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Meniscal Signal Grading

Page 5.6.2 · Meniscal MRI · imaging page

The grading of internal meniscal signal, which is the most consequential and most misused scheme in knee imaging.

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The same topic at 4 depths

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Meniscal Signal Grading

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For Medical Students

What It Shows

Internal meniscal signal is graded by whether it is a focal spot, a linear band that does not reach a surface, or a line that reaches an articular surface. 5.6.1 covers the normal meniscus.

Only the last of those is a tear. The first two are degenerate change within an intact meniscus. 5.6.3 covers horizontal tears.

That distinction is the single most useful thing in this chapter, because the first two are reported and the third is treated.

How It Is Acquired

Assess internal signal on the proton density or intermediate weighted images, where degenerate tissue and a tear both produce signal against a dark background. 5.2.15 covers proton density imaging.

Confirm on a fluid sensitive sequence, since a tear filled with fluid is bright there. 5.2.16 covers fat suppression.

Decide, for each area of signal, whether it reaches an articular surface, which is the question the grade turns on.

Check it on two consecutive slices and in both planes, because signal on one slice in one plane is usually artefact or partial volume. 5.5.1 covers the standard protocol.

Record the location by horn and by zone, since the zone predicts whether a tear can heal. 2.5.3 covers the vascular supply.

What Abnormal Looks Like

A focal area of increased signal not reaching a surface is intrasubstance degeneration, present in a large share of adult menisci and not a tear.

A linear area not reaching a surface is the same process, further along, and still not a tear.

A line reaching an articular surface is a tear, and its direction relative to the circumferential fibres is what the following pages are about. 5.6.5 covers radial tears.

The appearance is read against the patient's age, because the proportion of menisci with internal signal rises steadily with it.

Key Learning Points

  • Signal is graded by whether it reaches an articular surface.
  • Only a line reaching a surface is a tear.
  • Focal and linear internal signal are degenerate change in an intact meniscus.
  • A finding on one slice in one plane is usually artefact or partial volume.
  • The appearance is read against the patient's age.

MRCS Level

How It Is Acquired

In a viva this is the question that separates a candidate who has read a scan from one who has read a report.

The expected points are the sequence, the surface rule, the two slice rule, the two plane rule, the zone and the age.

Say that a tear is signal reaching an articular surface, and say it in those words. 5.6.1 covers the normal meniscus.

Say that you would want the finding on two consecutive slices. 5.1.6 covers imaging pitfalls.

Say that intrasubstance degeneration is common and is not a surgical finding. 4.9.11 covers the examination in the worn knee.

What Abnormal Looks Like

The reasoning expected is that a tear is a discontinuity in the circumferential collagen, and a discontinuity that does not reach a surface does not release the hoop tension. 3.6.2 covers hoop stress.

That is why the surface rule is mechanical rather than radiological: a tear matters because it lets the meniscus fail to carry load, and an intrasubstance change does not. 3.6.3 covers the circumferential fibres.

A tear reaching a surface can displace, and displacement is what produces the mechanical symptoms. 4.9.8 covers mechanical locking.

The zone determines healing because the blood supply is peripheral, which is why a peripheral vertical tear is repairable and a central radial tear often is not. 2.5.3 covers the vascular supply.

How Good It Is

The grading is reliable between readers for the surface rule and less reliable for the degenerate grades.

Its best contribution is separating the meniscus that needs an operation from the one that does not, which is the decision the whole chapter serves. 5.20.2 covers reporting meniscal tears.

It is undermined by the prevalence of degenerate signal in older knees, where a reported finding is more likely incidental than causal. 4.9.11 covers the examination in the worn knee.

No figures appear for any accuracy comparison on this page, under decision 028, and no prevalence figure appears either, for the same reason.

Key Learning Points

  • A tear is a discontinuity that releases hoop tension; degeneration is not.
  • The surface rule is mechanical rather than radiological.
  • A surface reaching tear can displace, which produces mechanical symptoms.
  • The zone predicts healing because the blood supply is peripheral.
  • Degenerate signal in older knees is more likely incidental than causal.

FRCS Level

How Good It Is

At decision level the grade decides whether a meniscus is part of the problem, and the commonest clinical error in the knee follows from getting it wrong.

A surface reaching tear in a young knee with a matching examination is a surgical finding, and the imaging and the examination agree. 4.9.1 covers the clinical presentation.

Intrasubstance degeneration reported in an older knee with anterior pain is not a surgical finding, and operating on it is the error the literature has spent twenty years on. 4.9.11 covers the examination in the worn knee.

A degenerate surface reaching tear in an arthritic knee is a real tear and still usually not the symptom source, which is the hardest version of the judgement. 5.6.11 covers meniscal extrusion.

The grade is read with the cartilage and the alignment, because a tear in a worn compartment is part of the wear. 5.11.1 covers normal cartilage.

Where It Misleads

The commonest error is treating reported internal signal as a tear. 5.6.1 covers the normal meniscus.

A grade is quoted from a report without the surface question having been answered, which the report may not state explicitly. 5.20.2 covers reporting meniscal tears.

A tear is attributed to a recent injury when the signal pattern is degenerate and long standing. 5.6.8 covers complex tears.

Partial volume averaging at the free edge creates apparent surface contact where there is none. 5.1.6 covers imaging pitfalls.

The magic angle effect raises signal in obliquely oriented collagen and is one of the specific causes of a false grade. 5.2.20 covers the magic angle effect.

Key Learning Points

  • Treating reported internal signal as a tear is the commonest error.
  • A report may not state whether the signal reaches a surface.
  • A degenerate pattern is not explained by a recent injury.
  • Partial volume at the free edge creates false surface contact.
  • The magic angle effect is a specific cause of a false grade.

Fellowship Level

Where It Misleads

At fellowship level the grading scheme's problem is that it describes structure and is used to decide about symptoms.

The surface rule identifies a mechanically significant tear and says nothing about whether that tear hurts, which is the gap every decision crosses. 4.9.11 covers the examination in the worn knee.

In a degenerate knee the meniscus, the cartilage and the bone are all abnormal, so attributing the pain to the meniscus because it was graded is a choice rather than a finding. 5.11.7 covers bone marrow lesions.

Interobserver agreement falls for the degenerate grades, so a grade from another centre is not a measurement. 5.20.8 covers common reporting errors.

Extrusion is a better mechanical marker than the grade in the worn knee and is reported less often. 5.6.11 covers meniscal extrusion.

The scheme predates the sequences in use, so the grades were defined on images that are not the ones being graded. 5.21.1 covers the meniscal imaging literature.

Key Learning Points

  • The surface rule identifies mechanical significance, not symptoms.
  • In a degenerate knee attributing pain to the meniscus is a choice.
  • Agreement falls for the degenerate grades, so a grade is not a measurement.
  • Extrusion is a better mechanical marker in the worn knee and is reported less.
  • The scheme was defined on images that are not the ones being graded.

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.

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