Swollen Knee Causes and Treatment

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A swollen knee, also known as a knee effusion or "water on the knee," can be caused by a blow to the knee, an overuse injury, or an underlying infection or disease. To determine the cause, a healthcare provider may order imaging tests or perform arthrocentesis. This is a procedure in which fluid is drawn from the knee with a needle to check for signs of blood, infection, or other abnormalities.

Removing the fluid may also reduce pain and stiffness. Once the underlying cause is found, treatment can be administered. This may involve rest, knee immobilization, antibiotics, painkillers, steroid injections, and possibly surgery.

This article looks at the symptoms and causes of a swollen knee, including how the underlying cause is diagnosed and treated.

Verywell / Brianna Gilmartin

Swollen Knee Symptoms

The knee joint is surrounded by a capsule. The 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, leading to a knee effusion.

A knee effusion can vary in severity and, depending on the underlying cause, may involve:

  • Aching knee pain
  • Intense pain when bearing weight on the joint
  • Difficulty moving the joint
  • A feeling of heaviness in the knee
  • A feeling that the knee is "giving out" while walking
  • Joint disfigurement
  • Joint redness and warmth
  • Fever

In some cases, fluids seeping from the joint space can form a fluid-filled sac on the back of the knee called a Baker's cyst (popliteal cyst). A Baker's cyst is painless and often feels like a water-filled balloon. However, if the cyst break opens, there may be pain, swelling, and bruising.

Knee Effusion or Edema?

A swollen knee isn't always due to effusion. It can also be caused by the buildup of fluids outside of the joint space, called edema. With knee edema, swelling occurs primarily in the soft tissues of the knee.

Edema can occur alongside knee effusion as inflammation in the joint space causes fluids to build the overlying tissues of the knee, making the skin look and feel spongy.

Knee edema can also occur independently, most often when the lower legs are affected. Known as peripheral edema, the condition is mainly caused by diseases like heart failure, liver failure, or kidney failure that affect circulation, causing blood and fluid to pool in the lower extremities.

Peripheral edema can cause puffiness, warmth, and a feeling of heaviness, typically without pain. Both knees are generally affected.

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
  • Intense pain
  • Knee joint deformity
  • Inability to place weight on the injured knee

What Causes Knee Swelling?

The causes of knee effusion can be broadly categorized as traumatic injuries, overuse injuries, infections, inflammatory conditions, and tumors.

The knee is comprised of many different tissues and structures, including:

  • Bones (three in each knee, including the patella or kneecap)
  • Cartilage (that covers bone joints so they move easily)
  • Tendons (that connect bones and muscles)
  • Ligaments (that connect and support bone joints)
  • Bursa (fluid-filled sacs that cushion joints)
  • Synovial membranes (that produces synovial fluid)

Knee effusion can occur when any of these structures are damaged, infected, or diseased.

Traumatic Injury

A traumatic injury like a blow to the knee, a crush injury, a severe fall, or the twisting of the knee can damage the knee and cause effusion.

Examples include:

Overuse Injury

Overuse injuries, also known as repetitive use injuries, are those in which long-time stress on a joint causes inflammation, pain, and the loss of joint movement. Overuse knee injuries are commonly associated with sports like running or occupations that require heavy lifting or squatting.

Examples include:

Infections

Infection of the knee, often bacterial, can occur when a germ enters the body through a break in the skin (such as after an open fracture, knee surgery, or a knee injection). It can also be caused by a systemic (whole-body) infection in which a germ travels through the bloodstream and invades a joint.

These most commonly include:

  • Cellulitis (caused when a local bacterial skin infection spreads and invades surrounding tissues)
  • Septic arthritis (caused when a systemic bacterial, viral, or fungal infection invades synovial fluid and joint tissues)

Inflammatory Conditions

Knee effusion can be caused by inflammatory conditions that cause chronic (persistent) or acute (sudden, severe) inflammation. Inflammation is the body's natural response to injury, infection, or disease, characterized by swelling, redness, and pain.

Many of the inflammatory conditions affecting the knee are autoimmune, meaning that the immune system mistakenly targets normal tissues with inflammation. Other conditions cause knee inflammation due to knee joint deterioration, causing bone to rub against bone.

Examples include:

  • Knee osteoarthritis (also known as "wear-and-tear arthritis")
  • Rheumatoid arthritis (an autoimmune arthritis typically affecting joints on both sides of the body)
  • Gout (a type of arthritis caused by the formation of uric acid crystals in joint spaces)
  • Pseudogout (a condition mimicking gout caused by the formation of calcium deposits)
  • Psoriatic arthritis (an autoimmune arthritis that commonly affects people with psoriasis)
  • Lupus (a systemic autoimmune disease that frequently targets joints)

Tumors

There are certain tumors, benign (non-cancerous) and malignant (cancerous, that can affect the knee or nearby structures, causing effusion.

These include:

  • Enchondroma (a benign bone tumor that mainly affects cartilage)
  • Chondroblastoma (a benign yet aggressive tumor affecting cartilage, primarily in children)
  • Chondromyxoid fibroma (an aggressive, benign bone tumor characterized by joint swelling)
  • Osteosarcoma (a rare type of bone cancer that mainly develops near the knees)
  • Synovial sarcoma (a rare cancer of synovial membranes that mainly occurs near the knees)

Diagnosis

The first step in treating an effusion is to pinpoint the cause. Your healthcare provider will first review your medical history, including recent accidents or history of trauma, and conduct a physical exam.

The physical exam can reveal a lot about the underlying cause. For instance, when the swelling is within the knee joint, the kneecap will usually be well-defined and easily felt under the skin (although it may seem pushed out). When the swelling is in the overlying soft tissues, the kneecap may hidden beneath the swelling.

A "straight leg raise" may also be performed in which you sit on an examining table and straighten your leg as far as you can. The range of motion of the knee (or lack of) can suggest whether an acute injury or other disease mechanism is involved.

Based on the outcome of the physical exam, the healthcare provider may order different tests and procedures, including:

  • Joint aspiration: Also known as arthrocentesis, this involves the insertion of a needle into the joint space to extract a sample of synovial fluid. The evaluation of the fluid may reveal signs of infection, inflammation, blood, or uric acid crystals.
  • Imaging studies: This typically involves an X-ray of the knee or a computed tomography (CT) scan in which multiple X-rays are composited to create a three-dimensional image of the internal knee structure.

How to Treat Knee Swelling

The treatment of knee swelling varies based on the underlying cause, whether it be an injury, infection, inflammatory condition, or tumor.

RICE Protocol

Knee swelling caused by an acute injury or flare-up is typically treated with the RICE protocol. RICE is an acronym for:

  • Rest: This involves the avoidance of unnecessary walking and weight-bearing activities.
  • Ice application: This involves icing the knee with an ice pack or cold compress.
  • Compression: This involves the use of a compression bandage to reduce knee pain and swelling.
  • Elevation: This reduces pain and swelling by elevating the leg above the heart.

A knee brace and a mobility device like a pair of crutches may be needed if the injury is severe. Physical therapy may be recommended to restore strength and range of motion after an injury, fracture, or period of prolonged immobilization.

Medications

Different medications may be prescribed to treat different causes of knee swelling. These include:

  • Nonsteroidal anti-inflammatory drugs (NSAIDs): These over-the-counter (OTC) and prescription drugs help ease pain and inflammation caused by many knee conditions.
  • Tylenol (acetaminophen): This OTC drug can help ease pain without the side effects of NSAIDs.
  • Antibiotics: These prescription drugs are delivered by mouth or injection into the knee to help clear bacterial infections.
  • Corticosteroids: These prescription drugs, also known as steroids, may be delivered by knee injection to ease pain caused by overuse injuries, arthritis, or autoimmune diseases.

Other drugs may be prescribed to manage chronic conditions, such as disease-modifying antirheumatic drugs (DMARDs) used for the treatment of autoimmune diseases like lupus or rheumatoid arthritis.

Surgery

Surgery to fully restore function to the knee after a fracture or the rupture of a knee tendon or ligament. The surgery can take two forms:

  • Open surgery: This accesses the knee through a large incision in the skin. For compound fractures, the bone fragments are repositioned (reduced) into their normal alignment with plates and screws and held securely in place with internal or external fixation.
  • Knee arthroscopy: This is a minimally invasive procedure that uses small "keyhole" incisions and specialized narrow tools to access the knee joint. It can used for conditions ranging from osteoarthritis and ACL tears to the removal (resection) of cancerous and non-cancerous tumors.

Summary

Knee effusion, also known as "water on the knee," is the swelling of the knee caused by trauma, overuse injuries, infections, inflammatory or autoimmune diseases, and cancerous or non-cancerous tumors. It can be diagnosed with a physical exam, imaging studies, and joint aspiration. Treatment is directed by the findings of the lab and imaging tests.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Jonathan Cluett, MD
Jonathan Cluett, MD, is board-certified in orthopedic surgery. He served as assistant team physician to Chivas USA (Major League Soccer) and the United States men's and women's national soccer teams.