Symptom Checker

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Persistent Knee Swelling

In general, one or both knees may be swollen due to

  • increased joint fluid

  • blood in the joint

  • enlargement of the bones near the knee joint

  • swelling near the knee joint (without an actual joint problem)--for example, bursitis can cause swelling directly on top of the kneecap

  • fluid behind the knee, called a Baker's cyst.

Each of these will be briefly reviewed. There are other, rarer, causes that will not be covered here.

Let's start with Increased Joint Fluid:

Normally, there is a tiny amount of fluid in the knee which serves as a lubricant and to deliver nutrition to the cartilage that lines the joint. When there is any type of arthritis present, fluid may accumulate in the joint. This is sometimes called "water on the knee." In addition to arthritis, structural problems in the knee (such as torn cartilage or ligament injury) may stimulate extra fluid formation.

Your doctor may recommend a procedure (called arthrocentesis, or joint aspiration) in which a sample of fluid is removed from the knee and analyzed for the presence of white blood cells, red blood cells, infection, or crystals.

This is helpful because certain diseases can be suggested or even proven by the results of this analysis:

  • high white blood cell count in joint fluid suggests the presence of marked inflammation (making infection a larger concern).

  • a┬álarge number of red blood cells suggests there's been bleeding into the joint.

  • many common bacterial joint infections can be identified by analysis of the fluid.

  • gout or pseudogout, the most common forms of "crystal-induced" arthritis, can be proven by seeing the appropriate crystal (uric acid crystals in gout; calcium crystals in pseudogout).

Would you like to continue learning about other causes of joint swelling (next up: blood in the joint) or would you prefer to move on to questions about your particular situation?

Blood in the joint

Information specific to my situation

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