Psoriatic Arthritis

For Medical Students

What It Shows

Psoriatic arthritis is an inflammatory arthropathy with asymmetrical joint involvement, inflammation at tendon and ligament attachments, and bone formation alongside erosion. 5.5.11 covers the synovium.

In the knee it shows synovitis, enthesitis at the patellar and quadriceps attachments, and new bone at those sites. 5.5.10 covers the tendons.

It matters because the productive bone response is the feature that distinguishes it, and it is the feature most often overlooked. 5.3.18 covers osteophytes.

How It Is Acquired

Use fluid sensitive sequences through the tendon attachments as well as the joint. 5.2.17 covers the STIR imaging.

Assess the quadriceps and patellar tendon insertions specifically. 5.5.10 covers the tendons.

Assess for marrow change at the attachments rather than only in the subchondral bone. 5.13.1 covers bone marrow oedema.

Obtain radiographs for the new bone, which they show better than a scan. 5.3.2 covers the lateral view.

Compare the two sides, since involvement is asymmetrical. 5.20.1 covers structured reporting.

What Normal Looks Like

Tendon attachments with no marrow or soft tissue oedema. 5.5.10 covers the tendons.

No new bone at the insertions. 5.3.2 covers the lateral view.

Symmetry between the knees. 5.20.1 covers structured reporting.

A thin synovial lining with physiological fluid. 5.5.11 covers the synovium.

What Abnormal Looks Like

Oedema at a tendon or ligament attachment, with marrow change in the bone behind it. 5.13.1 covers bone marrow oedema.

New bone at an insertion, which is the productive response. 5.3.2 covers the lateral view.

Synovitis that is asymmetrical between the knees. 5.14.1 covers synovitis.

Erosion with adjacent new bone, which is the combination that names the disease. 5.5.6 covers the bones.

Soft tissue swelling beyond the joint capsule. 4.3.8 covers swelling.

MRCS Level

How It Is Acquired

In a viva the expected answer looks at the entheses.

The expected points are the attachments, the marrow behind them, the new bone, the asymmetry, and the radiographs.

Say that enthesitis rather than synovitis is the characteristic lesion. 5.5.10 covers the tendons.

Say that erosion with new bone separates this from rheumatoid. 5.14.2 covers rheumatoid arthritis.

Say that the skin and the nails carry more diagnostic weight than the knee. 5.1.2 covers clinical examination versus imaging.

What Abnormal Looks Like

The reasoning expected is that the disease acts where fibrous tissue attaches to bone rather than through the synovium, which is why the pattern is asymmetrical and why bone responds by forming as well as resorbing. 5.5.10 covers the tendons.

That also explains why a knee can show attachment change with a relatively quiet joint. 5.14.1 covers synovitis.

The same mechanism produces appearances at the patellar tendon that mimic overuse tendinopathy in an athlete. 5.4.6 covers the patellar tendon.

Treatment changes the inflammation and not the new bone, so the structural record persists. 5.1.7 covers evidence-based imaging.

How Good It Is

Magnetic resonance imaging is the best available test of enthesitis and the radiograph the best record of the new bone, and together they describe a pattern rather than make a diagnosis.

Its best contribution is the attachment sites with their marrow change. 5.13.1 covers bone marrow oedema.

It is limited because enthesitis also follows overuse, because the pattern overlaps the other spondyloarthropathies, and because the knee is rarely where the diagnosis is made. 5.1.6 covers imaging pitfalls.

No figures appear for accuracy on this page, under decision 028, and no enthesitis scores or erosion counts appear either: those are published quantities from particular scoring systems and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the imaging decides whether a knee problem belongs to the arthropathy or to something mechanical alongside it. 5.1.3 covers choosing the correct investigation.

Enthesitis at the patellar tendon in a psoriatic patient is a medical problem rather than a surgical one. 5.4.6 covers the patellar tendon.

Asymmetrical joint space loss keeps a partial replacement on the table where rheumatoid would not. 5.3.17 covers joint space narrowing.

New bone at the insertions changes the surgical approach and the closure. 5.3.2 covers the lateral view.

Skin involvement over an incision is a planning issue the imaging does not report. 5.1.2 covers clinical examination versus imaging.

Where It Misleads

The commonest error is reading enthesitis as tendinopathy in a patient whose diagnosis is not yet known. 5.5.10 covers the tendons.

New bone is read as a degenerative osteophyte. 5.3.18 covers osteophytes.

Asymmetry is read as a mechanical cause. 5.20.6 covers reporting osteoarthritis.

Marrow change at an attachment is read as a stress reaction. 5.13.2 covers stress reactions.

A quiet joint is read as excluding the arthropathy. 5.14.1 covers synovitis.

Fellowship Level

Where It Misleads

At fellowship level the patient often arrives as an athlete with tendon pain and no diagnosis. 5.4.6 covers the patellar tendon.

Enthesitis at several attachments, or bilateral attachment change, is the pattern that should prompt a rheumatological referral rather than a tendon procedure. 5.1.3 covers choosing the correct investigation.

An injection or a tendon procedure into an inflammatory enthesis is the harm to avoid. 5.4.20 covers the tendon procedures.

Treated disease leaves the new bone and removes the oedema, so an old scan and a new one differ in ways that are not progression. 5.1.7 covers evidence-based imaging.

Imaging cannot distinguish the spondyloarthropathies from each other, and reporting as though it can is the overreach here. 5.20.8 covers common reporting errors.

The useful report names the attachments involved, the marrow change, the new bone, the asymmetry, and that the pattern is suggestive rather than diagnostic. 5.20.1 covers structured reporting.