Chapter 9: Joints (Articulations) – Structure and Function

Joints (Articulations)

Introduction

Joints, or articulations, are anatomical structures that connect bones within the skeletal system. They permit effective movement and provide protection for softer organs. Understanding joint anatomy and movement is essential for studying muscle actions and overall body mechanics.

• Joints link bones, enable movement, and protect organs.

• Joint structure and movement are foundational for muscle function.

Organizational Overview

Joints are classified based on their structure and the degree of movement they allow. The main categories include bony, fibrous, cartilaginous, and synovial joints.

• Bony joints (synostosis)

• Fibrous joints (synarthrosis)

  • Sutures

  • Gomphoses

  • Syndesmoses

• Cartilaginous joints

  • Synchondroses

  • Symphyses

• Synovial joints (diarthrosis)

I. Joints and Their Classification

Definition and Naming

A joint (articulation) is any point where two bones meet, regardless of movement. Joint names are typically derived from the bones involved (e.g., radioulnar joint).

Classification by Movement

• Diarthrosis: Freely movable joints

• Amphiarthrosis: Slightly movable joints

• Synarthrosis: Little or no movement

Classification by Structure

• Fibrous: Bones joined by collagen fibers

• Cartilaginous: Bones joined by cartilage

• Bony or Synovial: Bones fused or separated by a joint cavity

A. Bony Joint (Synostosis)

Bony joints occur when two bones fuse together, forming an immovable joint.

• Examples: Frontal and mandibular bones in infants, cranial sutures in elderly, first rib to sternum with age

• Immovable joint (synarthrosis)

B. Fibrous Joints (Synarthrosis)

Fibrous joints are connected by collagen fibers and are generally immovable.

• Sutures: Bind skull bones together; classified as serrate (interlocking), lap (overlapping), or plane (straight edges)

• Gomphoses: Attachment of a tooth to its socket, held by periodontal ligament

• Syndesmoses: Two bones connected by a ligament; slightly movable (amphiarthrosis), e.g., interosseous membranes between radius/ulna and tibia/fibula

C. Cartilaginous Joints

• Synchondrosis: Bones joined by hyaline cartilage (e.g., rib to sternum, epiphyseal plate in children); immovable

• Symphysis: Bones joined by fibrocartilage (e.g., pubic symphysis, intervertebral discs); slightly movable

II. Synovial Joints (Diarthrosis)

General Anatomy

Synovial joints are characterized by a joint cavity separating the bones, allowing for free movement.

• Articular capsule: Encloses joint cavity, lined by synovial membrane

• Synovial fluid: Slippery fluid for lubrication, nutrient distribution, and shock absorption

• Articular cartilage: Hyaline cartilage covering joint surfaces, reduces friction

• Meniscus: Pad of fibrocartilage between bones

• Fat pads: Adipose tissue for protection and packing

• Bursae: Small fluid-filled pockets reducing friction

• Tendon: Attaches muscle to bone

• Ligament: Attaches bone to bone, reinforces capsule

B. Levers

Joints and bones function as levers, facilitating movement when effort overcomes resistance. There are three classes of levers:

• First-Class Lever (RFE): Fulcrum between effort and resistance (e.g., nodding head)

• Second-Class Lever (FRE): Resistance between fulcrum and effort (e.g., standing on toes)

• Third-Class Lever (REF): Effort between resistance and fulcrum (most body joints, e.g., biceps at elbow)

Range of Motion (ROM)

Range of motion refers to the degrees through which a joint can move, determined by:

• Structure of articular surfaces

• Strength and tautness of ligaments, tendons, and capsule

• Action of muscles and tendons

• Muscle tone: State of tension in resting muscles

Axes of Rotation

• Multiaxial (triaxial) joints: Move in three planes (e.g., shoulder joint)

• Monoaxial or biaxial joints: Move in one or two planes

C. Types of Synovial Joints

Type	Structure	Example	Axes
Ball-and-Socket	Head fits in cuplike depression	Shoulder, hip	Multiaxial (3 planes)
Condyloid (Ellipsoid)	Oval convex surface fits into depression	Wrist, finger bases	Biaxial (2 planes)
Saddle	Each surface is saddle-shaped	Thumb base	Biaxial
Gliding (Plane)	Flat surfaces slide over each other	Clavicle, vertebrae	Monoaxial (limited)
Hinge	Convex surface fits into concave depression	Elbow, knee, fingers, toes	Monoaxial
Pivot	Projection fits into ringlike ligament	Turning head	Monoaxial

D. Movement of Synovial Joints

Flexion, Extension, and Hyperextension

• Flexion: Decreases joint angle

• Extension: Straightens joint, returns to anatomical position

• Hyperextension: Extension beyond 180 degrees

Abduction and Adduction

• Abduction: Movement away from midline

• Adduction: Movement toward midline

Elevation and Depression

• Elevation: Raises bone vertically (e.g., shrugging shoulders)

• Depression: Lowers bone (e.g., dropping shoulders)

Protraction and Retraction

• Protraction: Moves part anteriorly (e.g., thrusting jaw forward)

• Retraction: Moves part posteriorly

Circumduction

• One end stationary, other end moves in a circle

• Sequence: Flexion, abduction, extension, adduction

Supination and Pronation

• Supination: Rotation so palm faces forward; inversion/abduction of foot

• Pronation: Rotation so palm faces rear; eversion/abduction of foot

Rotation of Trunk and Head

• Right rotation of trunk; rotation of head

III. Anatomy of Selected Diarthroses

Shoulder Joint (Humeroscapular)

• Head of humerus articulates with glenoid fossa of scapula

• Most freely movable joint (greatest ROM)

• Ball-and-socket joint

• Stabilized by ligaments (glenohumeral, coracohumeral, etc.), rotator cuff muscles, tendons, and bursae

Elbow Joint

• Hinge joint

• Single joint capsule encloses humeroulnar and humeroradial joints

Hip Joint (Coxal)

• Head of femur articulates with acetabulum of coxae

• Ball-and-socket joint

• Stabilized by ligaments (iliofemoral, pubofemoral, ischiofemoral, acetabular, ligamentum teres)

Knee Joint

• Most complex diarthrosis

• Patellofemoral and tibiofemoral components

• Joint capsule strengthened by extracapsular and intracapsular ligaments

• Anterior and lateral cruciate ligaments limit sliding; collateral ligaments prevent rotation

Arthritis and Artificial Joints

Arthritis

• Osteoarthritis: Degeneration of articular cartilage, bone spurs, pain

• Rheumatoid arthritis: Autoimmune attack on synovial membrane, cartilage degradation, joint ossification

Arthroplasty

• Replacement of diseased joint with artificial device (prosthesis)

Summary Table: Joint Types and Movements

Joint Type	Movement	Example
Bony (Synostosis)	Immovable	Frontal bone fusion
Fibrous (Suture)	Immovable	Skull sutures
Fibrous (Gomphosis)	Immovable	Tooth in socket
Fibrous (Syndesmosis)	Slightly movable	Radius/ulna, tibia/fibula
Cartilaginous (Synchondrosis)	Immovable	Rib to sternum
Cartilaginous (Symphysis)	Slightly movable	Pubic symphysis
Synovial	Freely movable	Shoulder, knee, hip

Additional info: This guide expands on the provided notes with definitions, examples, and tables for clarity and completeness, suitable for college-level Anatomy & Physiology study.