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KneeSchool/Imaging Academy/Total Knee Replacement

Total Knee Replacement

Page 5.17.8 · Post-operative Imaging · imaging page

Loosening is a serial diagnosis, so no single film makes it, and the comparison only works where the projection is the same.

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.

Total Knee Replacement

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For Medical Students

What It Shows

Post-operative imaging of a total knee replacement shows the components, their position, their interfaces with bone and the surrounding soft tissue. 5.16.6 covers arthroplasty planning.

Radiographs in consistent projections are the test, with computed tomography where rotation or bone is the question. 5.3.1 covers the AP view.

It matters because the common questions are loosening, infection and malposition, and only the first is primarily radiographic. 5.14.7 covers septic arthritis.

How It Is Acquired

Obtain the same projections at every visit, a lateral and a skyline view included. 5.3.3 covers the skyline view.

Assess component position against the first post-operative film. 5.3.2 covers the lateral view.

Assess the interfaces for a progressive lucent line. 5.3.19 covers subchondral sclerosis.

Assess the patella and the extensor mechanism. 5.3.12 covers patellar fractures.

Use computed tomography for component rotation or bone loss. 5.16.8 covers the 3D reconstructions.

What Normal Looks Like

Components in the position the first post-operative film recorded. 5.3.2 covers the lateral view.

Interfaces with no progressive lucency across serial films. 5.3.19 covers subchondral sclerosis.

An intact extensor mechanism with the patella tracking. 5.3.3 covers the skyline view.

No periprosthetic fracture and no osteolysis. 5.5.6 covers the bones.

What Abnormal Looks Like

A progressive lucent line at a component interface. 5.3.19 covers subchondral sclerosis.

Component migration, or a change in position from the baseline film. 5.3.2 covers the lateral view.

Osteolysis, which is a focal loss of bone rather than a line. 5.5.6 covers the bones.

A periprosthetic fracture. 5.16.1 covers the fracture assessment.

Patellar fracture, maltracking or extensor disruption. 5.3.12 covers patellar fractures.

Key Learning Points

  • The first post-operative film is the baseline.
  • Loosening is progressive lucency across films.
  • Osteolysis is focal loss rather than a line.
  • The projection has to be the same every time.
  • The extensor mechanism is part of the assessment.

MRCS Level

How It Is Acquired

In a viva the expected answer compares with a baseline.

The expected points are the consistent projections, the baseline comparison, the interfaces, the extensor mechanism, and computed tomography for rotation.

Say that no single film diagnoses loosening. 5.3.19 covers subchondral sclerosis.

Say that infection cannot be excluded by imaging and is settled by aspiration. 5.4.17 covers the aspiration.

Say that rotation needs computed tomography and a stated reference. 5.16.6 covers arthroplasty planning.

What Abnormal Looks Like

The reasoning expected is that a lucent line at an interface is common and often stable, so the diagnostic information lies in the change rather than the appearance, which makes the baseline film the most valuable image in the series. 5.3.19 covers subchondral sclerosis.

Component rotation is a planning reference problem rather than a surveillance one, and the references disagree with each other. 5.16.6 covers arthroplasty planning.

Osteolysis reflects a particle response and behaves unlike the lucency of loosening. 5.5.6 covers the bones.

Pain with components in position and no interface change raises infection, the extensor mechanism and causes outside the knee. 5.14.7 covers septic arthritis.

How Good It Is

Serial radiography is the best available surveillance for a replaced knee and reports loosening only over time.

Its best contribution is component position, interface change and the extensor mechanism. 5.3.2 covers the lateral view.

It is limited by projection variation, by artefact in cross sectional imaging, and by being no test for infection. 5.14.7 covers septic arthritis.

No figures appear for accuracy on this page, under decision 028, and no lucency widths, rotation angles or survival figures appear either: those are published quantities from particular registries and this environment cannot check them.

Key Learning Points

  • The diagnostic information lies in the change.
  • The baseline film is the most valuable image.
  • Rotation references disagree with each other.
  • Osteolysis behaves unlike loosening.
  • Imaging is no test for infection.

FRCS Level

How Good It Is

At decision level the imaging decides whether a revision is for loosening, for malposition or for bone loss. 5.17.9 covers the revision arthroplasty.

Progressive lucency with migration is loosening and is a revision finding. 5.3.19 covers subchondral sclerosis.

Malrotation with a well fixed implant is a different revision and needs computed tomography with a named reference. 5.16.6 covers arthroplasty planning.

Osteolysis changes what the revision has to reconstruct. 5.5.6 covers the bones.

The examination and the pain pattern narrow which of those it is. 4.20.8 covers the total knee replacement examination.

Infection is excluded by aspiration before any revision is planned. 5.4.17 covers the aspiration.

Where It Misleads

The commonest error is diagnosing loosening on one film. 5.3.19 covers subchondral sclerosis.

A different projection between visits is read as migration. 5.1.6 covers imaging pitfalls.

Artefact at the interface is read as a lucent line. 5.2.19 covers the artefacts.

A well fixed implant is read as excluding infection. 5.14.7 covers septic arthritis.

Patellar maltracking is missed because no skyline view was taken. 5.3.3 covers the skyline view.

Key Learning Points

  • One film cannot diagnose loosening.
  • A projection change mimics migration.
  • Artefact mimics a lucent line.
  • Good fixation does not exclude infection.
  • No skyline view means the patella was not assessed.

Fellowship Level

Where It Misleads

At fellowship level the knee is painful, the films are inconsistent and the baseline is missing. 5.20.1 covers structured reporting.

Without a baseline the series cannot report loosening, so obtaining the first post-operative film matters more than any new study. 5.3.2 covers the lateral view.

Rotation measured on a replaced knee uses references the implant has altered, so the measurement describes the operation. 5.16.6 covers arthroplasty planning.

Metabolic imaging reports activity and cannot separate infection from aseptic loosening, which is the distinction wanted. 5.18.7 covers the PET-MRI.

Serial imaging after revision starts a new baseline, and recording that is part of the operation. 5.17.9 covers the revision arthroplasty.

The useful report gives the projections, the comparison film used, the component positions, the interface changes, the bone, the extensor mechanism and the artefact's effect. 5.20.1 covers structured reporting.

Key Learning Points

  • Without a baseline the series cannot report loosening.
  • Obtaining the first film beats any new study.
  • Rotation on a replaced knee describes the operation.
  • Metabolic imaging cannot separate the two causes.
  • A revision starts a new baseline.

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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