Chapter 8: Joints (Articulations) – Structure, Function, and Clinical Relevance

Joints (Articulations)

Introduction

Joints, also known as articulations, are sites where two or more bones meet. They are essential for providing the skeleton with mobility while also holding the skeleton together. Joints are classified both structurally and functionally, and their health is crucial for normal movement and daily activities.

Primary Joint Classifications

Structural Classifications

• Fibrous Joints: Bones joined by dense fibrous connective tissue; no joint cavity; most are immovable.

• Cartilaginous Joints: Bones united by cartilage; no joint cavity; not highly movable.

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

Functional Classifications

• Synarthroses: Immovable joints

• Amphiarthroses: Slightly movable joints

• Diarthroses: Freely movable joints

Fibrous Joints

• Sutures: Rigid, interlocking joints of the skull; immovable.

• Syndesmoses: Bones connected by ligaments (e.g., inferior tibiofibular joint, interosseous membrane between radius and ulna); movement varies by length of fibers.

• Gomphoses: Peg-in-socket joints (e.g., tooth in alveolar socket); immovable.

Cartilaginous Joints

• Synchondroses: Bar or plate of hyaline cartilage unites bones (e.g., epiphyseal plates, cartilage of 1st rib with manubrium of sternum); mostly immovable.

• Symphyses: Fibrocartilage unites bones in a symphysis joint (e.g., intervertebral joints, pubic symphysis); slightly movable.

Structural Characteristics of Synovial Joints

General Features

• Articular cartilage: Hyaline cartilage covering ends of bones; prevents crushing of bone ends.

• Joint (synovial) cavity: Small, fluid-filled potential space unique to synovial joints.

• Articular (joint) capsule: Two layers (outer fibrous layer and inner synovial membrane) that enclose the joint cavity.

• Synovial fluid: Viscous, slippery filtrate of plasma and hyaluronic acid; lubricates and nourishes articular cartilage; contains phagocytic cells.

• Reinforcing ligaments: Strengthen the joint; can be capsular, extracapsular, or intracapsular.

• Nerves and blood vessels: Nerve fibers detect pain and monitor joint position; capillary beds supply filtrate for synovial fluid.

Bursae and Tendon Sheaths

• Bursae: Bags of synovial fluid that reduce friction where ligaments, muscles, skin, tendons, or bones rub together.

• Tendon sheaths: Elongated bursae wrapped completely around tendons subjected to friction.

Stability of Synovial Joints

• Shape of articular surface: Shallow surfaces are less stable than ball-and-socket joints (minor role).

• Ligament number and location: More ligaments generally mean stronger joints (limited role).

• Muscle tone: Keeps tendons taut as they cross joints; most important factor for joint stability.

Range of Motion Allowed by Synovial Joints

• Nonaxial: Gliding movements only

• Uniaxial: Movement in one plane

• Biaxial: Movement in two planes

• Multiaxial: Movement in or around all three planes

Types of Movements at Synovial Joints

• Gliding: One flat bone surface glides or slips over another (e.g., intercarpal joints).

• Angular movements: Increase or decrease the angle between two bones (e.g., flexion, extension, abduction, adduction, circumduction).

• Rotation: Turning of a bone around its own long axis (e.g., rotation of atlas and axis, humerus, femur).

Special Movements

• Supination and pronation: Rotation of radius and ulna (supination: palms face anteriorly; pronation: palms face posteriorly).

• Dorsiflexion and plantar flexion: Upward and downward movement of the foot at the ankle.

• Inversion and eversion: Turning the sole of the foot medially or laterally.

• Protraction and retraction: Anterior and posterior movement in a transverse plane (e.g., mandible).

• Elevation and depression: Lifting or lowering a body part (e.g., shrugging shoulders, opening jaw).

• Opposition: Movement of the thumb to touch the tips of other fingers.

Major Synovial Joints of the Body

Synovial joints are classified into six types based on the shape of their articular surfaces and the movements they allow:

Type	Example	Movement	Axis
Plane	Intercarpal joints, joints between vertebral articular surfaces	Gliding	Nonaxial
Hinge	Elbow, interphalangeal joints	Flexion and extension	Uniaxial
Pivot	Proximal radioulnar joint, atlantoaxial joint	Rotation	Uniaxial
Condylar	Metacarpophalangeal (knuckle), wrist joints	Flexion/extension, abduction/adduction	Biaxial
Saddle	Carpometacarpal joint of thumb	Flexion/extension, abduction/adduction	Biaxial
Ball-and-socket	Shoulder, hip joints	Flexion/extension, abduction/adduction, rotation	Multiaxial

Clinical Implications

Common Joint Injuries and Disorders

• Bursitis: Inflammation of bursae, usually caused by blow or friction; treated with rest, ice, and anti-inflammatory drugs.

• Tendinitis: Inflammation of tendon sheaths, typically due to overuse; symptoms and treatment similar to bursitis.

• Arthritis: Over 100 types of inflammatory or degenerative diseases that damage joints.

  • Acute forms: Often caused by bacteria, treated with antibiotics.

  • Chronic forms: Osteoarthritis, rheumatoid arthritis, and gouty arthritis.

• Cartilage tears: Due to compression and shear stress; cartilage rarely repairs itself.

• Sprains: Reinforcing ligaments are stretched or torn; partial tears repair slowly due to poor vascularization.

• Dislocations (luxations): Bones forced out of alignment; accompanied by sprains and inflammation; must be reduced to treat.

• Subluxations: Partial dislocation of a joint.

Shoulder Dislocations

• Common due to high mobility of the shoulder joint.

• Structures reinforcing the joint are weakest anteriorly and inferiorly, making the head of the humerus prone to dislocation forward and downward.

• The glenoid cavity provides poor support when the humerus is rotated laterally and abducted (e.g., during a football tackle).

• Blows to the top and back of the shoulder can also cause dislocations.

Additional info: The notes above are expanded with standard anatomical terminology and clinical context for clarity and completeness.