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

Bones. Three bones meet to form your knee joint: your thighbone (femur), shinbone (tibia), and kneecap (patella).

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. 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 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. These include: Collateral Ligaments which 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 sideways motion of your knee and brace it against unusual movement. The cruciate ligaments 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 back and forth motion of your knee.

Tendons. Muscles are connected to bones by tendons. The quadriceps tendon connects the muscles in the front of your thigh to your patella. Stretching from your patella to your shinbone is the patellar tendon.

The most common knee injuries include fractures around the knee, dislocation, and sprains and tears of soft tissues, like ligaments. 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, your knee may catch or lock up. Many knee injuries cause instability — the feeling that your knee is giving way.