Post-operative Protocol

For Medical Students

What It Shows

A post-operative protocol accommodates metal, which distorts the local magnetic field and defeats the routine sequences. 5.2.19 covers artefacts.

It substitutes robust fat suppression for the frequency based kind and favours spin echo over gradient echo sequences. 5.2.17 covers short tau inversion recovery imaging.

It matters because the routine protocol fails exactly around the implant, which is where the question usually is. 5.17.8 covers total knee replacement imaging.

How It Is Acquired

Establish what hardware is present before the study is planned, which requires it to be on the request. 5.20.1 covers structured reporting.

Use inversion recovery fat suppression, which does not depend on field uniformity. 5.2.17 covers short tau inversion recovery imaging.

Prefer spin echo sequences, since gradient echo sequences are more sensitive to field distortion. 5.2.18 covers gradient echo imaging.

Apply metal artefact reduction techniques where the scanner offers them. 5.2.19 covers artefacts.

Expect residual artefact and read around it rather than through it. 5.7.12 covers tunnel assessment.

What Normal Looks Like

Suppressed fat around the implant, which is the protocol working. 5.2.16 covers fat suppression.

A graft or a repair with the expected post-operative signal for its age. 5.7.10 covers graft maturation.

Local signal loss immediately at a fixation device, which is hardware rather than a lesion. 5.2.19 covers artefacts.

Tunnels and fixation in their expected positions. 5.7.12 covers tunnel assessment.

What Abnormal Looks Like

Marrow signal around an implant, which raises infection or loosening. 5.14.7 covers septic arthritis.

A periprosthetic soft tissue collection. 5.17.8 covers total knee replacement imaging.

A discontinuous graft or a widened tunnel. 5.7.11 covers graft failure.

Failed fat suppression producing patchy brightness, which mimics all of the above. 5.2.16 covers fat suppression.

MRCS Level

How It Is Acquired

In a viva this is the protocol to name when asked how to image a knee with an implant.

The expected points are the hardware information, the inversion recovery suppression, the spin echo preference, the artefact reduction, and the residual artefact.

Say that the request must name the implant. 5.20.1 covers structured reporting.

Say that a routine study around metal has failed suppression. 5.2.16 covers fat suppression.

Say that the clinical question in a painful replacement is usually infection against aseptic change. 5.14.7 covers septic arthritis.

What Abnormal Looks Like

The reasoning expected is that frequency based fat suppression assumes a uniform field, and metal makes the field non-uniform, so the suppression fails locally and leaves bright fat that imitates oedema. 5.2.16 covers fat suppression.

Inversion recovery uses a relaxation property rather than a frequency, so it survives the distortion at the cost of signal and time. 5.2.17 covers short tau inversion recovery imaging.

Gradient echo sequences lack the refocusing pulse that compensates for inhomogeneity, so they degrade fastest near metal. 5.2.18 covers gradient echo imaging.

The diagnostic question around an implant depends on marrow and soft tissue signal, so the sequence choice is the diagnosis rather than a technicality. 5.14.7 covers septic arthritis.

How Good It Is

The post-operative protocol makes a metal bearing knee assessable and is often not used.

Its best contribution is reliable marrow assessment around an implant, which the routine protocol cannot give. 5.17.8 covers total knee replacement imaging.

It is limited by residual artefact, by longer scan times and by uneven availability of artefact reduction techniques. 5.2.19 covers artefacts.

No figures appear for accuracy on this page, under decision 028, and no inversion times or bandwidth values appear either: those are scanner specific protocol quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level this protocol decides whether a painful post-operative knee can be assessed at all.

A painful arthroplasty with a question of infection needs reliable marrow and soft tissue signal, which only this protocol gives. 5.14.7 covers septic arthritis.

A cruciate reconstruction assessment needs the tunnels and the graft, both next to fixation artefact. 5.7.12 covers tunnel assessment.

A meniscal repair assessment needs the detail sequences plus whatever artefact reduction is available. 5.6.13 covers meniscal repair imaging.

A study acquired on the routine protocol in a metal bearing knee should be repeated rather than reported, which is a decision worth making early. 5.2.16 covers fat suppression.

Where It Misleads

The commonest error is accepting a routine study in a metal bearing knee. 5.2.16 covers fat suppression.

Failed suppression is read as infection or loosening. 5.14.7 covers septic arthritis.

Metal signal loss is read as bone loss before a revision. 5.7.14 covers revision cruciate imaging.

The request does not name the implant, so the protocol cannot be chosen. 5.20.1 covers structured reporting.

Computed tomography is not considered where it would answer the bony question better. 5.16.8 covers three dimensional reconstructions.

Fellowship Level

Where It Misleads

At fellowship level this is the protocol whose correct use depends most on information the imaging department does not have.

The protocol cannot be selected without knowing the implant, and the request rarely names it, which makes a clinical communication failure into an imaging failure. 5.20.1 covers structured reporting.

Metal artefact reduction has advanced substantially and unevenly, so the same request answers differently in two hospitals and the report does not say which techniques were available. 5.2.19 covers artefacts.

Those techniques also alter the bone immediately around the metal, so the measurements made on them, tunnel width among them, are made on modified data. 5.7.12 covers tunnel assessment.

Infection against aseptic loosening remains the hardest question, and imaging contributes without settling it, so the aspirate and the bloods carry the decision. 5.14.7 covers septic arthritis.

The cheapest improvement available in this chapter is naming the implant on the request, and it is the one least often done. 5.1.3 covers choosing the correct investigation.