Comprehensive Notes on The Articular System and Joints
The Articular System
A joint (also called an articulation or arthrosis) is a point of contact between two bones, between bone and cartilage, or between bone and teeth.
Joints are classified structurally (anatomical characteristics) and functionally (type of movement permitted).
Structural classification depends on:
presence or absence of a space between articulating bones called the synovial cavity
type of connective tissue binding the bones together.
Structural Classification of Joints
Fibrous joints
Bones are bound by fibrous connective tissue rich in collagen fibers; lack a synovial cavity.
Cartilaginous joints
Bones are bound by cartilage; lack a synovial cavity.
Synovial joints
Bones forming the joint have a synovial cavity and are united by dense irregular connective tissue of an articular capsule, often with accessory ligaments.
Classification of Joints by Movement
Synarthroses (synarthrotic joints) – non-movable; most are fibrous (e.g., sutures in the skull).
Amphiarthroses (amphiarthrotic joints) – semimovable; mostly cartilaginous (e.g., intervertebral discs).
Diarthroses (diarthrotic joints) – movable joints; synovial joints.
Joints can be classified by movement or by structure. Non-movable joints are fibrous with dense regular connective tissue; semimovable joints are mostly cartilaginous; movable joints have a joint capsule composed of a synovial membrane.
Fibrous Joints
Sutures
Thin layer of dense fibrous connective tissue unites skull bones.
Example: coronal suture between the parietal and frontal bones.
Irregular, interlocking edges provide added strength and reduce fracture risk.
Syndesmoses
Greater distance between articulating bones and fibrous connective tissue between bones.
Example: distal tibiofibular joint.
Functionally classified as an amphiarthrosis.
Gomphoses
Cone-shaped peg fits into a socket (teeth in sockets).
Functionally classified as a synarthrosis (immovable).
Interosseous membrane
Sheet of dense irregular connective tissue binding neighboring long bones.
Allows slight movement (amphiarthrosis).
Example: between radius and ulna in the forearm.
Cartilaginous Joints
Synchondroses
Connecting material is hyaline cartilage.
Examples: Epiphyseal plates that connect the epiphysis and diaphysis of a growing bone; joint between the first rib and the manubrium of the sternum.
Symphyses
Ends of articulating bones covered with hyaline cartilage and joined by a pad of fibrocartilage (per the transcript: ends covered with hyaline cartilage; functionally amphiarthrosis).
Examples: Symphysis (e.g., intervertebral joints).
Function: amphiarthrosis (slightly movable).
Synovial Joints
Structure
Presence of a space called a synovial (joint) cavity between the articulating bones.
Freely movable joints.
Covered by articular cartilage (hyaline cartilage) that reduces friction and helps absorb shock.
Articular capsule unites the articulating bones.
Synovial fluid lubricates the joint and provides nutrients; helps remove metabolic wastes from chondrocytes within cartilage.
Accessory ligaments and articular discs help maintain stability and direct synovial fluid to areas of greatest friction.
Joint cavity and related structures
Periosteum
Fibrous capsule (outer layer of the joint capsule)
Synovial membrane (inner lining of the joint capsule)
Articular cartilage (hyaline cartilage on the ends of bones in the joint)
Extracapsular ligaments (outside the capsule)
Intracapsular ligaments (inside the capsule; example: knee ligaments)
Movement Permitted by Synovial Joints
Flexion – decreasing the joint angle on an anterior-posterior plane (e.g., bending the elbow or knee).
Extension – increasing the angle between the bones of a joint.
Abduction – moving a body part away from the main axis of the body (e.g., arm away from the body).
Adduction – moving a body part toward the main axis of the body (reverse of abduction).
Rotation – movement of a bone around its own axis (e.g., turning the head from side to side; twisting at the waist).
Supination – rotation of the forearm that results in the palm facing forward; hand-specific use (radius and ulna).
Pronation – rotation of the forearm so the palm faces backward or downward.
Circumduction – movement in which one end of a limb describes a cone-shaped path; involves flexion, extension, abduction, and adduction.
Inversion – turning the sole of the foot inward toward the midline.
Eversion – turning the sole of the foot outward away from the midline.
Elevation – movement of a bone upward (e.g., shrugging the shoulders, elevating the scapulae).
Depression – movement of a bone downward (e.g., lowering the shoulders after being shrugged).
Anatomy of Synovial Joints (Key Components)
Joint cavity contains synovial fluid to lubricate movement.
Extracapsular ligaments are outside the joint capsule; they help hold bones in position.
Intracapsular ligaments are inside the joint capsule (e.g., some knee ligaments) and control degree and direction of movement.
Types of Diarthroses (Synovial Joints)
Ball-and-socket joints
Multiaxial joints (allow movement in multiple axes and planes).
Examples: shoulder and hip joints.
Capable of all primary movements (flexion/extension, abduction/adduction, rotation, circumduction).
Condyloid joints
Biaxial joints (two axes), capable of flexion, extension, adduction, abduction, and circumduction.
Examples: radiocarpal (wrist) joint, metacarpophalangeal joints, and atlanto-occipital joints.
Gliding (plane) joints
Nonaxial joints; surfaces of two flat bones glide against each other; limited by ligaments.
Examples: intercarpal and intertarsal joints; movement is sliding.
Hinge joints
Uniaxial joints (one plane/one axis).
Examples: elbow, knee.
Movements: flexion and extension.
Pivot joints
Uniaxial joints with movement around a single axis; axis runs through the length of the bone.
Examples: radius with ulna (proximal radioulnar joint), atlas-axis joint.
Saddle joints
Unique biaxial joints (special version of a condyloid joint).
Found at the carpometacarpal joint of the thumb.
Movements include flexion/extension, abduction/adduction, and circumduction with limited rotation.
Illustrative Examples (Joint Types and Movements)
Pivot joint example: between C1 (atlas) and C2 (axis) vertebrae.
Ball-and-socket joint: hip joint.
Hinge joint: elbow.
Condyloid joint: between radius and carpal bones of the wrist; between phalanges and metacarpals; occipito-atlas and tibio-talar joints.
Saddle joint: between the trapezium (carpal) and the 1st metacarpal (thumb).
Plane (gliding) joint: between tarsal bones.
Axis and Movement Classifications (Movement Axes)
Nonaxial (plane/gliding): movements occur without a rotational axis.
Uniaxial: movement around a single axis (e.g., hinge joints, pivot joints).
Biaxial: movement around two axes (e.g., condyloid, saddle joints).
Multiaxial: movement around multiple axes (e.g., ball-and-socket joints).
The Axis and Joint Movements in Practice
The axis of rotation is typically along the length of the bone for pivot joints; for hinge joints, flexion/extension occur in one plane.
Rotation movements include lateral (external) and medial (internal) rotation.
Supination and pronation refer specifically to the forearm/hand movements; note that popular usage often misapplies these terms to the entire arm or wrist.
Radius/ulna rotation: supination places the palm up (radius and ulna parallel); pronation places the palm down (radius rotates over the ulna).
Condyloid and saddle joints are biaxial, enabling flexion, extension, abduction, adduction, and circumduction.
Movements of the Foot
Dorsiflexion – toes point upward (toward the shin).
Plantarflexion – toes point downward.
Inversion and eversion describe side-to-side movements of the sole relative to the midline; inversion is inward, eversion is outward. Note: inversion/eversion are distinct from forearm supination/pronation and are sometimes misnamed in popular usage.
The Elbow Joint (Illustrative Example)
The elbow is a hinge joint enclosed by a joint capsule that contains synovial fluid.
Bursae help lubricate movement around the joint.
The elbow’s structure supports flexion and extension with stable ligaments and capsule.
Muscles, Levers, and Movement (Musculoskeletal Mechanics)
Skeletal muscles produce movement when they contract.
Each muscle has two attachment points:
Origin: the stationary end during contraction.
Insertion: the moving end during contraction.
Synergists: muscles that cause the same action at a joint.
Antagonists: muscles that produce opposing actions.
Isotonic contraction: muscle tension causes movement (contraction with movement).
Isometric contraction: muscle tension increases but does not cause movement.
Flexor: muscle that pulls two limb bones toward each other.
Extensor: muscle that pulls two limb bones away from each other.
Levers: bones act as levers; a lever is a solid bar that pivots about a fixed point (the fulcrum).
Two opposing forces act on the lever: load (resistance) and effort (muscle force).
Key Terminology and References
Joint cavity: the space inside the synovial capsule that contains synovial fluid.
Synovial fluid: lubricates joints, provides nutrients, and removes wastes from chondrocytes within cartilage.
Articular cartilage: hyaline cartilage covering the ends of bones in a synovial joint; reduces friction and absorbs shock.
Articular capsule: fibrous outer layer plus synovial inner membrane surrounding the joint.
Accessory ligaments: stabilize joints and guide movement; include extracapsular and intracapsular ligaments where applicable.
Notation for angles: angle changes in joints are often described relative to the anatomical position, where the default angle is 180°, i.e., .
Connections to Foundational Principles and Real-World Relevance
Structural vs functional classification mirrors how joints are built and how they move, reflecting evolutionary adaptations for stability (fibrous) vs flexibility (synovial).
The variety of synovial joints explains the wide range of limb mobility required for daily activities and sports.
Understanding flexion/extension, abduction/adduction, circumduction, rotation, and special movements (supination/pronation, inversion/eversion, elevation/depression) supports assessments of range of motion and rehabilitation planning after injuries.
The concepts of synergists and antagonists underpin coaching, physical therapy, and athletic training strategies for efficient, safe movement patterns.
Quick Reference (Movements and Examples)
Flexion: decrease joint angle; e.g., bending the elbow.
Extension: increase joint angle; return toward anatomical position.
Abduction: move away from body axis; e.g., lifting arm laterally.
Adduction: move toward body axis; e.g., lowering arm toward torso.
Rotation: turning a bone about its axis; e.g., turning head side to side.
Supination: palm up (forearm); radius and ulna parallel.
Pronation: palm down (forearm); radius rotates over ulna.
Circumduction: cone-shaped movement combining flexion, extension, abduction, adduction.
Inversion/Eversion: foot sole movements relative to midline.
Elevation/Depression: vertical movements of body parts (e.g., shoulder girdle).
Specific joint notes: elbow is a hinge joint with a capsule and bursae; knee contains intracapsular ligaments; hip/shoulder are ball-and-socket (multiaxial).