Comprehensive Study Notes on the Articular System (Sistema Articular)

Definition and Functions of Joints

  • Definition of Joints (Articulaciones):

    • Joints constitute the articular system and are defined as the points of union between two or more bones, or between bones and cartilage.

  • Primary Functions of Joints:

    • Skeletal Integration: Connect the individual bones that compose the human skeleton into a functional framework.
    • Motion Regulation: Allow specific, controlled functional movements while restricting unwanted or excessive motion.
    • Load Transmission and Shock Absorption: Distribute, transmit, and dampen mechanical loads and compressive forces supported by the skeletal framework.
    • Role in Skeletal Growth: Facilitate longitudinal and circumferential bone growth (e.g., epiphyseal growth plates in developing long bones).

Functional and Structural Classification of Joints

  • Classification According to Degree of Movement (Functional Classification):

    • Synarthrosis (Sinartrosis): Immovable joints that provide high structural stability and protective function.
    • Amphiarthrosis (Anfiartrosis): Slightly movable (semimóviles) joints that balance limited mobility with structural support.
    • Diarthrosis (Diartrosis): Freely movable joints that permit extensive ranges of motion in various planes.

  • Classification According to Binding Tissue (Structural Classification):

    • Fibrous Joints (Articulaciones fibrosas): Articulating surfaces held directly together by dense fibrous connective tissue.
    • Cartilaginous Joints (Articulaciones cartilaginosas): Articulating bones connected by hyaline cartilage or fibrocartilage.
    • Synovial Joints (Articulaciones sinoviales): Articulating bones separated by a fluid-filled synovial cavity and enclosed by an articular capsule.

Structural Classes of Joints

  • Fibrous Joints (Articulaciones Fibrosas):

    • Sutures (Suturas):
    • Joined together by a thin layer or sheet of dense fibrous tissue.
    • Functional classification: Immovable (synarthrosis).
    • Location: Found exclusively between the flat bones of the skull (e.g., coronal suture joining the frontal and parietal bones).
    • Suture between skull bones
    • Syndesmosis (Sindesmosis):
    • Articulating bones united by a fibrous sheet, interosseous membrane, or bundle of ligamentous tissue.
    • Functional classification: Generally immovable (synarthrosis).
    • Examples: Tibiofibular syndesmosis (sindesmosis tibioperonea) connecting the distal tibia and fibula via the interosseous membrane and anterior tibiofibular ligament.
    • Syndesmosis between tibia and fibula
    • Gomphosis (Gonfosis):
    • Specialized peg-in-socket fibrous joint where the root of a tooth articulates with the alveolar socket (cavidad alveolar) of the maxilla or mandible.
    • Anchored by the periodontal ligament (membrane), a dense fibrous tissue lining the socket and tooth root.
    • Functional classification: Immovable (synarthrosis).
    • Gomphosis between tooth and socket of alveolar process

  • Cartilaginous Joints (Articulaciones Cartilaginosas):

    • Synchondrosis (Sincondrosis):
    • Articulating elements connected directly by hyaline cartilage.
    • Functional classification: Immovable (synarthrosis).
    • Primary function: Temporary skeletal growth plates (epiphyseal growth plates) in young individuals, which ossify upon skeletal maturity.
    • Symphysis (Sínfisis):
    • Articulating surfaces covered with hyaline cartilage and fused to an intervening pad or disc of fibrocartilage.
    • Functional classification: Partially movable (amphiarthrosis).

Detailed Anatomy of Synovial Joints

  • General Characteristics:

    • Articulating bones are separated by a distinct synovial joint cavity containing synovial fluid.
    • Functionally classified as diarthroses (freely movable joints).

  • Core Structural Components:

    • Articular Cartilage (Cartílago Articular):
    • Composed of hyaline cartilage lining the articulating surfaces of bones.
    • Avascular and uninnervated (sin inervación).
    • Creates a smooth, lubricated surface that facilitates movement and absorbs shock.
    • Fibrous Capsule (Cápsula Fibrosa):
    • Encloses the joint cavity.
    • Constructed from fibrous connective tissue.
    • Contains specialized localized thickenings termed intrinsic capsular ligaments.
    • May feature openings for tendons or projections of the synovial membrane.
    • Synovial Membrane (Membrana Sinovial):
    • Inner membrane lining the capsule.
    • Together with the outer fibrous capsule, forms the unified articular capsule (cápsula articular).
    • Synovial Cavity (Cavidad Sinovial):
    • Closed space bounded by the synovial membrane and articular cartilages.
    • Filled with synovial fluid: a serous fluid (aqueous solution) that acts as a lubricant and supplies nutrients to the avascular articular cartilage.
    • Functions as a potential space ("cavidad virtual"), normally collapsed into a thin layer with no visible space, distending only under pathological conditions.

  • Accessory and Adaptive Structures:

    • Adaptors (Adaptadores): Specialized fibrocartilaginous structures that increase structural congruence between two mismatched articulating surfaces:
    • Articular discs (discos articulares)
    • Menisci (meniscos)
    • Glenoid/acetabular labrums or rims (rodetes)
    • Ligaments (Ligamentos): Cords of fibrous tissue that reinforce joint stability and restrict excessive movement range:
    • Extracapsular ligaments
    • Intracapsular ligaments
    • Intrinsic capsular ligaments
    • Synovial Bursae (Bolsas Serosas):
    • Sacs lined with membranes containing fluid similar to synovial fluid.
    • Reduce friction between neighboring structures (muscles, bones, tendons, joints).
    • May maintain direct communication with the main synovial joint cavity.
    • Fat Pads (Cuerpos Adiposos):
    • Accumulations of adipose tissue filling empty spaces near the joint, adapting dynamically to shifting joint positions.

  • Structural Illustration:   Anatomy of a synovial joint

Biomechanics and Categories of Joint Movements

  • Fundamental Categories of Movement:

    • Joint movements fall into four primary categories: gliding, rotation, angular movements, and special movements.

  • Gliding / Sliding Movements (Deslizamiento):

    • Occur when relatively flat articular surfaces slide past one another.
    • Range of motion is strictly limited by surrounding articular capsule structures, associated ligaments, and adjacent bony contours.

  • Rotational Movements (Rotación):

    • Movement of a bone (or body segment) revolving around its own longitudinal axis.
    • Medial (Internal) Rotation: Rotation that directs the anterior surface of the moving limb inward toward the midline of the body.
    • Lateral (External) Rotation: Rotation that directs the anterior surface of the moving limb outward away from the midline of the body.
    • Example: Arm rotation via movement of the humerus at the glenohumeral joint.
    • Shoulder rotation medial and lateral

  • Angular Movements (Movimientos Angulares):

    • Characterized by an increase or decrease in the anatomical angle between the longitudinal axes of articulating bones:
    • Flexion (Flexión): Decreases the angle between articulating bones; occurs in an anterior direction within the sagittal plane.
    • Extension (Extensión): Increases the angle between articulating bones, returning a body part to the anatomical position; occurs in a posterior direction within the sagittal plane (Exceptions: knee, ankle, and foot).
    • Hyperextension (Hiperextensión): Continuation of extension beyond the standard anatomical position; generally directed posteriorly (Exceptions: knee, ankle, foot).
    • Lateral Flexion (Flexión Lateral): Movement of axial structures (head, trunk) laterally to the left or right, creating an angle with the vertical axis within the frontal (coronal) plane.
    • Abduction (Abducción): Movement of a limb away from the median sagittal plane of the body within the frontal (coronal) plane.
    • Adduction (Aducción): Movement of a limb toward the median sagittal plane of the body within the frontal (coronal) plane.
    • Circumduction (Circunducción): Circular movement of the distal end of an extremity combining flexion, abduction, extension, adduction, and rotation in sequence; not restricted to a single plane.
    • Angular movements flexion extension hyperextension
    • Anatomical planes of movement

  • Degrees of Freedom (Grados de Libertad):

    • Defined as the number of independent spatial planes in which a joint can perform movements.

Types of Synovial Joints

  • Overview:

    • Synovial joints are classified into six structural types based on articulating surface geometry and movement capabilities.
    • Types of synovial joints overview

  • 1. Planar / Gliding Joint (Artrodia):

    • Articular surfaces are flat or slightly curved.
    • Movements: Non-axial or planar gliding sliding motion.
    • Example: Joint between the navicular, second cuneiform, and third cuneiform bones of the tarsus in the foot.
    • Planar joint between tarsal bones

  • 2. Hinge Joint (Tróclea / Gínglimo):

    • Convex surface of one bone fits into the concave surface of another bone.
    • Movement: Uniaxial movement allowing motion in a single plane (flexion and extension).
    • Example: Hinge joint between the trochlea of the humerus and the trochlear notch of the ulna at the elbow.
    • Hinge joint at the elbow

  • 3. Pivot Joint (Trocoide):

    • Rounded or pointed surface of one bone articulates within a ring formed partly by another bone and partly by a ligament.
    • Movement: Uniaxial rotation around a central axis.
    • Example: Joint between the head of the radius and the radial notch of the ulna, secured by the annular ligament.
    • Pivot joint between radius and ulna

  • 4. Condyloid / Ellipsoid Joint (Condílea):

    • Convex oval-shaped projection fits into an oval concave depression.
    • Movement: Biaxial movement permitting motion in two planes (flexion/extension, abduction/adduction, and circumduction).
    • Example: Joint between the radius and the scaphoid and lunate bones of the carpus at the wrist.
    • Condyloid joint at the wrist

  • 5. Saddle Joint (Silla de Montar / Encaje Recíproco):

    • Articular surface of one bone is saddle-shaped, and the opposing bone fits into it like a rider.
    • Movement: Biaxial movement (flexion/extension, abduction/adduction, and circumduction).
    • Example: Joint between the trapezium of the carpus and the first metacarpal bone of the thumb.
    • Saddle joint of the thumb

  • 6. Ball-and-Socket Joint (Enartrosis):

    • Spherical ball-like surface fits into a cup-like socket.
    • Movement: Multiaxial movement across all planes (flexion/extension, abduction/adduction, rotation, and circumduction).
    • Example: Joint between the head of the femur and the acetabulum of the hip bone.
    • Ball-and-socket hip joint

Factors Limiting Joint Movement

  • Bony Limitations (Límites Óseos):

    • Shape and direct impingement of articulating bony processes restrict movement.
    • Example: The olecranon process of the ulna locking into the olecranon fossa of the humerus limits elbow extension.

  • Ligamentous Limitations (Límites por Ligamentos):

    • Mechanical orientation and tension of capsular and extracapsular ligaments limit movement direction and amplitude.
    • Example: The iliofemoral ligament restricts hip hyperextension.

  • Soft Tissue Resistance (Límites por Tejidos Blandos):

    • Contact between adjacent soft tissue masses (such as muscle or adipose tissue) limits maximum bending.
    • Example: Adipose tissue of the posterior thigh pressing against the calf limits knee flexion.

  • Muscular Limitations (Límites Musculares):

    • Passive tension and extensibility of opposing (antagonist) muscles restrict joint motion.
    • Example: Tight hamstring muscles limit forward hip flexion when the knee is held in extension.