Articular cartilage is the smooth, white connective tissue that covers the ends of bones wherever they meet to form a joint. It’s only 2 to 4 mm thick, but it performs a remarkable job: allowing your joints to glide with almost zero friction while absorbing the forces of every step, squat, and jump you take. What makes this tissue unusual, and clinically important, is that it contains no blood vessels, no nerves, and no lymphatic drainage. That means it can’t signal pain directly when damaged, and it has an extremely limited ability to heal itself.
Where Articular Cartilage Is Found
Articular cartilage lines the surface of every freely movable joint in your body, the type of joint anatomists call a diarthrodial joint. Your knees, hips, shoulders, ankles, elbows, wrists, and the small joints of your fingers and toes all have it. The knee is the most commonly discussed location because it bears enormous loads and is especially vulnerable to cartilage injuries, but the same tissue exists in every joint that allows a wide range of motion.
What It’s Made Of
Despite feeling firm and rubbery, articular cartilage is mostly water. Water accounts for 65% to 80% of the tissue’s total mass. The rest is a dense scaffolding called the extracellular matrix, built primarily from two components: collagen fibers (10% to 20% of cartilage mass) and large sugar-protein molecules called proteoglycans (10% to 15%). Nearly all of the collagen is type II, a variety uniquely suited to resisting tension and maintaining the tissue’s shape.
Scattered throughout this matrix are the cells responsible for maintaining it: chondrocytes. These cells are surprisingly sparse. Cartilage is one of the least cellular tissues in the body, which is a key reason it repairs so poorly after injury. Chondrocytes produce and maintain the collagen and proteoglycan network around them, but there simply aren’t enough of them to mount a significant repair response when damage occurs.
Four Layers With Different Jobs
If you could slice articular cartilage and look at it under a microscope, you’d see four distinct zones stacked from the joint surface down to the underlying bone.
- Superficial zone: The outermost layer, in direct contact with the joint fluid. Its chondrocytes are flattened, and its collagen fibers run parallel to the surface, creating a smooth, low-friction barrier that resists shearing forces.
- Middle zone: A thicker transitional layer where collagen fibers are arranged more randomly and cells are rounder. This zone acts as a bridge between the surface and the deeper, load-bearing layers.
- Deep zone: Collagen fibers here are oriented vertically, perpendicular to the bone surface, making this zone especially good at resisting compressive loads pushed down through the joint.
- Calcified zone: The bottom layer, where cartilage gradually transitions into bone. It anchors the deep zone’s collagen fibers into the subchondral bone below. Numerous protrusions and interlacing structures in this zone give excellent resistance to shear forces, preventing cartilage from peeling away from the bone.
How It Handles Mechanical Loads
Articular cartilage is an exceptional shock absorber, and the reason comes down to how water interacts with the solid matrix around it. When you put weight on a joint, the load compresses the cartilage and pressurizes the fluid trapped inside. That pressurized fluid carries a significant portion of the total load, reducing the stress on the collagen-proteoglycan framework. At the same time, the proteoglycans carry a negative electrical charge that causes them to repel one another, creating a natural springiness that resists compression.
Three internal forces work together: the stress in the deformed solid matrix, the pressure within the fluid phase, and the frictional drag as fluid flows through the matrix. Together, these produce a viscoelastic response, meaning cartilage behaves like both a spring and a shock absorber depending on how quickly the load is applied.
The friction performance is striking. Under normal conditions, the coefficient of friction between two cartilage surfaces stays between 0.002 and 0.01, as long as the fluid within the tissue is still pressurized and supporting the load. For comparison, ice sliding on ice has a coefficient around 0.03. Cartilage-on-cartilage is smoother than ice.
How Cartilage Gets Its Nutrients
Because articular cartilage has no blood supply, its cells depend on an indirect feeding system. Nutrients can theoretically reach chondrocytes through two routes: diffusion from the synovial fluid that bathes the joint, or diffusion upward from blood vessels in the bone beneath. In children and adolescents, both routes contribute. In adults, however, a calcified barrier between cartilage and bone effectively blocks the subchondral route, making synovial fluid the dominant, and essentially the only, source of nutrition for mature cartilage.
This is one reason joint movement matters so much for cartilage health. When you bend and straighten a joint, the compression and release cycles act like a pump, pushing waste products out and drawing nutrient-rich synovial fluid in. Prolonged immobility starves the tissue. Research shows that loss of synovial fluid nutrition leads to far more severe cartilage degeneration than loss of the subchondral bone route.
Why It Heals So Poorly
Articular cartilage is famously bad at repairing itself. Two features explain this: it has very few cells, and it has no blood supply to deliver the inflammatory and growth signals that kickstart healing in other tissues. When you cut your skin, blood rushes to the site, forms a clot, and delivers stem cells and growth factors. None of that happens in cartilage.
When cartilage is damaged and left untreated, the body does attempt a patch job, but the replacement tissue is fibrocartilage, a scar-like material that lacks the organized structure, smoothness, and durability of the original. Fibrocartilage wears down faster under load, which is why untreated cartilage injuries often lead to early-onset osteoarthritis.
What Happens in Osteoarthritis
Osteoarthritis is fundamentally a disease of articular cartilage breakdown. The process starts when chondrocytes, normally quiet maintenance cells, become abnormally activated. They shift their behavior and begin producing enzymes that chew through the collagen and proteoglycan matrix around them. The cartilage surface develops tiny cracks and fibrillations. Chondrocytes cluster together in a failed repair attempt. The zone of calcification creeps upward, and blood vessels from the bone below begin penetrating into tissue that was previously sealed off.
The breakdown products from degraded cartilage further fuel the cycle, triggering more enzyme production, more inflammation-like signaling, and eventually chondrocyte death. Once the collagen network is destroyed, it cannot be rebuilt to its original state. This irreversibility is what makes osteoarthritis a progressive condition and why so much clinical attention focuses on catching cartilage damage early.
Current Repair Options
Because cartilage won’t regenerate on its own, several surgical strategies have been developed to address focal defects, particularly in younger, active patients where joint replacement isn’t appropriate.
Microfracture is the simplest approach: a surgeon pokes small holes through the damaged area into the underlying bone marrow, allowing blood and stem cells to fill the defect. The result is a fibrocartilage patch, functional but not as durable as native cartilage. Autograft transplantation takes small plugs of healthy cartilage and bone from a less weight-bearing area of the joint and moves them to the damaged site. It works well for smaller defects but is limited by how much donor tissue is available and the risk of creating problems at the harvest site.
Autologous chondrocyte implantation, or ACI, was a landmark technique that involves harvesting a patient’s own cartilage cells, growing them in a lab, and reimplanting them. While considered a major milestone in cartilage surgery, ACI frequently produces fibrocartilage rather than true hyaline cartilage, with inferior mechanical properties compared to the original tissue. Newer variations using scaffold membranes have improved on the original technique but still face this limitation.
No current method perfectly recreates the layered, fluid-pressurized, nearly frictionless surface of healthy articular cartilage. Each technique offers trade-offs between durability, invasiveness, and the quality of the repair tissue produced.