Unit 8 — Joints (Articulations)

Major Joints: Locations and Body Movements

Joints, or articulations, are points where two or more bones meet, allowing for movement and providing structural support. Each major joint in the human body is associated with specific locations and types of movement.

• Shoulder (Glenohumeral Joint): Located between the scapula and humerus; allows for flexion, extension, abduction, adduction, rotation, and circumduction.

• Elbow: Between humerus, radius, and ulna; permits flexion and extension.

• Hip (Coxal Joint): Between pelvis and femur; allows for flexion, extension, abduction, adduction, rotation, and circumduction.

• Knee: Between femur, tibia, and patella; allows for flexion, extension, and slight rotation.

• Ankle: Between tibia, fibula, and talus; allows for dorsiflexion and plantarflexion.

• Wrist: Between radius and carpal bones; allows for flexion, extension, abduction, and adduction.

Example: The shoulder joint is a ball-and-socket joint, providing the greatest range of motion in the body.

Main Functions of Joints

Joints serve three primary functions in the human body:

• Movement: Facilitate motion between bones.

• Support: Maintain the structural integrity of the skeleton.

• Protection: Help protect internal organs by limiting excessive movement.

Classification of Joints: Structure and Function

Joints are classified by both their structural composition and their functional capacity for movement.

• Structural Classification:

  • Fibrous Joints: Bones joined by dense connective tissue; no joint cavity.

  • Cartilaginous Joints: Bones joined by cartilage; no joint cavity.

  • Synovial Joints: Bones separated by a fluid-filled joint cavity; most movable type.

• Functional Classification:

  • Synarthrosis: Immovable joints (e.g., sutures in the skull).

  • Amphiarthrosis: Slightly movable joints (e.g., intervertebral discs).

  • Diarthrosis: Freely movable joints (e.g., shoulder, hip).

Example: The knee is a synovial, diarthrotic joint.

Subclasses of Joints and Examples

Each structural category contains specific subclasses, each with unique features and examples.

• Fibrous Joints:

  • Suture: Immovable; found between skull bones.

  • Gomphosis: Peg-in-socket; teeth in alveolar sockets.

  • Syndesmosis: Bones connected by ligaments; distal tibiofibular joint.

• Cartilaginous Joints:

  • Synchondroses: Bones united by hyaline cartilage; epiphyseal plate.

  • Symphyses: Bones united by fibrocartilage; pubic symphysis, intervertebral discs.

• Synovial Joints:

  • Plane (Gliding): Flat surfaces; intercarpal joints.

  • Hinge: Cylindrical end fits into trough; elbow, knee.

  • Pivot: Rounded end fits into ring; proximal radioulnar joint.

  • Condylar (Ellipsoid): Oval articular surface; wrist.

  • Saddle: Both surfaces concave and convex; thumb (carpometacarpal joint).

  • Ball & Socket: Spherical head fits into cup; shoulder, hip.

Structure of a Typical Synovial Joint and Functions of Synovial Fluid

Synovial joints are characterized by a complex structure that allows for extensive movement.

• Articular Cartilage: Covers bone ends, reducing friction.

• Joint (Synovial) Cavity: Space containing synovial fluid.

• Articular Capsule: Encloses joint cavity; consists of fibrous and synovial layers.

• Synovial Fluid: Lubricates joint, nourishes cartilage, reduces friction.

• Reinforcing Ligaments: Strengthen and stabilize joint.

Functions of Synovial Fluid:

• Lubrication of articular surfaces

• Nutrient distribution to cartilage

• Shock absorption

Location, Structure, and Function of Ligaments, Tendons, Meniscus, Bursae, Labrum, and Tendon Sheaths

Accessory structures support and protect joints, enhancing their function.

• Ligaments: Dense connective tissue connecting bone to bone; stabilize joints.

• Tendons: Connect muscle to bone; transmit force for movement.

• Meniscus: Crescent-shaped fibrocartilage in knee; improves fit and absorbs shock.

• Bursae: Fluid-filled sacs; reduce friction between tissues.

• Labrum: Fibrocartilaginous rim; deepens socket in shoulder and hip.

• Tendon Sheaths: Tubular bursae surrounding tendons; facilitate smooth movement.

Joint Configurations and Planes of Movement

Joints are classified by the number of axes around which movement occurs.

• Nonaxial (Gliding): Movement in one plane; intercarpal joints.

• Uniaxial: Movement around one axis; hinge (elbow), pivot (radioulnar).

• Biaxial: Movement around two axes; condylar (wrist), saddle (thumb).

• Multiaxial: Movement around multiple axes; ball & socket (shoulder, hip).

Example: The hip joint is multiaxial, allowing movement in all directions.

Types of Movements at Synovial Joints

Synovial joints permit a variety of movements, classified as angular, nonangular, and special movements.

• Angular Movements: Change the angle between bones (flexion, extension, abduction, adduction).

• Nonangular Movements: Gliding movements; bones slide past each other.

• Special Movements: Unique to certain joints (e.g., rotation, circumduction, pronation, supination, inversion, eversion, opposition).

Example: Flexion at the elbow decreases the angle between the forearm and upper arm.

Articulation and Range of Motion

Articulation refers to the site where two bones meet. Range of motion (ROM) is the extent of movement possible at a joint, determined by several factors.

• Factors Determining Degree of Movement:

  • Shape of articulating surfaces

  • Strength and tension of ligaments

  • Action of muscles and tendons

  • Presence of accessory structures (meniscus, labrum)

Example: The shoulder joint has a wide range of motion due to its ball-and-socket configuration and loose ligaments.

Joint Classification Table

The following table summarizes the structural and functional classification of joints, with examples:

Key Definitions

• Articulation: The site where two bones meet.

• Range of Motion (ROM): The extent of movement possible at a joint.

• Synovial Fluid: Viscous fluid within synovial joints, providing lubrication and nourishment.

Additional info: Academic context and examples have been expanded for clarity and completeness.