The knee is one of the largest and most complex joints in the body. The knee joins the thigh bone (femur) to the shin bone (tibia). The smaller bone (fibula) that runs alongside the tibia and the kneecap (patella) are the other bones that comprise the knee joint.

Tendons connect the knee bones to the leg muscles that move the knee joint. Ligaments join the knee bones and provide stability to the knee:

  • The anterior cruciate ligament prevents the femur from sliding backward on the tibia (or the tibia sliding forward on the femur).
  • The posterior cruciate ligament prevents the femur from sliding forward on the tibia (or the tibia from sliding backward on the femur).
  • The medial and lateral collateral ligaments prevent the femur from sliding side to side.


Two C-shaped pieces of cartilage called the medial and lateral menisci to act as shock absorbers between the femur and tibia.

Listed below are some of the common knee conditions requiring medical treatment.

Osteoarthritis is the most common form of arthritis, and often affects the knees. Caused by aging and wear and tear of cartilage, osteoarthritis symptoms may include knee pain, stiffness, and swelling.

Effusion is fluid buildup inside the knee, usually from inflammation. Any form of arthritis or injury may cause a knee effusion.

Meniscal Tear
Damage to a meniscus, the cartilage that cushions the knee, often occurs with twisting the knee. Large tears may cause the knee to lock.

ACL (anterior cruciate ligament) strain or tear
The ACL is responsible for a large part of the knee’s stability. An ACL tear often leads to the knee “giving out,” and may require surgical repair.

PCL (posterior cruciate ligament) strain or tear
PCL tears can cause pain, swelling, and knee instability. These injuries are less common than ACL tears, and physical therapy (rather than surgery) is usually the best option.

MCL (medial collateral ligament) strain or tear
This injury may cause pain and possible instability to the inner side of the knee.

Chondromalacia patella (also called patellofemoral syndrome)
This is an irritation of the cartilage on the underside of the kneecap (patella), causing knee pain. This is a common cause of knee pain in young people.

Patellar subluxation
The kneecap slides abnormally or dislocates along the thigh bone during activity; knee pain around the kneecap results.

Patellar tendonitis
This is an inflammation of the tendon connecting the kneecap (patella) to the shin bone. This occurs mostly in athletes from repeated jumping.

Knee bursitis
Symptoms include pain, swelling, and warmth in any of the bursae of the knee. Bursitis often occurs from overuse or injury.

Baker’s cyst
This is a collection of fluid in the back of the knee. Baker’s cysts usually develop from a persistent effusion as in conditions such as arthritis.

If your knee is severely damaged by arthritis or injury, it may be hard for you to perform simple activities, such as walking or climbing stairs. You may even begin to feel pain while you are sitting or lying down.

If nonsurgical treatments like medications and using walking supports are no longer helpful, you may want to consider total knee replacement surgery. Joint replacement surgery is a safe and effective procedure to relieve pain, correct leg deformity, and help you resume normal activities.

Knee replacement surgery — also known as knee arthroplasty (ARTH-row-plas-tee) — can help relieve pain and restore function in severely diseased knee joints. The procedure involves cutting away damaged bone and cartilage from your thighbone, shinbone, and kneecap and replacing it with an artificial joint (prosthesis) made of metal alloys, high-grade plastics, and polymers. Most knee replacements can be expected to last 15 years or more.

Artificial knees can wear out
Another risk of knee replacement surgery is the failure of the artificial joint. Daily use wears on even the strongest metal and plastic parts. Joint failure risk is higher if you stress the joint with high-impact activities or excessive weight.

Although today’s implants are designed to last many years, it’s possible that at some point in the future — typically 15 to 20 years or more — your prosthetic will break or wear out. If you’re overweight or you engage in high-impact activities such as running or court sports, the device may fail sooner.

When a knee replacement no longer functions correctly, revision surgery is often required. During this procedure, a surgeon replaces the old device with a new one. Revision surgery isn’t something to take lightly. It’s more complicated than a primary (or initial) total knee replacement (TKR) and entails many of the same risks.