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KneeSchool/Imaging Academy/Radial Tears

Radial Tears

Page 5.6.5 · Meniscal MRI · imaging page

A tear running across the meniscus, which cuts the fibres that carry the load and is the pattern that matters most mechanically.

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The same topic at 4 depths

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Radial Tears

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For Medical Students

What It Shows

A radial tear runs from the free edge towards the capsule, across the circumferential fibres rather than along them. 5.6.4 covers vertical tears.

Cutting those fibres releases the hoop tension, so the meniscus stops converting load into circumferential tension and stops protecting the cartilage. 3.6.2 covers hoop stress.

It matters because a complete radial tear functionally divides the meniscus even when most of the tissue is still present. 3.6.12 covers the effects of radial tears.

How It Is Acquired

Look on the images perpendicular to the tear, which for a posterior horn radial tear means the sagittal images. 5.5.1 covers the standard protocol.

Look for the tear in the coronal plane as well, where a full thickness radial tear produces a gap in the meniscus. 5.6.1 covers the normal meniscus.

Establish whether it is complete, which means reaching the capsular margin, because that is the mechanical threshold. 3.6.12 covers the effects of radial tears.

Look for the cleft sign, a vertical gap, and for the ghost sign, where a segment of meniscus appears absent on one slice. 5.6.1 covers the normal meniscus.

Assess extrusion, because a radial tear releases the hoop and the meniscus then moves out of the joint. 5.6.11 covers meniscal extrusion.

What Abnormal Looks Like

A linear area of signal running from the free edge outwards, seen as a gap in the meniscus where it is complete.

A segment of meniscus apparently missing on one slice, which is the same tear seen in the plane of the gap.

Extrusion of the meniscus beyond the tibial margin, which accompanies a complete radial or root tear and is the mechanical consequence. 5.6.11 covers meniscal extrusion.

Cartilage loss and bone marrow signal in the same compartment, because the compartment is now unprotected. 5.11.7 covers bone marrow lesions.

Key Learning Points

  • A radial tear runs across the circumferential fibres.
  • Cutting them releases the hoop tension.
  • A complete radial tear functionally divides an intact looking meniscus.
  • Completeness means reaching the capsular margin.
  • Extrusion is the mechanical consequence to look for.

MRCS Level

How It Is Acquired

In a viva this is the mechanically important tear and the expected answer reaches hoop stress quickly.

The expected points are the plane, completeness, the cleft and ghost appearances, extrusion, and the compartment's cartilage.

Say that a radial tear cuts the circumferential fibres and releases hoop tension. 3.6.2 covers hoop stress.

Say that you would assess extrusion in the same study. 5.6.11 covers meniscal extrusion.

Say that a complete radial tear is mechanically closer to a meniscectomy than to a longitudinal tear. 3.6.5 covers load sharing.

What Abnormal Looks Like

The reasoning expected is that the meniscus works as a ring under tension, so a cut across the ring is a different failure from a split along it. 3.6.3 covers the circumferential fibres.

Once the ring is divided the meniscus extrudes under load rather than resisting it, which is why extrusion and radial tears appear together. 3.6.10 covers meniscal extrusion.

The compartment then carries load through a smaller contact area at higher pressure, which is the mechanism of the cartilage loss that follows. 3.6.5 covers load sharing.

Repair is harder because the fibres being approximated run across the repair rather than along it. 3.6.13 covers meniscal repair biomechanics.

How Good It Is

Radial tears are detected less reliably than longitudinal ones, because the tear plane and the imaging plane can coincide.

Its best contribution is the associated extrusion, which is conspicuous and indicates the mechanical state better than the tear's visibility. 5.6.11 covers meniscal extrusion.

It is weakest where the tear is incomplete, since a partial radial tear releases less tension and is less visible.

No figures appear for any accuracy comparison on this page, under decision 028, and the comparison is the claim being made.

Key Learning Points

  • The meniscus works as a ring under tension.
  • A cut across the ring is a different failure from a split along it.
  • A divided ring extrudes under load rather than resisting it.
  • The compartment then loads a smaller area at higher pressure.
  • Radial tears are detected less reliably than longitudinal ones.

FRCS Level

How Good It Is

At decision level a complete radial tear is a mechanical diagnosis and the decision is about the compartment rather than the tear.

A complete radial tear in a young knee is a repair consideration despite the difficulty, because the alternative is a functionally meniscus deficient compartment. 3.6.13 covers meniscal repair biomechanics.

The same tear with established cartilage loss is part of a worn compartment, and the decision moves to the compartment. 5.11.2 covers chondral defects.

Extrusion accompanying the tear is the finding that predicts the compartment's course, and it is reported inconsistently. 5.6.11 covers meniscal extrusion.

A radial tear at the root is a different lesion with its own page, because the root is where the hoop is anchored. 5.6.9 covers root tears.

Where It Misleads

The commonest error is missing a complete radial tear because the imaging plane runs along the tear. 5.5.1 covers the standard protocol.

The ghost appearance is read as a previous meniscectomy. 5.6.12 covers the post operative meniscus.

Completeness is not assessed, so a partial and a complete tear are reported alike. 3.6.12 covers the effects of radial tears.

Extrusion is not measured, so the mechanical consequence goes unreported. 5.6.11 covers meniscal extrusion.

A radial tear in a degenerate meniscus is read as traumatic because the pattern is associated with injury in young knees. 5.6.2 covers signal grading.

Key Learning Points

  • The tear is missed when the imaging plane runs along it.
  • The ghost appearance is read as a previous meniscectomy.
  • An unassessed completeness reports a partial and a complete tear alike.
  • Unmeasured extrusion leaves the mechanical consequence unreported.
  • A degenerate radial tear is read as traumatic.

Fellowship Level

Where It Misleads

At fellowship level the radial tear is where imaging and mechanics come closest and the clinical decision is still hard.

A complete radial tear makes the compartment behave as meniscus deficient, so the decision is about protecting cartilage rather than about the tear. 3.6.5 covers load sharing.

Repair of a radial tear is mechanically unfavourable and is still often the better option in a young knee, which is a judgement the imaging informs and does not make. 3.6.13 covers meniscal repair biomechanics.

Partial radial tears sit on a continuum, and where along it a tear becomes mechanically complete is not visible on a scan. 3.6.12 covers the effects of radial tears.

Extrusion is measured differently in different places and is not standardised, so the best mechanical marker available is also the least comparable. 5.6.11 covers meniscal extrusion.

A radial tear with bone marrow signal in the compartment is a knee already losing cartilage, and whether the tear or the wear came first is not answerable from the images. 5.11.7 covers bone marrow lesions.

Key Learning Points

  • A complete radial tear makes the compartment behave as meniscus deficient.
  • Repair is mechanically unfavourable and often still the better option.
  • Where a partial tear becomes mechanically complete is not visible.
  • Extrusion is the best marker available and the least standardised.
  • Which came first, the tear or the wear, is not answerable from the images.

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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