The knee is the largest joint in your body and the one that bears most of your weight. Because of this, it’s a tough—and sometimes problem-prone—joint.

Learn more: Types of Arthritis in the Knee

When you're feeling knee pain, how do you know what caused it? Your doctor's diagnosis is the ultimate answer, but there are some clues to get you started on determining the cause.

Knee osteoarthritis develops as a joint’s cartilage wears down. Watch: Knee Osteoarthritis Video

Osteoarthritis

According to the CDC, nearly half of all people will experience symptoms of knee osteoarthritis at some point in their lives.

See What Is Knee Osteoarthritis?

That's why it's important to know the symptoms of knee osteoarthritis:

  • Intermittent knee pain that gradually gets worse
  • Knee pain that appears or gets worse when strain is put on the joint
  • Pain in just one knee
  • Knee stiffness, especially after a period of inactivity
  • Knee swelling
  • A knee that locks or gives out
  • See a complete guide to symptoms of knee osteoarthritis.
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Bursitis

When the thin, fluid-filled sac that protects the joint (known as a bursa) becomes infected or inflamed, this is bursitis.

Learn more: Knee (Prepatellar) Bursitis

Many of the symptoms of knee bursitis are similar to osteoarthritis, but there are a few distinctive characteristics:

  • A swollen region on the knee that’s “squishy” to the touch
  • Tenderness when pressure is put on the knee
  • Warmth or redness of the knee
  • Fever or illness, which is a symptom of an infected bursa (also known as septic bursitis)
  • See a complete list of knee bursitis symptoms.

Knee pain from injury

Damage to the muscles, ligaments, tendons, and cartilage of the knee can cause pain if these tissues are overused or receive a blow or other injury.

This is particularly true in the case of these common knee injuries:

  • A tear in the knee’s anterior cruciate ligament (ACL)
  • A patellar tendon injury, also known as “jumper’s knee”
  • Damage to the tendon between the kneecap and femur (thigh bone), known as patellofemoral pain syndrome (PPS) or “runner’s knee”
  • A tear in the pads of cartilage in the knee known as the meniscus

Visit the knee injury section on Sports-health.com for in-depth, doctor authored and reviewed articles about these and other types of knee injuries.

Rheumatoid arthritis

Rheumatoid arthritis is a system-wide condition in which the immune system is attacking the joints and causing inflammation. So when rheumatoid arthritis shows up in the knee, chances are the smaller joints in the hands and feet have already been affected by pain and inflammation.

Learn more: What Is Rheumatoid Arthritis?

Rheumatoid arthritis in the knees can cause:

  • Pain and inflammation in both knees at the same time
  • Knees that are swollen, red, or warm to the touch
  • Stiffness in the morning, which can last 30 minutes or more despite light activity
  • General fatigue
  • Fever
  • See a complete list of symptoms for rheumatoid arthritis.

If you’re experiencing knee pain that’s not relieved by a few weeks of self-care using NSAID pain medications and the RICE (rest, ice, compression, elevation) method, make an appointment to see your doctor so you can receive the right diagnosis and treatment.

Learn more

Q&A: Diagnosing a Swollen Knee

Knee Pain and Arthritis