Normal 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. 1.2.2 covers the articular cartilage.

It appears as a layer of intermediate to high signal over the dark subchondral plate, with a smooth surface against joint fluid. 5.5.7 covers the cartilage.

It matters because the normal layer is not uniform in signal, and that normal variation is what gets reported as early degeneration. 5.11.3 covers fissuring.

How It Is Acquired

Use a cartilage sensitive sequence with good contrast between cartilage and fluid. 5.2.18 covers gradient echo imaging.

Use thin slices, because a thin layer is averaged with what lies either side of it. 5.2.5 covers magnification.

Assess the surface separately from the substance, since they are different findings. 5.11.2 covers chondral defects.

Assess all three compartments, which means reviewing the axial plane for the patellofemoral joint. 5.12.6 covers patellofemoral cartilage injury.

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

What Normal Looks Like

A smooth layer of even thickness with a sharp interface against joint fluid. 5.5.7 covers the cartilage.

A signal variation through the layer's depth, reflecting its zonal collagen organisation, which is normal. 1.2.2 covers the articular 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 the expected answer gives the normal appearance before any abnormality, because the normal is the harder half.

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

Say that zonal signal variation is normal. 1.2.2 covers the articular cartilage.

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

Say that the routine knee protocol understates chondral loss. 5.5.7 covers the cartilage.

What Normal Looks Like

The reasoning expected is that cartilage has an organised zonal structure, with collagen running tangentially at the surface and radially at depth, so its water content and the orientation of its fibres both change with depth and the signal changes with them. 1.2.2 covers the articular cartilage.

That organisation is what gives cartilage its mechanical properties, so the signal variation is a sign of a working tissue rather than a damaged one. 3.5.2 covers the contact pressures.

Thinness is the technical problem, since 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 behave as a mechanical unit, which is why a marrow lesion sits beneath a defect rather than beside it. 5.11.7 covers bone marrow lesions.

How Good It Is

Magnetic resonance imaging images the only tissue nothing else images, and images its shape rather than its mechanics.

Its best contribution is the normal appearance, which is what makes an abnormal one interpretable. 5.5.7 covers the cartilage.

It is limited because the layer is thin, its normal signal varies, and the mechanical state that matters is not what is measured. 5.18.2 covers T2 mapping.

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

FRCS Level

How Good It Is

At decision level a confident normal cartilage assessment is what allows a joint preserving operation to be offered.

Preserved cartilage in a compartment is what a meniscal repair or a cruciate reconstruction is protecting. 5.6.13 covers meniscal repair imaging.

Preserved cartilage surrounding a focal defect is what makes a repair procedure reasonable. 5.11.8 covers cartilage repair procedures.

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

Cartilage is assessed unloaded, so the thickness reported is not the thickness the joint works with. 5.18.5 covers weight bearing imaging.

Where It Misleads

The commonest error is reading normal zonal signal variation as early degeneration. 1.2.2 covers the articular cartilage.

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

Cartilage is assessed on a ligament protocol. 5.5.1 covers the standard protocol.

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

Thickness is compared between studies acquired differently. 5.21.3 covers the cartilage imaging literature.

Fellowship Level

Where It Misleads

At fellowship level normal cartilage is where the gap between what is imaged and what matters opens, and the rest of this chapter lives inside it.

Morphological imaging shows the layer's shape, and the property that determines whether a compartment survives is the matrix's mechanical state, which shape does not report. 3.5.2 covers the contact pressures.

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

Early matrix change precedes any morphological change, so the period in which intervention might alter the course is the period the routine study cannot see. 5.18.3 covers T1 rho imaging.

Cartilage is also imaged unloaded while the tissue deforms under load, so even the shape reported is not the shape the joint uses. 5.18.5 covers weight bearing imaging.

The honest framing is that the only modality that images cartilage images its shape, unloaded, against a normal that varies with depth. 5.5.7 covers the cartilage.