The Skeletal System: Joints

Introduction

Joints, or articulations, are the sites where two or more bones meet. They are essential for providing mobility to the skeleton and for holding the skeleton together. Understanding the structure and function of joints is crucial for comprehending movement and for managing injuries such as ankle sprains.

Structural and Functional Classification of Joints

Structural Categories of Joints

Joints are classified structurally based on the material binding the bones together and the presence or absence of a joint cavity. The three main structural categories are:

• Fibrous Joints

• Cartilaginous Joints

• Synovial Joints

Fibrous Joints

• Definition: Bones are joined by dense fibrous connective tissue; no joint cavity is present.

• Mobility: Most are immovable.

• Types of Fibrous Joints:

  • Sutures: Rigid, interlocking joints found in the skull.

  • Syndesmoses: Bones connected by ligaments (e.g., inferior tibiofibular joint, interosseous membrane between radius and ulna).

  • Gomphoses: Peg-in-socket joints (e.g., tooth held in its socket).

Cartilaginous Joints

• Definition: Bones are united by cartilage; no joint cavity is present.

• Mobility: Most are slightly movable.

• Types of Cartilaginous Joints:

  • Synchondroses: Bar or plate of hyaline cartilage unites bones (e.g., epiphyseal plate, joint between first rib and manubrium).

  • Symphyses: Fibrocartilage unites bones (e.g., intervertebral joints, pubic symphysis).

Synovial Joints

• Definition: Bones are separated by a fluid-filled joint cavity.

• Mobility: Freely movable; most joints of the limbs are synovial.

• Key Features:

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

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

  • Articular capsule: Two layers—an outer fibrous layer (strengthens joint) and an inner synovial membrane (produces synovial fluid).

  • Synovial fluid: Viscous, slippery fluid that lubricates and nourishes articular cartilage; contains phagocytic cells.

  • Reinforcing ligaments: Strengthen the joint.

  • Nerves and blood vessels: Detect pain, monitor joint position, and supply nutrients.

  • Bursae and tendon sheaths: Reduce friction where ligaments, muscles, skin, or tendons rub together.

Functional Categories of Joints

Joints are also classified functionally by the amount of movement they allow:

• Synarthroses: Immovable joints

• Amphiarthroses: Slightly movable joints

• Diarthroses: Freely movable joints

Movements of Synovial Joints

Types of Movement

Synovial joints allow several types of movement, classified by the axes and planes in which movement occurs:

• Nonaxial: Slipping movements only (e.g., intercarpal joints)

• Uniaxial: Movement in one plane (e.g., elbow)

• Biaxial: Movement in two planes (e.g., wrist)

• Multiaxial: Movement in or around all three planes (e.g., shoulder)

General Types of Movements

• Gliding: One flat bone surface glides or slips over another (e.g., intercarpal joints, intertarsal joints, articular processes of vertebrae).

• Angular Movements: Change the angle between two bones.

  • Flexion: Decreases the angle of the joint (usually in the sagittal plane).

  • Extension: Increases the angle of the joint.

  • Hyperextension: Extension beyond the anatomical position.

  • Abduction: Movement away from the midline (frontal plane).

  • Adduction: Movement toward the midline (frontal plane).

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

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

  • Medial rotation: Toward the midline.

  • Lateral rotation: Away from the midline.

• Special Movements:

  • Supination and Pronation: Rotation of the forearm (supination: palms face anteriorly; pronation: palms face posteriorly).

  • Dorsiflexion and Plantar Flexion: Dorsiflexion: bending foot upward; plantar flexion: pointing toes downward.

  • Inversion and Eversion: Inversion: sole faces medially; eversion: sole faces laterally.

  • Protraction and Retraction: Protraction: moving a body part forward (e.g., mandible); retraction: moving it backward.

  • Elevation and Depression: Elevation: lifting a body part superiorly (e.g., shrugging shoulders); depression: lowering a body part.

  • Opposition: Movement of the thumb to touch the tips of other fingers.

Types of Synovial Joints

Synovial joints are classified by the shape of their articular surfaces and the types of movement they allow. There are six main types:

Clinical Relevance

• Understanding joint structure and function is essential for diagnosing and treating joint injuries, such as sprains and dislocations.

• Joint stability is influenced by the shape of articular surfaces, the number and location of ligaments, and muscle tone.

Additional info: The notes above have been expanded for clarity and completeness, including definitions, examples, and a summary table of synovial joint types, as would be expected in a college-level Anatomy & Physiology course.