Swelling

For Medical Students

What It Tests

Looking at a swollen knee establishes three things: whether the swelling is inside the joint or outside it, how large it is, and how the knee is being held because of it.

Swelling inside the joint is an effusion, and the chapter on effusion assessment covers the tests that confirm and grade it. 4.8.3 covers the first of them.

This page covers the inspection. An effusion has a shape, and the shape is visible before any manoeuvre. 4.6.3 covers the palpation.

How It Is Performed

Expose both knees and compare them from the front with the patient supine.

Look for loss of the normal hollows on either side of the patella, which is the first visible sign of fluid inside the joint.

Look at the suprapatellar region, because a large effusion fills the pouch above the patella and gives the thigh a horseshoe outline.

Look at the position the knee is being held in, since an effused knee rests in slight flexion. 4.3.5 covers the supine assessment.

Distinguish localised swelling from generalised. A lump over the tibial tubercle, over the patella, or in the popliteal fossa is not an effusion. 4.6.10 covers the popliteal fossa.

Note the timing from the history, because an immediate swelling and one appearing overnight point at different structures. 4.2.3 covers swelling as a symptom.

What a Positive Means

Loss of the parapatellar hollows with a knee held in slight flexion indicates an effusion.

A tense, large effusion filling the suprapatellar pouch in an acutely injured knee suggests a haemarthrosis, which narrows the diagnosis sharply. 4.8.7 covers it.

Swelling localised in front of the patella is prepatellar rather than intra-articular, and the knee moves freely with it.

Swelling in the popliteal fossa is a cyst or a mass and is read standing with the knee extended. 4.6.10 covers that examination.

A swollen limb rather than a swollen knee points away from the joint, and the calf, the ankle and the foot are compared before anything else. 4.3.6 covers skin changes.

MRCS Level

How It Is Performed

In a viva the swelling is the finding most candidates reach for and the one most often described without being localised, and localising it is the mark.

The expected points are the comparison, the parapatellar hollows, the suprapatellar pouch, the resting position, and the distinction between intra-articular and extra-articular.

Say that you would confirm the effusion by palpation rather than resting on the appearance, and name the tests. 4.8.5 covers the sweep test.

Say that you would ask when it appeared, because the timing is the single most useful piece of information about a swollen knee. 4.2.3 covers the history.

Say that you would look at the other joints if the swelling were recurrent or bilateral, since that moves the diagnosis towards an arthropathy. 4.8.8 covers chronic effusions.

What a Positive Means

The reasoning expected is that the knee's capsule has a large suprapatellar extension, so intra-articular fluid fills a predictable space and produces a predictable outline. 4.8.1 covers normal fluid physiology.

The parapatellar hollows disappear first because they are the shallowest part of that space, which is why a small effusion is visible there before anywhere else.

An effused knee rests in slight flexion because that is the position of greatest capsular volume and therefore of least pressure, and the same mechanism inhibits the quadriceps. 4.8.2 covers the clinical significance.

Timing separates the causes. Immediate tense swelling after an injury is blood, and blood in a knee comes from a short list of structures. 4.8.7 covers it.

How Good It Is

Inspection detects a moderate or large effusion reliably and a small one poorly, which is why the palpation tests exist. 4.8.3 covers the most sensitive of them.

It is good at distinguishing intra-articular from extra-articular swelling, which is its most useful contribution and the one most often skipped.

It is unreliable in an obese patient, where the contours are hidden and the palpation tests carry the assessment. 4.23.4 covers those patients.

It cannot distinguish blood from synovial fluid, which the history and, where it is needed, aspiration do. 4.8.7 covers the acute haemarthrosis.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the appearance of the swelling contributes urgency rather than diagnosis, and urgency is sometimes the more important contribution.

A tense effusion in an acutely injured knee is a haemarthrosis, and that narrows the diagnosis to a short list and raises the question of whether the knee needs imaging now. 4.8.7 covers it.

A hot, red, swollen knee is a different pathway entirely and the inspection is where it is recognised. 4.3.6 covers skin changes.

A recurrent effusion in a knee with no mechanical findings moves the assessment towards an inflammatory or crystal cause, and operating on it achieves nothing. 4.8.10 covers crystal arthropathy.

An effusion also affects every other part of the examination, by inhibiting the quadriceps and limiting the range, so its presence changes how the rest of the findings are interpreted. 4.7.1 covers active motion.

Where It Misleads

The commonest error is calling any swollen knee effused without localising the swelling.

The second is relying on the appearance for a small effusion, which inspection misses. 4.8.3 covers the test that does not.

The third is reading a limited range in an effused knee as a contracture, when it is the effusion. 4.7.7 covers contractures.

The fourth is reading quadriceps failure in an effused knee as weakness, when it is inhibition. 4.8.2 covers it.

The fifth is attributing a swollen limb to the knee.

Fellowship Level

Where It Misleads

In referred practice a persistent effusion is the finding that most often means the referral's diagnosis is wrong.

A knee with a recurrent effusion, normal ligaments and no mechanical symptoms is an inflammatory, crystal or synovial problem, and repeated arthroscopy for it is a recognised pattern of futile treatment. 4.8.9 covers synovitis.

An effusion after an arthroplasty or a reconstruction raises low grade infection, and it has to be excluded rather than attributed to activity. 4.3.6 covers the skin.

A chronically effused knee has a quadriceps that cannot be rehabilitated while the effusion persists, which is why addressing the effusion precedes rehabilitation rather than accompanying it. 4.8.2 covers the mechanism.

An effusion that appears only after specific activity is a mechanical finding and is worth timing in the history rather than chasing in the clinic, since the knee may be dry when examined. 4.2.3 covers the history.

In medicolegal work the timing of the first swelling after an injury is often the most useful single fact in the record, and the examination finding months later contributes little to it. 4.23.7 covers that setting.