Partial Knee Replacement

For Medical Students

What It Shows

Post-operative imaging of a partial knee replacement shows the implant, its interfaces with bone, and the compartments that were left alone. 5.16.6 covers arthroplasty planning.

Radiographs in a consistent projection are the test. 5.3.1 covers the AP view.

It matters because progression in an unreplaced compartment is a common reason for revision, so the untouched compartments are the surveillance target. 5.3.17 covers joint space narrowing.

How It Is Acquired

Obtain radiographs in the same projections every time, a skyline view included. 5.3.3 covers the skyline view.

Assess the implant position and the bone interfaces. 5.5.6 covers the bones.

Assess the unreplaced compartments for progression. 5.3.17 covers joint space narrowing.

Assess the alignment on a standing film where the question is mechanical. 5.3.6 covers the long leg alignment films.

Use computed tomography where artefact defeats the question of fixation. 5.16.8 covers the 3D reconstructions.

What Normal Looks Like

An implant in its intended position with no lucent line at the interface. 5.5.6 covers the bones.

Unreplaced compartments with preserved joint space. 5.3.17 covers joint space narrowing.

Alignment consistent with the correction the implant provides. 5.3.7 covers the mechanical axis.

A patellofemoral joint unchanged from the pre-operative films. 5.12.7 covers the patellofemoral arthritis.

What Abnormal Looks Like

A progressive lucent line at a bone interface across serial films. 5.3.19 covers subchondral sclerosis.

Implant migration or subsidence into bone. 5.5.6 covers the bones.

Progression in an unreplaced compartment. 5.3.17 covers joint space narrowing.

A periprosthetic fracture, which the radiograph reports. 5.16.1 covers the fracture assessment.

Patellofemoral progression where that compartment was left. 5.3.3 covers the skyline view.

MRCS Level

How It Is Acquired

In a viva the expected answer looks at the compartments that were not replaced.

The expected points are the consistent projections, the implant interfaces, the unreplaced compartments, the alignment, and computed tomography for fixation.

Say that progression elsewhere is a common reason for revision. 5.3.17 covers joint space narrowing.

Say that loosening is a serial diagnosis rather than a single film one. 5.3.19 covers subchondral sclerosis.

Say that the examination carries the function and the pain pattern. 4.20.7 covers the partial knee replacement examination.

What Abnormal Looks Like

The reasoning expected is that a partial replacement leaves native compartments carrying load, so the operation's success depends on tissue it did not touch, which is why surveillance looks outward rather than at the implant. 5.3.17 covers joint space narrowing.

Alignment decides how that remaining load is shared, so a knee corrected too far loads the compartment it was meant to spare. 5.3.7 covers the mechanical axis.

A lucent line is normal at some interfaces, and progressive lucency is the finding, which is why one film cannot diagnose loosening. 5.3.19 covers subchondral sclerosis.

Pain with a well positioned implant and preserved compartments raises causes outside the joint. 5.1.2 covers clinical examination versus imaging.

How Good It Is

Serial radiography is the best available surveillance for a partial replacement and is only as good as the consistency of its projections.

Its best contribution is the unreplaced compartments, the interfaces over time and the alignment. 5.3.6 covers the long leg alignment films.

It is limited by projection variation, by artefact where cross sectional imaging is needed, and by an inability to assess infection. 5.14.7 covers septic arthritis.

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

FRCS Level

How Good It Is

At decision level the imaging decides between revising to a total replacement and doing nothing. 5.17.8 covers the total knee replacement.

Progression in another compartment with a sound implant is a conversion rather than a revision of the implant. 5.3.17 covers joint space narrowing.

Loosening or subsidence is an implant problem and needs the bone assessed. 5.17.9 covers the revision arthroplasty.

Alignment explains a pattern of progression and belongs in the plan. 5.3.7 covers the mechanical axis.

Unexplained pain is aspirated before it is revised. 5.4.17 covers the aspiration.

Where It Misleads

The commonest error is reading a single film's lucency as loosening. 5.3.19 covers subchondral sclerosis.

Projection differences between visits are read as migration. 5.1.6 covers imaging pitfalls.

Progression in another compartment is reported as implant failure. 5.3.17 covers joint space narrowing.

The patellofemoral joint is omitted because no skyline view was taken. 5.3.3 covers the skyline view.

A normal looking implant is read as excluding infection. 5.14.7 covers septic arthritis.

Fellowship Level

Where It Misleads

At fellowship level the knee arrives painful, with films from several departments in several projections. 5.3.1 covers the AP view.

A serial comparison needs one projection, so the first useful act is often to repeat the series properly. 5.20.1 covers structured reporting.

Conversion to a total replacement is planned from these films and from the bone they show. 5.16.6 covers arthroplasty planning.

Infection in a partial replacement presents the same way as loosening and is settled by aspiration. 5.14.7 covers septic arthritis.

Metabolic imaging has been used for the loosening question and cannot separate it from infection. 5.18.7 covers the PET-MRI.

The useful report gives the projections used, the implant position, the interfaces with their change over time, the unreplaced compartments, and the alignment. 5.20.1 covers structured reporting.