The knee joint represents a sophisticated hinge connecting the thigh bone (femur) to the shin bone (tibia), with the kneecap (patella) situated at the front. This complex structure allows for a range of motions, including bending and straightening the leg, while also accommodating slight rotational movements. Beyond the bones, soft tissues are responsible for enabling this intricate movement and ensuring the joint’s stability. These soft tissues are engineered to endure significant forces generated during everyday actions such as walking, running, and jumping.
The Knee Ligaments
Ligaments in the knee are strong, fibrous bands of connective tissue that serve to connect bones to other bones, providing stability and limiting excessive joint motion. Within the knee, four primary ligaments maintain the joint’s structural integrity. The anterior cruciate ligament (ACL) is positioned inside the knee joint, crossing from the femur to the tibia. Its main function involves preventing the tibia from shifting too far forward relative to the femur and stabilizing the knee against rotational forces.
The posterior cruciate ligament (PCL) also crosses within the knee joint, located behind the ACL. This ligament primarily prevents the tibia from sliding backward underneath the femur. Together, the ACL and PCL are known as the cruciate ligaments due to their cross-shaped arrangement within the joint.
On the inner side of the knee, the medial collateral ligament (MCL) connects the femur to the tibia. Its role is to resist forces that attempt to push the knee inward, protecting against valgus (knock-kneed) stress. Conversely, the lateral collateral ligament (LCL) is found on the outer side of the knee, connecting the femur to the fibula. The LCL counteracts forces that try to push the knee outward, protecting against varus (bow-legged) stress.
The Meniscus and Articular Cartilage
Within the knee, specialized cartilage structures play a role in cushioning the joint and facilitating smooth movement. The menisci are two C-shaped pieces of fibrocartilage, known as the medial meniscus and the lateral meniscus, positioned between the femur and the tibia. These structures act as shock absorbers, distributing weight across the joint surfaces and improving the fit between the rounded ends of the femur and the flatter top of the tibia. This improved congruence contributes to overall joint stability.
Articular cartilage is smooth, white, and slippery tissue that covers the ends of the bones within the joint, including the ends of the femur and tibia, as well as the back surface of the patella. Its primary function is to provide a low-friction surface, allowing the bones to glide against each other during movement. This smooth interface minimizes wear and tear on the bone surfaces.
The Knee Tendons
Tendons are cord-like tissues that connect muscles to bones, enabling the transmission of force necessary for movement. In the knee, these structures are important for the extensor mechanism, which is responsible for straightening the leg. The quadriceps tendon connects the quadriceps muscles located on the front of the thigh to the upper pole of the patella. This connection allows the quadriceps to exert force on the kneecap.
Extending from the bottom of the patella to the tibia is the patellar tendon. Although sometimes referred to as a ligament due to its bone-to-bone connection, it functions as a continuation of the extensor mechanism. The quadriceps tendon and patellar tendon work with the quadriceps muscle to pull on the lower leg, facilitating the extension of the knee joint. This coordinated action is for activities like standing, walking, and kicking.
The Supportive Synovial Structures
Beyond the primary ligaments, cartilage, and tendons, other soft tissue components contribute to the knee’s healthy function, primarily by ensuring a low-friction internal environment. The synovial membrane, also known as the synovium, is a thin layer of tissue that lines the inner surface of the joint capsule, but does not cover the articular cartilage. This membrane has a role producing synovial fluid, which is secreted into the joint space.
Synovial fluid is a viscous, clear liquid that serves multiple purposes within the knee. It acts as a lubricant, reducing friction between the articular cartilage surfaces during movement. This lubrication is comparable to oil in an engine, allowing smooth gliding and preventing excessive wear. The fluid also provides nourishment to the articular cartilage, which lacks its own direct blood supply.
Bursae are small, fluid-filled sacs located around the knee joint in areas subject to high friction. These sacs are typically found between tendons and bones, or between skin and bone, serving as cushions. Their presence allows tendons and other soft tissues to glide smoothly over bony prominences without irritation or damage.