Chapter 8: Articulations (Joints) – ANP College Study Guide

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Joints and Articulations

Definition and Function

Joints, also known as articulations, are the locations where two or more bones meet. They may also occur between bone and cartilage or between bone and teeth. The primary functions of joints are to provide both stability and mobility to the skeleton. Most joints represent a compromise between these two requirements, with some joints being highly mobile but less stable, and others being very stable but less mobile.

Mobility and stability of joints

Stability: Prevents excessive movement and protects internal structures.
Mobility: Allows for movement necessary for daily activities and locomotion.

Classification of Joints

Functional Classification

Joints are classified functionally based on the degree of movement they permit:

Synarthrosis: Immovable joints (e.g., sutures of the skull).
Amphiarthrosis: Slightly movable joints (e.g., intervertebral discs).
Diarthrosis: Freely movable joints (e.g., shoulder, hip).

Structural Classification

Joints are also classified structurally based on how the bones are held together:

Fibrous Joints: Bones joined by dense connective tissue; mostly immovable or slightly movable.
Cartilaginous Joints: Bones joined by cartilage; allow limited movement.
Synovial Joints: Bones separated by a fluid-filled cavity; freely movable.

Fibrous Joints

Fibrous joints include sutures, syndesmoses, and gomphoses. These joints are typically immovable or allow very limited movement.

Types of fibrous joints: gomphosis, suture, syndesmosis

Cartilaginous Joints

Cartilaginous joints are classified as either synchondroses (contain hyaline cartilage) or symphyses (contain fibrocartilage).

Synchondroses: epiphyseal plate, costochondral joints Symphyses: intervertebral disc, symphysis pubis

Synovial Joints

Synovial joints are the most common and most movable type of joint in the body. They are characterized by a joint cavity filled with synovial fluid.

Types of Synovial Joints

Six Subtypes of Synovial Joints

Synovial joints are classified into six types based on the shapes of their articulating surfaces and the movements they allow:

Planar (Gliding) Joint: Allows sliding or gliding movements.
Hinge Joint: Permits flexion and extension (e.g., elbow, knee).
Pivot Joint: Allows rotational movement (e.g., atlas/axis).
Condyloid (Ellipsoid) Joint: Permits movement in two planes (e.g., wrist).
Saddle Joint: Allows movement in two planes, with greater freedom than condyloid (e.g., thumb).
Ball-and-Socket Joint: Allows movement in multiple axes and planes (e.g., shoulder, hip).

Types of synovial joints: ball-and-socket, hinge, pivot, gliding, saddle, condyloid

Synovial Joint Types Table

Type	Movement	Examples
Gliding (Planar)	Slight monaxial or multiaxial	Acromioclavicular, intercarpal, vertebrocostal
Hinge	Monaxial (flexion/extension)	Elbow, knee, ankle, interphalangeal
Pivot	Monaxial (rotation)	Atlas/axis, proximal radioulnar
Ellipsoid (Condyloid)	Biaxial	Radiocarpal, metacarpophalangeal
Saddle	Biaxial	First carpometacarpal (thumb)
Ball-and-Socket	Triaxial	Shoulder, hip

Synovial joint types table Ellipsoid and saddle joints Ball-and-socket, hinge, pivot, gliding, saddle, condyloid joints

Action Terms: Movements at Joints

Basic Movements

Joints allow a variety of movements, each with specific terminology:

Flexion: Decreases the angle between two bones.
Extension: Increases the angle between two bones.
Abduction: Movement away from the midline.
Adduction: Movement toward the midline.
Circumduction: Circular movement combining flexion, extension, abduction, and adduction.
Rotation: Movement around a longitudinal axis (medial/lateral).
Elevation/Depression: Raising or lowering a body part.
Supination/Pronation: Rotational movement of the forearm.
Dorsiflexion/Plantarflexion: Movements at the ankle.
Inversion/Eversion: Movements of the sole of the foot.
Protraction/Retraction: Moving a body part forward or backward.

Flexion of the trunk Extension of the trunk Abduction, adduction, circumduction Flexion of the leg Extension of the leg Lateral and medial rotation Elevation and depression of mandible Supination and pronation of forearm Dorsiflexion and plantarflexion Eversion of the foot Protraction and retraction of mandible

Synovial Joint Structures

General Anatomy of Synovial Joints

Synovial joints have a unique structure that allows for free movement:

Joint cavity: Space between articulating bones filled with synovial fluid.
Synovial fluid: Lubricates and nourishes the joint.
Articular cartilage: Covers bone surfaces, reducing friction and absorbing shock.
Articular capsule: Encloses the joint cavity; consists of fibrous and synovial layers.
Synovial membrane: Produces synovial fluid.
Ligaments: Strengthen and stabilize the joint.

General anatomy of synovial joint

Accessory Structures

Bursa: Fluid-filled sacs that reduce friction between tissues.
Tendon sheath: Tubular bursae surrounding tendons.
Tendons: Connect muscle to bone, contributing to joint stability.
Ligaments: Connect bone to bone, reinforcing the joint.
Menisci: Pads of fibrocartilage that improve fit and absorb shock.

The Knee Joint

Structures of the Knee Joint

The knee is a complex synovial joint with several important structures:

Anterior cruciate ligament (ACL): Prevents anterior displacement of the tibia.
Posterior cruciate ligament (PCL): Prevents posterior displacement of the tibia.
Lateral collateral ligament (LCL): Stabilizes the lateral aspect of the knee.
Medial collateral ligament (MCL): Stabilizes the medial aspect of the knee.
Lateral and medial meniscus: C-shaped cartilage pads that cushion and stabilize the joint.

Joint Disorders

Common Disorders

Sprain: Stretching or tearing of ligaments.
Dislocation: Displacement of bones at a joint.
Osteoarthritis: Most common form; chronic, slow-progressive, irreversible. Results from aging, with softening and degeneration of cartilage, development of bony spurs, pain, and restricted movement.
Rheumatoid Arthritis: Chronic, rapid onset, autoimmune disorder affecting 2.5% of adults. Characterized by variable flare-ups; inflammation controlled by steroids and aspirin.

Comparison Table: Osteoarthritis vs. Rheumatoid Arthritis

Feature	Osteoarthritis	Rheumatoid Arthritis
Onset	Slow, progressive	Rapid, variable
Cause	Aging, wear-and-tear	Autoimmune
Cartilage	Degeneration, bony spurs	Inflammation, erosion
Population	Older adults	2.5% of adults
Treatment	Pain management	Steroids, aspirin

Note: Osteoarthritis is not related to osteoporosis.

Summary

Joints are essential for movement and stability in the human body. Understanding their classification, structure, and function is fundamental for anatomy students. Synovial joints, with their complex anatomy and wide range of movements, are particularly important. Awareness of common joint disorders aids in clinical understanding and application.