Cartilage is a tough, flexible connective tissue that gives parts of the body structure while allowing them to move smoothly. In joints, it forms a remarkably resilient surface that helps bones glide against one another and absorbs some of the forces generated by walking, running, lifting, and other movements.
But cartilage is more than a simple “cushion.” Its unusual composition and organization explain both its mechanical strength and one of its most important limitations: once damaged, it generally has a poor capacity to repair itself.
What is cartilage?
Cartilage is a type of connective tissue made primarily of specialized cells called chondrocytes embedded in a surrounding material known as the extracellular matrix. Unlike bone, cartilage does not contain blood vessels within its tissue. It is also generally less mineralized and more flexible than bone.
The extracellular matrix is what gives cartilage much of its physical character. It contains water, collagen fibers, and other large molecules called proteoglycans. Different proportions and arrangements of these components produce different types of cartilage with different mechanical properties.
Cartilage occurs throughout the body, not just in joints. It helps shape the nose and outer ear, supports the windpipe, contributes to the structure of the rib cage, and plays important roles in the growth and development of bones.
There are three major types: hyaline cartilage, elastic cartilage, and fibrocartilage. Articular cartilage—the cartilage covering the ends of bones in movable joints—is a specialized form of hyaline cartilage.
How cartilage protects and moves joints
In a typical synovial joint, such as the knee or hip, the ends of the bones are covered by a thin layer of articular cartilage. The opposing cartilage surfaces provide a smooth interface for movement.
Articular cartilage has a distinctive layered structure. Near the surface, collagen fibers are arranged in a way that helps the tissue withstand forces generated as joint surfaces move against each other. Deeper layers contain collagen arranged differently and progressively become more mineralized near the underlying bone.
Water is particularly important. Cartilage can contain a large amount of water, which is held within its molecular network. When a joint is compressed, some of this water shifts within the matrix. The interaction between water, proteoglycans, and collagen allows cartilage to resist compression without behaving like a rigid material.
This combination gives cartilage several jobs at once:
- Reducing friction: Its smooth surface helps joint surfaces move relative to one another.
- Distributing loads: It spreads forces over a broader area rather than concentrating them at a single point.
- Absorbing mechanical stress: Its water-rich matrix and molecular structure allow it to deform under load and recover when the load is reduced.
- Protecting underlying bone: The cartilage layer helps prevent direct bone-on-bone contact during normal joint movement.
Synovial fluid also contributes to joint lubrication. It is produced within the joint and contains molecules that help reduce friction at the cartilage surface. Cartilage and synovial fluid therefore work together rather than serving as interchangeable forms of cushioning.
The different types of cartilage
Although all cartilage shares basic characteristics, its structure is adapted to where it is found and what forces it must handle.
Hyaline cartilage
Hyaline cartilage is the most widespread type. It contains a relatively fine network of collagen and has a smooth, firm appearance.
It forms much of the cartilage in the respiratory tract, contributes to the rib cage, and covers the surfaces of many movable joints. It also forms much of the cartilage model involved in the development of the skeleton before and during bone formation.
Articular cartilage is a specialized form of hyaline cartilage adapted to repeated loading and low-friction movement.
Elastic cartilage
Elastic cartilage contains abundant elastic fibers in addition to its collagen network. These fibers allow it to bend and return toward its original shape.
It is found in structures that need both support and flexibility, including much of the external ear and the epiglottis, a flap that helps protect the airway during swallowing.
Fibrocartilage
Fibrocartilage is particularly well suited to resisting substantial compression and tension. It contains prominent bundles of collagen fibers, giving it greater toughness than typical hyaline cartilage.
It is found in structures such as the intervertebral discs and the menisci of the knee, where it helps withstand and distribute mechanical forces. The fibrocartilage of these structures is not identical to articular cartilage and should not be treated as though all cartilage behaves the same way.
Why cartilage heals slowly
One of cartilage’s defining biological features is its limited blood supply. Most cartilage has no blood vessels running through it, which restricts the delivery of oxygen, nutrients, and repair-related cells.
Chondrocytes can survive in this environment because cartilage has a specialized extracellular matrix and relatively low metabolic demands. But the same characteristics that help cartilage function mechanically also make substantial repair difficult.
The degree of healing depends strongly on the type and location of the injury. Some cartilage injuries can provoke a limited repair response, particularly where an injury reaches underlying tissues that have a blood supply. Other injuries, especially damage confined to the deeper layers of articular cartilage, have little capacity for spontaneous restoration of normal cartilage structure.
This distinction matters because a defect may fill with repair tissue without recreating the highly organized cartilage that was originally present. The resulting tissue can have different mechanical properties and may not perform as effectively as healthy articular cartilage.
What happens when cartilage wears down?
Cartilage does not simply disappear from normal use in a predictable, uniform way. It is living tissue that continually undergoes maintenance and remodeling. Problems arise when tissue breakdown and mechanical or biological stress outpace its ability to maintain itself.
In osteoarthritis, changes occur throughout the joint rather than exclusively in cartilage. Articular cartilage can become structurally damaged and lose some of its normal organization. At the same time, the underlying bone, joint lining, and other tissues can change.
As the disease progresses, the joint may become painful and stiff, and movement can become more difficult. Although osteoarthritis is often described casually as “wear and tear,” that phrase is incomplete. Age, previous joint injury, abnormal joint mechanics, body weight, genetics, and other biological and mechanical factors can all influence the development and progression of joint disease.
Cartilage itself has few or no pain-sensing nerve endings in the healthy articular layer. Consequently, cartilage damage does not necessarily produce pain in direct proportion to the amount of cartilage loss. Pain in an arthritic joint can arise from other structures, including the bone and tissues surrounding the joint.
What can damage cartilage?
Cartilage can be affected by both sudden injuries and longer-term processes.
A traumatic injury may damage articular cartilage directly or injure structures that work with it, such as the meniscus or ligaments. Repeated abnormal loading can also contribute to joint damage, particularly when the joint’s normal mechanics have been altered.
Inflammatory joint diseases can damage cartilage through persistent inflammation and changes in the joint environment. Certain metabolic and genetic conditions can also affect cartilage or the structures that support it.
Age is another important factor, but aging and disease are not synonymous. Cartilage changes over time, yet significant osteoarthritis is not an inevitable consequence of getting older.
Cartilage versus bone
Cartilage and bone are both connective tissues, but they are built for different mechanical and biological roles.
Bone is rigid, mineralized tissue with a strong blood supply and a capacity for substantial remodeling. Cartilage is generally more flexible, contains much more water, and relies heavily on its extracellular matrix for mechanical behavior.
Their differences are especially important during skeletal development. In many parts of the developing skeleton, cartilage provides a temporary framework that is gradually replaced by bone through endochondral ossification. Growth plates, which allow long bones to lengthen during childhood and adolescence, also depend on specialized cartilage.
In adults, cartilage continues to serve structural and mechanical functions even though most of the skeleton has become bone.
How cartilage is studied and evaluated
When a joint is painful or movement is limited, clinicians do not usually diagnose cartilage damage from symptoms alone. They consider the history and physical examination and may use imaging or other tests depending on the suspected problem.
X-rays are useful for evaluating bone and can show changes such as narrowing of the joint space that may indicate loss of cartilage, although cartilage itself is not directly visible on a standard X-ray.
MRI can provide much more information about soft tissues and can visualize articular cartilage, menisci, ligaments, bone marrow, and other structures. It is therefore particularly useful when the question involves a specific cartilage injury or other soft-tissue abnormality.
Importantly, an imaging finding does not always explain a person’s symptoms by itself. Joint pain and function depend on the condition of the entire joint and on factors beyond the appearance of cartilage.
Can damaged cartilage grow back?
In most adults, extensively damaged articular cartilage does not reliably regenerate into fully normal cartilage on its own. That limitation has made cartilage repair an important area of orthopedic medicine and tissue research.
Treatment depends on the type of damage, its location, symptoms, age, activity level, joint alignment, and other factors. Some cartilage problems are managed without surgery, using approaches aimed at controlling symptoms and maintaining strength, mobility, and joint function. Physical therapy can be important because the muscles and other tissues around a joint influence how forces are transmitted through it.
For selected cartilage defects, orthopedic procedures may attempt to stimulate repair, replace damaged tissue with a patient’s own cartilage, or use transplanted or engineered tissue. These approaches differ considerably in their indications, techniques, and ability to reproduce the structure and long-term performance of native articular cartilage.
There is no single treatment that restores every damaged cartilage surface to its original state. The biology of cartilage—and the fact that joint disease often involves more than cartilage alone—makes treatment more complicated than simply replacing a worn-out layer.
Why cartilage is such an unusual tissue
Cartilage succeeds mechanically because its structure is carefully matched to its job. Its collagen network provides tensile strength, proteoglycans and water help it resist compression, and its smooth surface supports efficient joint motion. Chondrocytes maintain this surrounding matrix, even though they operate in a tissue with limited access to the bloodstream.
That same biology creates a fundamental trade-off. Cartilage is excellent at handling repeated mechanical stress, but it has limited resources for repairing major structural damage. Understanding that balance explains why healthy cartilage can function for decades while serious cartilage injuries can become persistent problems.
For the joints to work well, cartilage is only one part of a larger system. Healthy movement also depends on the bones, muscles, ligaments, tendons, joint lining, synovial fluid, and the nervous system coordinating how the joint is loaded and controlled. Cartilage is the specialized interface that allows those components to work together with remarkably little friction under substantial mechanical forces.
