The knee functions as a modified hinge joint, connecting the femur, tibia, and patella, allowing for the essential movements of bending and straightening the leg. Cartilage is the strong, flexible connective tissue that lines the joint surfaces. This specialized tissue acts as a protective layer and shock absorber, ensuring the bones glide smoothly past one another and dissipating the forces placed upon the joint.
The Two Primary Types of Knee Cartilage
The knee joint contains two distinct forms of cartilage, each with a different structure and mechanical role. Hyaline cartilage, often called articular cartilage, is a smooth, shiny tissue that covers the ends of the bones, providing a low-friction surface for movement. Fibrocartilage is tougher and denser due to its thick collagen fibers, built for resilience and impact absorption, and forms the menisci in the knee.
Mapping the Articular Cartilage
Articular cartilage is distributed across the articulating surfaces of the three bones that form the knee joint. This smooth, thin layer coats the rounded femoral condyles and the corresponding flat top surface of the tibial plateau. It also covers the back surface of the patella, which slides within a groove on the femur. This slick, elastic coating allows the bones to move with minimal resistance, reducing friction and distributing compressive loads across the joint surfaces.
The Menisci Location and Role
The menisci are two C-shaped wedges of fibrocartilage located internally between the femur and the tibia, resting on the tibial plateau. There is one on the inner side (medial meniscus) and one on the outer side (lateral meniscus) of the knee. Their wedge-like shape deepens the shallow surface of the tibia, improving the fit between the bones. The menisci function as dynamic shock absorbers, distributing the weight-bearing load across the joint and contributing to overall stability.
Why Cartilage Damage is Biologically Significant
Cartilage tissue presents a unique biological challenge because it is isolated from the body’s repair systems. This tissue is avascular, meaning it lacks a direct blood supply, and its cells receive nutrients slowly through diffusion from the surrounding joint fluid. This lack of blood flow is the primary reason damaged cartilage cannot heal or regenerate effectively. Furthermore, healthy cartilage is aneural, containing no nerve endings, which is why damage often goes unnoticed initially. Pain only registers when the underlying bone or surrounding innervated tissues become irritated due to the loss of the protective layer, leading to the progressive condition of osteoarthritis.