Joints: Structure, Classification, and Movements

Joints (Articulations)

Introduction

Joints, or articulations, are the sites where two or more bones meet. They play a crucial role in providing the skeleton with mobility and holding the bones together. Understanding the structure and function of joints is essential for diagnosing and treating musculoskeletal disorders.

Classification of Joints

Structural Classification

• Fibrous Joints: Bones joined by dense fibrous connective tissue; no joint cavity; mostly immovable.

• Cartilaginous Joints: Bones united by cartilage; no joint cavity; not highly movable.

• Synovial Joints: Bones separated by a fluid-filled joint cavity; freely movable (diarthroses).

Functional Classification

• Synarthroses: Immovable joints.

• Amphiarthroses: Slightly movable joints.

• Diarthroses: Freely movable joints.

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

Fibrous Joints

General Features

• Bones joined by dense fibrous connective tissue.

• No joint cavity present.

• Most are immovable, but some allow limited movement depending on the length of connective tissue fibers.

Types of Fibrous Joints

• Sutures: Found only in the skull; bones are tightly bound by minimal fibrous tissue.

• Syndesmoses: Bones connected by ligaments; movement varies with fiber length (e.g., distal tibiofibular joint).

• Gomphoses: Peg-in-socket joints (e.g., teeth in alveolar sockets).

Cartilaginous Joints

General Features

• Bones united by cartilage (hyaline or fibrocartilage).

• No joint cavity present.

• Not highly movable.

Types of Cartilaginous Joints

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

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

Synovial Joints

General Structure

• Articular cartilage: Hyaline cartilage covering bone ends.

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

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

• Synovial fluid: Lubricates and nourishes articular cartilage.

• Reinforcing ligaments: Strengthen the joint.

• Nerves and blood vessels: Monitor joint position and supply nutrients.

Bursae and Tendon Sheaths

• Bursae: Flattened fibrous sacs lined with synovial membrane; reduce friction between tissues.

• Tendon sheaths: Elongated bursae that wrap around tendons.

Factors Influencing Stability of Synovial Joints

• Shape of articular surfaces (minor role).

• Number and position of ligaments (limited role).

• Muscle tone (most important factor): Keeps tendons taut and stabilizes joints.

Movements at Synovial Joints

Types of Movements

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

• Angular movements: Increase or decrease the angle between two bones (e.g., flexion, extension, abduction, adduction, circumduction).

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

• Special movements: Include supination, pronation, inversion, eversion, protraction, retraction, elevation, depression, opposition.

Table: Movements at Synovial Joints

Movement	Description	Example
Flexion	Decreases the angle of the joint	Bending the elbow
Extension	Increases the angle of the joint	Straightening the knee
Abduction	Movement away from the midline	Raising the arm laterally
Adduction	Movement toward the midline	Bringing fingers together
Rotation	Turning a bone around its own axis	Turning the head side to side
Supination/Pronation	Rotational movement of the forearm	Turning the palm up/down
Inversion/Eversion	Movements of the sole of the foot	Turning sole medially/laterally
Protraction/Retraction	Anterior/posterior movement in a transverse plane	Jutting jaw forward/backward
Elevation/Depression	Lifting/lowering a body part	Shrugging shoulders
Opposition	Touching thumb to fingers	Grasping objects

Types of Synovial Joints

Synovial joints are classified based on the shape of their articular surfaces and the movements they allow.

• Plane: Flat articular surfaces; allow gliding movements (e.g., intercarpal joints).

• Hinge: Cylindrical projection fits into a trough-shaped surface; allow flexion and extension (e.g., elbow joint).

• Pivot: Rounded end of one bone fits into a ring; allows rotation (e.g., proximal radioulnar joint).

• Condylar (Ellipsoid): Oval articular surface fits into a complementary depression; allows all angular movements (e.g., knuckle joints).

• Saddle: Articular surfaces have both concave and convex areas; allow greater freedom of movement (e.g., thumb carpometacarpal joint).

• Ball-and-socket: Spherical head of one bone fits into a round socket; allows multiaxial movement (e.g., shoulder and hip joints).

Additional info: The stability and range of motion of synovial joints are determined by the shape of the articular surfaces, the number and position of ligaments, and the tone of surrounding muscles.