Chapter 8: Joints (Articulations) – Structured Study Notes

Joints (Articulations)

Definition and Functions

A joint, also known as an articulation, is the site where two or more bones meet. Joints serve two primary functions:

• Movement: Allowing various types of motion between bones.

• Stability: Holding the skeleton together.

Joints are classified in two main ways: structural and functional.

Functional Classification of Joints

Functional classification is based on the degree of movement permitted:

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

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

• Diarthrosis: Freely movable joints (e.g., most synovial joints).

Synovial joints are functionally classified as diarthroses.

Structural Classification of Joints

Structural classification is based on what holds the bones together:

• Fibrous joints: Bones united by dense fibrous connective tissue.

• Cartilaginous joints: Bones united by cartilage.

• Synovial joints: Bones separated by a joint cavity filled with synovial fluid.

Only synovial joints possess a joint cavity.

Fibrous Joints

Types and Characteristics

• Sutures: Found in the skull; immovable.

• Syndesmoses: Bones connected by ligaments; slight movement possible.

• Gomphoses: Peg-in-socket joints (e.g., tooth and periodontal ligament).

Most fibrous joints are functionally synarthroses (immovable).

Cartilaginous Joints

Types and Characteristics

• Synchondrosis: Bones joined by hyaline cartilage; functionally synarthrosis. Example: Epiphyseal plate in growing bones.

• Symphysis: Bones joined by fibrocartilage; functionally amphiarthrosis. Example: Intervertebral discs, pubic symphysis.

Synovial Joints

Features and Functions

Synovial joints are characterized by the presence of a joint cavity and are all diarthroses (freely movable). They have six distinguishing features:

• Articular cartilage: Covers bone surfaces, reducing friction.

• Joint cavity: Space containing synovial fluid.

• Articular (joint) capsule: Encloses the joint cavity.

• Synovial membrane: Produces synovial fluid.

• Synovial fluid: Lubricates, nourishes, and absorbs shock.

• Reinforcing ligaments: Strengthen and stabilize the joint.

Synovial fluid is essential for lubrication, nutrient distribution, and shock absorption.

Friction-Reducing Structures

• Bursae: Flattened sacs filled with synovial fluid, reduce friction between tissues.

• Tendon sheath: Elongated bursa that wraps around a tendon.

Factors Influencing Joint Stability

• Shape of articular surfaces: Deeper or more congruent surfaces provide greater stability.

• Number and location of ligaments: More ligaments generally increase stability.

• Muscle tone: The tension of muscles crossing the joint is a primary stabilizer.

Shoulder (glenohumeral joint): Most mobile, least stable major joint. Hip: Most stable ball-and-socket joint.

Joint Movements

Types of Movements

• Gliding: Sliding movements (e.g., plane joints in the wrist).

• Angular Movements:

  • Flexion: Decreases the angle between bones.

  • Extension: Increases the angle between bones.

  • Hyperextension: Extension beyond anatomical position.

  • Abduction: Movement away from the midline.

  • Adduction: Movement toward the midline.

  • Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

• Rotation: Bone turns around its longitudinal axis.

• Special Movements:

  • Forearm: Supination (palm up), Pronation (palm down).

  • Foot: Plantar flexion, Dorsiflexion, Inversion, Eversion.

  • Jaw: Elevation (up), Depression (down).

  • Thumb: Opposition (touching thumb to fingers).

Types of Synovial Joints

Classification and Examples

Type	Movement	Example
Plane	Gliding only	Intercarpal joints
Hinge	Flexion/Extension	Elbow
Pivot	Rotation	Radioulnar joint
Condyloid	Biaxial	Metacarpophalangeal joints
Saddle	Biaxial (thumb)	Carpometacarpal joint of thumb
Ball-and-socket	Multiaxial	Shoulder, Hip
Modified hinge	Complex movement	TMJ (temporomandibular joint)

Selected Major Joints

Shoulder (Glenohumeral Joint)

• Type: Ball-and-socket

• Highly mobile, less stable

• Stabilized primarily by rotator cuff muscles

Hip Joint

• Type: Ball-and-socket

• Deep socket (acetabulum) provides stability

• More stable than shoulder

Knee Joint

• Type: Modified hinge

• Menisci (fibrocartilage) cushion and stabilize

• Three C's: Collateral ligaments, Cruciate ligaments, Cartilage (meniscus)

Temporomandibular Joint (TMJ)

• Joint between mandible and temporal bone

• Most easily dislocated major joint

Injuries and Disorders of Joints

• Sprain: Ligament tear

• Dislocation (luxation): Bones forced out of alignment

• Subluxation: Partial dislocation

• Bursitis: Inflammation of a bursa

• Tendonitis: Inflammation of a tendon

• Arthritis: Joint inflammation

  • Osteoarthritis: Degenerative, "wear and tear"

  • Rheumatoid arthritis: Autoimmune disorder

  • Gouty arthritis: Uric acid crystal deposition

Summary of Key Concepts

• Structural classification: What holds bones together (fibrous, cartilaginous, synovial)

• Functional classification: Amount of movement allowed (synarthrosis, amphiarthrosis, diarthrosis)

• Only synovial joints have a joint cavity

• Diarthroses are synonymous with synovial joints

• Shoulder is the most mobile but least stable major joint

• Muscle tone is the primary stabilizer of joints