The knee is the body's largest joint. It's the place where three bones meet: the tibia, the femur and the patella. The knee is a "hinge" joint. It allows the leg to bend in one direction only. Let's take a closer look at the main parts of the knee's anatomy.

The anterior cruciate ligament, commonly called the ACL, is a thick, elastic band of tissue that runs from the bottom of the femur to the top of the tibia. It helps stabilize the knee joint. The ACL can become stretched or torn when the knee is twisted or hyperextended. For reasons that are not fully understood, ACL injuries are much more common in women than in men.

This injury is a tearing of the ACL ligament in the knee joint. The ACL ligament is one of the bands of tissue that connects the femur to the tibia. An ACL tear can be painful. It can cause the knee to become unstable.

This condition occurs when a bone's normal blood supply is disrupted. The affected bone cells die and the dead bone weakens. The bone may begin to fracture and collapse, leading to arthritis.

This bulge, also called a "popliteal cyst," forms behind the knee. It's a swollen bursa. A bursa is a fluid-filled sac that helps reduce friction between soft tissues and bones. There are several of these sacs in the knee and in other joints. A swollen bursa can be uncomfortable, or even painful.

This is a swelling of a fluid-filled sac called a "bursa." It's on the inner side of your knee, between the tibia and the tendons that attach to your hamstring muscle. You have similar sacs near other large joints throughout your body. They act as cushions between your bones and your soft tissues. Normally they have a small amount of fluid inside them. But sometimes they can swell. We call that "bursitis."

This is a type of chronic, long-lasting, pain. In most cases, it develops in an arm or a leg that you have previously injured. With CRPS, you may have unexplained pain that won't go away. It may be severe, and it may spread.

This condition is a blood clot that forms in a vein deep inside the body. This type of clot most commonly develops in the legs. This condition is dangerous, because the clot can break free and travel through the bloodstream to the lungs.

The hamstrings are three powerful muscles that travel along the back of your thigh. They are the semitendinosus, the semimembranosus and the biceps femoris muscles. They help bend your knee and extend your leg behind your body. Because these muscles handle high loads of stress, they are susceptible to stretching and tearing. This is called a muscle "strain." Hamstring strains are common among runners, dancers, and athletes who play sports that require sudden starts and stops.

This is a problem on the outer side of your thigh. It's an inflammation of the iliotibial band. That's a thick band of tissue that spans from your hip to your shinbone. When this band becomes in inflamed, it can hurt.

Joint pain is a common problem for many people. It can keep you from doing the things you want to do. It can make it hard for you to be as active as you like. Let's take a moment to learn about the causes of joint pain, and what you can do about it.

Our knees take a lot of force. They support the weight of the body. They keep us upright and stable. They cushion us when we walk and run. So it's not unusual to feel knee pain at some time in our lives. Let's learn about knee pain, and what you can do about it.

This is a stretching or tearing of a ligament on the outer side of your knee. The lateral collateral ligament, commonly called the "LCL", connects the femur to the fibula. The LCL helps stabilize your knee. This ligament, along with the medial collateral ligament, helps prevent excessive side-to-side movement of your knee joint. It helps keep the upper and lower leg aligned properly.

This is a stretching or tearing of a ligament on the inner side of your knee. The medial collateral ligament, commonly called the "MCL", is connected to the femur and to the tibia. The MCL helps stabilize your knee. This ligament, along with the lateral collateral ligament, helps prevent excessive side-to-side movement of your knee joint. It helps keep the upper and lower leg aligned properly.

This is a common injury of the knee. Your knee joint is cushioned by two c-shaped wedges of cartilage called the "menisci." Each individual cushion is called a "meniscus." This injury is a tear of one of these cushions.

This is a type of injury that causes knee pain in children. It's an inflammation of a growth plate in the tibia. Growth plates are places where new bone tissue forms. They are found in the long bones of growing children. But growth plates are weaker than the surrounding bone. That makes them easier to injure.

Knee pain may keep you from being as active as you like. And it may come from a gradual breakdown of your knee's cartilage. That's a protective tissue on the ends of your bones. In a healthy knee, the bones glide smoothly against each other. But in a knee with osteoarthritis, cartilage begins to wear away. Bone rubs against bone. Bony bumps we call "bone spurs" may form.

This condition is characterized by the death of an area of cartilage and bone in the knee joint. The dead section may remain in place, forming a lesion, or it may loosen and partially detach from the surrounding bone. It may break away completely and float around inside the joint.

This is a break of the patella. That's the small bone in the front of your knee often called the "kneecap." The kneecap protects the joint and helps link your thigh muscles to your lower leg. A fracture can cause pain and other problems.

This is a pain you feel just below your patella (the bone we commonly call the "kneecap"). It's an injury to your patellar tendon, which connects the bottom of the kneecap to the shinbone.

This is a tear of a large tendon in the front of your knee. We call it the "patellar tendon." It connects the bottom of the kneecap to the top of the shinbone. It helps you straighten your leg. You can have a tear that doesn't go all the way through the tendon, or you can tear the tendon completely. A complete tear is a disabling injury.

This is a problem with the alignment of the patella (the bone at the front of the knee, commonly called the "kneecap"). With this disorder, the patella shifts out of its normal track as you bend or extend your knee.

This is a pain you feel in the front of your knee. It involves the patella. That's the bone we commonly call the "kneecap." The patella slides up and down in a groove on your femur as you bend and extend your knee. If you have this syndrome, you may have injured the soft tissues that support and cushion your kneecap. Or, you may have some damage to the cartilage on the underside of the kneecap.

Strong bands of tissue called "ligaments" help stabilize the bones that form the knee joint. One of these ligaments is called the "PCL." It helps connect the femur to the tibia. If you stretch or tear a PCl, your knee may become unstable.

This condition is an inflammation of the prepatellar bursa, a fluid-filled sac that covers the front of the kneecap. Prepatellar bursitis results in pain and swelling at the front of the knee.

This condition is a tear of the tendon that connects the patella to the quadriceps muscles of the thigh. The quadriceps muscle is used to straighten the leg from the bent position. A complete rupture of the quadriceps tendon is a disabling injury.

This is an infection in the knee joint. It causes your knee to become painfully inflamed. Without proper treatment, it can become a chronic problem. It can permanently damage your knee.

This is pain you feel in the front of one or both of your lower legs. It can be a problem for runners, dancers, gymnasts and other active people.

This condition is a break in the femur at the knee joint. The fracture can range from a small crack to a full break that allows part of the bone to separate. This injury can involve the cartilage on the base of the femur in the knee joint, and may increase the risk of developing knee arthritis later in life.

This is a break of the shinbone. That's the larger of the two bones in the lower leg. Tibias are strong bones that support most of your body's weight.

This condition is a fracture at the top of the tibia, also called the shin bone. This fracture usually involves both bone and cartilage, so there is a high risk of developing arthritis from injury to the cartilage cells.

This fracture happens in the center of the knee, between two cushions of cartilage known as the "menisci." It involves a bony protrusion we call the "tibial spine." It's where the anterior cruciate ligament (commonly called the "ACL") attaches to the tibia. A tibial spine fracture can cause the knee to become unstable.