The knee is a very complex joint that bends to allow for walking, as well as a range of other motions, and it also bears a large proportion of the body’s weight. The knee joint is considered to be a hinge joint because its structure and range of motion resembles a hinge. The knee is also a joint that is prone to injury because of its structure and the large amounts of force it undergoes on a daily basis. The knee is prone to many different kinds of injuries, and understanding the anatomical structure of the knee is critical to understanding these injuries.
The knee joint is comprised of four different bones. The femur, or thigh bone, and the tibia, or shin bone, come together to make the functional aspect of the knee joint. The fibula, a thin bone, runs parallel to the tibia on the outer aspect of the leg. The fibula attaches to the tibia near the region of the knee joint. The patella, or knee cap, lies over the knee joint. The actual function of the knee cap is to provide protection to the joint and reduce friction between the bones and muscles of the knee.
The bones of the knee joint are supported by ligaments. The knee joint has four ligaments that all serve to stabilize the knee, plus a ligament that attaches the knee cap. If any of these ligaments are torn, the stability of the knee joint becomes compromised. Many sports injuries of the knee involve injury to these ligaments. The medial and lateral collateral ligaments of the knee lie on the sides of the knee. The medial collateral ligament attaches the femur to the tibia on the inside of the knee joint and the lateral collateral ligament attaches the femur to the head of the fibula on the outside of the knee joint. The posterior and anterior cruciate ligaments are located on the inside of the knee joint, and they provide most of the stability of the knee joint. They both attach the two main bones of the knee joint, the tibia and femur, and prevent the knee joint from sliding backwards or forwards or rotating too far. The anterior cruciate ligament is also called the ACL, and an ACL tear is a common sports injury. The patellar ligament connects the knee cap to the tibia; the knee cap is not connected via a ligament to the femur.
The two major muscle groups of the knee joint are the quadriceps muscles and the hamstrings. There are four quadriceps muscles that come together to form a common tendon and three hamstrings. Muscle tendons have a unique role in providing structural support to the front of the knee joint. The tendon of the quadriceps muscles in the front of the thigh not only keeps the patella in place, but also allows the quadriceps muscles to pull the leg into a straightened position (extension) when they contract. The quadriceps tendon extends past the patella to connect the knee cap to the tibia, but at this point it is called the patellar ligament because it connects a bone to another bone. The hamstrings make up the back of the thigh and they have the opposite action of the quadriceps muscles: to bend the knee joint.
The knee joint has cartilage between the femur and the tibia. There are two menisci (singular= “meniscus”) or fibrous cartilaginous discs, that attach to the top of the tibia. The one on the outside (lateral) aspect of the knee joint is called the lateral meniscus, and the inner one is called the medial meniscus. The knee joint also contains articular cartilage on the joint surfaces of the other bones that comprise the knee joint. Joint cartilage allows the knee joint to move without friction or pain. Osteoarthritis is a condition that can damage the joint cartilage and cause joint pain and stiffness in the knee. The menisci absorb shock and cushion the knee joint. The knee joint is surrounded by a capsule of fibrous connective tissue. The knee joint capsule itself is lined with a membrane called a synovial membrane and filled with a type of viscous fluid called synovial fluid, which acts as a shock absorber and friction reducer. People with arthritis may also have a decreased amount of synovial fluid in their joint capsules.
Another type of structure that is present within the knee joint is called a bursa. There are thirteen bursae associated with the knee joint. Bursae are fluid-filled sacs that reduce friction between various structures in a joint, for example, friction between a ligament or tendon and a bone. When a bursa becomes inflamed, a painful condition called bursitis may occur.