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The knee joint is the largest joint in the body, consisting of 4 bones and an extensive network of ligaments and muscles.
Bones of the knee joint
The knee is made up of four main bones- the femur or thigh bone, the tibia which is known as the shin bone, the fibula or outer shin bone and patella or kneecap. The main movements of the knee joint occur between the femur, patella and tibia.
Each are covered in articular cartilage which is an extremely hard, smooth substance designed to decrease the frictional forces as movement occurs between the bones.
The patella lies in an indentation at the lower end of the femur known as the intercondylar groove.
At the outer surface of the tibia is the fibula, a long thin bone that travels right down to the ankle joint. The function of the tibia is predominantly load bearing whilst the function of the fibula is predominantly to provide a surface for muscles to attach to.
The knee joint capsule
The joint capsule is a thick ligamentous structure that surrounds the entire knee. Inside this capsule is a specialized membrane known as the synovial membrane which provides nourishment to all the surrounding structures. The synovial membrane produces synovial fluid which lubricates the knee joint. Other structures include the infrapatellar fat pad and bursa which function as cushions to exterior forces on the knee. The capsule itself is strengthened by the surrounding ligaments.
Ligaments of the knee joint
The stability of the knee owes greatly to the presence of its ligaments. Each has a particular function in helping to maintain optimal knee stability in a variety of different positions.
The medial Collateral Ligament or MCL for sort is a band that runs between the inner surfaces of the femur and the tibia. It resists forces acting from the outer surface of the knee called valgus forces and prevents the knee from collapsing inwards. The medial knee ligament has two parts to it; an inner part which attaches to the cartilage meniscus on the inside of the knee and an outer part with attaches to the tibia bone.
The lateral Collateral Ligament or LCL is on the outside of the knee and joins the outer surface of the femur to the head of the fibula. It resists impacts from the inner surface of the knee known as varus forces.
The anterior Cruciate Ligament or ACLis one of the most important structures in the knee, not least because injury to it may require extensive surgery and rehabilitation. The cruciate ligaments are so called because they form a cross in the middle of the knee joint. The ACL, travels from the anterior or front of the tibia to the posterior or back of the femur and prevents the tibia moving forward. It is most commonly injured in twisting movements.
The posterior Cruciate Ligament or PCL attaches from the posterior or back surface of the tibia to the anterior or front surface of the femur and in doing so wraps around the ACL. The posterior cruciate ligament prevents the knee joint from being bent back the wrong way.
Each knee joint has two crescent-shaped cartilage menisci. These lie on the medial or inner and lateral or outer edges of the top surface of the tibia bone. They are essential components, acting as shock absorbers for the knee as well as allowing for correct weight distribution between the tibia and the femur.
Muscle groups surrounding the knee joint
The two main muscle groups of the knee knee joint are the quadriceps and the hamstrings. Both play a vital role, both moving and stabilizing the knee joint.
The quadriceps muscle group is made up of four different individual muscles which join together forming the quadriceps tendon. This thick tendon connects the muscle to the patella which in turn connects to the tibia via the patella tendon. Contraction of the quadriceps pulls the patella upwards and extends the knee. The quadriceps muscles consist of the biceps femoris, vastus medialis, vastus intermedius and vastus lateralis muscles.
The hamstring muscles at the back of the thigh function in flexing or bending the knee as well as providing stability on either side of the joint line. The hamstring muscles consist of the biceps femoris, semitendinosus and semimembranosus. See knee joint muscles for more detailed information.