Some possible causes of acute knee swelling include:

Bursitis

Bursitis occurs when the fluid-filled sacs in the knee, called the bursae, become irritated.

An inflammatory response in the knee then causes swelling. The swelling puts pressure on other parts of the knee and can cause pain.

The American Academy of Orthopaedic Surgeons suggest that the most common causes of bursitis are frequent kneeling or blunt force trauma to the knee.

To diagnose this, a doctor examines the knee and may order scans, such as an X-ray or MRI. If bursitis is the cause, treatment may include a combination of:

• compression
• using ice packs
• elevating the knee
• resting
• taking nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, for pain relief

Arthritis

Arthritis is another common cause of swelling in the knees. In a person with arthritis, there is underlying chronic inflammation, but they may still experience the five key markers of acute inflammation.

According to the Arthritis Foundation, several types of arthritis can cause knee swelling, including:

• osteoarthritis, the most common form, which results from a breakdown of cartilage in the knee
• rheumatoid arthritis, an inflammatory form, in which the immune system mistakenly attacks the joints
• juvenile arthritis, which occurs in people aged 16 and under
• reactive arthritis, a chronic type that commonly follows an infection of the genitals or urinary tract
• infectious arthritis, which occurs when an infection causes swelling in a joint
• gout, a form of arthritis that results from a buildup of uric acid in the blood

If a doctor suspects arthritis, they will ask about other symptoms and check the person’s medical history. They may also order tests, such as blood work, and imaging, such as an X-ray.

The best treatment plan depends on the type of arthritis but might include:

• pain relief medication, such as NSAIDs
• lifestyle changes, such as exercise
• physical therapy
• corticosteroids
• disease-modifying antirheumatic drugs
• assistive devices
• surgery

Acute injuries

Acute injuries may result from sports, exercise, or accidents involving blunt force trauma to the knee. Some common forms of acute knee injury include:

• sprains to the ligaments or muscles surrounding the knee from a sudden twist, turn, or hit
• tendonitis from overuse of a tendon or joint
• torn cartilage from trauma

A doctor may use imaging, such as an X-ray, CT, or MRI, to assess the damage.

Treatment may involve a combination of:

• resting
• using braces or other assistive devices
• taking NSAIDs
• receiving physical or occupational therapy
• undergoing surgery

Infections

Infections such as the following can cause swelling in the knee:

• lupus, a chronic condition that causes the immune system to attack healthy tissues, including those in the joints
• Lyme disease, which is spread by ticks and causes flu-like symptoms, a rash, and joint swelling

Testing can help a doctor determine whether an infection is responsible for the swelling. They may prescribe antibiotics if the infection is bacterial, as with Lyme disease.