Articulations: Structure and Function of Joints

Chapter 9: Articulations

Introduction to Articulations

Articulations, also known as joints, are the sites where two or more bones meet. They play a crucial role in providing mobility to the skeleton and holding bones together. The structure of a joint determines its function and the degree of movement it allows.

Joints: Classification

Structural Classification of Joints

Joints are classified based on the material binding the bones and the presence or absence of a joint cavity.

• Fibrous joints: Bones held together by dense connective tissue; no joint cavity.

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

• Synovial joints: Bones held together by a fluid-filled joint cavity; most freely movable.

Functional Classification of Joints

• Synarthrosis: Immovable joint.

• Amphiarthrosis: Slightly movable joint.

• Diarthrosis: Freely movable joint.

Fibrous Joints

Characteristics of Fibrous Joints

Fibrous joints lack a synovial cavity and are held together by dense connective tissue. They allow little or no movement.

• Sutures: Found between bones of the skull.

• Syndesmoses: Bones connected by a ligament; e.g., distal tibiofibular joint.

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

Cartilaginous Joints

Characteristics of Cartilaginous Joints

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

• Synchondroses: Connecting tissue is hyaline cartilage; e.g., epiphyseal plate.

• Symphyses: Ends of articulating bones are covered with hyaline cartilage, but a broad disc of fibrocartilage connects the bones; e.g., intervertebral joints, pubic symphysis.

Synovial Joints

Characteristics of Synovial Joints

Synovial joints are the most movable type of joint in the body. They have a synovial cavity and are characterized by the following structures:

• Articular capsule: Encloses the joint cavity; consists of an outer fibrous capsule and an inner synovial membrane.

• Synovial fluid: Lubricates the joint, nourishes articular cartilage, and reduces friction.

• Articular cartilage: Covers the ends of bones, providing a smooth surface for movement.

Bursae and Tendon Sheaths

These structures reduce friction in synovial joints:

• Bursae: Sac-like structures containing synovial fluid; found between skin and bone, tendons and bones, muscles and bones.

• Tendon sheaths: Tubular bursae that wrap around tendons, especially those subjected to friction.

Types of Movements at Synovial Joints

Gliding Movements

Gliding occurs when flat bone surfaces move back and forth or side to side over one another. No significant angular or rotational movement occurs.

Angular Movements

Angular movements change the angle between articulating bones. Types include:

• 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

Rotation is the movement of a bone around its longitudinal axis. Examples include turning the head side to side or rotating the arm.

Special Movements

• Elevation: Lifting a body part superiorly (e.g., closing the mouth).

• Depression: Moving a body part inferiorly (e.g., opening the mouth).

• Protraction: Moving a body part anteriorly (e.g., thrusting the mandible forward).

• Retraction: Moving a body part posteriorly (e.g., pulling the mandible backward).

• Inversion: Turning the sole of the foot medially.

• Eversion: Turning the sole of the foot laterally.

• Dorsiflexion: Bending the foot upward at the ankle.

• Plantar flexion: Bending the foot downward at the ankle.

• Supination: Rotating the forearm so the palm faces anteriorly.

• Pronation: Rotating the forearm so the palm faces posteriorly.

Types of Synovial Joints

Classification Based on Movement

Synovial joints are classified according to the shapes of their articulating surfaces and the types of movement they allow:

• Plane (gliding) joints: Permit back-and-forth and side-to-side movements; e.g., intercarpal joints.

• Hinge joints: Permit flexion and extension; e.g., elbow, knee.

• Pivot joints: Permit rotation; e.g., proximal radioulnar joint.

• Condyloid joints: Permit flexion, extension, abduction, adduction, and circumduction; e.g., wrist joint.

• Saddle joints: Permit a greater range of movement than condyloid joints; e.g., thumb (carpometacarpal joint).

• Ball-and-socket joints: Permit movement in all directions; e.g., shoulder, hip.

Summary Table: Types of Joints

Joint Type	Structural Features	Movement Allowed	Examples
Fibrous	No synovial cavity; dense connective tissue	Immovable or slightly movable	Sutures, syndesmoses, gomphoses
Cartilaginous	No synovial cavity; cartilage connects bones	Immovable or slightly movable	Synchondroses, symphyses
Synovial	Synovial cavity; articular capsule and cartilage	Freely movable	Shoulder, hip, knee, elbow

Summary Table: Types of Synovial Joints

Type	Movement	Example
Plane	Gliding	Intercarpal joints
Hinge	Flexion/Extension	Elbow, knee
Pivot	Rotation	Proximal radioulnar joint
Condyloid	Flexion/Extension, Abduction/Adduction	Wrist joint
Saddle	Greater range than condyloid	Thumb (carpometacarpal joint)
Ball-and-socket	All directions	Shoulder, hip

Key Terms and Definitions

• Articulation: The site where two bones meet.

• Synovial fluid: Viscous fluid within synovial joints that reduces friction and nourishes cartilage.

• Bursa: Fluid-filled sac that reduces friction between tissues.

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

• Flexion: Decreasing the angle between two bones.

• Extension: Increasing the angle between two bones.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Rotation: Movement around a longitudinal axis.

Applications and Examples

• Elbow joint: Hinge joint allowing flexion and extension.

• Shoulder joint: Ball-and-socket joint allowing movement in all directions.

• Intervertebral joints: Symphysis type cartilaginous joints allowing slight movement.

Additional info: Academic context and definitions have been expanded for clarity and completeness.