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Different views of the normal anatomy of the knee.

The knee is the largest joint in the body, and one of the most easily injured. It is made up of four main structures: bones, cartilage, ligaments, and tendons.

• Bones. Three bones meet to form your knee joint: the femur (thighbone), the tibia (shinbone), and the patella (kneecap). The patella sits in front of the joint to provide some protection.
• Articular cartilage. The ends of the femur and tibia, and the back of the patella are covered with articular cartilage. This slippery substance helps your knee bones glide smoothly across each other as you bend or straighten your leg.
• Meniscus. Two wedge-shaped pieces of meniscal cartilage act as shock absorbers between your femur and tibia. Different from articular cartilage, the meniscus is tough and rubbery to help cushion and stabilize the joint. When people talk about torn cartilage in the knee, they are usually referring to a torn meniscus.
• Ligaments. Bones are connected to other bones by ligaments. The four main ligaments in your knee act like strong ropes to hold the bones together and keep your knee stable.
  • Collateral ligaments. These are found on the sides of your knee. The medial collateral ligament is on the inside of your knee, and the lateral collateral ligament is on the outside. They control the side-to-side motion of your knee.
  • Cruciate ligaments. These are found inside your knee joint. They cross each other to form an X with the anterior cruciate ligament in front and the posterior cruciate ligament in back. The cruciate ligaments control the front and back motion of your knee.
• Tendons. Muscles are connected to bones by tendons. The quadriceps tendon connects the muscles in the front of the thigh to the patella. The patellar tendon, on the other hand, runs from the patella to the tibia.

The most common knee injuries include tears of soft tissues (e.g., ligaments, meniscus), fractures, and dislocation. In many cases, injuries involve more than one structure in the knee.

Pain and swelling are the most common signs of knee injury. In addition, the knee may catch or lock. Some knee injuries (e.g., ACL tear) cause instability — the feeling that your knee is giving way.

Anterior Cruciate Ligament (ACL) Injuries

The anterior cruciate ligament is often injured during sports activities. ACL injuries are more likely to occur in athletes who participate in cutting and pivoting sports like soccer, football, and basketball. Changing direction rapidly or landing from a jump incorrectly can tear the ACL.

About half of all injuries to the anterior cruciate ligament occur along with damage to other structures in the knee, such as articular cartilage, meniscus, or other ligaments.

Learn more about ACL injuries:

• Anterior Cruciate Ligament (ACL) Injuries
• ACL Injury: Does It Require Surgery?

Posterior Cruciate Ligament (PCL) Injuries

The posterior cruciate ligament is often injured from a blow to the front of the knee, while the knee is bent. This often occurs in motor vehicle crashes and sports-related contact. Posterior cruciate ligament tears tend to be partial tears with the potential to heal on their own.

Learn more about PCL injuries:

• Posterior Cruciate Ligament Injuries

Collateral Ligament Injuries

Injuries to the collateral ligaments are usually caused by a force that pushes the knee sideways. These are often contact injuries.

Injuries to the MCL are usually caused by a direct blow to the outside of the knee and are often sports-related.

Blows to the inside of the knee that push the knee outwards may injure the lateral collateral ligament (LCL). Lateral collateral ligament tears occur less frequently than other knee injuries.

Learn more about collateral ligament injuries:

• Collateral Ligament Injuries

Meniscal Tears

Acute meniscal tears often happen during sports. Tears in the meniscus can occur when twisting, cutting, pivoting, or being tackled.

Meniscal tears may also occur as a result of arthritis or aging. Even an awkward twist when getting up from a chair may be enough to cause a tear if the menisci have weakened with age.

Learn more about meniscal tears:

• Meniscus Tears

Tendon Tears

The quadriceps and patellar tendons can be stretched and torn. Although anyone can injure these tendons, tears are more common among middle-aged people who play running or jumping sports. Falls, direct force to the front of the knee, and landing awkwardly from a jump are common causes of knee tendon injuries.

Learn more about tendon tears:

• Patellar Tendon Tear
• Quadriceps Tendon Tear

Fractures

The most common bone broken around the knee is the patella. The ends of the femur and tibia where they meet to form the knee joint can also be fractured. Many fractures around the knee are caused by high energy trauma, such as falls from significant heights and motor vehicle collisions.

Learn more about common fractures of the knee:

• Patellar (Kneecap) Fractures
  Distal Femur (Thighbone) Fractures of the Knee
  Fractures of the Proximal Tibia (Shinbone)

When you are first injured, the RICE protocol can help speed your recovery:

• Rest. Avoid putting weight on the painful knee.
• Ice. use cold packs for 20 minutes at a time, several times a day. Do not apply ice directly on the skin.
• Compression. Lightly wrap the injured area in a soft bandage or ace wrap.
• Elevation. To reduce swelling, elevate the injury higher than your heart while resting.

Be sure to seek treatment as soon as possible, especially if you:

• Hear a popping noise and feel your knee give out at the time of injury
• Have severe pain
• Cannot move the knee
• Begin limping
• Have swelling at the injury site

The type of treatment your doctor recommends will depend on several factors, such as the severity of your injury, and your age, general health, and activity level.