Cartilage Injury

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Student MRCS FRCS Fellow

For Medical Students

What It Shows

Patellofemoral cartilage injury is damage to or loss of the cartilage on the patellar facets or in the trochlear groove. 5.11.1 covers the normal cartilage.

Magnetic resonance imaging shows it as thinning, a surface defect, internal signal change or full thickness loss down to bone. 5.11.2 covers chondral defects.

It matters because this joint carries high contact pressures over small areas, so a small defect sits where the load is. 3.12.3 covers contact pressures.

How It Is Acquired

Use axial images through the patella as the primary plane, because the facets face sideways. 5.5.1 covers the standard protocol.

Use a sequence with cartilage contrast rather than a fluid sensitive one alone. 5.5.7 covers the cartilage.

Assess the median ridge and each facet separately, since the pattern differs between them. 3.12.2 covers contact areas.

Assess the trochlear groove on axial and sagittal images, because the groove curves away from both. 2.12.1 covers the trochlea.

Assess the bone under any defect, since the marrow change often marks it first. 5.11.7 covers bone marrow lesions.

What Normal Looks Like

A smooth cartilage surface of even thickness across each facet. 5.11.1 covers the normal cartilage.

Cartilage that is thicker over the patella than elsewhere in the knee, which is normal rather than swollen. 5.5.7 covers the cartilage.

A continuous interface between the cartilage and the bone beneath it. 5.11.4 covers delamination.

No marrow signal change under the surface. 5.11.7 covers bone marrow lesions.

What Abnormal Looks Like

Surface irregularity or fissuring of the cartilage. 5.11.3 covers fissuring.

A partial thickness defect with sloping or undercut edges. 5.11.2 covers chondral defects.

Full thickness loss with the bone exposed. 5.11.2 covers chondral defects.

Separation of cartilage from the bone with an intact surface above it. 5.11.4 covers delamination.

An osteochondral injury after a dislocation, which is a different mechanism from wear. 5.11.5 covers osteochondral lesions.

Key Learning Points

Axial images through the patella are the primary plane.
Each facet and the median ridge are assessed separately.
Patellar cartilage is normally thicker than elsewhere.
Separation can sit under an intact surface.
Injury after a dislocation differs from wear.

MRCS Level

How It Is Acquired

In a viva the expected answer names the plane and the sequence before the grade.

The expected points are the axial plane, the cartilage sensitive sequence, the facets separately, the trochlear groove, and the bone beneath.

Say that the grading system used must be named in the report. 5.20.5 covers reporting cartilage lesions.

Say that the depth of a defect and its area are separate facts. 5.11.2 covers chondral defects.

Say that the marrow change under a defect often precedes a visible surface change. 5.11.7 covers bone marrow lesions.

What Abnormal Looks Like

The reasoning expected is that these surfaces are curved and thin where the trochlea turns, so the same defect is easier to see on the patella than in the groove. 2.12.1 covers the trochlea.

Cartilage signal alters where the surface lies at a particular orientation to the field, and this joint reaches that orientation. 5.2.20 covers the magic angle effect.

A defect under the median ridge is foreshortened on axial images and is missed. 3.12.2 covers contact areas.

Cartilage loss with no history of injury sits in a different argument from cartilage loss after a dislocation. 5.12.5 covers MPFL injury.

How Good It Is

Magnetic resonance imaging is the best available test of this cartilage without an arthroscopy and the least reliable application of cartilage imaging in the knee.

Its best contribution is a full thickness defect with its location and the state of the bone beneath. 5.11.2 covers chondral defects.

It is limited by the curved thin surfaces, by signal effects at particular orientations, and by grading that varies between readers. 5.2.20 covers the magic angle effect.

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

Key Learning Points

Curved thin surfaces make the groove harder than the patella.
Signal at particular orientations alters the cartilage appearance.
A median ridge defect is foreshortened on axial images.
Depth and area are separate facts.
The grading system must be named.

FRCS Level

How Good It Is

At decision level the grade and the location decide whether a cartilage procedure is discussed at all, which puts the weight on the least reproducible part of the report. 5.17.5 covers cartilage procedures.

A contained full thickness defect on one facet is a different conversation from diffuse loss across the joint. 5.11.8 covers cartilage repair procedures.

Loss on both the patella and the trochlea changes the options available. 3.12.10 covers patellofemoral arthritis.

The alignment and the bony measurements belong in the same read, because a defect under an abnormal load has a cause. 5.16.4 covers the TT-TG measurement.

A grade assigned without naming its system cannot be compared with the next scan. 5.20.5 covers reporting cartilage lesions.

Where It Misleads

The commonest error is reading a patellar cartilage change as the cause of anterior pain. 3.12.9 covers patellofemoral pain.

Cartilage change on the patella is common in the asymptomatic middle aged knee. 3.12.10 covers patellofemoral arthritis.

Signal change with an intact surface is reported as a defect. 5.11.3 covers fissuring.

A partial volume effect where the trochlea curves is read as loss. 5.5.7 covers the cartilage.

Separation from the bone is missed because the surface above it is intact. 5.11.4 covers delamination.

Key Learning Points

Patellar cartilage change is common in the asymptomatic knee.
It is reported as the cause of pain more often than it is.
Signal change under an intact surface is over-called.
Partial volume where the trochlea curves mimics loss.
Separation hides under an intact surface.

Fellowship Level

Where It Misleads

At fellowship level the patient arrives with a graded report and an operation already proposed on the strength of it.

The grade on the referring scan and the grade at arthroscopy disagree often enough that the operative plan should carry both. 5.17.5 covers cartilage procedures.

Quantitative cartilage measures are offered as objective and are acquired differently in each department. 5.18.2 covers T2 mapping.

A repaired surface looks abnormal on every sequence for a long time, so a post-operative scan read as a failure may be reading the repair. 5.11.8 covers cartilage repair procedures.

Automated grading inherits this joint's difficulty and reports it with the same confidence it reports the femoral condyle. 5.18.8 covers artificial intelligence in imaging.

The useful report gives the depth, the location by facet, the area, the state of the bone, and the system used. 5.20.1 covers structured reporting.

Key Learning Points

The imaging grade and the arthroscopic grade disagree often.
Quantitative measures vary between departments.
A repair looks abnormal long after it has worked.
Automated grading carries this joint's difficulty forward.
The report should name depth, facet, area, bone and system.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 028 rather than an omission: the comparative standing of the modalities is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page. Any measurement here, including a sequence parameter, an angle or a millimetre, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.

Safety. KneeSchool is educational and does not give individual medical advice. Seek urgent assessment for a knee that cannot bear weight after an injury, a hot swollen knee with a fever, or new numbness, weakness or coldness in the leg or foot.