Joints and Bone Structure: Anatomy & Physiology Study Guide

Study Guide - Smart Notes

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🦴 Biaxial Synovial Joints

Joint Classification

Biaxial synovial joints allow two types of movements, requiring four total movements to complete their range of motion.

Joint Type	Description	Movements
Condylary Joint	Round surface fitting into another surface (not perfectly round)	Flexion, extension, abduction, adduction
Saddle Joint	Two saddle-shaped surfaces connecting together	Flexion, extension, abduction, adduction

Abduction vs. Adduction

Abduction and adduction are fundamental joint movements describing motion relative to the midline of the body.

Abduction: Movement away from the midline of the body
Adduction: Movement toward the midline of the body
Example: Jumping jacks demonstrate abduction and adduction of arms and legs; fingers and toes can also perform these movements.

⚽ Ball and Socket Joints

Structure and Mobility

Ball and socket joints are multi-axial joints with the greatest range of motion.

Location	Structure	Characteristics
Shoulder	Head of humerus (less rounded) + glenoid cavity (flat)	High mobility, low stability - easy to dislocate
Hip	Head of femur (perfectly round) + acetabulum (deep socket)	Low mobility, high stability - securely locked in place

Movements possible: Flexion/extension, abduction/adduction, plus additional multi-axial movements.

🔔 Additional Joint Movements

Protraction and Retraction

Protraction: Moving forward (pro = before)
Retraction: Moving backward (re = behind)
Examples: Jaw movements, shoulder movements during running, scapular movements

Elevation and Depression

Elevation: Moving upward
Depression: Moving downward
Important: Some movements require the opposite movement first (e.g., must elevate shoulders before you can depress them)

🦶 Special Foot Movements

Dorsiflexion and Plantarflexion

Movement	Action	Memory Aid
Dorsiflexion	Toes point upward	Taking foot off the gas pedal
Plantarflexion	Toes point downward	Pressing foot on the gas pedal or doing calf raises

Embryology note: Legs twist during development, making the top of foot the dorsal surface.

Inversion and Eversion

Inversion: Sole of foot turns inward
Eversion: Sole of foot turns outward
Common in: Soccer players use these movements for ball control

🦴 Limb Bone Equivalents and Orientation

Upper vs. Lower Limb Bones

Region	Upper Limb	Lower Limb	Notes
Arm	Humerus	Femur	Both have ball-and-socket joints
Forearm/Antibrachium	Radius (thumb side) Ulna (pinky side)	Tibia (big toe side) Fibula (pinky toe side)	Leg is backwards!
Hand/Foot	Manus (hand)	Pes (foot)	From wrist/ankle to fingertips/toes

🔗 Limb Attachment to Axial Skeleton

Lower Limb Attachment

Direct attachment through sacrum (part of vertebral column)
Pelvic girdle firmly connects to sacrum
Femur attaches laterally to body

Functions of lateral attachment:

Stability - prevents tightrope walking effect
Propulsion - lower limbs drive forward movement
Automatic torso movement - no conscious effort needed

Upper Limb Attachment

Indirect attachment via clavicle to sternum
Scapula connects to torso through muscle only
Sternoclavicular joint is only direct connection point

Attachment Comparison

Feature	Lower Limb	Upper Limb
Attachment Type	Direct (bone-to-bone)	Indirect (muscle-mediated)
Stability	High	Lower
Range of Motion	Limited	Full 360° movement

💡 Evolutionary Context

Quadrupeds to Bipeds

Humans evolved from quadrupeds (four-legged animals) to bipeds (two-legged). This transition required:

Lower limbs to become primary weight-bearing and propulsion structures
Upper limbs to become free-moving for manipulation and tool use
Pelvic girdle to provide stable base for upright walking
Pectoral girdle to allow maximum mobility for reaching and grasping

🦴 Pelvic Ligaments

Sacrospinous Ligament

The sacrospinous ligament connects two key anatomical structures:

Origin: Sacrum
Insertion: Ischial spine
This ligament transforms the greater sciatic notch into a foramen (opening) that allows passage of the sciatic nerve, which innervates the posterior leg.

Sacrotuberous Ligament

Origin: Sacrum
Insertion: Ischial tuberosity
This ligament creates the lesser sciatic foramen. Structures exit through the greater sciatic foramen.

🔬 Joint Structure Comparison

Joint	Components	Description
Acromioclavicular joint	Acromion (scapula) + Acromial end of clavicle	Where scapula and clavicle meet
Sternoclavicular joint	Clavicle + Sternum	Only direct bone-to-bone attachment of upper limb to axial skeleton

Anatomical Relationships

The pectoral girdle (scapula + clavicle) functions similarly to the pelvic girdle (coxa bones)
Both create circular support structures for the cylindrical torso
Unlike the pelvis, the pectoral girdle uses two separate bones rather than a composite structure
Additional upper limb attachment occurs via muscle connections, not bone-to-bone junctions

⚖️ Joint Stability vs. Mobility Trade-off

Key Principle

Stability and mobility are inversely related in joints – complete stability requires immobility, while full mobility sacrifices stability.

Joint classifications by movement:

Synarthroses: Immobile but highly stable (e.g., skull sutures)
Amphiarthroses: Slightly mobile with moderate stability (e.g., symphysis joints)
Diarthroses: Fully mobile but less stable (synovial joints)

💪 Three Factors for Joint Health in Synovial Joints

Factor	What You Can Change	Clinical Application
Muscle tone	✔️ Yes	Patients with joint pain need to strengthen surrounding muscles
Ligament number/location	❌ No	Hypermobile patients have fewer ligaments than average
Articular surface shape	❌ No	Determines joint classification but cannot be modified

Muscle tone is crucial: Toned muscles spanning a joint provide stability through tension, making exercise essential for joint health.

🦾 Joint Motion Fundamentals

Important distinction: The skeletal system doesn't move independently – muscles contract and pull on bones to create joint movement.

Motion types are critical for future muscle studies because each muscle has:

Action (what movement it creates)
Two attachment points (origin and insertion)
Innervation (nerve supply)

🔄 Uniaxial Joint Movements

Pivot Joints

Structure: One bone acts as a rod, another rotates around it
Example: C1 and C2 vertebrae
Motion: Rotation around a single axis

Rotation types:

Supination: Palm rotates to anterior surface (like holding soup)
Pronation: Palm rotates to posterior surface (showing off ring)
Note: Elbow performs these movements, not the wrist

Circumduction vs. Rotation: Circumduction creates a circular motion, while rotation moves around a fixed axis point.

Hinge Joints

Structure: One bone fits into a trough of another
Examples: Elbow and ankle joints
Motion: Flexion and extension only (like a door hinge)

🦴 Bone Structure: Osteon vs Trabeculae

Osteon (Compact Bone Unit)

Structure: Perfect cylinder with concentric layers
Central feature: Haversian canal with blood vessels running parallel to bone axis
Composition:
- Osteocytes in lacunae arranged in layers
- Lamellae (extracellular matrix layers) of calcium + collagen
- Pattern: cells → calcium → collagen → repeat

Trabeculae (Spongy Bone Unit)

Structure: Irregular rod-shaped units
No central canal: Blood vessels access through open spaces
Additional features:
- Endosteum wrapping (connective tissue barrier)
- Osteogenic cells beneath endosteum
- Osteoblasts and osteoclasts for size modification
Key advantage: Can increase/decrease in size (unlike osteons)

🧱 Bone Classification Systems

Bones are classified by shape and structure, which determines their function and location in the body.

Long bones: Femur, humerus – support weight and facilitate movement
Short bones: Carpals, tarsals – provide stability and some movement
Flat bones: Skull, ribs – protect internal organs
Irregular bones: Vertebrae – complex shapes for specialized functions

*Additional info: Some explanations and examples have been expanded for clarity and completeness. All tables have been recreated and formatted for study purposes.*