Articular cartilage is mostly water, making up 70% to 80% of the tissue by weight. The remaining solid portion is a dense mesh of collagen fibers, large sugar-protein molecules called proteoglycans, and a small population of living cells called chondrocytes. These components work together to create a material that is simultaneously slippery, resilient, and strong enough to absorb the forces of daily movement across your joints.
Water: The Dominant Ingredient
It seems counterintuitive that a tissue tough enough to cushion your knees during a run is mostly liquid, but water is the single largest component of articular cartilage. The concentration varies by depth: near the joint surface, water content can reach 80%, while deeper layers are closer to 65%. This water isn’t just sitting there passively. It’s bound up with electrically charged molecules in the matrix, and when the joint bears weight, the water resists being squeezed out. That resistance is a major source of cartilage’s ability to handle compressive loads.
Because cartilage has no blood vessels, this water also serves as the delivery system for nutrients. Oxygen and glucose reach the cartilage cells primarily by diffusing through the water from the synovial fluid that bathes the joint. The rhythmic loading and unloading of the joint during walking may help move fluid in and out of the tissue, though for small molecules like glucose and oxygen, simple diffusion does most of the work.
Collagen: The Structural Scaffold
Collagen provides the tensile backbone of cartilage, preventing the tissue from being pulled apart or sheared during movement. Type II collagen accounts for 90% to 95% of the collagen present. Its molecules are arranged as triple-helix chains that bundle into tightly packed fibrils roughly 50 nanometers in diameter. These fibrils are reinforced by smaller collagen types (IX and XI) that crosslink to the type II network and help stabilize it. Type IX collagen, for instance, covalently attaches to type II fibrils with projections that stick outward, connecting to other matrix components and keeping the whole network organized.
The orientation of these collagen fibers changes depending on depth. At the surface, fibers run parallel to the joint face, forming a tough protective sheet that resists shear forces. In the middle zone, they angle obliquely. In the deep zone, they stand perpendicular to the joint surface, anchoring the cartilage to the underlying bone. This layered architecture means the tissue responds differently to forces depending on which direction they come from.
Proteoglycans: The Compression Absorbers
If collagen handles tension, proteoglycans handle compression. The most abundant proteoglycan in articular cartilage is aggrecan, a massive molecule with a protein core and hundreds of sugar chains branching off it. These sugar chains carry strong negative electrical charges. In the watery environment of the cartilage matrix, those negative charges repel each other, causing the aggrecan molecule to swell and spread out, pulling in water as it does.
When you step down and compress the joint, you push those negatively charged chains closer together. Their mutual repulsion increases, generating a force that pushes back against the load. At the same time, the collagen network restrains the proteoglycans from swelling indefinitely, creating a constant tug-of-war. This balance between proteoglycan swelling and collagen tension is what gives cartilage its springy, resilient quality. Without enough aggrecan, the tissue loses its ability to bounce back from compression. Without intact collagen, the proteoglycans would swell unchecked and the tissue would fall apart.
Chondrocytes: The Maintenance Crew
Chondrocytes are the only living cells in articular cartilage, and they fill just 5% to 10% of the tissue’s total volume. They are round or oval, scattered throughout the matrix, and they don’t contribute mechanically to the tissue’s strength. Instead, their job is to build and maintain everything around them. They produce new collagen and proteoglycans, and they break down old or damaged matrix components. In healthy cartilage, this cycle of construction and breakdown stays in balance.
The challenge is that chondrocytes work slowly and in isolation. Cartilage has no blood supply and no nerve endings. When damage occurs, chondrocytes can’t recruit immune cells or healing factors the way other tissues do. This is a major reason cartilage injuries heal poorly or not at all, and why conditions like osteoarthritis, where the breakdown outpaces repair, tend to progress over time.
Minor Matrix Components
Beyond the big three of collagen, proteoglycans, and water, the cartilage matrix contains smaller molecules that fine-tune its behavior. Type VI collagen concentrates in the thin shell immediately surrounding each chondrocyte, forming a protective microenvironment called the pericellular matrix. Small proteoglycans like decorin and biglycan interact with the collagen fibrils, influencing how they form and how they connect to one another. Growth factors embedded in the matrix help regulate chondrocyte activity, signaling cells to ramp up or slow down production as conditions change.
How Thick Cartilage Actually Is
All of this complex architecture is packed into a remarkably thin layer. In the knee, one of the most studied joints, the average cartilage thickness on the femur (thighbone) is about 2.3 mm. Specific areas range from roughly 2.0 mm on the outer (lateral) condyle to 2.5 mm on the inner back surface of the bone. Younger adults tend to have thicker cartilage; one study of people under 30 found femoral cartilage between 3.6 mm and 4.3 mm. Thickness varies by joint, by location within a joint, and by individual factors like age, body weight, and activity level.
That a layer of tissue thinner than a stack of two coins can distribute the forces of walking, running, and jumping across a lifetime speaks to how precisely its components are engineered. The water resists compression, the collagen resists tension, the proteoglycans create osmotic pressure, and the chondrocytes quietly maintain all of it. When any one of those components degrades, the entire system begins to falter.