Why Is My Knee Swollen or Stiff? Common Causes and When to See a Doctor

Knee swelling or stiffness can be uncomfortable, frustrating, and sometimes concerning—especially when it appears suddenly or doesn’t go away. Whether it happens after an injury, exercise, or seemingly without reason, these symptoms are your body’s way of signaling that something isn’t quite right inside the joint.

Understanding what may be causing knee swelling or stiffness can help you decide when rest is enough—and when it’s time to see an orthopedic specialist.

What Causes Knee Swelling?

Knee swelling occurs when excess fluid builds up in or around the joint. This can happen for several reasons, including:

1. Injury or Trauma

A fall, twist, or direct blow to the knee can cause swelling due to inflammation or internal damage. Common injuries include:

• Meniscus tears

• Ligament injuries (ACL, MCL, PCL)

• Cartilage damage

Swelling from an injury may appear immediately or develop over several hours.

2. Overuse or Repetitive Stress

Activities like running, kneeling, squatting, or jumping can irritate the knee joint over time. Overuse injuries often cause gradual swelling and discomfort, especially after activity.

3. Arthritis

Osteoarthritis is one of the most common causes of knee swelling, particularly in adults over 40. As cartilage wears down, inflammation increases, leading to:

• Swelling

• Stiffness

• Aching or grinding sensations

Rheumatoid arthritis and other inflammatory conditions can also cause persistent knee swelling.

4. Fluid Buildup (Knee Effusion)

Sometimes referred to as “water on the knee,” effusion can result from injury, arthritis, or infection. The knee may feel tight, puffy, or difficult to bend fully.

What Causes Knee Stiffness?

Knee stiffness often occurs alongside swelling but can also happen on its own. Common causes include:

1. Inflammation

Inflammation inside the joint limits smooth movement, making the knee feel tight—especially after sitting, sleeping, or inactivity.

2. Cartilage or Meniscus Damage

Damage inside the knee can interfere with normal joint motion, leading to stiffness, locking, or catching sensations.

3. Arthritis-Related Changes

As arthritis progresses, joint surfaces become less smooth, reducing flexibility and increasing stiffness, particularly in the morning or after long periods of rest.

4. Reduced Mobility or Muscle Tightness

Tight muscles around the knee—especially the quadriceps and hamstrings—can contribute to stiffness and reduced range of motion.

When Should You See a Doctor for Knee Swelling or Stiffness?

While mild symptoms may improve with rest and activity modification, you should schedule an evaluation if you experience:

• Swelling that doesn’t improve after a few days

• Persistent or worsening stiffness

• Pain that interferes with walking or daily activities

• A feeling of instability or weakness in the knee

• Locking, catching, or limited range of motion

• Swelling after an injury or sports activity

Early diagnosis can help prevent further damage and lead to more effective treatment options.

How Knee Swelling and Stiffness Are Treated

Treatment depends on the underlying cause but may include:

• Activity modification and rest

• Physical therapy to improve strength and mobility

• Anti-inflammatory treatments

• Joint injections

• Advanced imaging to identify structural problems

• Surgical options when conservative care isn’t effective

An orthopedic specialist can determine the best course of treatment based on your symptoms, activity level, and overall joint health.

Don’t Ignore Ongoing Knee Symptoms

Knee swelling and stiffness are common, but they’re not something you should ignore—especially if they keep coming back or limit your movement. Addressing the problem early can help you stay active, reduce pain, and protect your knee for the long term.

If you’re experiencing ongoing knee swelling or stiffness, a professional evaluation can provide answers—and a clear plan to get you moving comfortably again.