Portage Learning A&P Module 4, Articulations

Joint (articulation)

Any place where adjacent bones (or bone and cartilage) meet to form a connection.

Classification of joints

Two ways joints are classified: structurally (by tissue/space) and functionally (by mobility).

Structural classes of joints

Fibrous, cartilaginous, synovial.

Functional classes of joints

Synarthrosis (immovable), amphiarthrosis (slightly movable), diarthrosis (freely movable).

Synarthrosis

Immovable; e.g., skull sutures, manubriosternal joint (often limited/none).

Amphiarthrosis

Slightly movable; e.g., intervertebral discs, pubic symphysis.

Diarthrosis

Freely movable; all synovial joints; mostly in the appendicular skeleton.

Basic joint motions

Linear (gliding), angular (incl. circumduction), rotation.

Uniaxial joint

Moves in one plane/axis; e.g., elbow (hinge).

Biaxial joint

Moves in two planes; e.g., metacarpophalangeal (knuckle).

Multiaxial (triaxial) joint

Moves in three planes incl. rotation; e.g., shoulder, hip.

Fibrous joint

Unites bones in a fibrous joint with dense fibrous connective tissue; no joint cavity.

Types of fibrous joints

Suture, syndesmosis, gomphosis.

Suture

Location: between skull bones; function: strong protection; mobility class: synarthrosis.

Fontanelle

Broad CT regions in infant skull; allow birth molding and rapid brain growth.

Synostosis

Fusion 'joined by bone'; frontal/maxillary halves fuse in childhood; late-life fusion of major sutures.

Syndesmosis

Two parallel bones linked by ligament or interosseous membrane; e.g., radius-ulna, tibia-fibula.

Syndesmosis functional class

Amphiarthrosis; forearm membrane flexible (allows rotation), leg membrane stabilizing (little motion).

Interosseous membrane

Unites bones + broad surface for muscle attachment.

Gomphosis

Tooth-to-socket joint; periodontal ligament anchors root; synarthrosis.

Cartilaginous joints

Unites bones in cartilaginous joints with hyaline cartilage or fibrocartilage; no joint cavity.

Types of cartilaginous joints

Synchondrosis and symphysis.

Synchondrosis

Bone joined to hyaline cartilage (temporary or permanent); synarthrosis.

Temporary synchondrosis

Classic example: Epiphyseal plate (growth plate): hyaline cartilage between diaphysis and epiphysis.

Synostosis

Fusion of growth plate to bone (plate replaced by bone at maturity).

Childhood pelvic synchondroses

Ilium-ischium-pubis fuse into single coxal bone (synostosis).

Permanent synchondrosis

Key thoracic example: First sternocostal joint (1st rib to manubrium).

Symphysis

Bones joined by fibrocartilage; strong yet slightly movable; amphiarthrosis.

Narrow symphyses

Examples: Pubic symphysis; manubriosternal joint.

Wide symphyses

Major example: Intervertebral discs; unite vertebrae, allow small motion, provide cushioning.

Synovial joint

Uniquely defined by a fluid-filled joint cavity between articulating bone surfaces.

Functional class of all synovial joints

Diarthrosis (freely movable).

Articular (joint) capsule

Two layers: outer fibrous layer; inner synovial membrane.

Articular cartilage

Hyaline cartilage covering articulating surfaces; reduces friction, protects bone.

Synovial membrane

Produces synovial fluid: lubricates, nourishes avascular cartilage, absorbs shock.

Diffusion from synovial fluid

Aided by compression/relaxation 'pumping.'

Ligaments

Three positional types: Extrinsic (outside capsule), intrinsic (part of capsule), intracapsular (inside capsule).

Tendons

Role at joints; muscle to bone; dynamic stabilizers that resist forces across joints.

Fat pads

Fill spaces and cushion.

Articular disc vs meniscus

Disc: small, oval (e.g., TMJ, sternoclavicular). Meniscus: larger, C-shaped (knee) for stability + shock absorption.

Bursa

Small fluid-filled sac reducing friction where tissues rub.

Tendon sheath

Elongated bursa around a tendon; reduces friction during motion.

Pivot joint

Movement: rotation around one axis; example: atlantoaxial (C1-C2), proximal radioulnar.

Hinge joint

Movement: flexion/extension in one plane; examples: elbow, interphalangeal, knee (primarily hinge).

Condyloid (ellipsoid) joint

Movement: biaxial: flex/extend + abduct/adduct; examples: MCP (knuckles), radiocarpal (wrist).

Saddle joint

Movement: biaxial with added range; example: 1st carpometacarpal (thumb).

Plane (gliding) joint

Movement: sliding/translation; examples: intercarpal, intertarsal, acromioclavicular.

Ball-and-socket joint

Movement: multiaxial including rotation; examples: shoulder, hip.

Shoulder joint

Bones involved: clavicle, scapula, humerus (glenohumeral joint).

Shoulder mobility vs stability

Highly mobile but less stable due to large humeral head vs shallow glenoid cavity; relies on ligaments + muscles.

Glenohumeral ligaments

Reinforce capsule, connect glenoid to humeral head.

Coracoacromial ligament

Connects coracoid process to acromion of scapula.

Coracoclavicular ligament

Connects coracoid process of scapula to clavicle.

Glenoid labrum

Fibrocartilage rim that deepens glenoid socket.

Rotator cuff

Muscle/tendon group providing primary dynamic stability to shoulder.

Hip joint

Bones: head of femur + acetabulum; type: multiaxial ball-and-socket.

Hip vs shoulder stability

Hip = deeper socket, stronger ligaments, less motion.

Acetabular labrum

Fibrocartilage lip deepening socket for femoral head.

Iliofemoral, pubofemoral, ischiofemoral

Major hip ligaments; tighten in extension to stabilize upright posture.

Knee joint

Type: modified hinge joint; motion: flexion and extension.

Patella

Bone type: sesamoid bone in quadriceps tendon; role: protects tendon, increases leverage.

Menisci

Names: medial & lateral; role: fibrocartilage for shock absorption + stability.

Medial meniscus injury

More common due to strong attachment to tibial collateral ligament → less mobility.

Collateral ligaments

Names: fibular (lateral) and tibial (medial).

Cruciate ligaments

Names: ACL & PCL; role: prevent anterior/posterior displacement of femur on tibia.

shoulder joint

Most dislocated major joint; reason: shallow socket and high mobility.

'Terrible triad' of the knee

Tibial (medial) collateral ligament, medial meniscus, and ACL.

Rotator cuff tear risk factors

Age >60, repetitive lifting or abduction sports, trauma.

Bursitis

Definition: inflammation of bursa due to overuse, trauma, pressure, RA, gout, infection.

Osteoarthritis

Main cause: wear-and-tear degeneration of articular cartilage, common after 60.

Hands, spine, knees, hips, feet.

Osteoarthritis most affected joints

Rheumatoid arthritis

Mechanism: autoimmune inflammation destroying synovial joints + cartilage.

Gout

Cause: uric acid crystal buildup; typical location: often big toe.

Aging + spine disc changes

Loss of water in nucleus pulposus → decreased height and elasticity.

Bulging vs herniated disc

Bulging = annulus protrudes; Herniated = nucleus pulposus breaks through annulus.

Flexion

To bend. Decreases the angle between the bones of a joint. Examples: Elbow, knee, neck, ankle. Joint Type: Hinge, Condyloid, Saddle, Ball-and-Socket.

Extension

To stretch out. Increases the angle between the bones of a joint. Examples: Elbow, knee, neck, ankle. Joint Type: Hinge, Condyloid, Saddle, Ball-and-Socket.

Hyperextension

Beyond or excessive. Extension beyond anatomical position (angle >180°). Examples: Wrist, neck. Joint Type: Hinge, Condyloid, Saddle, Ball-and-Socket.

Abduction

To lead away. Movement away from the midline. Examples: Arms, legs, digits. Joint Type: Condyloid, Saddle, Ball-and-Socket.

Adduction

To lead toward. Movement toward the midline. Examples: Arms, legs, digits. Joint Type: Condyloid, Saddle, Ball-and-Socket.

Gliding

Nearly flat bone surfaces slide or glide over each other. Example: Carpal bones. Joint Type: Plane Joint.

Circumduction

"Circ" = circle. Move distal part of an appendage in a circle while the other end remains stationary (no rotation). Examples: Shoulder, hip, knuckle. Joint Type: Condyloid, Ball-and-Socket.

Medial Rotation

"Rota" = revolve. Rotation toward the midline (internal rotation). Examples: Neck, arm, leg. Joint Type: Pivot, Ball-and-Socket.

Lateral Rotation

Rotation away from the midline (external rotation). Examples: Neck, arm, leg. Joint Type: Pivot, Ball-and-Socket.

Pronation

"Pronate" = lying face down. Rotation of forearm so the palm faces posteriorly; radius crosses over ulna. Example: Radioulnar joint. Joint Type: Pivot.

Supination

"Supine" = lying on back. Rotation of forearm so palm faces anteriorly; radius and ulna are parallel. Example: Radioulnar joint. Joint Type: Pivot.

Inversion

Turns sole of foot medially. Example: Foot.

Eversion

Turns sole of foot laterally. Example: Foot.

Dorsiflexion

Flexion at ankle lifting toes upward. Example: Foot.

Plantar Flexion

Extension at ankle pointing toes downward. Example: Foot.

Opposition

Thumb moves toward palm or other fingers. Example: Thumb.

Reposition

Movement returning thumb to anatomical position. Example: Thumb.

Protraction

Anterior movement in the horizontal plane (forward). Example: Scapulothoracic joint.

Retraction

Posterior movement in the horizontal plane (backward). Example: Scapulothoracic joint.

Elevation

Movement superiorly (upward). Examples: Scapula, mandible (chewing).

Depression

Movement inferiorly (downward). Examples: Scapula, mandible.

Lateral Flexion

Bending of the vertebral column to the side. Example: Neck or trunk.

Sutures, Syndesmosis, Gomphosis

3 types of fibrous joints

Synchondrosis, Symphysis

2 types of cartilaginous joints

Amphiarthrosis

Term for slightly mobile joint