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KneeSchool/Imaging Academy/Weight-Bearing MRI

Weight-Bearing MRI

Page 5.18.5 · Advanced Imaging · imaging page

Imaging the loaded knee, which closes the largest gap in knee imaging and closes it in a narrow range of positions.

Pipeline status. Editorial draft. Evidence verification did not run. Not consultant reviewed. Not for clinical use.

The same topic at 4 depths

Each tier deepens the one below it and never repeats it. Choose the level you want.

Weight-Bearing MRI

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For Medical Students

What It Shows

Weight-bearing magnetic resonance imaging scans the knee while the joint is carrying load. 5.2.12 covers the basic principles.

It needs either an upright scanner or a device that compresses the joint inside a conventional one. 5.5.1 covers the standard protocol.

It matters because symptoms occur under load and every conventional study images the joint unloaded. 5.1.2 covers clinical examination versus imaging.

How It Is Acquired

Use an upright or open scanner, or apply axial load inside a conventional bore. 5.5.1 covers the standard protocol.

Record the position and the load applied, because the findings depend on both. 5.20.1 covers structured reporting.

Allow time under load before acquiring, since the tissues respond over minutes. 5.11.1 covers the normal cartilage.

Accept lower image quality than a conventional study gives. 5.2.19 covers the artefacts.

Acquire the matched unloaded study, because the comparison is the result. 5.18.1 covers the quantitative MRI.

What Normal Looks Like

Cartilage that thins under load and recovers afterwards. 5.11.1 covers the normal cartilage.

A meniscus that moves and deforms without extruding further. 3.6.10 covers meniscal extrusion.

A patella that engages in the groove as flexion and load increase. 3.12.1 covers patellar tracking.

Joint space that narrows symmetrically with load. 5.3.17 covers joint space narrowing.

What Abnormal Looks Like

Extrusion that appears or increases only under load. 5.6.11 covers meniscal extrusion.

A patella that displaces under load with a normal unloaded position. 5.12.4 covers patellar subluxation.

Cartilage contact where the unloaded study showed a preserved space. 5.3.17 covers joint space narrowing.

A ligament that lengthens under load with an intact appearance at rest. 5.5.9 covers the ligaments.

Key Learning Points

  • The study loads the joint while imaging it.
  • Symptoms are loaded and conventional imaging is not.
  • The position and the load applied must be recorded.
  • Image quality is worse than in a conventional study.
  • The matched unloaded study is what gives the result.

MRCS Level

How It Is Acquired

In a viva the expected answer names the mismatch the technique addresses.

The expected points are the upright or loaded acquisition, the recorded position and load, the time under load, the image quality, and the matched comparison.

Say that the finding is the difference between loaded and unloaded. 5.18.1 covers the quantitative MRI.

Say that the achievable positions are few and are not the symptomatic ones. 5.18.6 covers the dynamic MRI.

Say that availability is the practical constraint. 5.1.8 covers the future of knee imaging.

What Abnormal Looks Like

The reasoning expected is that soft tissues deform under load and recover without it, so an unloaded study records the recovered state rather than the one that hurts. 3.5.4 covers the effects of flexion.

Meniscal extrusion is the clearest example, because the load that produces it is absent in the scanner. 3.6.10 covers meniscal extrusion.

Patellar instability occurs near extension under load, and a loaded study comes closer to that than any static one. 5.12.4 covers patellar subluxation.

Load applied axially inside a bore is not the same as standing, so the technique closes part of the gap rather than all of it. 5.3.17 covers joint space narrowing.

How Good It Is

Weight-bearing magnetic resonance imaging addresses the single largest mismatch in knee imaging, in a few positions and with reduced image quality.

Its best contribution is a finding that appears only under load, with extrusion the established example. 5.6.11 covers meniscal extrusion.

It is limited by availability, by image quality, by the restricted positions, and by load that is applied rather than natural. 5.1.8 covers the future of knee imaging.

No figures appear for accuracy on this page, under decision 028, and no load values, cartilage deformation values or positional angles appear either: those are published quantities tied to particular devices and this environment cannot check them.

Key Learning Points

  • An unloaded study records the recovered state.
  • Extrusion is the clearest load dependent finding.
  • Instability occurs near extension under load.
  • Axial load inside a bore is not standing.
  • It closes part of the gap rather than all of it.

FRCS Level

How Good It Is

At decision level a loaded study changes a decision where a finding is suspected and absent on the conventional one. 5.1.3 covers choosing the correct investigation.

Extrusion alongside a root tear is the situation where it carries most weight. 5.6.9 covers root tears.

A patellofemoral case with normal static measurements is the other. 5.12.4 covers patellar subluxation.

A weight bearing radiograph answers the alignment and joint space question more cheaply. 5.3.5 covers the Rosenberg view.

Availability means the question is usually settled by examination instead. 4.15.2 covers the J sign.

Where It Misleads

The commonest error is comparing a loaded study with an unloaded one from another occasion. 5.18.1 covers the quantitative MRI.

Reduced image quality is read as pathology in a structure that is simply less well seen. 5.2.19 covers the artefacts.

A position achievable in the device is reported as the symptomatic position. 5.18.6 covers the dynamic MRI.

Load held briefly is read as the steady state. 5.11.1 covers the normal cartilage.

Normal loaded deformation is read as cartilage loss. 5.11.2 covers chondral defects.

Key Learning Points

  • Loaded and unloaded studies have to be matched.
  • Lower image quality is read as pathology.
  • The achievable position is not the symptomatic one.
  • Brief loading is not the steady state.
  • Normal deformation is read as loss.

Fellowship Level

Where It Misleads

At fellowship level the request is usually for a finding the referrer already believes in. 5.1.6 covers imaging pitfalls.

Devices differ in how load is applied, so values and findings are device specific. 5.18.1 covers the quantitative MRI.

Upright scanners run at lower field strength, so a comparison with a conventional study confounds load with image quality. 5.2.12 covers the basic principles.

The literature is small and the positive studies are the ones published, which is a general problem with a technique this scarce. 5.1.7 covers evidence-based imaging.

Combining load with quantitative mapping is the current direction and compounds both standardisation problems. 5.18.2 covers the T2 mapping.

The honest position is that a loaded study asks a question the rest of this section cannot ask, and that almost nobody can obtain one. 5.1.8 covers the future of knee imaging.

Key Learning Points

  • The request usually seeks a finding already believed in.
  • Findings are device specific.
  • Lower field strength confounds load with image quality.
  • The published literature favours positive studies.
  • The right question, and almost nobody can ask it.

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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