A swollen knee is a common problem that can affect the young as well as old. Many people refer to it as "water on the knee" because of its often spongy appearance. Determining the cause of a swollen knee can sometimes be challenging.
A swollen knee may be an acute condition caused by a traumatic injury or a chronic one that has developed slowly over time. The location of the swelling can also vary, sometimes occurring within the knee joint and, at others, in the soft tissues surrounding the knee.
The Knee Joint
The knee joint is surrounded by a capsule. This capsule forms the "joint space" where a small amount of lubricating fluid (called synovial fluid) keeps the knee moving easily. Certain conditions can cause this fluid to accumulate. When this happens, the knee can swell, a condition typically referred to as a knee effusion.
When to Seek Urgent Care
Go to the nearest emergency room or urgent care center if you experience a traumatic knee injury accompanied by:
A popping sound at the time of the injury
Rapid swelling of the knee
Knee joint deformity
Inability to place weight on the injured knee
The first step in treating an effusion is to pinpoint the cause. Your healthcare provider will first look at the physical appearance of the knee itself.
When the swelling is within the knee joint, the kneecap is usually well-defined and easily felt under the skin (although it may seem pushed out a bit). When the swelling is in the soft tissue, the kneecap may not be visible or easily felt.
Based on the outcome of the physical exam, the healthcare provider can then explore some of the more typical causes of knee effusion.
Fluid Outside the Knee Joint
The most common cause of excessive fluid in the soft tissue surrounding the knee is prepatellar bursitis. This is inflammation of a fluid-filled sac (called the bursa) which cushions the kneecap (called the patella). The buildup can be seen and felt at the top of the kneecap. It is not something you would see under the knee.
An injury such as a contusion (soft tissue bruise) may also cause localized swelling. In some cases, the buildup of blood and fluid may mimic an acute injury of the knee joint.
Fluid Inside the Knee Joint
If the knee joint is the area of effusion, we typically explore three possible causes: an acute injury, a chronic condition, and an acute condition not related to an injury.
Acute injuries are those that have occurred within the past 24 to 48 hours, resulting in rapid swelling of the knee. In this instance, we would determine whether the fluid in the knee is bloody or non-bloody:
Blood in the knee fluid is usually caused by either a torn anterior cruciate ligament (ACL) in the knee or a fracture of the bone and cartilage of the knee. When bleeding is the source of the swelling, the onset will be rapid and intense, usually within minutes.
Non-bloody fluid can be caused by a ligament sprain or a meniscus tear in the rubbery disk that cushions the knee. The swelling is typically slower and often only noticed hours or days after the injury. The volume of fluid can be significant but is not typically as profound as a blood accumulation.
Chronic effusions are characterized by the gradual onset of swelling. The swelling can often fluctuate as the symptoms come and go. In addition to aging-related wear-and-tear, there are two common causes for a chronic knee effusion:
Osteoarthritis can cause excess fluid production in response to underlying inflammation. With knee osteoarthritis, the affected knee is often larger than the other. Swelling tends to worsen with activity, particularly when the knee bears weight. The pain will often disappear once the knee is relaxed.
Rheumatoid arthritis, an autoimmune form of arthritis, can cause the same effect. Rheumatoid arthritis will most often affect multiple joints accompanied by a greater persistence of swelling due to the ongoing, underlying inflammation.
Acute Onset Without Injury
Rapid onset of swelling with no injury is a broad category wherein the accumulation of fluid is not due to an injury or a chronic condition, such as:
Infection can result in joint fluid accumulation, often as a result of surgery, a knee wound, or systemic infection that spreads to the joint. Treatment can be a problem as the body has a tough time clearing infection from this space. Surgery may be required to fully clean out a septic infection.
Gout and pseudogout involve a buildup of crystals in the knee fluid. With gout, the uric acid used to transport waste can accumulate and crystallize in various joints of the body, causing intense swelling and pain. With pseudogout, the culprit is calcium crystals.