Swelling

For Medical Students

What It Tests

Swelling after a knee injury tests how fast fluid accumulated, and the speed separates bleeding from a reactive effusion.

That distinction matters because blood in the joint means a structure with a blood supply has torn, and the list of candidates is short.

This is the clearest example in the chapter of a question only the history can answer. By the time anyone examines or images the knee, the timing has already happened. 4.1.3 covers the principle.

How It Is Performed

Ask how long after the injury the knee swelled, and press for a number of hours rather than accepting soon.

Ask how big it became and whether it has settled, because a swelling that recurs behaves differently from one that resolved.

Ask whether the knee has swollen before, and whether swelling follows activity, since that pattern points to a degenerate or inflammatory cause rather than an acute tear.

Distinguish swelling of the joint from swelling around it. A patient pointing to the front of the knee may be describing a prepatellar swelling rather than an effusion.

What a Positive Means

Swelling within a few hours of injury means a haemarthrosis, and the common causes are an anterior cruciate rupture, a patellar dislocation, an osteochondral fracture or a peripheral meniscal tear. 4.8.7 covers the examination.

Swelling developing over a day or more is a reactive effusion and points towards a meniscal or cartilage injury rather than a cruciate.

Swelling after activity in an older knee points to degenerative disease.

Swelling with warmth, redness and systemic symptoms is a different problem entirely and needs excluding before anything else. 4.8.10 covers crystal arthropathy.

MRCS Level

What a Positive Means

The expected reasoning at this level is from the timing to a short differential and then to what must be excluded.

An immediate haemarthrosis in a young patient after a pivoting injury is an anterior cruciate rupture until proven otherwise.

The same presentation after a patellar dislocation event has a different structural cause and the patient will usually describe the kneecap moving. 4.2.22 covers it.

An immediate haemarthrosis with an inability to weight bear raises an osteochondral fracture, which changes the urgency because a loose fragment may be fixable.

A locked knee with an effusion is a surgical problem rather than a clinic problem. 4.2.5 covers locking.

How Good It Is

Timing of swelling is among the strongest single questions in the knee history, and the reason is that the mechanism is biological rather than interpretive: bleeding is fast and synovial reaction is slow.

It performs better than any examination finding for the same question, because the examination can confirm that fluid is present and cannot date it. 4.8.1 covers normal fluid physiology.

It performs better than imaging too, for the same reason.

Where it performs badly is in the patient who cannot remember, which is common after a frightening injury, and in the patient who was not watching the knee. A negative answer is much weaker than a positive one.

FRCS Level

How Good It Is

At decision level the timing of swelling shapes the urgency more than the diagnosis, because the diagnosis will be made anyway and the window may not wait.

An immediate haemarthrosis with a mechanical block is the one knee presentation in this chapter that should not wait for an elective pathway.

A recurrent effusion that follows activity in a knee with structural findings is a better predictor of a poor outcome from a mechanical intervention than any single imaging feature, because it indicates a reactive joint.

A knee that has stopped swelling, in a patient whose activity has not changed, has usually settled rather than been cured, and the distinction matters when counselling.

Where It Misleads

The commonest error is accepting a vague answer. Soon, quickly and straight away all get recorded as immediate and they do not mean the same thing.

The second is attributing a chronic effusion to the most visible structural finding. A degenerate knee with an effusion and a meniscal tear on imaging has two findings and the effusion is not evidence that the tear is responsible. 4.9.11 covers it.

The third is missing infection. A hot, swollen, painful knee with systemic features is not a mechanical problem and the history is where that is caught, particularly after a recent injection or operation. 4.2.31 covers previous injections.

And a knee that swells after every attempt at rehabilitation is giving information about the rehabilitation rather than about the structure.

Fellowship Level

Where It Misleads

Recurrent effusion is one of the commonest reasons a knee is referred after surgery, and the history separates the causes better than imaging.

An effusion that appeared after an operation and never settled behaves differently from one that settled and returned, and the second is more likely to be mechanical.

An effusion with pain and stiffness after arthroplasty needs infection excluded before any mechanical explanation is entertained, and the history of the early post-operative period is where the suspicion is raised. 4.20.8 covers the assessment.

An effusion after a meniscal repair or a cartilage procedure may reflect a graft or repair that is being loaded beyond what it can tolerate, which is a rehabilitation question rather than a structural one. 4.20.2 covers meniscal repair.

The limit of this page is that every discriminating claim on it is standard teaching with no figure attached, and the figures were refused rather than retrieved under decision 023.