Articulations

Introduction

An articulation, or joint, is where two or more bones meet. Joints are classified structurally or functionally.

Structural Classification of Joints

Based on anatomical characteristics:

  • Presence/absence of a synovial cavity
  • Type of connective tissue

Three types:

  • Fibrous: Held by dense connective tissue; no synovial cavity.
  • Cartilaginous: Held by cartilage; no synovial cavity.
  • Synovial: Held by dense connective tissue of an articular capsule; possess a synovial cavity.

Functional Classification of Joints

Based on movement permitted:

  • Synarthrosis: Immovable.
  • Amphiarthrosis: Slightly movable.
  • Diarthrosis: Freely movable.

Each structurally classified joint will also be classified by function.

Fibrous Joints

Articulating bones held closely together by dense connective tissue; permit little to no movement.

Three types:

  • Suture
  • Syndesmosis and gomphosis
  • Interosseous membrane

Fibrous Joints: Examples

  • Suture: Functionally synarthroses in adults, amphiarthroses in infants/children.
  • Syndesmosis: Functionally amphiarthroses, except gomphoses (synarthroses).
  • Interosseous Membrane: Functionally amphiarthroses.

Cartilaginous Joints

Articulating bones tightly connected by hyaline or fibrocartilage.

Two types:

  • Synchondrosis
  • Symphysis

Cartilaginous Joints: Examples

  • Synchondrosis: Functionally synarthroses (e.g., epiphyseal plates).
  • Symphysis: Functionally amphiarthroses (e.g., pubic symphysis).

Synovial Joints

All synovial joints have a synovial cavity, allowing function as a diarthrosis.

Distinguishing structures:

  • Articular hyaline cartilage: Reduces friction, absorbs shock.
  • Articular capsule.
  • Synovial fluid.

Accessory structures:

  • Ligaments and articular menisci.
  • Bursae and tendon sheaths.

Articular Capsule

Two layers enclosing the synovial cavity:

  • Outer fibrous membrane: Dense connective tissue, stabilizes joint.
  • Inner synovial membrane: Areolar connective tissue, secretes synovial fluid. Articular fat pads cushion within some joints.

Synovial Fluid

Forms a thin film over articular joint structures; functions include:

  • Reducing friction by lubrication.
  • Absorbing shock.
  • Supplying oxygen and nutrients to articular cartilage.
  • Removing wastes. Phagocytic cells remove microbes and debris.

Accessory Structures

  • Ligaments: Resist strain, hold bones together.
  • Articular menisci: Stabilize joint, direct synovial fluid flow.
  • Bursae: Cushion adjacent body parts, alleviate friction.
  • Tendon sheaths: Tubelike bursae around tendons, prevent friction.

Synovial Joint Movement Terminology

Indicates direction/relationships during movement.

Four categories:

  • Gliding movements.
  • Angular movements.
  • Rotation.
  • Special movements.

Gliding Movements

Flat bone surfaces move back and forth or side to side; limited range, no significant angle alteration.

Angular Movements

Angle between articulating bones increases/decreases.

Principle movements:

  • Flexion: decrease in angle
  • Extension: increase in angle
  • Lateral flexion: flexion along frontal plane
  • Hyperextension: extension beyond anatomical position
  • Abduction: movement away from midline
  • Adduction: movement toward midline
  • Circumduction: circular movement

Rotation

Bone revolves around its longitudinal axis; defined relative to midline.

  • Medial rotation: anterior surface turned toward midline.
  • Lateral rotation: anterior surface turned away from midline.

Special Movements

  • Elevation: upward movement.
  • Depression: downward movement.
  • Protraction: anterior movement.
  • Retraction: posterior movement.
  • Inversion: soles of feet medially.
  • Eversion: soles of feet laterally.
  • Dorsiflexion: bending foot superiorly.
  • Plantar flexion: bending foot inferiorly.
  • Supination: palm anteriorly.
  • Pronation: palm posteriorly.
  • Opposition: thumb touching fingertips.

Categories of Synovial Joints

Shapes allow variety of movements.

Major categories:

  • Plane joint
  • Hinge joint
  • Pivot joint
  • Condyloid joint
  • Saddle joint
  • Ball-and-socket joint

Examples of Synovial Joints

  • Plane: flat surfaces (e.g., intercarpal, intertarsal).
  • Hinge: convex into concave (e.g., knee, elbow).
  • Pivot: round into ring (e.g., atlanto-axial, radioulnar).
  • Condyloid: oval into oval (e.g., wrist, metacarpophalangeal).
  • Saddle: saddle-shaped (e.g., carpometacarpal of thumb).
  • Ball-and-Socket: ball into cup (e.g., shoulder, hip).

Major Synovial Joints of Body

  • Shoulder: Ball-and-socket (humerus and scapula). Stability from rotator cuff muscles. Permits flexion, extension, abduction, adduction, medial/lateral rotation, circumduction.
  • Elbow: Hinge joint (humerus, ulna, radius). Permits flexion/extension.
  • Hip: Ball-and-socket (femur and hip). Permits rotation and angular movement; strong ligaments limit range.
  • Knee: Largest, most complex. Permits flexion, extension, slight rotation.