Effusion Assessment

For Medical Students

What It Tests

Palpating for an effusion establishes whether there is fluid inside the joint and roughly how much, and it does so more sensitively than looking. 4.3.8 covers the inspection.

The effusion chapter covers the individual tests and their order of sensitivity. This page covers where the assessment sits in the palpation sequence and why it comes early. 4.8.3 covers the most sensitive test.

It comes early because it needs a relaxed knee, and because its result changes the interpretation of every finding that follows. 4.8.2 covers the clinical significance.

How It Is Performed

With the patient supine and the knee extended, place one hand above the patella and milk the suprapatellar pouch downwards.

Feel for fluid shifting into the parapatellar hollows with the other hand, which is the basis of the sweep test. 4.8.5 covers it.

Tap the patella gently with the knee extended and feel for a fluid impulse underneath it. 4.8.4 covers the patellar tap.

For a small effusion, stroke upwards along the medial side of the joint and watch for a bulge appearing on the medial side as the fluid returns. 4.8.3 covers the bulge sign.

For a large tense effusion, feel whether the patella can be lifted away from the femur, which indicates a substantial volume. 4.8.6 covers ballottement.

What a Positive Means

A fluid impulse under the patella indicates an effusion of moderate size or more.

A visible bulge appearing after a sweep indicates a small effusion, and this is the finding inspection misses. 4.8.3 covers it.

A tense effusion in an acutely injured knee indicates blood rather than synovial fluid, which narrows the diagnosis. 4.8.7 covers the acute haemarthrosis.

An effusion with warmth and tenderness raises infection or inflammation, and the temperature assessment is what raises it. 4.6.2 covers it.

No palpable effusion in a knee the patient describes as swelling means the swelling is extra-articular, intermittent, or something other than fluid. 4.2.3 covers swelling as a symptom.

MRCS Level

How It Is Performed

In a viva the expected answer names more than one test and knows which is for a small effusion and which for a large one.

The expected points are the milked suprapatellar pouch, the sweep, the patellar tap, the bulge sign for a small effusion, and ballottement for a large one.

Say that the bulge sign is the most sensitive and the patellar tap the least, because that ordering is the content of the question. 4.8.3 covers it.

Say that you would assess the effusion before the painful parts of the examination, since a guarding patient cannot be assessed for fluid.

Say that an effusion inhibits the quadriceps and limits the range, so its presence changes how the later findings are read. 4.8.2 covers it.

What a Positive Means

The reasoning expected is that the knee's capsule is a single space with a large suprapatellar extension, so fluid can be moved from one part of it to another and that movement is what the tests detect. 4.8.1 covers normal fluid physiology.

A small volume can only be detected by moving it into a shallow space, which is why the bulge sign works and the tap does not. 4.8.3 covers the test.

A large volume lifts the patella off the femur, which is why ballottement works for a tense effusion and is uninformative for a small one. 4.8.6 covers it.

The effusion's effect on the quadriceps is reflex inhibition rather than mechanical, which is why it resolves as the effusion settles rather than needing strengthening. 4.8.2 covers the mechanism.

How Good It Is

Palpation is considerably more sensitive than inspection and the tests differ substantially among themselves, which is why the chapter treats them separately. 4.8.3 covers the most sensitive.

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

It is unreliable in an obese patient, where the fluid cannot be moved into a palpable space, and in a very tense knee where nothing moves at all. 4.23.4 covers those patients.

It is reproducible for a moderate effusion and poorly reproducible for a small one, which is worth knowing when a recorded effusion disappears at the next visit.

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 presence of an effusion changes the interpretation of the examination and the timing of a plan more often than it changes the plan itself.

An effused knee has an inhibited quadriceps and a limited range, so a ligament reconstruction is timed after the effusion settles rather than during it. 4.7.7 covers contractures.

A recurrent effusion with no mechanical findings moves the assessment to an inflammatory or crystal cause, and the examination is what raises it. 4.8.10 covers crystal arthropathy.

A tense effusion in an acute knee changes the urgency and sometimes prompts aspiration, which is both diagnostic and relieving. 4.8.7 covers it.

Following the effusion after surgery is a useful index of how a knee is settling and is more informative than the range alone. 4.7.10 covers post-operative range.

Where It Misleads

The commonest error is assessing the effusion after the painful tests, by which point the knee is guarding.

The second is using one test, which misses either the small effusions or the tense ones.

The third is reading a limited range in an effused knee as a contracture. 4.7.7 covers that distinction.

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

The fifth is recording an effusion without a size, which makes the next comparison impossible. 4.1.8 covers the record.

Fellowship Level

Where It Misleads

In referred practice the effusion is frequently the finding that reframes the referral, because a persistently effused knee is usually not a mechanical problem.

A knee with a recurrent effusion, normal ligaments and no mechanical symptoms has an inflammatory, crystal or synovial cause, and arthroscopy for it is a recognised futility. 4.8.9 covers synovitis.

An effusion after a reconstruction or an arthroplasty raises low grade infection, and in the absence of another explanation it is excluded rather than observed. 4.6.2 covers temperature.

A chronically effused knee cannot be rehabilitated, so addressing the effusion precedes the quadriceps work rather than accompanying it. 4.3.9 covers wasting.

A knee that effuses only after specific activity is a mechanical finding whose timing belongs in the history, since the clinic knee may be dry. 4.2.3 covers it.

In medicolegal work the documented presence or absence of an effusion at presentation is a frequently disputed and frequently decisive fact, which is a reason to record it with a size. 4.23.7 covers that setting.