The knee is a complex hinge joint, recognized as the largest joint in the human body. It connects the thigh bone to the shin bone, enabling a wide range of movements. Understanding its internal structure is valuable for appreciating its function.
The Bones of the Knee
Three primary bones form the knee joint. The femur, or thigh bone, contributes to the upper part of the knee. Below it lies the tibia, the larger of the two lower leg bones, commonly known as the shin bone. These two bones articulate at the tibiofemoral joint, which is the main weight-bearing part of the knee.
The third bone is the patella, or kneecap, a thick, triangular bone positioned at the front of the knee. It sits within the quadriceps tendon and articulates with a groove on the femur, forming the patellofemoral joint. While the fibula, the smaller bone in the lower leg, runs alongside the tibia, it does not directly form part of the knee joint’s articulation.
Cartilage and Menisci
Within the knee, cartilage provides smooth surfaces for movement and absorbs impacts. Articular cartilage, a slick, white, flexible connective tissue, covers the ends of the femur, tibia, and the back of the patella. It reduces friction between the bones and helps absorb shock during movement. It relies on nutrient diffusion as it lacks a direct blood supply.
Complementing the articular cartilage are the menisci, two C-shaped pads of fibrocartilage located between the femur and tibia. The medial meniscus is on the inner side of the knee, while the lateral meniscus is on the outer side. These menisci distribute the body’s weight across the joint, deepen the joint cavity to improve stability, and act as shock absorbers.
Ligaments and Tendons
The knee’s stability and controlled movement rely on strong, fibrous connective tissues. Ligaments are bands that connect bones to other bones, providing essential support and limiting excessive motion. The knee has four main ligaments: the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL).
The ACL and PCL are located inside the knee, crossing each other to form an “X,” and control forward and backward movement of the tibia relative to the femur. The MCL is on the inner side of the knee, connecting the femur to the tibia, while the LCL is on the outer side, connecting the femur to the fibula. These collateral ligaments prevent the knee from moving too far sideways.
Tendons connect muscles to bones and facilitate movement. The quadriceps tendon connects the quadriceps muscles to the top of the patella, and the patellar tendon extends from the bottom of the patella to the tibia. Together, these tendons enable the leg to straighten at the knee.
Lubrication and Cushioning
Smooth and efficient knee function depends on internal fluid and specialized sacs that reduce friction and provide cushioning. Synovial fluid, a viscous, clear fluid, fills the joint cavity. This fluid is produced by the synovial membrane, a thin lining inside the joint capsule that covers surfaces not coated by articular cartilage. It acts as a lubricant, allowing bone surfaces to glide easily with minimal friction. It also nourishes the articular cartilage, which lacks its own blood supply, by transporting nutrients and removing waste products.
Bursae are small, fluid-filled sacs located around the knee joint. These bursae are situated where tendons, muscles, and bones might rub against each other, reducing friction and cushioning these structures during movement.