Swollen Knee
How quickly a knee swells is one of the most useful pieces of information about it. Swelling that comes on within an hour or two of an injury usually means bleeding inside the joint, which points towards a torn ACL, a fracture or a displaced meniscal tear. Swelling that builds over a day or more is more typical of a meniscal tear or an arthritic knee reacting. Swelling with fever or feeling unwell is treated as an emergency until proven otherwise.
Also described as: fluid on the knee, water on the knee, knee effusion, puffy knee.
Go to A&E now if
- •A hot, very painful, swollen knee when you also feel unwell or feverish - this can be an infected joint and is treated as an emergency
- •A leg that is cold, pale, numb or that you cannot feel properly below the knee
- •An obvious deformity after an injury, or a knee that gives way completely and cannot take any weight at all
What causes it
ACL rupture
Bleeding into the joint after an ACL tear typically makes the knee tight and swollen within a few hours of the injury, often with a pop felt at the time.
More on ACL reconstructionMeniscal tear
More often swells overnight or over the following day, with pain along the joint line, and tends to swell again whenever the knee is worked hard.
More on meniscus repairOsteoarthritis
An arthritic knee swells intermittently, often after a long day or a lot of walking, and settles with rest. This is a flare rather than a new injury.
More on knee arthritis clinicKneecap dislocation
A first-time dislocation usually leaves a markedly swollen knee, and is worth assessing because a fragment of cartilage can be knocked off as the kneecap relocates.
More on patella instability and MPFL reconstructionInfection in the joint
An infected knee is hot, very painful, and usually comes with feeling unwell or feverish. This is the one cause of a swollen knee that needs same-day emergency care rather than a clinic appointment.
Worth being seen within a week if
- •A knee that swelled within a few hours of an injury
- •Swelling that keeps coming back without an obvious reason
- •Swelling with locking, catching or giving way
- •A swollen knee after a previous knee operation
How it is assessed
- •How fast the swelling came on after any injury, which narrows the likely cause more than almost anything else.
- •Examination for the amount of fluid, the range of movement and the stability of the joint.
- •Weight-bearing X-rays where arthritis is likely.
- •MRI where a ligament, meniscal or cartilage injury is suspected.
- •Drawing off fluid where it is needed to relieve pressure or to establish a diagnosis.
Mr Sam Nahas is a consultant knee surgeon in Reading, and sees new knee problems within a week at Circle Reading Hospital. No GP referral is needed for self-pay patients.
Book a consultationThis page is general information about knee symptoms and does not replace individual medical advice. If you are worried about your knee, please be assessed.