The knee joint is the most complicated joint in the body. It consists of three articulations– two tibiofemoral and one patellofemoral. As is characteristic of synovial joints, the knee joint has a thin articular capsule that partially surrounds this joint cavity. Additionally, several ligaments, muscles, and cartilaginous structures support the movement of the knee.
A total of seven ligaments support the knee joint. The patellar ligament, which is also attached to the quadriceps femoris group of muscles, is the major ligament that stabilizes the knee joint. Two other ligaments, the lateral and medial collateral ligaments, prevent excessive sideways movement of the femur at the knee joint. The joint's posterior surface contains two popliteal ligaments between the femur, the tibia, and the fibula. Unlike these ligaments, the anterior and posterior cruciate ligaments, or ACL and PCL respectively, are present within the articulating capsule. These ligaments cross each other between the condyles of the femur. While the anterior cruciate ligament prevents hyperextension of the knee, the posterior cruciate ligament supports the knee during weight bearing, and together they provide rotational stability to the knee.
Like other synovial joints of the body, the knee joint has fibrocartilage structures between the articulating bones. These articular discs, called the lateral and medial menisci, are large, C-shaped structures that serve several functions, such as shock absorption, cushioning between the bones, and smooth movements between the articulating bones.
Additional structures such as bursae prevent friction between the bones of the joint and the overlying muscle tendons or skin. A bursa is a thin connective tissue sac filled with lubricating liquid. Within a knee joint, the suprapatellar bursa separates the tendon of the large anterior thigh muscle from the distal femur just above the knee, and the prepatellar bursa allows the skin to move smoothly over the patella.
This text is partially adapted from Openstax, Anatomy and Physiology 2e, 9.4 Synovial Joints
