Ballottement

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For Medical Students

What It Tests

Ballottement tests for a large or tense effusion by pressing the patella and feeling it float, bounce or refuse to move at all.

It is the test at the opposite end of the volume range from the bulge sign. One detects the smallest effusion, this one characterises the largest. 4.8.3 covers the bulge sign.

Its real value is identifying a tense effusion, because tension rather than volume is what makes an effusion painful and occasionally urgent. 4.8.7 covers the acute haemarthrosis.

How It Is Performed

Lay the patient supine with the knee extended and the quadriceps relaxed.

Place a hand above the patella to compress the suprapatellar pouch gently, which loads the retropatellar space.

Press the patella downwards with the other hand and release, feeling for it floating back.

Note how far the patella can be displaced. A patella that sinks and floats indicates a large effusion; one that cannot be displaced at all indicates a tense one.

Note whether the manoeuvre is painful, since a tense effusion is tender and that is part of the finding.

What a Positive Means

A patella that sinks through fluid and floats back indicates a large effusion.

A patella that cannot be displaced in a visibly swollen knee indicates a tense effusion, which in an acute knee is a reason to consider aspiration both for diagnosis and for relief. 4.8.7 covers it.

A tense effusion with warmth and an inability to move the knee raises infection, and the pathway changes. 4.6.2 covers temperature.

A tense effusion in an acutely injured knee is blood under pressure, which narrows the diagnosis to a short list. 4.8.7 covers it.

A large but compliant effusion suggests a slower accumulation and a chronic process. 4.8.8 covers chronic effusions.

Key Learning Points

The test characterises a large or tense effusion by how far the patella can be displaced.
It is at the opposite end of the volume range from the bulge sign.
Tension rather than volume is what makes an effusion painful.
A patella that cannot be displaced in a swollen knee indicates a tense effusion.
A large compliant effusion suggests a slower accumulation and a chronic process.

MRCS Level

How It Is Performed

In a viva this test is where the useful answer talks about tension rather than volume, which is the distinction the question is usually probing.

The expected points are the extended relaxed knee, the compressed pouch, the downward pressure and release, the degree of displacement, and the tenderness.

Say that tension is the clinically important variable and that it reflects the rate of accumulation. 4.8.1 covers the physiology.

Say that a tense effusion in an acute knee may warrant aspiration, and say why it is both diagnostic and relieving. 4.8.7 covers it.

Say that you would exclude infection in a tense warm knee before anything else. 4.6.2 covers temperature.

What a Positive Means

The reasoning expected is that the capsule is compliant up to a volume and stiff beyond it, so the same volume produces very different tensions depending on how fast it accumulated. 4.8.1 covers normal fluid physiology.

A tense effusion holds the patella firmly against the femur, which is why it cannot be displaced and why the test reads tension rather than volume.

Raised intra-articular pressure inhibits the quadriceps and limits the range, and the effect is proportional to the pressure rather than to the volume. 4.8.2 covers the significance.

A rapidly accumulating effusion is usually blood, because arterial and subchondral bleeding is faster than synovial production. 4.8.7 covers it.

How Good It Is

Ballottement is reliable for a large or tense effusion and useless for a small one, which makes it the complement of the bulge sign rather than a competitor. 4.8.3 covers the bulge sign.

Its finding is more clinically consequential than a volume estimate, because tension is what drives pain and what prompts aspiration.

It is reproducible, since the finding is a clear mechanical impression rather than a subtle visual one.

It cannot say what the fluid is, and in a tense knee that question matters more than usual, which is the argument for aspiration. 4.8.7 covers it.

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

Key Learning Points

The capsule is compliant up to a volume and stiff beyond it.
The same volume produces different tensions depending on how fast it accumulated.
A tense effusion holds the patella against the femur, so the test reads tension.
The quadriceps inhibition is proportional to the pressure rather than to the volume.
A rapidly accumulating effusion is usually blood.

FRCS Level

How Good It Is

At decision level a tense effusion contributes to urgency and to the decision to aspirate, which is a genuine and immediate contribution.

A tense effusion in an acutely injured knee narrows the differential and makes aspiration reasonable for diagnosis and for relief. 4.8.7 covers it.

A tense warm effusion raises infection, and the pathway that follows is urgent and different. 4.6.2 covers temperature.

A tense effusion in a knee listed for an operation is a reason to defer, because the examination is unreliable and the quadriceps is inhibited. 4.8.2 covers the significance.

A large but compliant effusion in a chronic knee contributes to the diagnosis rather than the urgency, and the examination's job is to establish which kind it is. 4.8.8 covers chronic effusions.

Where It Misleads

The commonest error is using this test alone and recording a dry knee when there is a small effusion. 4.8.3 covers the right test.

The second is reading an undisplaceable patella as a normal finding rather than as a tense effusion.

The third is attributing a tense warm knee to an injury without excluding infection.

The fourth is treating a tense effusion's quadriceps inhibition as weakness. 4.7.8 covers the lag.

The fifth is recording tense or not rather than a size and a tension, which loses half the finding. 4.1.8 covers the record.

Key Learning Points

A tense effusion in an acutely injured knee makes aspiration reasonable.
A tense warm effusion raises infection and an urgent pathway.
A tense effusion is a reason to defer an operation.
An undisplaceable patella in a swollen knee is a tense effusion, not a normal finding.
Recording tense or not loses half the finding.

Fellowship Level

Where It Misleads

In referred practice a tense effusion is usually an acute problem and a large compliant one is usually a chronic problem, and the examination separates them in seconds.

A patient with a tense effusion after a reconstruction has either bleeding or infection, and both need addressing rather than observing. 4.6.2 covers temperature.

A patient with a large compliant effusion and a normal mechanical examination has a synovial process, and the chronicity is in the compliance rather than in the history. 4.8.9 covers synovitis.

A patient with a tense effusion and a locked knee has two findings that together raise a displaced tear with bleeding, and the urgency comes from the combination. 4.9.8 covers mechanical locking.

In a knee with a scarred capsule the compliance is reduced, so a modest volume can be tense and the usual relationship does not hold. 4.2.30 covers previous surgery.

In medicolegal work the recorded tension of an effusion at presentation is more informative than its recorded size, and it is almost never documented. 4.23.7 covers that setting.

Key Learning Points

A tense effusion is usually acute and a large compliant one usually chronic.
A tense effusion after a reconstruction is bleeding or infection.
A large compliant effusion with a normal mechanical examination is a synovial process.
A tense effusion with a locked knee raises a displaced tear with bleeding.
A scarred capsule means a modest volume can be tense.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 023 rather than an omission. The comparative standing of these tests is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page, and chapter 3.17 lists the studies the figures would come from. Any measurement here, including a flexion angle, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.

Safety. KneeSchool is educational and does not give individual medical advice. Seek urgent assessment for a knee that cannot bear weight after an injury, a hot swollen knee with a fever, or new numbness, weakness or coldness in the leg or foot.