Septic Arthritis

For Medical Students

What It Shows

Septic arthritis is infection within the joint. 5.5.11 covers the synovium.

Imaging shows an effusion, synovial thickening and sometimes bone change, and every one of those appears in the uninfected knees of this chapter. 5.14.1 covers synovitis.

It matters because the diagnosis is made on an aspirate, so any imaging that delays the aspirate causes harm. 5.4.17 covers the aspiration.

How It Is Acquired

Aspirate the joint. 5.4.17 covers the aspiration.

Obtain radiographs to establish a baseline and to look for bone change. 5.3.1 covers the AP view.

Use ultrasound to confirm an effusion and to guide the aspiration where the joint is difficult. 5.4.17 covers the aspiration.

Use magnetic resonance imaging where the question is bone involvement, a collection outside the joint, or an alternative diagnosis. 5.5.6 covers the bones.

Do not wait for a scan before aspirating. 5.1.3 covers choosing the correct investigation.

What Normal Looks Like

A small physiological effusion with a thin lining. 5.5.11 covers the synovium.

No periarticular bone change. 5.5.6 covers the bones.

No collection outside the capsule. 5.4.4 covers the posterior knee.

A normal radiograph, which is usual early and excludes nothing. 5.3.1 covers the AP view.

What Abnormal Looks Like

An effusion with marked synovial thickening and surrounding soft tissue oedema. 5.14.1 covers synovitis.

Marrow change on both sides of the joint, which raises bone involvement. 5.13.1 covers bone marrow oedema.

Erosion or cortical loss, which is late. 5.5.6 covers the bones.

A collection outside the capsule or inside a bursa. 5.5.12 covers the bursa.

Gas within the joint, which is specific and rare. 5.2.22 covers the CT acquisition.

MRCS Level

How It Is Acquired

In a viva the expected answer aspirates first and says why.

The expected points are the aspirate, the baseline radiograph, ultrasound for guidance, magnetic resonance imaging for bone and collections, and that nothing waits for a scan.

Say that no imaging finding is diagnostic. 5.1.6 covers imaging pitfalls.

Say that an acute crystal attack cannot be told apart on imaging. 5.14.4 covers crystal arthropathy.

Say that a replaced or operated knee raises the suspicion rather than lowering it. 5.17.9 covers the revision arthroplasty.

What Abnormal Looks Like

The reasoning expected is that infection produces the synovium's standard response, so it looks like everything else in this chapter, and the only specific findings are late ones or rare ones. 5.14.1 covers synovitis.

Gas and frank bone destruction are specific and arrive too late to help the decision. 5.5.6 covers the bones.

Imaging's real contribution is finding what the aspirate cannot reach: a collection outside the joint, bone infection, or a different diagnosis altogether. 5.5.12 covers the bursa.

In a child the same presentation sits alongside transient synovitis and bone infection, so the bone has to be imaged. 5.15.1 covers the normal paediatric anatomy.

How Good It Is

Imaging is the best available way to find a collection and to assess the bone, and it is no test at all for the presence of infection.

Its best contribution is the bone, the collection outside the joint, and the alternative diagnosis. 5.5.6 covers the bones.

It is limited absolutely: a normal scan does not exclude infection and an abnormal one does not establish it. 5.1.6 covers imaging pitfalls.

No figures appear for accuracy on this page, under decision 028, and no cell counts or enhancement thresholds appear either: those are laboratory and published quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the imaging never decides whether to washout, and it often decides how. 5.1.3 covers choosing the correct investigation.

Bone involvement changes the operation from a joint washout to a bone procedure. 5.5.6 covers the bones.

A collection outside the capsule needs its own drainage. 5.4.4 covers the posterior knee.

An operated or replaced knee needs the implant and its interfaces assessed, which is where artefact is worst. 5.17.9 covers the revision arthroplasty.

Serial imaging has a role after treatment and none before it. 5.20.7 covers reporting the post-operative knee.

Where It Misleads

The commonest and most dangerous error is using a scan to decide against aspirating. 5.4.17 covers the aspiration.

A crystal attack is treated as infection, or infection as an attack. 5.14.4 covers crystal arthropathy.

An inflammatory flare in a known arthropathy conceals a superimposed infection. 5.14.2 covers rheumatoid arthritis.

Marrow change is read as bone infection when it is reactive. 5.13.1 covers bone marrow oedema.

A normal radiograph is reported in a way that reassures. 5.3.1 covers the AP view.

Fellowship Level

Where It Misleads

At fellowship level the knee has usually been operated on, aspirated and partly treated before it arrives. 5.20.7 covers reporting the post-operative knee.

Partial treatment makes both the aspirate and the imaging harder to interpret, and the imaging does not recover what the antibiotics obscured. 5.1.6 covers imaging pitfalls.

Around an implant the artefact sits exactly at the interface the question concerns. 5.17.9 covers the revision arthroplasty.

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

Imaging after treatment lags the clinical course, so a persisting appearance is not persisting infection. 5.13.1 covers bone marrow oedema.

The honest position is that this page's whole content is a boundary: imaging supports the management of septic arthritis and plays no part in diagnosing it. 5.4.17 covers the aspiration.