The meniscus is a crescent-shaped pad of fibrocartilage found within a joint, most notably the knee. This tissue provides mechanical support and stability to the joint surfaces. While the term can refer to similar structures in the wrist or jaw, it is primarily associated with the two menisci in the knee joint. This article explores the location, biological makeup, and essential biomechanical roles of this frequently injured component.
Anatomical Definition and Placement
A pair of menisci is situated within the knee joint, positioned between the rounded bottom of the thigh bone (femur) and the flat top of the shin bone (tibia). These structures are often called “semi-lunar” cartilages due to their half-moon or C-shaped appearance. The menisci deepen the shallow socket of the tibial plateau, which improves the fit between the two bones.
The knee contains two menisci: the medial and the lateral. The medial meniscus is located on the inner side and is shaped like an open “C.” The lateral meniscus is found on the outer side and is more circular, resembling a tight “C.”
The primary difference between them is their attachment and mobility. The medial meniscus is firmly attached to the joint capsule and the medial collateral ligament, making it less mobile and more susceptible to injury during twisting. In contrast, the lateral meniscus is not directly attached to the lateral collateral ligament and has a looser connection, allowing it to shift more freely on the tibial plateau.
Biological Composition and Tissue Structure
The meniscus is composed of fibrocartilage, a tough tissue characterized by a high concentration of collagen fibers embedded in a dense extracellular matrix. This matrix is about 72% water by weight; the dry weight consists mainly of collagen and proteoglycans. The collagen is predominantly Type I, accounting for up to 90% of the dry weight, and provides the tensile strength needed to resist pulling forces.
The cells maintaining this matrix are called fibrochondrocytes, which synthesize and maintain the intricate network of collagen fibers. These fibers are organized mainly in a circumferential pattern to resist the “hoop stresses” created when the joint is loaded. This architecture allows the meniscus to convert vertical compressive forces into outward-directed tensile forces.
The meniscus has a limited blood supply, which significantly impacts its ability to heal. In adults, only the outer 10% to 25% of the tissue, known as the red-red zone, receives a direct blood supply. The inner two-thirds, called the white-white zone, is avascular and receives nutrition solely through diffusion from the synovial fluid. The peripheral outer zone also contains sensory nerve endings, contributing to the joint’s proprioception, or sense of position.
Essential Functions in Joint Movement
The meniscus performs several functions essential for the stability and health of the knee joint. One primary role is load transmission and distribution. The menisci transmit a significant portion of the compressive load, handling 50% of the load in the medial compartment and 70% in the lateral compartment.
By increasing the contact area between the rounded femoral condyles and the flat tibial plateau, the menisci spread the weight across a larger surface. This action dramatically reduces stress on the articular cartilage, the smooth tissue covering the ends of the bones. Without this distribution, contact stress can increase by over 100%, accelerating wear and tear on the joint surfaces.
The menisci also function as effective shock absorbers, cushioning the joint against impact forces during activities like running or jumping. During movement, the wedge-shaped profile contributes to joint stabilization. This shape helps prevent excessive forward and backward movement of the femur on the tibia, especially in knees with ligamentous laxity.
A final function involves joint lubrication and nutrition. The menisci aid in the circulation of synovial fluid, the joint’s natural lubricant. By pushing the fluid over the articular cartilage surfaces, the menisci assist in delivering nutrients and removing waste products, which maintains cartilage health.