Biomechanics of Articular Cartilage Study Guide

The Concept and Functional Unit of the Joint

A joint is defined as the functional unit constituted by a set of elements that allow two or more bones to be united with one another. This union serves a fundamental and essential purpose: facilitating movement. While joints unify the skeletal structure, their primary classification is based on the range or amplitude of motion they allow. There are three major groups in this classification hierarchy. The first is Sinartrosis, also known as fibrous joints, which exhibit the least amount of movement; a primary example is the sutures of the skull. The second is Anfiartrosis, or cartilaginous joints, which allow for a medium range of motion, such as the intervertebral joints. The third is Diartrosis, or synovial joints, which provide the widest range of movement, exemplified by the knee joint.

A complete joint structure consists of several distinct anatomical components, including the articular capsule, ligaments that reinforce the structure, the synovial membrane, articular cartilage, menisci, and fibrocartilage rings (labrums/rodetes). When a joint is healthy, it operates without noise, without friction, and without pain. The loss of cartilage does not necessarily lead to immediate luxation, but it significantly alters movement and biomechanical stability.

Detailed Classification and Types of Joints

Sinartrosis (Fibrous Joints) are characterized by being immobile and held together by fibrous tissue. Within this category, there are specific sub-types based on the tissue involved: Sinostosis refers to the fusion of two or more bones; Sincondrosis (derived from "khondro-" meaning cartilage) are temporary joints where bones are held together by hyaline cartilage, such as the epiphyseal growth plates connecting the epiphysis and diaphysis in growing bones; and Sindesmosis, where fibrous tissue connects bones, such as the interosseous membrane of the distal tibiofibular joint.

Anfiartrosis (Cartilaginous Joints) are semi-mobile and occur where bone and cartilage meet, such as in the vertebrae. In these joints, the articular surfaces are covered with hyaline cartilage and are joined by resistant fibrous tissue and fibrocartilage. Diartrosis (Synovial Joints) offer an extensive range of movement. Their structure includes an articular capsule and reinforcing ligaments, an internal synovial membrane that secretes synovial fluid, and hyaline cartilage that coats the bony surfaces. Furthermore, they contain menisci and labrums made of fibrocartilage, which serve to increase articular congruence and surface area.

Definition and Characteristics of Articular Cartilage

Articular cartilage is a specialized type of elastic connective tissue that is uniquely characterized by being avascular (lacking blood vessels), aneural (lacking nerves), and lacking lymphatic vessels. It is composed primarily of an extracellular matrix and specialized cells known as chondrocytes. Macroscopically, young cartilage appears white with a slight bluish tint, while in adults, it takes on a yellowish hue. It is characterized by being white and shiny in appearance. Because it lacks a direct blood supply, it receives its nutrition through the synovial fluid and the underlying bone tissue. The outer part of the cartilage, known as the perichondrium (present in elastic cartilage), provides vital support to the chondrocytes.

This tissue is found coating joints, at the junctions between ribs and the sternum, as reinforcement in the trachea and bronchi, in the external ear, and in the nasal septum. It is also found in vertebrate embryos and cartilaginous fish. The thickness of the cartilage varies depending on its location and the specific joint; for instance, it is approximately $5\,mm$ thick in the patella (rótula) and as thin as $1\,mm$ in the phalanges.

The Function and Biomechanics of Articular Cartilage

The primary functions of articular cartilage include reducing friction between bones, transferring and distributing mechanical loads across different articular positions, and providing a lubricated surface that allows bones to slide and rotate over one another with minimal wear. Biomechanically, it increases the area of load distribution, which reduces overall mechanical stress. It also accommodates articular surfaces, such as fitting the femoral condyles into the tibial cavities, thereby cushioning the dynamics of movement, such as walking, running, and jumping.

Articular cartilage prevents wear caused by friction. Because it has no nerves, the cartilage itself does not produce pain; the sensation of pain in joint conditions is typically provoked by neighboring structures when the cartilage is damaged. If the structure is compromised due to age or trauma, it leads to Artrosis (Osteoarthritis), where chondrocytes are lost, viscosity decreases, and the cartilage eventually wears away completely.

Composition of the Articular Cartilage Matrix

The cartilage is composed of an organic matrix, an inorganic matrix, and a cellular component. The organic matrix contains Type II collagen fibers, which make up approximately $60\%$ of the dry weight and provide the cartilage with its structural resistance. It also contains a gel of proteoglycans that have an affinity for water; these proteoglycans provide the elasticity needed to resist intermittent compression and shear forces, allowing the tissue to withstand external loads and mechanical stress.

The inorganic matrix is primarily composed of water. When pressure is applied to the cartilage, water can be squeezed out of the tissue. This water content provides turgency to the cartilage. The cellular component consists of chondrocytes, which represent only $2\%$ of the total tissue volume. These cells reside in a mesh-like structure and are responsible for manufacturing proteoglycans.

The Role of Collagen in Structural Integrity

Collagen is the most abundant protein in the human body, accounting for $25\%$ of total body protein. Production begins to decline after the age of $25$ . In articular cartilage, Type II collagen is the predominant form. The structural hierarchy begins with collagen fibrils forming fibers, which then form bundles, and finally, these bundles form laminae. Collagen is notably resistant to traction.

Different types of collagen serve specific roles across body tissues:

Type I: Found in bones, tendons, ligaments, and skin. It aids in sliding. Bone (demineralized) is $90\%$ collagen, while tendons are $80-90\%$ and skin is $50-70\%$ .
Type II: Found mainly in cartilage ( $50-70\%$ of dry weight) and eye structures. It functions to cushion and distribute loads.
Type III: Found in the liver, lungs, and arteries. It is associated with compression resistance.
Type IV: Located in kidneys and various internal organs.
Type V: Found on cell surfaces, hair, and the placenta.

Under tension, collagen fibers exhibit a specific behavior: in the first phase of low loading, there is significant deformity; as load increases, the load-deformity relationship becomes proportional until the point of rupture.

Microscopic Zoning of Articular Cartilage

Cartilage structure is organized into distinct microscopic zones. The Superficial Zone consists of flattened cells and disordered fibrils at the very top, followed by collagen fibers forming bundles parallel to the movement plane. This surface is undulating with grooves parallel to the direction of motion. The Transitional Zone contains more rounded cells and disordered collagen fibers. Finally, the Calcified Zone contains disoriented fibers with signs of degeneration. This layer becomes calcified and resembles bone tissue.

Categorization of Cartilage Tissues

Cartilage is categorized into three types based on tissue structure and location:

Hyaline or Articular Cartilage: The most abundant type. Found in the ribs, nasal skeleton, larynx, trachea, bronchi, and bone ends (knees, coxofemoral joints).
Elastic Cartilage: Distinguished by a layer of perichondrium. Found in the epiglottis, external ear, and Eustachian tubes.
Fibrous Cartilage (Fibrocartilage): A transition between connective tissue and hyaline cartilage, containing both chondrocytes and fibroblasts. Found in intervertebral discs, articular borders, menisci, and labrums.

Biomechanical Behavior: Viscoelasticity and Resilience

The most important mechanical property of cartilage is viscoelasticity. This means its response to a load depends on the speed at which the load is applied. Cartilage is often compared to a water-soaked sponge. It possesses a solid phase (the matrix and collagen with elastic pores) and a liquid phase (interstitial fluid/water).

When compression is applied, local pressure increases, and liquid is exuded toward non-compressed areas and the interarticular space. The collagen permits deformity relative to the duration of the load. Upon the withdrawal of compression or load, rehydration occurs as water flow reverses, allowing the cartilage to recover its original configuration. Because cartilage has low permeability, its response varies:

Constant, prolonged loading (e.g., standing): Fluid is progressive expelled, leading to progressive deformity. If followed by adequate rest, the tissue recovers via its viscoelastic properties. Constant pressure without relief can lead to ulcers or wear even without high weight.
Point or intermittent loading (e.g., jumping): There is insufficient time for fluid exudation. The tissue undergoes immediate deformity and recovery, exhibiting pure elastic behavior.

Cartilage is also described as resilient, meaning it has the capacity to recover its natural shape after mild pressure before reaching the point of permanent deformation.

Degeneration and Mechanical Failure

Cartilage degeneration can lead to Artrosis (Osteoarthritis), a chronic progressive disease of cartilage wear. This wear causes bony contact (choque óseo), resulting in loss of mobility, pain, and deformity. Proper alignment of the limbs is essential to prevent asymmetric wear; for instance, valgus deformity (X-shaped legs) can focus stress on specific areas of the joint.

Mechanical failure of the cartilage can be categorized as:

Acute Failure: Caused by a high external force acting on a small articular contact area.
Chronic Failure (Fatigue): Caused by cyclic stress on already damaged cartilage, or by forces applied over a prolonged time without sufficient recovery periods.

The Influence of Activity and Inactivity

Weight, obesity, intense activity, and overuse can accelerate cartilage wear. However, cartilage requires load and movement to remain healthy. Loading and unloading cycles facilitate the supply of nutrients from the synovial fluid and the removal of waste products, effectively improving tissue lubrication. Conversely, immobilization and disuse lead to atrophy and degeneration. Because cartilage lacks blood flow, it has an incomplete and difficult capacity for regeneration once damaged. Maintenance of joint alignment and physiological movement levels is essential for preserving the tissue.