Normal Fluid Physiology

For Medical Students

What It Tests

Understanding the normal fluid state is what makes the effusion tests interpretable, because every one of them depends on moving a volume from one part of the capsule to another. 4.8.5 covers the test that does it most directly.

A normal knee contains only a small volume of synovial fluid, enough to lubricate and nourish the cartilage and not enough to be detectable by hand.

The capsule, by contrast, has a large potential volume including the suprapatellar pouch, so a knee can accommodate a substantial effusion before it becomes tense. 4.3.8 covers the appearance.

How It Is Performed

This page is not a manoeuvre. It is the physiology the manoeuvres rest on, and the examination it informs is the effusion assessment. 4.6.3 covers that assessment.

In practice the relevant steps are establishing that the knee's parapatellar hollows are present, which indicates a normal small volume, and establishing that the suprapatellar region is not full.

A normal knee should also move freely into full extension, since a knee holding fluid rests in slight flexion. 4.3.5 covers the supine assessment.

A normal knee should hold the leg straight against gravity, since even a modest effusion inhibits the quadriceps. 4.7.8 covers extension lag.

Comparing the two knees is what establishes a patient's normal, because the contours vary between people. 4.1.2 covers the principle.

What a Positive Means

Present parapatellar hollows, a flat suprapatellar region and a knee that rests straight is a knee with a normal fluid volume.

Loss of the hollows is the earliest visible change, because they are the shallowest part of the capsular space. 4.8.3 covers the test that uses that.

A knee resting in slight flexion with no other explanation suggests fluid, since flexion is the position of greatest capsular volume and therefore of least pressure. 4.8.2 covers the significance.

A knee that cannot hold the leg straight with no weakness and no tendon problem suggests fluid, through reflex inhibition of the quadriceps. 4.7.8 covers it.

A knee with a normal fluid volume and a complaint of 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 this physiology is the reasoning behind the tests, and giving it explains why one test is more sensitive than another.

The expected points are the small normal volume, the large potential volume, the suprapatellar pouch's communication with the joint, and the pressure volume relationship.

Say that the tests work by moving fluid, which is why a sweep followed by an observation detects less fluid than a tap does. 4.8.5 covers the sweep.

Say that the capsule's compliance explains why a knee can hold a large volume painlessly and why a rapidly accumulating volume is painful at a smaller size. 4.8.7 covers the acute haemarthrosis.

Say that the suprapatellar pouch's communication is what allows fluid to be milked downwards, and that it is occasionally separate, which changes the examination. 4.6.3 covers the assessment.

What a Positive Means

The reasoning expected is that synovial fluid is produced by the synovium and cleared through the lymphatics, so an effusion represents production exceeding clearance rather than a fixed volume. 4.8.9 covers synovitis.

The capsule is compliant up to a point and then stiff, which is why a slowly accumulating effusion is painless and a rapidly accumulating one is tense and painful at a smaller volume. 4.8.7 covers it.

Flexion increases the capsular volume, which is why the knee adopts flexion and why extension is the position in which an effusion is most uncomfortable. 4.5.4 covers flexion deformity.

Raised intra-articular pressure inhibits the quadriceps through a reflex arc, which is why an effusion produces weakness that resolves rather than weakness that needs strengthening. 4.8.2 covers that mechanism.

How Good It Is

The physiology is established and the clinical application is robust, which makes this page unusual in a section where most pages describe an imperfect test.

What varies between patients is the capsular compliance and the baseline contours, which is why every effusion test is a comparison. 4.1.2 covers the principle.

The composition of the fluid cannot be determined by examination, which is what aspiration is for, and the distinction between blood and synovial fluid is clinically important. 4.8.7 covers the acute haemarthrosis.

The physiology also explains the limits of the tests, which is why it is worth knowing rather than only the manoeuvres. 4.8.3 covers the most sensitive.

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 physiology contributes to interpretation rather than to a decision, and the interpretation changes several decisions.

A tense effusion in an acute knee is a small volume accumulating fast, which narrows the differential and raises the urgency. 4.8.7 covers it.

A large comfortable effusion is a larger volume accumulating slowly, which points at a chronic process. 4.8.8 covers chronic effusions.

The quadriceps inhibition explains why an effused knee is not ready for a reconstruction and why rehabilitation stalls while the effusion persists. 4.7.8 covers the lag.

The capsular volume explains why an effused knee loses flexion before extension, which is worth knowing when interpreting a post-operative range. 4.7.4 covers flexion assessment.

Where It Misleads

The commonest error is treating the volume as the finding when the rate of accumulation is the more informative variable. 4.8.7 covers it.

The second is attributing quadriceps weakness in an effused knee to the muscle.

The third is attributing lost flexion in an effused knee to stiffness. 4.7.7 covers contractures.

The fourth is assuming an effusion means an intra-articular injury, when inflammatory and crystal causes produce the same thing. 4.8.10 covers crystal arthropathy.

The fifth is drawing a conclusion about fluid composition from the examination.

Fellowship Level

Where It Misleads

In referred practice the physiology is most useful in explaining the knee that keeps effusing and has nothing mechanical wrong with it.

A knee producing fluid faster than it clears it has a synovial process, and that is an inflammatory or crystal question rather than a mechanical one. 4.8.9 covers synovitis.

A knee whose effusion fills a popliteal cyst has a valve like communication, so the cyst's behaviour reflects the joint's production and treating the cyst alone recurs. 4.6.10 covers the popliteal fossa.

A knee that has been aspirated repeatedly without the cause being addressed will refill, which is a predictable consequence of the physiology rather than a treatment failure. 4.8.8 covers chronic effusions.

In a knee with a previous synovectomy or a chronically scarred capsule the compliance is altered, so the usual relationship between volume and tension does not hold. 4.2.30 covers previous surgery.

In the chronically effused knee the quadriceps cannot be rebuilt while the effusion persists, which sets the order of treatment and is the most practical consequence on this page. 4.3.9 covers wasting.