The knee joint, which connects the thigh bone (femur) to the shin bone (tibia), relies on specialized tissues to manage the constant forces of movement and weight-bearing. The ends of these bones are covered in a smooth, protective layer of cartilage that allows for gliding motion. However, the C-shaped meniscus is often mistakenly identified as being made of the same material. Understanding the true function and nature of the meniscus requires examining the specific type of cartilage that forms it.
Defining Hyaline Cartilage
Hyaline cartilage is the most common type of cartilage found throughout the human body. It is characterized by its smooth, translucent, and glassy appearance. This tissue is primarily composed of fine, evenly distributed Type II collagen fibers within a firm, hydrated gel-like matrix. Hyaline cartilage contains a high water content, which gives it a shock-absorbing property while maintaining a low-friction surface.
The primary function of this material is to provide a slick surface for bones to articulate, ensuring smooth joint movement. Locations where hyaline cartilage is found include the ends of the long bones in movable joints, connecting the ribs to the breastbone, and forming the structural support for the nose and trachea. It is an avascular tissue, meaning it lacks a direct blood supply, and receives its nutrients through diffusion from the surrounding synovial fluid.
The True Composition of the Meniscus
The meniscus is not composed of hyaline cartilage but is instead made of a tougher, more resilient material known as fibrocartilage. This tissue is specifically adapted to withstand the enormous compressive and tensile forces that occur within the knee joint. The fundamental difference lies in the type of collagen that forms the structural backbone of the tissue.
Fibrocartilage contains dense, thick bundles of Type I collagen fibers, accounting for approximately 90% of its total collagen content. This high concentration of Type I collagen provides much higher tensile strength and a denser structure compared to the Type II collagen found in hyaline cartilage. The matrix is organized into a complex network, including strong circumferential fibers that resist “hoop stresses” when the knee is loaded.
This specialized composition allows the meniscus to function as a spacer and stabilizer. While hyaline cartilage provides a smooth, gliding surface, fibrocartilage is built for load distribution and absorbing the impact of movement. The crescent-shaped menisci act as wedges to improve the fit (congruity) between the rounded femur and the flatter tibia, distributing weight over a larger surface area.
Structure and Healing Implications
The unique fibrocartilaginous structure of the meniscus directly dictates its capacity for healing after injury. The blood supply to the meniscus is not uniform, which creates distinct zones with varying healing potential.
The outermost third of the meniscus, known as the “red zone,” is the only portion that receives a direct blood supply from the surrounding knee capsule. Tears in this peripheral zone have the greatest chance of healing naturally or with surgical repair.
Moving inward, the central two-thirds of the meniscus is largely avascular, termed the “white zone.” Tears that occur exclusively in this white zone generally do not heal because healing factors cannot reach the site of injury. A transitional area, the “red-white zone,” sits between the two, having a partial blood supply and intermediate healing potential.
The healing prognosis for a meniscal tear is highly dependent on its location within these zones. Tears in the well-supplied red zone are often candidates for surgical repair to preserve the tissue’s function. Conversely, tears isolated to the avascular white zone frequently require a partial removal of the damaged tissue, known as a meniscectomy, because repair is unlikely to be successful.