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KneeSchool/Imaging Academy/Arthroplasty Planning

Arthroplasty Planning

Page 5.16.6 · CT of the Knee · imaging page

Geometry a scan gives precisely, and the soft tissue balance that decides the operation and appears nowhere on it.

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.

Arthroplasty Planning

Choose a depth

For Medical Students

What It Shows

Arthroplasty planning establishes implant size, bone resection and the reference axes the components will be set to. 5.3.7 covers the mechanical axis.

It is built from radiographs, with computed tomography added for rotation, bone loss and the landmarks a projection cannot separate. 5.2.22 covers the CT acquisition.

It matters because implant position is set from these references, and the references disagree with each other. 2.1.8 covers the imaging anatomy.

How It Is Acquired

Start from a standing long leg film for the alignment. 5.3.6 covers the long leg alignment films.

Add a lateral and a skyline view for the patellofemoral joint and the posterior condyles. 5.3.3 covers the skyline view.

Add computed tomography where rotation, a bone defect or a previous implant is part of the question. 5.17.9 covers the revision arthroplasty.

Identify the rotational reference landmarks and record which were used. 2.1.4 covers the lateral femoral condyle.

Record the bone stock on both sides of the joint. 5.3.19 covers subchondral sclerosis.

What Normal Looks Like

A standing film containing the hip and the ankle. 5.3.6 covers the long leg alignment films.

The posterior condyles and the epicondylar landmarks identifiable. 2.1.4 covers the lateral femoral condyle.

Bone stock adequate on both sides with no contained defect. 5.3.19 covers subchondral sclerosis.

The patellofemoral joint assessed on a view of its own. 5.12.7 covers the patellofemoral arthritis.

The rotational reference stated rather than assumed. 5.20.1 covers structured reporting.

What Abnormal Looks Like

Bone loss that changes the resection level and the implant choice. 5.17.9 covers the revision arthroplasty.

Landmarks obscured by deformity or by a previous implant. 5.2.19 covers the artefacts.

Rotational references that disagree with one another in the same knee. 2.1.4 covers the lateral femoral condyle.

A deformity whose apex sits outside the joint, which the implant cannot correct. 5.3.10 covers the deformity analysis.

Patellofemoral wear that changes the patellar part of the operation. 5.12.7 covers the patellofemoral arthritis.

Key Learning Points

  • Alignment comes from the standing film.
  • Computed tomography adds rotation, bone loss and landmarks.
  • The rotational references can disagree within one knee.
  • Bone stock decides the resection and the implant.
  • The patellofemoral joint needs a view of its own.

MRCS Level

How It Is Acquired

In a viva the expected answer separates geometry from balance.

The expected points are the standing film, the lateral and skyline views, the rotational landmarks, the bone stock, and the reason for a scan.

Say that the scan is not routine and is requested for a stated question. 5.1.3 covers choosing the correct investigation.

Say that soft tissue balance is intraoperative rather than radiological. 5.5.9 covers the ligaments.

Say that whether a deformity is correctable is a clinical assessment. 5.3.10 covers the deformity analysis.

What Abnormal Looks Like

The reasoning expected is that the components are positioned relative to bony references, so a reference that cannot be identified reliably becomes an error in implant position. 2.1.8 covers the imaging anatomy.

The rotational references were each defined on different anatomy, which is why they disagree and why the one used has to be named. 2.1.4 covers the lateral femoral condyle.

Bone loss decides whether the reconstruction resurfaces or rebuilds, which changes the implant entirely. 5.17.9 covers the revision arthroplasty.

A flexion contracture and the ligament state decide the balance, and neither appears on a static study. 5.5.9 covers the ligaments.

How Good It Is

Imaging gives arthroplasty geometry well and gives nothing about the balance the operation is judged by.

Its best contribution is the alignment, the bone stock and the rotational landmarks. 5.3.6 covers the long leg alignment films.

It is limited by landmark variability, by the disagreement between references, and by the soft tissues deciding the result. 2.1.4 covers the lateral femoral condyle.

No figures appear for accuracy on this page, under decision 028, and no resection values, implant sizes or rotational angles appear either: those are published quantities tied to particular systems and this environment cannot check them.

Key Learning Points

  • An unreliable reference becomes an implant position error.
  • The references were defined on different anatomy.
  • Bone loss decides resurfacing against rebuilding.
  • Balance is intraoperative rather than radiological.
  • Landmark variability is the main limitation.

FRCS Level

How Good It Is

At decision level the imaging decides the implant, the constraint and whether the operation is a primary one at all. 5.17.8 covers the total knee replacement.

A preserved compartment with the others worn raises a partial replacement. 5.17.7 covers the partial knee replacement.

Bone loss and a previous implant move the question to revision. 5.17.9 covers the revision arthroplasty.

Extra-articular deformity decides whether a correction precedes the replacement. 5.3.10 covers the deformity analysis.

The patellofemoral joint decides the patellar part of the operation and is the compartment most often under-imaged. 5.3.3 covers the skyline view.

Where It Misleads

The commonest error is planning rotation from a reference the scan could not identify clearly. 2.1.4 covers the lateral femoral condyle.

Supine scanning loses the deformity the standing film showed. 5.3.6 covers the long leg alignment films.

A previous implant produces artefact across the landmarks being measured. 5.2.19 covers the artefacts.

An implant size templated from a projection is magnified by the geometry. 5.2.5 covers magnification.

A corrected alignment on paper is read as a correctable deformity in the knee. 5.5.9 covers the ligaments.

Key Learning Points

  • A poorly identified reference makes rotation unreliable.
  • Supine scanning loses the standing deformity.
  • A previous implant obscures the landmarks.
  • Projection magnification affects templating.
  • Correctability is clinical rather than radiographic.

Fellowship Level

Where It Misleads

At fellowship level the knee usually carries an implant already, and the planning question is what can come out and what has to be rebuilt. 5.17.9 covers the revision arthroplasty.

Artefact is worst at the bone implant interface, which is the interface the decision rests on. 5.2.19 covers the artefacts.

Bone loss classifications were defined on radiographs, and a scan reclassifies them. 5.3.19 covers subchondral sclerosis.

Serial imaging across departments to different protocols compares protocols as much as it compares loosening. 5.20.7 covers reporting the post-operative knee.

Automated segmentation for planning fixes one set of landmark rules and carries its error into every case alike. 5.18.8 covers artificial intelligence in imaging.

The useful report gives the standing alignment, the bone stock with any defect, the rotational reference used, the patellofemoral joint, and what the artefact prevented. 5.20.1 covers structured reporting.

Key Learning Points

  • Artefact is worst at the interface the decision rests on.
  • A scan reclassifies radiographic bone loss grades.
  • Serial imaging across departments compares protocols.
  • Automated segmentation carries one error into every case.
  • The report should name the rotational reference used.

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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