Articular cartilage, a smooth, rubbery tissue between two and four millimeters thick, cushions the ends of bones wherever they meet at a joint. This tissue is remarkably slippery and resilient, able to absorb impact and distribute load across your knees, hips, shoulders, and every other movable joint in the body. But cartilage does not work alone. Synovial fluid, menisci, fat pads, and even the bone just beneath the cartilage surface all contribute to a cushioning system that is far more sophisticated than a simple rubber bumper.
What Articular Cartilage Actually Is
Articular cartilage is a dense, white connective tissue that caps the ends of bones inside a joint. Unlike most tissues in your body, it has no blood vessels, no lymphatic drainage, and no nerves.1PubMed Central. Articular cartilage degradation in osteoarthritis That makes it exceptional for absorbing shock quietly, but it also means the tissue has very limited ability to repair itself once damaged.
The tissue is mostly water, collagen fibers, and large sugar-protein molecules called proteoglycans. The collagen provides structural scaffolding, while the proteoglycans trap water and give cartilage its ability to spring back after being compressed. The cells that maintain all of this are called chondrocytes, and they are the sole residents of articular cartilage. Chondrocytes regulate the balance between building new matrix material and breaking down old material, keeping the tissue functional throughout your life.2PubMed Central. Role of Chondrocytes in Cartilage Formation, Progression of Osteoarthritis and Cartilage Regeneration They manage this by controlling the production of both structural components and specialized enzymes that break down worn-out matrix.3The Journal of Rheumatology. The Immunological Facets of Chondrocytes in Osteoarthritis: A Narrative Review
When cartilage sustains a small amount of damage, chondrocytes near the injured area ramp up their activity and try to lay down replacement material.4Biorheology: The Official Journal of the International Society of Biorheology. The role of chondrocyte–matrix interactions in maintaining and repairing articular cartilage This repair response works reasonably well for minor wear. The problem arises when the damage is large or chronic, because without a blood supply, the tissue cannot mount the kind of inflammatory healing response that a cut on your skin or a broken bone can.
The Lubrication System That Keeps Cartilage Working
Cartilage alone would grind itself apart within weeks without lubrication. That job falls to synovial fluid, a viscous liquid that fills the joint capsule. The key ingredient giving synovial fluid its slippery, egg-white consistency is hyaluronic acid, a large sugar molecule.5PubMed Central. Hyaluronan and synovial joint: function, distribution and healing Biomechanical testing in whole joints shows that hyaluronic acid produces less friction than saline or other comparison fluids, confirming its superiority as a lubricant.6PubMed Central. Influence of hyaluronic acid on intra-articular friction – a biomechanical study in whole animal joints
But hyaluronic acid doesn’t act in isolation. Another molecule, lubricin, coats the cartilage surface and works with hyaluronic acid in a way that neither could achieve alone. Lubricin appears to anchor hyaluronic acid near the cartilage surface, boosting the local viscosity right where surfaces slide past each other.7PLOS ONE. Elastoviscous Transitions of Articular Cartilage Reveal a Mechanism of Synergy between Lubricin and Hyaluronic Acid The result is a lubrication system that adapts to conditions. Under light loads, a thick fluid film separates the two cartilage surfaces entirely. Under heavier loads, the cartilage squeezes out fluid and the surfaces come closer together, but the hyaluronic acid-lubricin complex gets physically trapped in the pores of the cartilage’s collagen network, forming a kind of boundary lubricant that prevents direct surface-to-surface grinding and eliminates wear damage.8PubMed Central. Adaptive mechanically controlled lubrication mechanism found in articular joints
This adaptive behavior is part of why healthy joints can handle everything from gentle finger movements to the repeated high-impact forces of running. The lubrication shifts modes depending on the load, keeping friction extraordinarily low across a wide range of activities.
How Cartilage Feeds Itself Without Blood Vessels
Most living tissues receive oxygen and nutrients from blood vessels running through them. Cartilage has none. Instead, it relies on the very mechanical forces that it absorbs. When you walk, run, or simply stand and shift your weight, the cartilage compresses and then rebounds. During compression, fluid is squeezed out of the tissue. During rebound, fluid is drawn back in. This pumping action moves nutrients from the synovial fluid through the porous matrix and delivers them to the chondrocytes buried inside.9PubMed Central. The basic science of articular cartilage: structure, composition, and function
This means that moderate, regular movement is not just harmless to your cartilage — it is the mechanism by which cartilage stays fed. Prolonged immobility, such as being bedridden or casting a joint for weeks, can starve cartilage of nutrients. This is also why astronauts and researchers studying weightlessness worry about joint health in zero gravity, and why exercise research matters for understanding cartilage maintenance.
Other Structures That Share the Cushioning Load
Articular cartilage is the primary cushion, but several other structures pitch in, especially in joints that bear heavy loads.
Menisci
The knee contains two crescent-shaped pads of fibrocartilage called menisci, wedged between the femur and tibia. Unlike the smooth articular cartilage capping the bone ends, menisci are tougher and more fibrous, designed to spread load across a wider area. Biomechanical testing confirms that menisci contribute to shock absorption during both repetitive loading and sudden impacts.10PubMed Central. Knee Joint Menisci Are Shock Absorbers: A Biomechanical In-Vitro Study on Porcine Stifle Joints They also dissipate energy during cyclic motion, helping to stabilize the knee and protect the articular cartilage underneath from excess stress.11Osteoarthritis and Cartilage. Mechanisms of energy dissipation and relationship with tissue composition in human meniscus
When a meniscus tears, the consequences illustrate just how much work it does. A complete radial tear in the lateral meniscus significantly increases peak contact pressure on the underlying cartilage and reduces the contact area, concentrating force on a smaller patch of bone.12PubMed. Different effects of the lateral meniscus complete radial tear on the load distribution and transmission functions depending on the tear site Over time, this uneven loading accelerates cartilage wear and raises the risk of osteoarthritis.
Fat Pads
Tucked behind the patellar tendon in the knee is a structure called the infrapatellar fat pad, or Hoffa’s fat pad. Its exact role is still debated, but it appears to act as a secondary cushion, distributing and dampening mechanical stresses during joint movement.13Journal of the Mechanical Behavior of Biomedical Materials. Investigation of biomechanical response of Hoffa’s fat pad and comparative characterization Similar fat pads exist in other joints, including the heel and elbow, where they fill gaps and absorb forces that cartilage alone does not handle.
Subchondral Bone
Just beneath the articular cartilage lies a thin plate of dense bone called the subchondral bone. Research on impact compression shows that both cartilage and subchondral bone absorb shock, but the bone absorbs far more energy when loads become extreme enough to cause fracture. This might spare the cartilage from immediate injury under a single hard impact, though it can set the stage for post-traumatic arthritis later.14Journal of the Mechanical Behavior of Biomedical Materials. Shock absorbing ability of articular cartilage and subchondral bone under impact compression
Why Cartilage Damage Doesn’t Always Hurt Right Away
Because articular cartilage contains no nerves, you can have significant cartilage damage and feel nothing at all. The pain people associate with worn cartilage comes from surrounding structures: the bone underneath, the joint lining, ligaments, and the fat pad, all of which are richly innervated.1PubMed Central. Articular cartilage degradation in osteoarthritis This is why early cartilage loss often goes unnoticed. By the time a joint hurts, the damage may have progressed far enough to irritate neighboring tissues or to expose the nerve-rich bone beneath.
This silent damage is one reason many doctors now advocate for MRI-based cartilage assessments in people at risk. Quantitative MRI techniques can detect changes in cartilage composition — shifts in water content, proteoglycan density, and collagen integrity — before the cartilage visibly breaks down on a standard scan.15PubMed Central. Quantitative MRI techniques of cartilage composition These methods offer a window into whether your cartilage cushion is healthy or quietly deteriorating.
How Aging Wears Down the Cushion
Cartilage changes with age in ways that make it progressively less resilient. The tissue thins, the collagen network becomes stiffer, and proteoglycans break down. On a cellular level, chondrocytes decline in number and become less responsive. They enter a state of senescence in which they produce abnormal proteins and fail to repair damage the way younger cells would.16PubMed Central. Effects of aging on articular cartilage homeostasis The matrix they are supposed to maintain accumulates chemically modified proteins from a process called non-enzymatic glycation, which stiffens the collagen and makes it more brittle.17Osteoarthritis and Cartilage. Aging and osteoarthritis: the role of chondrocyte senescence and aging changes in the cartilage matrix
At the chemical level, aging cartilage loses water content in its deeper layers and accumulates certain sulfate molecules, further altering its mechanical properties.18Annals of the Rheumatic Diseases. Variation of chemical composition with age in human femoral head cartilage The net result is a cushion that compresses less easily, recovers more slowly, and is more vulnerable to cracking and fraying. These aging changes do not inevitably lead to arthritis, but they lower the threshold at which injury, obesity, or genetic susceptibility can tip a joint into disease.
When the Cushion Breaks Down Entirely
Osteoarthritis is the most common result of severe cartilage loss. In osteoarthritis, the balance between building and breaking down matrix tips decisively toward destruction. Specialized enzymes chew through the proteoglycans and collagen that give cartilage its structure. The most important of these are the aggrecanases, particularly one called ADAMTS-5, which attacks aggrecan, the main proteoglycan in cartilage. Another enzyme, MMP-13, targets collagen.19PubMed Central. Proteases involved in cartilage matrix degradation in osteoarthritis Together, these enzymes dismantle the two pillars of cartilage structure, and the tissue cannot rebuild fast enough to keep up.
This enzymatic destruction begins early, often before a person feels any symptoms. The aggrecanases active in osteoarthritis are the same family of enzymes implicated in rheumatoid arthritis, and they cut the aggrecan molecule at a specific location that researchers have pinpointed precisely.20PubMed Central. Aggrecanases and cartilage matrix degradation Understanding that cut site has become a target for drug development, though no therapy has yet managed to fully halt cartilage destruction once it starts.
Exercise and Cartilage Thickness
Given that movement is how cartilage receives its nutrients, it’s not surprising that exercise affects cartilage health. Research on plyometric (jump-based) training in mice found that repeated jumping sessions led to measurably thicker articular cartilage in the knee, with low-frequency dynamic compression appearing to stimulate the chondrocytes to produce more matrix. The jumping may also have increased the transport of growth-promoting hormones into the cartilage.21npj Microgravity. Plyometric training increases thickness and volume of knee articular cartilage in mice Earlier animal studies had shown similar results with moderate running.
The key word is moderate. Extremely heavy or prolonged loading without adequate recovery can damage cartilage faster than it can repair itself. The sweet spot appears to be regular, varied activity with enough rest in between for the tissue to rehydrate and recover. For people worried about their joints, the evidence leans firmly away from the old idea that running or jumping inevitably wears out your knees. Reasonable exercise appears to help cartilage, not harm it.
Cartilage in Children and How It Changes
In children, the story of cartilage at bone ends is more complex because of growth plates. A growth plate is a region of cartilage near the end of a long bone where the bone actually lengthens. The chondrocytes in the growth plate multiply, enlarge, and then die in a controlled sequence, after which they are replaced by bone-forming cells that convert the cartilage into permanent bone tissue.22PubMed Central. The growth plate: a physiologic overview This process of forming cartilage and then replacing it with bone is what drives a child’s height increase over years.23PubMed. The role of the growth plate in longitudinal bone growth
Once growth is complete in late adolescence, the growth plates close and are replaced entirely by bone. The articular cartilage on the joint surface, however, remains for life. This is the cartilage that cushions the bone ends in adults. The distinction matters because injuries to a child’s growth plate can affect bone length and alignment, while injuries to the articular surface affect the cushioning and lubrication system that protects the joint throughout adulthood.
Approaches to Replacing Damaged Cartilage
Because cartilage heals so poorly on its own, regenerating or replacing it has been a major goal of biomedical research. One of the most promising directions involves hydrogels, which are water-swollen polymer networks that can be shaped to fill a cartilage defect. These gels can mimic the mechanical, swelling, and lubricating behavior of real cartilage, and when loaded with chondrocytes, they encourage the cells to produce new matrix. In laboratory settings, researchers have engineered cartilage tissue inside hydrogels that approaches the mechanical properties of native cartilage.24PubMed Central. Hydrogels for the repair of articular cartilage defects
A persistent challenge is getting any engineered tissue to bond with the surrounding native cartilage. One recent approach used a modified alginate gel that chemically adheres to the tissue around it, producing stronger bonds than previous designs. The gel also incorporated sulfate groups to better imitate the chemical makeup of real cartilage, and chondrocytes planted inside it responded by increasing their production of key structural proteins.25PubMed. Tyrosinase-crosslinked, tissue adhesive and biomimetic alginate sulfate hydrogels for cartilage repair These lab advances have not yet translated into routine clinical treatments, but the gap between engineered and real cartilage continues to narrow.
Cartilage Thickness Varies Across Species
Articular cartilage is not the same everywhere in the animal kingdom. A comparative study of salamanders found that aquatic species had significantly thicker articular cartilage in their limb bones than terrestrial species — roughly double the thickness as a proportion of bone diameter in the upper arm bone.26Frontiers in Ecology and Evolution. Variation in Articular Cartilage Thickness Among Extant Salamanders and Implications for Limb Function in Stem Tetrapods The likely explanation is that aquatic species rely more on cartilage for skeletal support and joint function, while land-dwelling species shift more of the structural load to mineralized bone. This pattern offers clues about how the earliest land-walking vertebrates transitioned from water to land, gradually reducing their cartilage and building up bone as their joints adapted to the demands of gravity.
In humans, cartilage thickness also varies by joint and by location within a joint. Weight-bearing areas of the knee tend to have thicker cartilage than non-weight-bearing areas, and the patella (kneecap) has some of the thickest cartilage in the body. These differences reflect the loads each area routinely handles, a principle that holds from salamanders to professional athletes.