Joints and Bone Structure: Anatomy & Physiology Study Notes

Joints: Classification and Movements

Biaxial Synovial Joints

Biaxial synovial joints allow movement in two planes, enabling four total types of movement. These joints are essential for complex motion in the body.

• Condylary Joint: A round surface fits into another surface (not perfectly round). Movements: Flexion, extension, abduction, adduction.

• Saddle Joint: Two saddle-shaped surfaces connect. Movements: Flexion, extension, abduction, adduction.

Abduction vs. Adduction

• 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.

Ball and Socket Joints

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

• 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: Moving forward (e.g., jaw or scapula).

• Retraction: Moving backward.

• Elevation: Moving upward (e.g., shoulder shrug).

• 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: Toes point upward (taking foot off the gas pedal).

• Plantarflexion: Toes point downward (pressing foot on the gas pedal or doing calf raises).

• Inversion: Sole of foot turns inward.

• Eversion: Sole of foot turns outward.

• Example: Soccer players use inversion/eversion 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:

1. Stability: Prevents tightrope walking effect.

2. Propulsion: Lower limbs drive forward movement.

3. 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
Primary Function	Support & locomotion	Manipulation & reach
Injury Risk	Stable	Can be completely severed

Evolutionary Context

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

• Origin: Sacrum

• Insertion: Ischial spine

• This ligament transforms the greater sciatic notch into a foramen (opening) that allows passage of the sciatic nerve.

Sacrotuberous Ligament

• Origin: Sacrum

• Insertion: Ischial tuberosity

• This ligament creates the lesser 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 joints.

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.

• 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 does not move independently – muscles contract and pull on bones to create joint movement.

• Action: What movement a muscle 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

• 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

  • Lameliae (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)