Chapter 8: Joints – Structure, Classification, and Clinical Implications

Joints (Articulations)

Introduction

Joints, also known as articulations, are sites where two or more bones meet. They play a crucial role in providing the skeleton with mobility while also holding bones together for structural support.

Primary Joint Classifications

Structural Classification

• 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.

Functional Classification

• Synarthroses: Immovable joints

• Amphiarthroses: Slightly movable joints

• Diarthroses: Freely movable joints

Fibrous Joints

• Sutures: Rigid, interlocking joints of the skull

• Syndesmoses: Inferior tibiofibular joint; interosseous membrane connecting radius and ulna

• Gomphoses: Holds tooth in socket

Cartilaginous Joints

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

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

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 thick; outer fibrous layer and inner synovial membrane

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

• Reinforcing ligaments: Strengthen and stabilize the joint

• Nerves and blood vessels: Detect pain, monitor joint position and stretch; capillary beds supply filtrate for synovial fluid

Bursae and Tendon Sheaths

• Bursae: Bags of synovial fluid that act as lubricating "ball bearings"; reduce friction where ligaments, muscles, skin, tendons, or bones rub together

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

Factors Determining Joint Stability

• Shape of articular surface: Shallow surfaces are less stable than ball-and-socket joints

• Ligament number and location: More ligaments generally mean a stronger joint

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

Range of Motion and Types of Movements in Synovial Joints

Range of Motion

• Nonaxial: Gliding movements only

• Uniaxial: Movement in one plane

• Biaxial: Movement in two planes

• Multiaxial: Movement in or around all three planes

General Types of Movements

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

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

• Rotation: Turning of bone around its own long axis (e.g., rotation between C1 and C2 vertebrae, rotation of humerus and femur)

Special Movements

• Supination and Pronation: Rotation of radius and ulna; supination = palms face anteriorly, pronation = palms face posteriorly

• Dorsiflexion and Plantar Flexion: Dorsiflexion = bending foot toward shin, plantar flexion = pointing toes

• Inversion and Eversion: Inversion = sole of foot faces medially, eversion = sole faces laterally

• Protraction and Retraction: Movement in lateral plane (e.g., mandible jutting out or pulled back)

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

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

Major Synovial Joints of the Body

Types of Synovial Joints

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

Clinical Implications

Common Joint Injuries and Disorders

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

• Tendonitis: Inflammation of tendon sheaths, typically caused by overuse; symptoms and treatment similar to bursitis

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

  • Acute forms: Caused by bacteria, treated with antibiotics

  • Chronic forms: Osteoarthritis, Rheumatoid arthritis, 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: Bones forced out of alignment; accompanied by sprains, inflammation, and difficulty moving joint; must be reduced to treat

• Shoulder dislocations: Common due to high mobility; weakest support anteriorly and inferiorly; glenoid cavity provides poor support when humerus is rotated laterally and abducted; blows to top and back of shoulder can also cause dislocations

• Subluxations: Partial dislocation of a joint

Example: Shoulder Dislocation

• Occurs when the head of the humerus is displaced from the glenoid cavity

• Common in contact sports or falls

• Requires medical intervention to restore normal alignment

Additional info: The notes have been expanded to include definitions, examples, and clinical context for each joint type and movement. The table summarizes the six major synovial joint types, their examples, and movement axes.