Articular Surface: Function, Damage, and Health

Articular surfaces are the smooth, cartilage-covered ends of bones that meet inside every moveable joint in your body. They allow your knees, hips, shoulders, and fingers to glide with remarkably little friction, absorb impact from every step and jump, and distribute loads so that no single point on the bone takes too much punishment. When articular cartilage is healthy, its performance outstrips any bearing material engineers have created. When it is damaged, the consequences cascade quickly, because cartilage has almost no ability to repair itself.

What Articular Cartilage Is Made Of

Articular cartilage looks simple from the outside, but its internal architecture is surprisingly layered. It is organized into three distinct zones: a thin superficial zone at the very top, a thicker middle zone, and a deep zone that sits just above the underlying bone.¹ Each zone has collagen fibers oriented differently: parallel to the surface up top (to resist shear), randomly arranged in the middle, and perpendicular to the bone at the bottom (to anchor everything in place). This layered design is what gives cartilage its combination of flexibility and strength.

The tissue itself is mostly water, collagen, and a family of large molecules called proteoglycans. Proteoglycans like aggrecan act as tiny molecular sponges, trapping water within the cartilage matrix and giving it the ability to resist compression. Other proteoglycans, including biglycan, decorin, and perlecan, help maintain cartilage health and participate in repair processes when damage occurs.² The cells that live within this matrix, called chondrocytes, make up less than five percent of the tissue’s volume. They are responsible for building and maintaining everything around them, but they are sparse and relatively inactive compared to cells in most other tissues. This low cellularity is one of the main reasons cartilage heals so poorly.

Beneath the cartilage lies a calcified transition zone and then the subchondral bone. In healthy joints, there is limited molecular traffic between the cartilage and the bone. But in osteoarthritis, vascular channels and marrow cavities break through the boundary between the two, increasing the transport of signaling molecules and accelerating the disease process.³

How Joints Stay So Slippery

The friction inside a healthy joint is astonishingly low. Measured friction coefficients can start as low as about 0.01, which is lower than ice sliding on ice.⁴ The secret is not one lubrication system but several working in concert.

The dominant mechanism is fluid pressurization. When you load a joint, the water trapped inside the cartilage matrix pressurizes and supports most of the applied force. Experimental work has shown that this pressurized fluid can carry close to 90 percent of the load, leaving very little direct contact between the solid components of the opposing surfaces.⁵ As fluid slowly seeps out under sustained loading, friction rises. In laboratory experiments, friction coefficients climbed from around 0.01 at the start of loading to about 0.24 over the course of minutes to hours, while fluid load support dropped from roughly 89 percent down to about 9 percent.⁴ This is why standing motionless for very long periods can feel uncomfortable, while walking, which cyclically reloads and unloads the cartilage, keeps the fluid pressurized and the joint well lubricated.

On top of fluid pressurization, two key molecules work at the cartilage surface itself. Lubricin (also called PRG4) coats the outermost layer, and hyaluronic acid fills the synovial fluid bathing the joint. Research has shown that lubricin helps concentrate hyaluronic acid near the articular surface, locally boosting viscosity and preventing the joint from slipping into a high-friction state where the solid surfaces actually rub together.⁶ Computational models confirm that fluid pressurization also helps generate a self-sustaining lubrication cycle under shear loading, not just compression.⁷

The Load-Bearing Sweet Spot

Articular cartilage thrives under the right amount of mechanical stress. Loading drives nutrients into the tissue and flushes waste products out, while simultaneously providing the mechanical cues that chondrocytes need to maintain the matrix around them. Across different joints and different species, static peak contact stresses cluster in a range of roughly 0.5 to 5.0 MPa, suggesting that joints across the animal kingdom have evolved toward a common mechanical design window.⁸

Chondrocytes sense these forces through receptors and structures in their cell membranes, including ion channels, hair-like projections called primary cilia, and proteins called integrins that connect the cell to the surrounding matrix. These sensors convert physical stress into biochemical signals that regulate what genes the cell turns on or off.⁹ When the load is within the normal range, the result is healthy tissue maintenance. When it exceeds that range, the response can flip. Research has shown that mechanical overload reduces levels of estrogen receptor-alpha in chondrocytes, a receptor that appears to play a protective role. This may be one reason why excessive loading tips the balance toward cartilage breakdown.¹⁰

The upper cartilage zones play a disproportionate role in protecting the bone underneath. When the superficial and middle zones are experimentally compromised, cartilage stiffness drops by roughly 39 percent and energy absorption in the underlying bone jumps by about 32 percent, making the bone far more vulnerable to damage under impact.¹¹ In other words, the surface layers of cartilage are not just a frictionless coating; they are a critical shock absorber.

How Cartilage Damage Begins

Cartilage breakdown rarely starts with a single dramatic event, though a bad injury can accelerate the process. In osteoarthritis, the earliest visible changes are subtle: cell death in the superficial zone, reduced staining of the surrounding matrix (indicating proteoglycan loss), and fine roughening of the surface called fibrillation. In a rodent model, these changes appeared within days of a destabilizing injury and progressed over two weeks to deeper fissuring and outright matrix loss.¹²

A central driver of degeneration is the activity of enzymes, particularly matrix metalloproteinases (MMPs) and enzymes in the ADAMTS family, which actively chop apart collagen and proteoglycans. Inflammatory signals from cytokines ramp up production of these enzymes in chondrocytes and surrounding synovial cells, creating a feedback loop where inflammation triggers matrix destruction, and matrix fragments trigger more inflammation.¹³

Traumatic joint injuries follow a related but distinct path. Both high-energy injuries like fractures through the joint surface and lower-energy injuries like ligament tears can lead to post-traumatic osteoarthritis. The mechanisms include direct chondrocyte death from impact, dysfunction of the cells’ energy-producing mitochondria, production of damaging reactive oxygen species, and inflammatory cascades in both the cartilage and the synovial membrane.¹⁴

Why Cartilage Barely Heals on Its Own

Most tissues in the body repair themselves by recruiting blood vessels and stem cells to the injury site. Cartilage lacks a blood supply. It gets its nutrients entirely from synovial fluid and from the slow diffusion of molecules through its dense matrix. When the damage stays within the cartilage itself and does not penetrate down to the bone, the healing response is minimal. Experimental studies in animal models have shown that purely cartilage-level defects produce very little cellular activity at the injury site.¹⁵

When the defect does penetrate through to the bone, blood and marrow cells rush in and produce a repair tissue. But the tissue that forms is fibrocartilage, a tougher, less smooth material that lacks the organized structure and low-friction properties of native articular cartilage. It is better than an open wound, but it wears down faster and does not perform as well mechanically. This distinction between shallow and deep defects is one of the fundamental frustrations in orthopedic medicine and shapes every treatment strategy.

Detecting Damage Before It Becomes Obvious

Standard X-rays can reveal advanced joint damage, when the space between bones has already narrowed and bone spurs have formed, but by that stage the cartilage is already severely degraded. Catching trouble earlier requires different tools.

Specialized MRI techniques have shown real promise. Two approaches, known as T1rho and T2 mapping, measure how water molecules in cartilage interact with the surrounding matrix. In people with osteoarthritis, both T1rho and T2 values are significantly elevated compared to healthy controls, and both correlate with disease severity.¹⁶ Of the two, T1rho appears more sensitive to the earliest biochemical changes that precede visible cartilage loss, while T2 mapping is more widely available and easier to add to a routine MRI protocol.¹⁷ After anterior cruciate ligament reconstruction, T1rho has shown particular sensitivity for tracking early degeneration in the medial compartment of the knee.¹⁸

Blood and urine markers are also being investigated. Urinary CTX-II, a fragment of type II collagen breakdown, has been associated with knee, hip, and hand osteoarthritis in a large meta-analysis, and adding it to standard risk factors like age, sex, and BMI modestly improved the prediction of knee OA progression.¹⁹ Serum COMP, another marker released as cartilage degrades, has shown potential for predicting OA progression.²⁰ Combining both markers in animal models has improved accuracy for both early diagnosis and severity staging.²¹ These biomarkers are not yet used as standalone diagnostic tests in routine clinical practice, but they are getting closer to clinical utility, particularly when combined with imaging.

On the mechanical side, indentation sensors that can be threaded through an arthroscope allow surgeons to measure cartilage stiffness directly during a procedure. Cadaver and clinical validation studies have confirmed that these instruments give reliable readings, offering a way to quantify softening that might not be visible on the cartilage surface.²² Miniaturized versions using fiber-optic force sensors are under development, potentially making real-time mechanical assessment of cartilage even more practical during surgery.²³

Surgical and Regenerative Options

Because cartilage cannot heal itself effectively, orthopedic medicine has developed a range of interventions that try to either stimulate a repair response or replace the damaged tissue outright.

Microfracture is one of the oldest and simplest approaches. A surgeon uses a small awl to punch tiny holes through the subchondral bone beneath the defect, allowing blood and marrow cells to fill the damaged area. The resulting tissue is fibrocartilage. A systematic review of the contemporary literature found that second-look arthroscopies generally revealed well-integrated fibrocartilaginous repair tissue.²⁴ It works best for smaller defects, and the repair tissue tends to wear over time because it lacks the organized structure of the original.

For larger defects, cell-based therapies aim to produce something closer to true articular cartilage. Matrix-induced autologous chondrocyte implantation (MACI) involves harvesting a patient’s own cartilage cells, growing them in a laboratory, seeding them onto a scaffold membrane, and then implanting the membrane into the defect. It is an established therapy for chondral defects in the knee.²⁵ The procedure requires two surgeries, one to harvest cells and one to implant, and recovery can take months. But the resulting tissue more closely resembles normal articular cartilage than what microfracture produces.

Research is pushing beyond these established methods. Scaffold-based tissue engineering uses biomaterial frameworks designed to mimic the natural cartilage matrix, providing a structure that guides cells toward producing cartilage-like tissue.²⁶ Both synthetic polymers and hybrid materials combining natural and synthetic components are being explored, with the central challenge being to create a temporary environment that lets cells behave as they would in native tissue.²⁷ Stem cell therapies using mesenchymal stem cells and induced pluripotent stem cells are also under intensive study, along with technologies like 3D bioprinting and organ-on-a-chip systems for testing potential treatments before they reach patients.²⁸ None of these newer approaches have yet matched the clinical track record of microfracture or MACI, but the pace of development is rapid.

Exercise and Cartilage Health

A common worry is that running or other impact exercise will wear out your cartilage. The evidence does not support that fear, at least for people with healthy joints. Systematic reviews have found that regular moderate-impact exercise does not raise the risk of osteoarthritis and may actually protect against cartilage degradation. Physical activity promotes the production of lubricin in the joint, improving the lubrication properties of the articular surface.²⁹

Interestingly, the type and duration of exercise matters at a tissue level. After longer bouts of cyclical loading like running or cycling, cartilage thickness actually increases briefly in the minutes following the exercise, suggesting that the tissue can adapt to sustained loads by absorbing fluid.³⁰ The key distinction is between moderate cyclical loading, which cartilage tolerates well, and chronic overloading or single massive impacts, which overwhelm the tissue’s ability to maintain itself.

How Joint Alignment Affects Where Damage Appears

If your legs are perfectly straight, the forces through your knee are shared roughly evenly between the inner (medial) and outer (lateral) compartments. But few people have perfectly neutral alignment. A bow-legged (varus) stance shifts load toward the medial compartment, while a knock-kneed (valgus) stance loads the lateral side more heavily. These alignment differences have measurable consequences for cartilage.

In a large observational study, knees with varus alignment had over three and a half times the odds of developing new medial cartilage damage compared to non-varus knees, even after adjusting for other risk factors. This effect held even when meniscal damage was excluded, and there was evidence of a dose effect: each additional degree of varus increased the risk.³¹ Conversely, varus alignment was associated with reduced risk of damage on the lateral side, and valgus alignment protected the medial compartment, confirming that the pattern is about load distribution, not generalized joint health.³²

The ratio of cartilage loss tells the story clearly. In neutrally aligned knees, the medial-to-lateral ratio of cartilage loss was about 1.4 to 1, a slight medial predominance. In varus knees, that ratio jumped to 3.7 to 1. In valgus knees, the pattern flipped to 1 to 6, with the lateral side losing far more cartilage.³³ This is why alignment correction, whether through insoles, bracing, or surgical osteotomy, is a real strategy for slowing compartment-specific cartilage loss. It does not regenerate cartilage, but it can take pressure off the compartment that is wearing out fastest.

Body Weight and Cartilage Metabolism

Excess body weight affects cartilage through both mechanical overloading and metabolic pathways. The mechanical part is intuitive: more weight means more force through weight-bearing joints with each step. But fat tissue is also metabolically active, releasing hormones called adipokines that can influence cartilage directly.

A study in obese subjects undergoing weight loss found that changes in two specific adipokines predicted cartilage preservation. Increases in adiponectin, an anti-inflammatory hormone that rises as people lose fat, were associated with less cartilage volume loss in the medial knee compartment. Decreases in leptin, a hormone tied to inflammation that drops with weight loss, were also associated with reduced cartilage loss.³⁴ This means weight loss protects cartilage through at least two channels: it physically unloads the joint and it shifts the hormonal environment toward less inflammation and more tissue preservation. Losing weight is probably the single most impactful non-surgical intervention for overweight people with early knee osteoarthritis, and the adipokine data helps explain why even modest weight loss can produce outsized improvements in joint symptoms.