Joints: Classification, Structure, and Function

Overview of Joint Classification

Joints, also known as articulations, are points where two or more bones meet. They are classified based on their structure and function, which determines their movement and stability.

• Structural Classification: Based on the material binding the bones and the presence or absence of a joint cavity.

• Functional Classification: Based on the amount of movement allowed by the joint.

Key Point: The two main types of joint classification are structure and function.

Structural Types of Joints

• Fibrous Joints: Bones are joined by dense fibrous connective tissue; no joint cavity. Examples: sutures, syndesmoses, gomphoses.

• Cartilaginous Joints: Bones are united by cartilage; no joint cavity. Examples: synchondroses, symphyses.

• Synovial Joints: Bones are separated by a fluid-filled joint cavity; most freely movable.

Functional Types of Joints

• Synarthrosis: Immovable joint (e.g., sutures in the skull).

• Amphiarthrosis: Slightly movable joint (e.g., symphysis).

• Diarthrosis: Freely movable joint (e.g., hip, shoulder).

Examples and Features of Joint Types

Fibrous Joints

• Suture: Immovable joint found between skull bones.

• Syndesmosis: Bones connected by ligaments; allows slight movement (e.g., distal tibiofibular joint).

• Gomphosis: Peg-in-socket joint (e.g., teeth in sockets).

Cartilaginous Joints

• Synchondrosis: Bones united by hyaline cartilage; mostly immovable (e.g., epiphyseal plate).

• Symphysis: Bones united by fibrocartilage; slightly movable (e.g., pubic symphysis).

Synovial Joints

Synovial joints are the most common and freely movable joints in the body. They have a unique structure that includes a joint cavity filled with synovial fluid.

• Joint Cavity: Space that contains synovial fluid.

• Articular Cartilage: Hyaline cartilage covering bone ends.

• Ligaments: Connect bone to bone, providing stability.

• Synovial Fluid: Viscous, egg-white consistency due to hyaluronic acid; lubricates and nourishes the joint.

• Bursae: Fluid-filled sacs that reduce friction (not always present).

Body Movements at Joints

Types of Movements

• Flexion: Decreases the angle between bones (e.g., bending fingers to make a fist).

• Extension: Increases the angle between bones.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Rotation: Bone turns around its own axis.

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

• Pronation: Opposite of supination; palm faces down.

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

• Inversion: Turning the sole of the foot medially.

• Eversion: Turning the sole of the foot laterally.

• Dorsiflexion: Lifting the foot upwards.

• Plantar Flexion: Pointing the foot downwards.

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

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

• Protraction: Moving a body part forward (e.g., jutting the jaw).

• Retraction: Moving a body part backward.

Types of Synovial Joints

Classification by Shape and Movement

Synovial joints are classified by the shapes of their articulating surfaces and the types of movement they allow.

Key Point: Ball-and-socket joints are the most freely movable type of synovial joint.

Selected Synovial Joints and Their Features

Major Synovial Joints

• Shoulder Joint: Most easily dislocated; stabilized by rotator cuff muscles and the tendon of the long head of the biceps.

• Elbow Joint: Stabilized by the trochlear notch and collateral ligaments.

• Knee Joint: Contains collateral ligaments; susceptible to sprains.

• Hip Joint: Ball-and-socket; strong but less mobile than the shoulder.

• Temporomandibular Joint (TMJ): Only partially enclosed in a joint capsule; allows both hinge and gliding movements.

Ligaments and Joint Stability

• Collateral Ligaments: Found in the elbow and knee; provide lateral stability.

• Tendon of the Long Head of the Biceps: Strengthens the shoulder joint.

Summary Table: Joint Types and Examples

Key Terms and Definitions

• Articulation: The site where two bones meet.

• Ligament: Connective tissue that connects bones and stabilizes joints.

• Synovial Fluid: Lubricating fluid within synovial joints, rich in hyaluronic acid.

• Bursa: Fluid-filled sac that reduces friction in joints.

• Flexion/Extension: Movements that decrease/increase the angle between bones.

• Abduction/Adduction: Movements away from/toward the midline.

Equations and Additional Information

• Joint Movement Axes: Joints are classified as uniaxial, biaxial, or multiaxial based on the number of axes they move around.

Example Equation:

Additional info: This study guide expands on brief quiz questions to provide a comprehensive overview of joint classification, structure, and function, suitable for college-level Anatomy & Physiology students.