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.1PubMed Central. Broadband scattering properties of articular cartilage zones and their relationship with the heterogenous structure of articular cartilage extracellular matrix 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.2PubMed Central. Proteoglycans in Articular Cartilage and Their Contribution to Chondral Injury and Repair Mechanisms 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.3PubMed Central. Elevated cross-talk between subchondral bone and cartilage in osteoarthritic joints
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.4PubMed Central. Experimental verification of the role of interstitial fluid pressurization in cartilage lubrication 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.5PubMed Central. The role of interstitial fluid pressurization in articular cartilage lubrication 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.4PubMed Central. Experimental verification of the role of interstitial fluid pressurization in cartilage lubrication 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.6PLOS ONE. Elastoviscous Transitions of Articular Cartilage Reveal a Mechanism of Synergy between Lubricin and Hyaluronic Acid Computational models confirm that fluid pressurization also helps generate a self-sustaining lubrication cycle under shear loading, not just compression.7Tribology International. Experimental and numerical investigation of the behaviour of articular cartilage under shear loading—Interstitial fluid pressurisation and lubrication mechanisms
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.8PubMed Central. Joint contact stress: a reasonable surrogate for biological processes?
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.9PubMed Central. Mechanotransduction pathways in the regulation of cartilage chondrocyte homoeostasis 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.10Bone Research. Mechanotransduction pathways in articular chondrocytes and the emerging role of estrogen receptor-α
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.11Journal of the Mechanical Behavior of Biomedical Materials. Shock absorbing ability in healthy and damaged cartilage-bone under high-rate compression 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.12Scientific Reports. Histological and immunohistochemical analyses of articular cartilage during onset and progression of pre- and early-stage osteoarthritis in a rodent model
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.13PubMed Central. The role of inflammatory mediators and matrix metalloproteinases (MMPs) in the progression of osteoarthritis
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.14PubMed Central. Post-traumatic osteoarthritis: A review of pathogenic mechanisms and novel targets for mitigation
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.15Biomaterials. Healing of chondral and osteochondral defects in a canine model: the role of cultured chondrocytes in regeneration of articular cartilage
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.16PubMed Central. In vivo T(1rho) and T(2) mapping of articular cartilage in osteoarthritis of the knee using 3 T MRI 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.17PubMed. Biochemical magnetic resonance imaging of knee articular cartilage: T1rho and T2 mapping as cartilage degeneration biomarkers After anterior cruciate ligament reconstruction, T1rho has shown particular sensitivity for tracking early degeneration in the medial compartment of the knee.18PubMed Central. Longitudinal Analysis of Knee Articular Cartilage Degeneration After Anterior Cruciate Ligament Reconstruction: Comparison of T1rho and T2 Mapping
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.19Osteoarthritis and Cartilage. Large scale meta-analysis of urinary C-terminal telopeptide, serum cartilage oligomeric protein and matrix metalloprotease degraded type II collagen and their role in prevalence, incidence and progression of osteoarthritis Serum COMP, another marker released as cartilage degrades, has shown potential for predicting OA progression.20Osteoarthritis and Cartilage. Cartilage oligomeric matrix protein, C-terminal cross-linking telopeptide of type II collagen, and matrix metalloproteinase-3 as biomarkers for knee and hip osteoarthritis (OA) diagnosis: a systematic review and meta-analysis Combining both markers in animal models has improved accuracy for both early diagnosis and severity staging.21PubMed Central. Combined detection of serum CTX-II and COMP concentrations in osteoarthritis model rabbits: an effective technique for early diagnosis and estimation of disease severity 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.22PubMed. Application of an indentation sensor for the arthroscopic measurement of articular cartilage stiffness Miniaturized versions using fiber-optic force sensors are under development, potentially making real-time mechanical assessment of cartilage even more practical during surgery.23Sensors and Actuators B: Chemical. Microindentation sensor system based on an optical fiber Bragg grating for the mechanical characterization of articular cartilage by stress-relaxation
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.24PubMed. Microfracture for cartilage repair in the knee: a systematic review of the contemporary literature 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.25PubMed Central. Minimum 10-Year Outcomes of Matrix-Induced Autologous Chondrocyte Implantation in the Knee: A Systematic Review 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.26PubMed Central. Biomaterial-based scaffolds in promotion of cartilage regeneration: Recent advances and emerging applications 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.27PubMed Central. Review of Synthetic and Hybrid Scaffolds in Cartilage Tissue Engineering 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.28PubMed Central. Advances in cartilage tissue regeneration: a review of stem cell therapies, tissue engineering, biomaterials, and clinical trials 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.29PubMed Central. How Physical Activity Affects Knee Cartilage and a Standard Intervention Procedure for an Exercise Program: A Systematic Review
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.30Journal of Bodywork and Movement Therapies. Effects of exercise mode in knee cartilage thickness 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.31PubMed Central. The Role of Varus and Valgus Alignment in the Initial Development of Knee Cartilage Damage by MRI: the MOST Study 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.32PubMed Central. Varus-Valgus Alignment: Reduced Risk for Subsequent Cartilage Loss in the Less Loaded Compartment
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.33PubMed. Patterns of femorotibial cartilage loss in knees with neutral, varus, and valgus alignment 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.34Osteoarthritis and Cartilage. Association of adipokines and joint biomarkers with cartilage-modifying effects of weight loss in obese subjects 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.