Bulge Sign

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For Medical Students

What It Tests

The bulge sign detects a small effusion by moving fluid out of one parapatellar hollow and watching it return.

It works because the parapatellar hollows are the shallowest part of the capsular space, so a small volume is visible there and nowhere else. 4.8.1 covers normal fluid physiology.

It is the most sensitive clinical test for an effusion, and it is the test that finds the fluid the inspection and the patellar tap both miss. 4.8.4 covers the tap.

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 any fluid into the parapatellar region.

Stroke firmly upwards along the medial side of the joint, from below the joint line to above the patella, which empties the medial hollow.

Watch the medial hollow while stroking downwards on the lateral side, and look for a bulge of fluid appearing medially.

The bulge is the sign. It appears a second or two after the lateral stroke and is subtle, so the eye has to be on the medial side before the stroke begins.

What a Positive Means

A visible bulge appearing in the medial hollow indicates a small effusion.

A positive bulge sign with a negative patellar tap indicates a small effusion rather than a contradiction, since the two tests detect different volumes. 4.8.4 covers the tap.

A positive sign in a knee with no visible swelling is the test earning its place, and it is the commonest useful version of the finding. 4.3.8 covers the inspection.

A positive sign in a knee with a normal examination otherwise means something is irritating the joint, even if nothing structural has been found. 4.8.9 covers synovitis.

A negative sign does not exclude an effusion in an obese patient, where the fluid cannot be moved into a visible space. 4.23.4 covers those patients.

Key Learning Points

The parapatellar hollows are the shallowest part of the capsular space.
The test moves fluid out of one hollow and watches it return.
It is the most sensitive clinical test for an effusion.
A positive bulge with a negative tap is a small effusion, not a contradiction.
A negative sign does not exclude an effusion in an obese patient.

MRCS Level

How It Is Performed

In a viva the bulge sign is the answer when asked how you would detect a small effusion, and knowing it is the most sensitive test is the mark.

The expected points are the extended relaxed knee, the milked pouch, the upward stroke medially, the downward stroke laterally, and watching the medial hollow.

Say that you would look before you stroke, since the bulge is transient and easily missed.

Say that the test is for a small effusion and that the tap and ballottement are for larger ones, which demonstrates that you know why several tests exist. 4.8.6 covers ballottement.

Say that you would perform the effusion tests before anything painful, since the knee has to be relaxed. 4.6.3 covers the assessment.

What a Positive Means

The reasoning expected is that a small volume of fluid distributes itself to the most dependent and compliant part of the capsule, and moving it mechanically is the only way to make it visible. 4.8.1 covers the physiology.

Stroking upwards displaces the fluid proximally, and the downward stroke on the other side returns it, so the test's two strokes do different jobs and the order matters.

The medial hollow is used because it is shallower than the lateral, which is why the bulge appears there and why the strokes are in that direction.

A larger effusion fills both hollows permanently, which is why a large effusion gives a negative bulge sign and a positive tap, and why the tests complement each other. 4.8.4 covers the tap.

How Good It Is

The bulge sign is the most sensitive clinical test for an effusion and consequently the least specific about volume, since it is positive across a wide range.

Its reproducibility is moderate, because the bulge is subtle and the examiner has to be watching the right place at the right moment.

It is insensitive in an obese knee and in a very tense knee, in the first because the fluid cannot be seen and in the second because it cannot be moved. 4.23.4 covers the first.

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

A small volume distributes to the most compliant part, so it must be moved to be seen.
The two strokes do different jobs, so the order matters.
The medial hollow is used because it is shallower than the lateral.
A large effusion fills both hollows permanently and gives a negative bulge sign.
It is insensitive in an obese knee and in a very tense one, for different reasons.

FRCS Level

How Good It Is

At decision level the bulge sign contributes by detecting what would otherwise be recorded as a dry knee, and that changes the interpretation of the rest of the examination.

A knee with a small effusion has an inhibited quadriceps and a provisional range, and recording the effusion is what makes those figures interpretable later. 4.8.2 covers the significance.

A persistent small effusion in a knee with no mechanical findings is the finding that moves the patient towards an inflammatory assessment, and without this test it is missed. 4.8.9 covers synovitis.

In the post-operative knee a small effusion explains a stalled range and a persisting lag, and addressing it is what allows the rehabilitation to progress. 4.7.10 covers post-operative range.

In the chronic knee a small effusion with joint line tenderness contributes to whether the surface is the problem. 4.9.11 covers meniscal examination in osteoarthritis.

Where It Misleads

The commonest error is not performing it, so small effusions go unrecorded.

The second is performing it after the painful parts of the examination, when the knee is guarding.

The third is reading a negative sign as a dry knee in an obese patient.

The fourth is reading it as a measure of volume, which it is not.

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

Key Learning Points

The test detects what would otherwise be recorded as a dry knee.
A small effusion makes the quadriceps findings and the range provisional.
A persistent small effusion with no mechanical findings prompts an inflammatory assessment.
In the post-operative knee it explains a stalled range.
It is not a measure of volume.

Fellowship Level

Where It Misleads

In referred practice the bulge sign is the test that explains the knee recorded as dry at every visit and still not settling.

A patient with a persistent small effusion through a rehabilitation programme has a joint that is being irritated, and finding the cause is more useful than intensifying the programme. 4.8.9 covers synovitis.

A patient with a stalled post-operative range and a small effusion has a treatable reason for the stall, and the effusion is frequently not recorded because only the inspection was performed. 4.7.10 covers the range.

A patient with a small effusion and a normal mechanical examination over months has a synovial or inflammatory process, and the repeated normal examinations are the clue rather than reassurance. 4.8.8 covers chronic effusions.

In a knee with a previous synovectomy or a scarred capsule the fluid may not move as the test expects, so a negative result is less informative. 4.2.30 covers previous surgery.

In medicolegal work a recorded small effusion is more useful than a recorded normal examination, because it is a positive observation, and its absence from early records is a common gap. 4.23.7 covers that setting.

Key Learning Points

It explains the knee recorded as dry at every visit and still not settling.
A persistent small effusion through rehabilitation means the joint is being irritated.
A stalled post-operative range with a small effusion has a treatable reason.
Repeated normal mechanical examinations with a small effusion are a clue, not reassurance.
A scarred capsule makes a negative result less informative.

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.