TL;DR: Most swollen knees can wait for a routine appointment. Four cannot: a hot, swollen, very painful knee with fever or feeling unwell; a newly swollen, warm or tender calf; a knee that has locked and will not straighten; and an injury after which you cannot bear weight. Those need same-day assessment or an emergency room. For everything else, how quickly the swelling appeared is the single most useful piece of information you can give a clinician.
If any of the four applies to you right now, stop reading and call us on (214) 233-3094, or go to your nearest emergency room. For a medical emergency, call 911.
The four that cannot wait
1. Hot, swollen, severely painful — with fever or feeling unwell
A joint that is hot to touch, markedly swollen, painful even at rest and almost impossible to move, particularly alongside fever, chills or simply feeling systemically unwell, raises the possibility of an infected joint. Septic arthritis is a surgical emergency and is treated with urgency for a reason: an infected joint can be damaged quickly. This is an emergency room presentation, not a clinic booking.
2. A newly swollen, warm or tender calf
Swelling that is in the calf rather than the knee — particularly if it is warm, tender, and came on without an obvious injury — can indicate a blood clot in a deep vein. That is a different problem from a knee problem and needs assessing the same day. It is worth knowing that a Baker’s cyst behind the knee can leak and mimic this, which is exactly why it needs someone to look rather than someone to guess.
3. A knee that has locked
Not stiff. Genuinely blocked — you cannot straighten it, and something mechanical is stopping you. That can mean a displaced fragment of meniscus trapped inside the joint, and it is not something to sit on for weeks. See what a meniscus tear on MRI does and does not mean.
4. An injury you cannot walk on
After an acute knee injury, widely used decision rules treat several findings as reasons to obtain an X-ray before anything else: being aged 55 or older, tenderness isolated to the kneecap, tenderness over the head of the fibula on the outer side just below the joint, inability to bend the knee to 90 degrees, and inability to take four steps bearing weight both immediately after the injury and at assessment. Any one of them is enough.
How fast it swelled tells you a great deal
This is the question a clinician will ask, so it is worth having the answer ready.
- Within minutes to a couple of hours after an injury — blood in the joint. This pattern is associated with more significant injuries, including ligament ruptures and fractures.
- Overnight, or the following day — the joint lining producing fluid in response to irritation. Common with meniscal injuries and with flares of osteoarthritis.
- Gradually, over days to weeks, without injury — usually an inflammatory or degenerative process rather than a mechanical one.
- Rapidly, severely, with no injury at all — worth considering crystal arthritis such as gout or pseudogout, and worth excluding infection. Get it seen.
When it is a flare rather than an emergency
A knee with osteoarthritis that puffs up after a long day, a lot of stairs or an unusually big walk is a knee telling you the dose exceeded what it currently tolerates. That is information, not a catastrophe. It usually settles over a few days with relative rest, and it is a reason to look at the loading in your program rather than to stop moving.
What it should not be is routine. A knee swelling every week is a knee whose plan needs revisiting — see the Knee Program.
What an evaluation does about it
We examine the knee, look at the fluid with in-office ultrasound, and decide whether aspirating it is useful — both to reduce pressure and, where infection or crystals are a possibility, to send the fluid for analysis. If imaging would change the plan, we order it. If a finding points to a surgical or rheumatology problem, we refer. Details in what happens at your knee evaluation.
Should I drain a swollen knee myself?
No. Aspiration is a sterile procedure done for a clinical reason, and the fluid itself is frequently the most useful diagnostic sample available. There is no home version of this.
Is it safe to walk on a swollen knee?
In the absence of the four red flags above, walking within comfort is generally fine and usually better than complete rest — with the caveat that the amount that swells it is, by definition, too much for today. If you cannot bear weight after an injury, that is one of the four. Get it looked at.
