What are the Most Common Causes of a Swollen Knee?

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  • Written By: wiseGEEK Writer
  • Edited By: O. Wallace
  • Last Modified Date: 10 August 2019
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It’s not necessarily easy to self-diagnose the causes of a swollen knee. A single knee may swell or appear to have “water” on it for a variety of reasons. There are relatively common reasons why the knee swells, but even knowing these, people should avoid self-diagnosis and head to the doctor for real diagnosis, particularly if the swelling persists. Most likely reasons a swollen knee occurs can include single injury/trauma to the knee, wearing down of the knee joint parts over time, or disease states like gout, infection, benign or cancerous tumors, and arthritic conditions.

Of these conditions, it’s probably easiest to determine cause of swelling, to a degree, if a traumatic injury has occurred. Sprains, fractures or tearing of knee cartilage that occurs suddenly may produce significant pain and swelling. Function of the knee joint can be reduced and many people might have trouble standing on the affected leg.

Though it’s easy to make an association between the injury and the swollen knee, it’s not necessarily possible to determine extent of injury. People who have had traumatic injury to the knee are advised to see a doctor for diagnosis. Early intervention and treatment may prevent problems down the line.


A lot of the time, a swollen knee seems to occur without any particular reason. It could be that fluid is accumulating in the joint space due to injuries that take place gradually or that result from overuse of the joint. People, who repeatedly use the knee for something like bicycling, might develop these injuries. Alternately conditions like bursitis could cause fluid to develop outside of the joint.

Another potential cause of knee swelling is conditions that are recurring or chronic. This is true of things like arthritis, which usually includes significant pain. People who suffer from diseases like gout suffer period build-ups of uric acid, which can crystallize around joints resulting in swelling and pain.

One swollen knee cause that many people don’t think of is viral, bacterial or fungal infection. A variety of diseases may affect joints. Such diseases include gonorrhea, HIV, mumps, hepatitis, Lyme disease, tuberculosis, and staph infections. When infection moves to the joint this is called infectious arthritis, and antibiotic treatment is needed right away.

Sometimes swelling can result from presence of tumors inside the knee joint. These tumors may or may not be benign. They often begin in the leg and spread to the knee, limiting movement. When benign, removal may solve the problem, and even if malignant, many times the cancer is effectively treated with removal of the tumor and radiation or chemotherapy.


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