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Articular cartilage

Articular cartilage is a specialized connective tissue that covers the ends of bones within synovial joints. It provides a smooth, lubricated surface for articulation and plays a critical role in absorbing loads during movement. Its structure, composition, and limited regenerative capacity make it a subject of significant interest in both anatomy and clinical medicine.

Anatomy of Articular Cartilage

Location in Synovial Joints

Articular cartilage is found at the articulating surfaces of bones that form synovial joints. It is absent in fibrous or cartilaginous joints. Common locations include the knee, hip, shoulder, and small joints of the hand. The presence of articular cartilage allows smooth motion with minimal friction.

Gross Appearance and Thickness

Macroscopically, articular cartilage appears smooth, white, and glistening. Its thickness varies depending on the joint and functional load, generally ranging from 1 to 6 mm. Joints subjected to high stress, such as the knee, typically have thicker cartilage compared to smaller joints.

Relationship with Subchondral Bone

Beneath the articular cartilage lies the subchondral bone, a thin layer of dense bone that supports and distributes forces. The interface between cartilage and bone is critical for transmitting mechanical loads. Damage at this junction often results in impaired joint function and degenerative changes.

Histological Structure

Four Structural Zones

Articular cartilage is organized into four distinct zones, each with unique cellular arrangements and extracellular matrix properties:

Superficial (Tangential) Zone: Contains flattened chondrocytes and collagen fibers aligned parallel to the surface, providing tensile strength and smooth articulation.
Middle (Transitional) Zone: Features round chondrocytes and obliquely arranged collagen fibers, offering resistance to compressive forces.
Deep (Radial) Zone: Contains columnar chondrocytes and vertically aligned collagen fibers, providing high resistance to compressive loading.
Calcified Zone: Characterized by mineralized matrix anchoring the cartilage to the subchondral bone.

Chondrocytes and Their Arrangement

Chondrocytes are the only cellular component of articular cartilage. Their shape and density vary across zones, reflecting functional specialization. In the superficial zone, they are flattened and numerous, whereas in the deep zone they are larger, organized in columns, and less abundant.

Extracellular Matrix Components

The extracellular matrix forms the bulk of articular cartilage and is primarily composed of collagen, proteoglycans, and water. Collagen fibers provide tensile strength, while proteoglycans contribute to compressive resistance. The high water content allows cartilage to withstand repetitive loading and maintain joint lubrication.

Composition of Articular Cartilage

Collagen Fibers

Collagen accounts for the majority of the structural framework of articular cartilage. Type II collagen is the most abundant, forming a fibrillar network that provides tensile strength and resists stretching. Other collagens, including types IX and XI, help organize and stabilize the matrix.

Proteoglycans and Glycosaminoglycans

Proteoglycans are macromolecules consisting of a protein core with glycosaminoglycan side chains, such as chondroitin sulfate and keratan sulfate. These molecules attract water and contribute to the osmotic properties of cartilage, allowing it to resist compressive forces during movement.

Water Content and Its Role

Water makes up 65 to 80 percent of the wet weight of articular cartilage. It facilitates the transport of nutrients and waste products, provides lubrication, and distributes mechanical loads across the joint surface. The dynamic movement of water within the matrix also contributes to viscoelastic behavior.

Other Non-Collagenous Proteins

Several non-collagenous proteins, such as cartilage oligomeric matrix protein (COMP) and link proteins, play essential roles in matrix organization and stability. These proteins help maintain interactions between collagen and proteoglycans, contributing to the structural integrity of the tissue.

Biomechanical Properties

Load-Bearing Functions

Articular cartilage serves as a load-bearing tissue, distributing compressive forces over a wider area to reduce stress on subchondral bone. This capacity protects the underlying bone from damage during weight-bearing activities such as walking and running.

Viscoelastic Behavior

The tissue demonstrates viscoelastic properties, meaning it exhibits both fluid-like and solid-like behavior. This allows articular cartilage to absorb shocks, dissipate energy, and return to its original shape after deformation. These properties are largely determined by the interaction of collagen, proteoglycans, and water within the matrix.

Lubrication and Low-Friction Mechanisms

Articular cartilage ensures low-friction movement of joint surfaces. Lubrication is achieved through synovial fluid and boundary molecules such as lubricin and hyaluronic acid. These mechanisms reduce wear and tear, enabling long-term joint function under repetitive motion and loading.

Development and Growth

Embryological Origin

Articular cartilage develops from mesenchymal condensations during embryogenesis. These mesenchymal cells differentiate into chondrocytes, which secrete extracellular matrix components, forming the early cartilaginous models of future synovial joints.

Postnatal Growth and Maturation

After birth, articular cartilage continues to grow through appositional growth, where new layers of matrix are deposited on the surface by chondrocytes. Maturation involves increased organization of collagen fibers and a reduction in cell density as the tissue adapts to mechanical demands.

Age-Related Changes

With aging, articular cartilage undergoes structural and biochemical changes. These include thinning of the tissue, reduced proteoglycan content, and diminished chondrocyte activity. Such changes compromise the mechanical properties of cartilage and predispose joints to degenerative conditions like osteoarthritis.

Nutrition and Metabolism

Avascular Nature of Articular Cartilage

Articular cartilage is avascular, meaning it lacks its own blood supply. This unique feature distinguishes it from most other connective tissues and contributes to its limited capacity for healing following injury.

Nutrient Supply via Synovial Fluid

Chondrocytes depend on diffusion of nutrients and oxygen from synovial fluid. Joint movement enhances this diffusion process by cyclically compressing and releasing the cartilage, which facilitates exchange of nutrients and waste products between the cartilage and synovial fluid.

Metabolic Activity of Chondrocytes

Although relatively low compared to other cell types, chondrocytes maintain continuous metabolic activity to preserve cartilage integrity. They synthesize and degrade matrix components in response to mechanical loading, growth factors, and cytokines. Imbalances in these processes can lead to cartilage breakdown and disease progression.

Physiological Functions

Shock Absorption

Articular cartilage acts as a shock absorber, distributing mechanical loads evenly across the joint surface. By compressing under pressure and returning to its original shape, it reduces stress on the underlying subchondral bone and surrounding soft tissues during activities such as walking, running, or jumping.

Joint Congruence and Stability

The smooth surface of articular cartilage enhances congruence between articulating bones, which contributes to joint stability. Proper alignment and distribution of forces help prevent abnormal joint mechanics and reduce the risk of injury.

Smooth Movement of Joints

The low-friction properties of articular cartilage, aided by synovial fluid, allow for effortless gliding of joint surfaces. This ensures efficient and pain-free motion, which is vital for maintaining mobility and functional independence in daily life.

Repair and Regeneration Capacity

Limited Healing Potential

Due to its avascular nature, articular cartilage has very limited self-healing capacity. Injuries that do not penetrate the subchondral bone rarely heal adequately, leading to persistent defects that may progress over time.

Mechanisms of Repair

When defects extend into the subchondral bone, repair occurs through the influx of mesenchymal stem cells from the bone marrow. However, the tissue that forms is fibrocartilage rather than true hyaline cartilage. Fibrocartilage lacks the same mechanical properties, making the repair often functionally inferior.

Factors Affecting Regeneration

Several factors influence the regenerative ability of articular cartilage, including:

Age of the individual, with younger patients showing better repair capacity
Size and depth of the lesion, as larger defects are more difficult to heal
Mechanical environment of the joint, where excessive load impairs healing
Biological factors such as growth factors and cytokine signaling

Pathological Conditions

Osteoarthritis

Osteoarthritis is the most common degenerative joint disorder characterized by progressive breakdown of articular cartilage. Loss of proteoglycans, fibrillation of the cartilage surface, and subchondral bone remodeling lead to pain, stiffness, and reduced joint mobility.

Chondromalacia

Chondromalacia refers to the softening and degeneration of articular cartilage, commonly affecting the patella. It presents with anterior knee pain, crepitus, and discomfort during activities such as climbing stairs. It is often associated with overuse or abnormal patellar tracking.

Traumatic Cartilage Injuries

Direct trauma or sports-related injuries can cause focal cartilage defects, often accompanied by ligament or meniscal damage. These injuries compromise joint function and increase the risk of early-onset osteoarthritis if not properly managed.

Inflammatory Joint Diseases

Conditions such as rheumatoid arthritis lead to inflammation of the synovial membrane, which in turn damages articular cartilage through the action of inflammatory cytokines and enzymes. This process contributes to joint erosion, deformity, and chronic disability.

Diagnostic Approaches

Clinical Examination

Diagnosis of articular cartilage disorders begins with a detailed clinical history and physical examination. Symptoms such as joint pain, swelling, and crepitus guide suspicion of cartilage pathology. Functional assessments evaluate the degree of mobility and stability impairment.

Imaging Techniques

X-ray: Useful for detecting secondary changes such as joint space narrowing and subchondral sclerosis but does not visualize cartilage directly.
MRI: The most sensitive imaging modality for cartilage assessment, providing detailed visualization of thickness, defects, and degeneration.
Ultrasound: Can assess cartilage thickness in superficial joints and detect associated synovial abnormalities.

Arthroscopy

Arthroscopy allows direct visualization of articular cartilage surfaces and assessment of lesion size and severity. It is often considered the gold standard for diagnosing focal cartilage defects and may be combined with therapeutic interventions.

Biochemical Markers

Emerging diagnostic approaches include the use of biochemical markers in serum, urine, or synovial fluid that reflect cartilage turnover. Examples include fragments of collagen type II and aggrecan. These markers are being investigated for their potential to monitor disease progression and treatment response.

Treatment and Management

Conservative Approaches

Conservative management of articular cartilage disorders focuses on symptom relief and preservation of joint function. Common strategies include:

Pharmacological agents such as analgesics and nonsteroidal anti-inflammatory drugs
Intra-articular injections, including corticosteroids and hyaluronic acid, to reduce inflammation and improve lubrication
Physical therapy aimed at strengthening periarticular muscles and improving joint stability
Lifestyle modifications, such as weight management and activity adjustments, to decrease joint load

Surgical Techniques

When conservative measures fail, surgical interventions may be considered to restore joint surface integrity. Common techniques include:

Microfracture: Drilling small holes in the subchondral bone to stimulate marrow-derived repair tissue
Osteochondral grafting: Transplanting cartilage and bone plugs from non-weight-bearing regions to the defect site
Autologous chondrocyte implantation (ACI): Harvesting chondrocytes, culturing them in vitro, and re-implanting them into the cartilage defect

Tissue Engineering and Regenerative Therapies

Recent advances focus on regenerative approaches to restore true hyaline cartilage. These include the use of stem cells, growth factors, and biomaterial scaffolds to enhance chondrogenesis. Tissue engineering holds promise for overcoming the limitations of traditional repair methods by promoting long-term regeneration of functional cartilage.

Comparative Anatomy and Evolutionary Perspective