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A swollen knee is a common problem. Many people call this "water on the knee." However, determining the cause of knee swelling can be a challenge. Swelling may be acute or chronic. It may be associated with a recent injury or may have a gradual onset. The swelling can be within the knee or around the knee. Determining the treatment of the swollen knee depends on what caused the problem. Here you will learn how to determine the cause of knee swelling.

A. People who have a swollen knee can either have fluid within the knee joint itself, or in the surrounding soft tissue. The first step in determining the cause of the fluid accumulation is to determine where the fluid is located.

The knee joint is surrounded by a capsule. This capsule forms the "joint space." The joint space normally has a small amount of lubricating fluid within the knee to help keep the knee moving easily. Some conditions cause an accumulation of this fluid. This is what most people consider a swollen knee, also called a knee effusion.

Fluid Inside The Knee Joint

If it is determined that fluid is inside the knee joint, the next step is to determine if there was an associated injury.

• Acute Injuries
  An acute injury is a traumatic event that has occurred recently (within the past 24-48 hours). Acute injuries are distinct from chronic conditions because the knee swelling developed suddenly, as a result of the injury.

  If the injury is acute, the next step is to determine the type of fluid within the knee. There are specific injuries that can cause bleeding in the knee, and others that may cause a sudden increase in the production of joint fluid.

  • Blood in the Knee
    Two conditions commonly cause the accumulation of blood within the knee; these are an ACL tear and a fracture of the bone and cartilage of the knee. These injuries allow blood to enter the joint and will create a large, swollen knee. When bleeding is the cause of knee swelling, the onset is rapid, and the swelling can be intense. Fluid usually accumulates within minutes of the injury.

  • Non-Bloody Fluid in the Knee
    Acute injuries that cause the accumulation of non-bloody fluid within the knee include meniscus tears and ligament sprains. The knee swelling seen with these injuries is acute in onset, but less rapid than blood accumulation. Typically patients with these injuries will notice fluid accumulation hours to days after the injury (rather than within minutes as seen with bleeding into the knee). The amount of fluid can be significant, but it is not typically tense as seen with blood accumulation.

• Chronic Conditions
  Chronic injuries cause a gradual onset of knee swelling. This fluid may fluctuate in amount and the symptoms may come and go. This is the most common type of swelling and often seen as a result of knee arthritis or wear-and-tear.
  • Knee Arthritis
    Knee arthritis causes the body to produce extra fluid in the knee joint. The amount of fluid tends to fluctuate over time. Patients with knee arthritis often notice the affected knee is larger than the other. The amount of fluid often corresponds with the amount of activity the patient has been doing--more significant activities cause more swelling.

• Rapid Onset of Swelling, No Injury
  The last broad category of swelling is the rapid onset of fluid within the joint, but no recent injury to the knee. The most common causes of this type of fluid accumulation are due to infection or gout.

  • Infection
    Infections can cause fluid to accumulate within the knee joint. Infections can be caused by contamination in the knee, such as from surgery or a wound to the knee, or a systemic infection that spreads to the joint. Infections inside a joint are problematic because your body has a hard time fighting infections within this space. Surgery may be needed to clean out the infection.

  • Gout and Pseudogout
    Gout is due to the accumulation of uric acid crystals within the fluid of your knee. Uric acid is a substance produced as part of digestion. In order to properly digest food, and rid our body of waste, our bodies produce substances such as uric acid to transport waste material. People with gout accumulate uric acid crystals within joints, leading to inflammation and swelling.

    Pseudogout is a similar problem, only a different type of crystals accumulates within the joint. In patients with pseudogout, calcium crystals simulate an inflammatory reaction within the joint, leading to a swollen knee.

Fluid Outside The Knee Joint

When fluid is outside of the actual joint space, and in the soft tissues surrounding the knee, the most common cause is prepatellar bursitis. This condition causes fluid accumulation in the bursa just on top of the kneecap. Usually the fluid can be felt on top of the kneecap, rather than underneath the kneecap.

Fluid can also accumulate in the soft tissue around the knee after an injury such as a contusion to the knee. A forceful injury to the soft tissue surrounding the knee can cause fluid and/or blood to accumulate, giving the appearance of a swollen knee.

Johnson, MW "Acute Knee Effusions: A Systematic Approach to Diagnosis" American Academy of Family Physicians; April 15,2000.