The knee is the joint where the bones of the lower and upper legs meet. The largest joint in the body, the knee moves like a hinge, allowing you to sit, squat, walk or jump.

The knee consists of three bones:

• femur – the upper leg bone, or thigh bone
• tibia – the bone at the front of the lower leg, or shin bone
• patella – the thick, triangular bone that sits over the other bones at the front of the knee, or kneecap.

The ends of the bones are covered with a layer of cartilage, a slick, elastic material that absorbs shock and allows the bones to glide easily against one another as they move.

Between the tibia and femur bone are two crescent-shaped pads of cartilage that reduce friction and disperse the weight of the body across the joint. They are:

• The lateral meniscus, situated at the outside of the knee.
• The medial meniscus, situated on the inside of the knee.

The bones are held together by a joint capsule, which consists of two distinct layers – an outer layer of dense connective tissue and an inner membrane, called the synovium, which secretes a fluid to lubricate the joint. 

The outer layer of the capsule is attached to the ends of the bones and is supported by these ligaments and tendons:

• quadriceps tendon, which attaches the quadriceps to the patella
• medial collateral ligament (MCL),  which gives stability to the inner part of the knee
• lateral collateral ligament (LCL), which stabilizes the outer part of the knee
• anterior cruciate ligament (ACL), which is located in the center of the knee and prevents excessive forward movement of the tibia
• posterior cruciate ligament (PCL), which is located in the center of the knee and prevents excessive backward shifting of the knee.

Two groups of muscles support the knee. They are:

• Hamstrings – muscles on the back of the thigh, which run from the hip to just below the knee and work to bend the knee.  
• Quadriceps – four muscles on front of the thigh that run from the hip to the knee and straighten the knee from a bent position.