Knee cartilage is a tough, smooth tissue that covers the surfaces where bones meet inside your knee joint. It serves as both a cushion and a gliding surface, allowing your knee to bend, straighten, and bear your body weight without bone grinding against bone. Your knee actually contains two distinct types of cartilage, each with a different structure and job.
Two Types of Cartilage in the Knee
The first type is articular cartilage, a slippery white tissue that coats the ends of the three bones forming the knee: the thighbone, the shinbone, and the kneecap. It creates the smooth surface that lets these bones glide past each other during movement. Articular cartilage is firm but slightly flexible, typically 2 to 4 millimeters thick depending on the location within the joint.
The second type is the meniscus cartilage. You have two menisci in each knee, C-shaped wedges that sit between the thighbone and shinbone like rubber washers. While articular cartilage is glassy and smooth, meniscus cartilage is tougher and more fibrous. It acts as a shock absorber, distributing the force of each step across a wider area so no single spot on the articular cartilage takes too much punishment. Meniscus tears are the most common knee injuries overall, which is why you’ll sometimes hear “cartilage tear” and “meniscus tear” used interchangeably, even though they’re technically different structures.
What Cartilage Is Made Of
Cartilage looks simple from the outside, but its internal structure is surprisingly complex. The bulk of it is extracellular matrix, a dense scaffold built primarily from a protein fiber called collagen and large molecules called proteoglycans. Collagen fibers give cartilage its shape and tensile strength, forming an interwoven framework that resists being pulled apart. Proteoglycans fill the spaces between those fibers and attract water, which makes up a large portion of the tissue’s total weight.
That water content is critical. When you step down on your knee, the pressurized fluid inside the cartilage helps bear the load, essentially turning the tissue into a hydraulic cushion. Scattered throughout the matrix are specialized cells called chondrocytes. These cells build and maintain the surrounding structure, but they’re relatively few in number and slow to work, which has major consequences for healing.
How Cartilage Keeps Your Knee Moving
Healthy articular cartilage, bathed in the joint’s natural lubricating fluid (synovial fluid), produces remarkably little friction. In laboratory measurements, the friction coefficient of cartilage lubricated by synovial fluid comes in around 0.02 to 0.03. For comparison, ice sliding on ice has a friction coefficient of about 0.03. Your knee joint is, in engineering terms, one of the most efficient bearings in existence.
This near-frictionless performance comes from two mechanisms working together. At the moment you load the joint, pressurized fluid inside the cartilage and between the joint surfaces carries the weight, so the solid parts of the tissue barely touch each other. Over longer periods of sustained pressure, lubricant molecules on the cartilage surface take over, forming a slippery boundary layer that prevents the surfaces from sticking. The combination means your knee can handle everything from a sudden jump landing to hours of standing with minimal wear on the tissue.
How Cartilage Gets Its Nutrients
One of the most important things to understand about cartilage is that it has no blood supply. Unlike bone, muscle, or skin, cartilage is completely avascular. It also lacks nerves and lymphatic vessels. This is why cartilage damage often progresses silently for years before you feel pain: the pain signals come from surrounding structures, not the cartilage itself.
Without blood vessels, chondrocytes get their oxygen and glucose through diffusion from the synovial fluid that fills the joint capsule. Small nutrient molecules seep into the cartilage the way a sponge absorbs water. There’s a common belief that joint movement “pumps” nutrients deeper into the tissue, and while cyclic loading does help move larger molecules around (increasing their transport rate by 30 to 100%), the small molecules that cartilage cells depend on most, like oxygen and glucose, travel effectively by diffusion alone. Still, regular movement keeps synovial fluid circulating across the cartilage surface, which is one reason prolonged immobility is bad for joint health.
Why Cartilage Heals So Poorly
The same features that make cartilage a brilliant engineering material also make it terrible at repairing itself. Because there are no blood vessels, the body can’t deliver the inflammatory cells and growth factors that normally kickstart tissue repair elsewhere. Chondrocytes are few, slow to divide, and embedded deep within a dense matrix that limits their ability to migrate to a damaged area. Once cartilage is lost, the body has very limited ability to replace it with tissue of the same quality.
Chondrocytes also rely on a cellular recycling process called autophagy to manage stress and stay healthy. When that recycling system breaks down, often due to aging or chronic overload, the cells begin to die off, accelerating the breakdown of the surrounding matrix. This creates a vicious cycle: less matrix means more mechanical stress on the remaining tissue, which damages more cells, which leads to further matrix loss.
How Cartilage Breaks Down Over Time
The gradual wearing away of knee cartilage is the hallmark of osteoarthritis, the most common joint disease worldwide. The knee, which absorbs enormous cumulative stress over a lifetime, is one of the joints most frequently affected. Cartilage breakdown follows a fairly predictable progression.
In the earliest stage, the cartilage shows minor surface roughening and early wear. You probably won’t feel anything at this point. By stage 2, you may start to notice occasional pain and stiffness, but enough cartilage remains to keep the bones separated. Stage 3 brings significant cartilage loss: pain becomes more frequent during walking, running, squatting, or kneeling, and the joint often feels stiff after sitting for a while. By stage 4, the cartilage is nearly or completely gone in parts of the joint, leaving bone to grind directly against bone. The knee may feel stiff, swollen, and painful with almost any weight-bearing activity.
Articular cartilage damage can also happen suddenly, from a sports injury or a hard impact. These acute injuries can present as thinning, cracks, flaps that catch during movement, or full-thickness defects where a piece of cartilage breaks away entirely. Acute damage in younger people doesn’t always progress to osteoarthritis, but it does increase the long-term risk, particularly if the injury also involves the meniscus or ligaments.
What Keeps Cartilage Healthy
Because cartilage can’t meaningfully regrow once lost, protecting what you have matters more than with almost any other tissue. The factors that accelerate cartilage loss are well established: excess body weight (every extra pound adds roughly three to four pounds of force across the knee with each step), repetitive high-impact loading without adequate recovery, prior joint injuries, and muscle weakness around the knee that shifts mechanical stress onto the cartilage.
Regular moderate exercise, particularly activities that load the joint without excessive impact, helps maintain cartilage health by promoting synovial fluid circulation and keeping the surrounding muscles strong. Strengthening the quadriceps and hamstrings gives the joint better shock absorption and stability, reducing the mechanical demand on the cartilage itself. Maintaining a healthy weight is one of the single most effective strategies for preserving knee cartilage over time, because it directly reduces the cumulative force the tissue has to absorb with every step you take.