What Causes Knee Swelling?

A person with a swollen knee.
A diagram of the knee, including ligaments.
Knee swelling is often accompanied by pain.
Knee injuries are common in sports.
A close up of the knee, with the ligaments in pink.
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  • Written By: Susan Grindstaff
  • Edited By: Heather Bailey
  • Last Modified Date: 23 August 2014
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Knee swelling is most commonly caused by blood or fluid either inside or around the knee joint. Accumulations of the fluids are usually caused by injury. This is especially true if the swelling in the knee is sudden. Aside from injury, conditions such as arthritis and gout can also lead to knee swelling.

There are four major ligaments associated with the knee joint, and the one that is most prone to injury or tearing is called the anterior cruciate ligament (ACL). ACL tears are often sports related, but can be sustained in many other activities. Most of the time, this ligament is torn when too much pressure is put on the knee when jumping or pivoting. Knee swelling from ACL tears is usually very sudden, and is caused by blood rushing to the inside of the knee. In some cases, ACL tears are so severe that reconstructive surgery is necessary, however, with minor tears, the knee often heals on its own within a couple of months.

In many cases, ligament sprains can also lead to swelling in and around the knee. Sprains are generally not as serious as tears, and do not usually require surgery. Most of the time sprains, though very painful, are completely healed within a few weeks time. Unlike ACL tears, swelling resulting from sprains may take several hours or even days to occur, and the fluid buildup in the knee is usually not bloody.


Arthritis is a condition that can affect one or both knees, and is typically accompanied by pain, stiffness, and swelling. This is because arthritis often causes fluid to build up in and around joints. People who suffer from knee arthritis usually have symptoms that come and go. In many cases, the amount of swelling also seems to correspond to the activity level of the arthritis sufferer. If they have been very active, they may see more swelling.

People who suffer from gout often develop deposits of crystallized uric acid in their knees, and these deposits frequently cause knee swelling. Uric acid is a compound that the body produces to aid in digestion, and is usually eliminated with body waste. People with gout, however, are unable to rid their bodies of the acid; instead, the acid accumulates inside their joints.

Most of the time, knee swelling is treated using non-steroidal anti-inflammatory medications, such as ibuprofen and aspirin. If these medications do not reduce swelling, it may be necessary for the patient to have steroids injected directly into the knee. Most of the time patients are advised to use their knee as little as possible until the swelling has gone down.


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