Patellar Tap

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For Medical Students

What It Tests

The patellar tap detects an effusion by pressing the patella towards the femur and feeling a fluid impulse underneath it.

It works when there is enough fluid to lift the patella off the femur, which means it needs a moderate volume to be positive. 4.8.1 covers normal fluid physiology.

It is the least sensitive of the effusion tests and the most frequently performed, which is the point worth taking from this page. 4.8.3 covers the most sensitive.

How It Is Performed

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

Place a hand above the patella and milk the suprapatellar pouch downwards, which brings fluid under the patella.

With the other hand, press the patella sharply downwards towards the femur and then release.

Feel for the patella sinking through fluid and then floating back up, which is the impulse that constitutes a positive test.

Compare with the other knee, since the patella's mobility varies between people. 4.1.2 covers the principle.

What a Positive Means

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

A negative tap does not exclude an effusion, and the bulge sign should be performed before concluding the knee is dry. 4.8.3 covers it.

A positive tap in an acutely injured knee indicates a substantial effusion, which in that setting is usually blood. 4.8.7 covers the acute haemarthrosis.

A patella that cannot be pressed down at all in a swollen knee suggests a very tense effusion, which is a different and more urgent finding. 4.8.6 covers ballottement.

A patella that feels as though it is grinding rather than floating suggests articular change rather than fluid. 4.6.5 covers patellar tenderness.

Key Learning Points

The test needs enough fluid to lift the patella off the femur.
It is the least sensitive of the effusion tests and the most frequently performed.
A negative tap does not exclude an effusion.
A positive tap in an acutely injured knee usually means blood.
A patella that cannot be pressed down suggests a very tense effusion.

MRCS Level

How It Is Performed

In a viva the patellar tap is a safe thing to mention and a poor thing to stop at, and knowing its place among the tests is what the question is about.

The expected points are the extended relaxed knee, the milked pouch, the sharp downward pressure, the impulse, and the comparison.

Say that the tap is for a moderate or large effusion and that the bulge sign is for a small one, and name both. 4.8.3 covers the bulge sign.

Say that you would relax the quadriceps, since a contracted quadriceps holds the patella against the femur and prevents the test working.

Say that you would milk the pouch first, because without it the fluid may not be under the patella. 4.6.3 covers the assessment.

What a Positive Means

The reasoning expected is that the patella sits directly on the trochlea, so fluid between them lifts it, and the test detects that lift. 2.4.1 covers the patellar anatomy.

A small volume distributes to the compliant parts of the capsule rather than under the patella, which is why a small effusion gives a negative tap. 4.8.1 covers the physiology.

Milking the pouch is what brings fluid into the retropatellar space, which is why the preparatory step matters as much as the tap itself.

A very tense effusion fills the space to the point that the patella cannot be displaced at all, which is why the test can be negative at both extremes of volume. 4.8.6 covers ballottement.

How Good It Is

The patellar tap is specific and insensitive, so a positive result is useful and a negative one is close to uninformative.

It is more reproducible than the bulge sign, because the finding is felt rather than seen, which is part of why it remains popular. 4.8.3 covers the alternative.

It fails at both extremes of volume, which is unusual among the tests in this chapter and worth knowing.

It cannot say what the fluid is, which no clinical test can. 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.

Key Learning Points

Fluid between the patella and the trochlea lifts the patella, and the test detects that lift.
A small volume distributes elsewhere, which is why a small effusion gives a negative tap.
Milking the pouch is what brings fluid into the retropatellar space.
The test can be negative at both extremes of volume.
It is specific and insensitive, so a negative result is close to uninformative.

FRCS Level

How Good It Is

At decision level the patellar tap confirms a moderate effusion quickly and should never be the only effusion test performed.

A positive tap in an acute knee contributes to the urgency, since a substantial effusion in that setting is blood and narrows the differential. 4.8.7 covers it.

A negative tap is not a dry knee, and a clinic that relies on it alone will record dry knees that are not, with consequences for the interpretation of the range and the quadriceps. 4.8.2 covers the significance.

In the post-operative knee the tap is adequate for following a moderate effusion and inadequate for detecting its persistence at a small volume. 4.7.10 covers post-operative range.

In a tense knee the inability to displace the patella is itself the finding and contributes to the decision about aspiration. 4.8.6 covers ballottement.

Where It Misleads

The commonest error is performing it alone and concluding the knee is dry.

The second is performing it without milking the pouch.

The third is performing it with the quadriceps contracted.

The fourth is reading grinding as fluid.

The fifth is recording an effusion as present or absent rather than with a size, which makes the next comparison impossible. 4.1.8 covers the record.

Key Learning Points

The tap confirms a moderate effusion and should never be the only test performed.
A positive tap in an acute knee contributes to the urgency.
A negative tap is not a dry knee.
In a tense knee the inability to displace the patella is itself the finding.
Recording present or absent rather than a size makes the next comparison impossible.

Fellowship Level

Where It Misleads

In referred practice the patellar tap is the test that produced the dry knee recorded in the notes, and repeating the assessment properly often changes the picture.

A patient with a persistent small effusion recorded as a dry knee has had the wrong test, and the finding matters because it explains a stalled range and a persisting lag. 4.8.3 covers the right test.

A patient with a tense effusion that cannot be tapped has a knee that may need aspirating both for diagnosis and for relief, and the examination is where that is established. 4.8.7 covers the acute haemarthrosis.

A patient whose patella grinds rather than floats has patellofemoral change, and the tap has incidentally produced a useful finding. 4.7.6 covers the painful arc.

In a knee with a patellar resurfacing or a previous patellar fracture the patella's behaviour is altered and the test is harder to interpret. 4.2.30 covers previous surgery.

In medicolegal work an effusion recorded as absent on the basis of a tap alone is a weaker observation than it looks, and that limit is worth stating when an opinion depends on it. 4.23.7 covers that setting.

Key Learning Points

The tap is often the test that produced the dry knee recorded in the notes.
A persistent small effusion recorded as dry explains a stalled range.
A tense effusion that cannot be tapped may need aspirating for diagnosis and relief.
A grinding patella is an incidental but useful finding.
An effusion recorded as absent on a tap alone is a weaker observation than it looks.

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.