Anatomy & Physiology Study Notes: Joint Classification and Structure

Joints (Articulations)

Definition and Function

• Joint (Articulation): A point of contact between two bones.

• Function: Enables skeletal mobility and holds the skeleton together.

Classification of Joints

Systems of Classification

• Structural Classification: Based on the material binding the bones and the presence or absence of a joint cavity. Includes fibrous, cartilaginous, and synovial joints.

• Functional Classification: Based on the amount of movement allowed by the joint. Includes synarthrosis (immovable), amphiarthrosis (slightly movable), and diarthrosis (freely movable).

Structural Joint Classifications

1. Fibrous Joints

Fibrous joints are connected by fibrous connective tissue and lack a synovial cavity. They permit little or no movement.

• Suture: Thin layer of dense fibrous connective tissue unites skull bones. Immovable; in adults, bones become ossified.

• Syndesmosis: Bones connected by a ligament. Permits slight movement; functionally classified as amphiarthrotic.

• Gomphosis: Cone-shaped peg fits into a socket (e.g., tooth in alveolar socket). Immovable joint.

2. Cartilaginous Joints

Cartilaginous joints are connected by cartilage and lack a synovial cavity. They permit little or no movement.

• Synchondrosis: Connecting material is hyaline cartilage. Immovable joint (e.g., epiphyseal plate).

• Symphysis: Connecting material is fibrocartilage. Slightly movable joint (e.g., intervertebral discs, pubic symphysis).

3. Synovial Joints

Synovial joints are separated by a fluid-containing synovial cavity and permit free movement. Most functionally classified as diarthrotic.

• General Structure:

  • Joint (synovial) cavity: Space filled with synovial fluid.

  • Articular cartilage: Hyaline cartilage covers ends of articulating bones; reduces friction and absorbs shock.

  • Articular capsule: Encloses synovial cavity; consists of two layers:

    • Fibrous capsule: Outer layer of dense irregular connective tissue; attaches to periosteum, provides strength.

    • Synovial membrane: Inner layer of areolar connective tissue; secretes synovial fluid.

  • Synovial fluid: Viscous, slippery fluid derived from blood filtration; lubricates and reduces friction, supplies nutrients, and removes waste.

  • Ligaments: Cross synovial cavity to strengthen joint, connect bone to bone.

  • Menisci: Pads of fibrocartilage found between articular bones; help maintain joint stability and direct synovial fluid flow.

  • Bursae: Flattened sacs filled with synovial fluid; reduce friction at synovial joints.

  • Tendon Sheaths: Elongated bursae wrapped around tendons; reduce friction where tendons pass over bones.

Types of Synovial Joints

• Plane (Gliding) Joint: Allows sliding movements (e.g., intercarpal joints).

• Hinge Joint: Allows flexion and extension (e.g., elbow).

• Pivot Joint: Allows rotation (e.g., proximal radioulnar joint).

• Condyloid Joint: Allows movement in two planes (e.g., wrist).

• Saddle Joint: Allows movement in two planes (e.g., thumb).

• Ball-and-Socket Joint: Allows movement in multiple axes (e.g., shoulder, hip).

Joint Injuries and Disorders

Common Injuries

• Sprain: Stretches or tears ligaments without dislocation.

• Strain: Stretched or partially torn muscle.

• Dislocation: Displacement of a bone from a joint.

Inflammatory and Degenerative Conditions

• Bursitis: Inflammation of the bursa, usually caused by blow or friction.

• Tendinitis: Inflammation of the tendons, usually caused by overuse.

Arthritis

• Rheumatoid Arthritis: Chronic autoimmune inflammatory disorder; immune system attacks cartilage and joint linings. Leads to synovial fluid accumulation, scar tissue formation, and eventual ossification. More common in females.

• Osteoarthritis: Non-inflammatory, degenerative joint disease; breakdown of articular cartilage and formation of bone spurs. Most common arthritis; progression is irreversible.

• Gouty Arthritis: Uric acid crystals deposited in joints, causing inflammation and pain. More common in males.

Joint Structure Diagram

• Ligament: Connects bone to bone, stabilizes joint.

• Joint Cavity: Contains synovial fluid.

• Fibrous Layer: Outer layer of articular capsule.

• Synovial Membrane: Inner layer, secretes synovial fluid.

• Articular Capsule: Encloses joint cavity.

• Periosteum: Connective tissue covering bone.

Joint Classification Table

The following table compares the functional and structural classification of major joints:

Joint	Functional Type (synarthrotic, amphiarthrotic, diarthrotic)	Structural Type (synovial, cartilaginous, fibrous)
Skull - suture	Synarthrotic	Fibrous
Tooth	Synarthrotic	Fibrous
Intervertebral (C3-L5 - between bodies)	Amphiarthrotic	Cartilaginous
Sternocostal #1	Synarthrotic	Cartilaginous
Sternocostal #2-#7	Diarthrotic	Synovial
Shoulder	Diarthrotic	Synovial
Elbow	Diarthrotic	Synovial
Radioulnar (proximal)	Diarthrotic	Synovial
Wrist	Diarthrotic	Synovial
Fingers	Diarthrotic	Synovial
Sacroiliac	Diarthrotic, Amphiarthrotic	Synovial
Pubic Symphysis	Amphiarthrotic	Cartilaginous
Hip	Diarthrotic	Synovial
Knee	Diarthrotic	Synovial
Tibiofibular (distal)	Synarthrotic	Fibrous
Ankle	Diarthrotic	Synovial
Toes	Diarthrotic	Synovial

Key Terms and Definitions

• Synarthrosis: Immovable joint.

• Amphiarthrosis: Slightly movable joint.

• Diarthrosis: Freely movable joint.

• Fibrous Joint: Bones joined by fibrous tissue.

• Cartilaginous Joint: Bones joined by cartilage.

• Synovial Joint: Bones separated by a fluid-filled cavity.

• Ligament: Connects bone to bone.

• Tendon: Connects muscle to bone.

• Bursa: Fluid-filled sac reducing friction.

• Meniscus: Cartilage pad improving joint fit and stability.

Formulas and Equations

• Synovial Fluid Production: $\text{Synovial Fluid Volume} = \text{Filtration Rate} \times \text{Time}$ Additional info: This formula is a general representation; actual production depends on joint activity and health.

Examples and Applications

• Example: The knee joint is a synovial, diarthrotic joint, allowing flexion and extension, stabilized by ligaments and menisci.

• Application: Understanding joint classification aids in diagnosing joint injuries and diseases such as arthritis.

Additional info: Some content was expanded for clarity and completeness, including definitions, examples, and the general formula for synovial fluid production.