Articulations (Joints): Chapter 8 - Structure, Classification, and Function

Chapter 8: Articulations

Introduction to Articulations

Articulations, or joints, are sites where two or more bones meet. They play a crucial role in providing mobility and holding the skeleton together. Understanding the structure and function of joints is essential for comprehending body movement and stability.

• Articulation: Site where two or more bones meet.

• Functions of joints:

  • Give the skeleton mobility in response to skeletal muscle contractions.

  • Hold the skeleton together, providing stability and protection.

  • Allow for bone growth in length during skeletal development (e.g., epiphyseal plate).

Classification of Joints

Functional Classification of Joints

Joints are classified functionally based on the amount of movement they allow.

• Synarthroses: Immovable joints (e.g., sutures of the skull).

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

• Diarthroses: Freely movable joints (e.g., most limb joints).

Structural Classification of Joints

Structural classification is based on the material binding bones together and the presence or absence of a joint cavity.

• Fibrous joints: Bones joined by dense fibrous connective tissue; no joint cavity.

• Cartilaginous joints: Bones united by cartilage; no joint cavity.

• Synovial joints: Bones separated by a fluid-filled joint cavity; all are diarthrotic.

Fibrous Joints

Fibrous joints are united by dense fibrous connective tissue and are mostly immovable (synarthrotic). The amount of movement depends on the length of connective tissue fibers.

• Sutures: Rigid, interlocking joints found only in the skull. Allow for growth during youth and ossify in adulthood (synostoses).

• Syndesmoses: Bones connected by ligaments. Movement varies depending on fiber length (e.g., tibiofibular joint is immovable; interosseous membrane between radius and ulna is more movable).

• Gomphoses: Peg-in-socket joints of teeth in alveolar sockets. The fibrous connection is the periodontal ligament.

Cartilaginous Joints

Cartilaginous joints unite bones with cartilage and do not have a joint cavity. They are not highly movable.

• Synchondroses: Bones united by hyaline cartilage (e.g., epiphyseal plates in children, joint between first rib and manubrium). Mostly synarthrotic.

• Symphyses: Bones united by fibrocartilage (e.g., intervertebral discs, pubic symphysis). Strong, flexible amphiarthroses.

Synovial Joints

Synovial joints are characterized by a fluid-filled joint cavity that separates the articulating bones. All synovial joints are diarthrotic (freely movable) and are the most common type of joint in the body.

• Articular cartilage: Hyaline cartilage covering bone ends to prevent crushing.

• Joint (synovial) cavity: Small, fluid-filled space.

• Articular (joint) capsule: Two layers (external fibrous layer and inner synovial membrane).

• Synovial fluid: Viscous, slippery filtrate of plasma and hyaluronic acid for lubrication and nourishment.

• Reinforcing ligaments: Capsular (intrinsic), extracapsular, and intracapsular ligaments.

• Nerves and blood vessels: Nerve fibers detect pain, monitor joint position, and stretch; blood vessels supply filtrate for synovial fluid.

Structures Associated with Synovial Joints

• Bursae: Flattened fibrous sacs lined with synovial membrane and containing synovial fluid. Reduce friction between ligaments, muscles, skin, tendons, or bones.

• Tendon sheaths: Elongated bursae wrapped around tendons to reduce friction.

• Menisci: Semilunar fibrocartilage pads that cushion and improve the fit of articulating surfaces.

Stabilizing and Supporting Factors of Synovial Joints

• Articular surfaces: Shape determines possible movements.

• Ligaments: Number and location contribute to stability.

• Muscle tone: Keeps tendons taut and is the most important stabilizing factor in many joints.

Joint Disorders

• Bursitis: Inflammation of a bursa, usually caused by a blow or friction. Treated with rest and ice; anti-inflammatory drugs if severe.

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

• Chronic forms of arthritis:

  • Osteoarthritis: Degenerative, most common, irreversible, related to aging.

  • Rheumatoid arthritis: Chronic, inflammatory, autoimmune disease of unknown cause.

  • Gouty arthritis: Deposition of uric acid crystals in joints, causing inflammation.

Movements at Synovial Joints

Movements are described based on the direction and plane in which the bones move relative to each other.

• Gliding: Flat bone surfaces glide or slip over one another (e.g., intercarpal joints).

• Angular movements: Increase or decrease the angle between two bones.

  • Flexion: Decreases the angle of the joint.

  • Extension: Increases the angle of the joint.

  • Hyperextension: Extension beyond the anatomical position.

  • Abduction: Movement away from the midline.

  • Adduction: Movement toward the midline.

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

• Rotation: Turning of a bone around its own long axis (e.g., rotation of the head, neck, or humerus).

• Special movements: Supination, pronation, dorsiflexion, plantar flexion, inversion, eversion, protraction, retraction, elevation, depression, opposition.

Summary Table: Characteristics of Body Joints

Structural Classification	Functional Classification	Movement Allowed
Fibrous	Synarthroses or Amphiarthroses	Immovable or slightly movable
Cartilaginous	Synarthroses or Amphiarthroses	Immovable or slightly movable
Synovial	Diarthroses	Freely movable

Examples and Applications

• Sutures: Found only in the skull; allow for growth during youth and fuse in adulthood.

• Syndesmoses: Distal tibiofibular joint (immovable); interosseous membrane between radius and ulna (more movement).

• Gomphoses: Teeth in alveolar sockets, held by the periodontal ligament.

• Synchondroses: Epiphyseal plates in children, first rib and manubrium.

• Symphyses: Intervertebral discs, pubic symphysis.

• Synovial joints: Shoulder, elbow, hip, knee, etc.

Key Terms

• Articulation

• Synarthrosis

• Amphiarthrosis

• Diarthrosis

• Synovial fluid

• Bursa

• Meniscus

Additional info: The notes have been expanded with academic context and examples to ensure completeness and clarity for exam preparation.