen orientation vary with depth and the signal varies with them. 5.11.1 covers normal cartilage.

That normal variation is the appearance most often read as early degeneration, which makes knowing it a prerequisite for reading cartilage at all. 5.11.3 covers cartilage fissuring.

Thinness is the technical problem: a layer comparable to the slice thickness is averaged with the fluid above and the plate below, which blurs the surface the assessment depends on. 5.2.5 covers magnification.

Cartilage and the subchondral plate form a mechanical unit, which is why a marrow lesion sits under a cartilage defect rather than beside it. 5.11.7 covers bone marrow lesions.

How Good It Is

Magnetic resonance imaging is the only modality that assesses cartilage and does it less well than the clinical question needs.

Its best contribution is a focal full thickness defect, which is read reliably and matters surgically. 5.11.2 covers chondral defects.

It is limited for early and partial thickness change, which is where the compartment question actually sits. 5.11.3 covers cartilage fissuring.

No figures appear for accuracy on this page, under decision 028, and no cartilage thickness values or grading criteria appear either: those are published quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the cartilage assessment decides whether a joint preserving operation has something to preserve.

A focal full thickness defect in a young patient with preserved surrounding cartilage is the cartilage repair picture. 5.11.8 covers cartilage repair procedures.

Diffuse partial thickness loss across a compartment is a different patient, and distinguishing the two is the decision this tissue supports. 5.11.2 covers chondral defects.

A cruciate or meniscal operation's prognosis depends on the compartment's cartilage, which makes this assessment part of every such decision. 5.6.13 covers meniscal repair imaging.

A routine protocol understates the loss, so a cartilage decision made on one is made on the wrong data. 5.5.1 covers the standard protocol.

Where It Misleads

The commonest error is assessing cartilage on a ligament protocol. 5.5.1 covers the standard protocol.

Normal zonal signal variation is read as early degeneration. 5.11.1 covers normal cartilage.

Partial volume averaging at the surface is read as a defect. 5.2.5 covers magnification.

The patellofemoral compartment is not assessed because the axial plane was not reviewed. 5.12.6 covers patellofemoral cartilage injury.

Loss is understated and a knee is judged suitable for an operation it is not. 5.11.2 covers chondral defects.

Fellowship Level

Where It Misleads

At fellowship level cartilage is the tissue where the gap between what imaging measures and what matters is widest in the whole section.

Morphological assessment detects a defect and says nothing about the matrix's mechanical state, and the mechanical state is what determines whether the compartment survives. 5.11.1 covers normal cartilage.

Compositional sequences measure matrix properties and have not entered routine practice, partly because their thresholds do not transfer between sequences and field strengths. 5.18.2 covers T2 mapping.

Morphological grading systems were derived on particular sequences, so applying them across protocols applies a rule to data it was not derived from, which is the pattern this chapter keeps finding. 5.21.3 covers the cartilage imaging literature.

Cartilage is also measured unloaded, and the tissue deforms under load, so the thickness reported is not the thickness the joint works with. 5.18.5 covers weight bearing imaging.

The honest position is that the only modality that images cartilage images its shape, and that the compartment question is about its mechanics. 5.11.2 covers chondral defects.