Skeletal System III

Why Calcium Homeostasis Matters

Muscle contraction (including heart)

Nerve Impulse transmission

Blood Clotting

Enzyme function and second-messenger systems

Bone Mineralization

Normal blood Calium Range

Typical range: ~ 9-11mg/dL (2.2-2.7mmol/L)

Homeostasis maintained by a balance between dietary intake, urinary excretion, and bone storage

Regulated by the Endocrine System

What is the Endocrine System?

Glands that secrete hormones into the blood stream

Hormones: Chemical messengers that affect distant organs

Parathyroid Gland

Thyroid Gland

Endocrine Glands are often Control Center

Bone as Calcium Reservoir

99% of body calcium stored as hydroxyapatite in bone

Bone constantly remodeled by 

• Osteoblasts → Build bone, deposit calcium

• Osteoclasts → Break down bone, Releases calcium

Balance between them keeps both bone and blood healthy

If Osteoclasts are too active, what happen to blood calcium levels?

Calcium will be rise

Parathyroid Hormone (PTH)

Secreted by parathyroid glands when blood Ca2+ is low (hypocalcemia)

Actions

• Bone: Stimulates osteoclast activity (via osteoblast signaling) → releases Ca2+

• Kidney: Increases Ca2+ reabsorption, stimulates calcitriol synthesis

• Intestine: Indirectly up absorption via calcitriol

Calcitriol (Active Vitamin D3)

Formed from vitamin D after liver & kidney activation (PTH stimulates final step)

Increases intestinal Ca2+ absorption

Works with PTH to raise blood Ca2+

Promotes bone deposition

PTH and Calcitriol work together … Sorta

Both work to increase blood calcium

• PTH causes it to be released from bone

• Calcitriol causes it to be absorbed from the gut and retained in blood by action on kidneys

Same goal, different effect at bone level

• PTH causes bone breakdown, Calcitriol causes bone deposition

PTH quickly raises blood calcium

Calcitriol maintains long term blood-calcium levels

Calcitonin

Secreted by thyroid ( parafollicular (C) cells) gland when blood Ca2+ is high (hypercalcemia)

Inhibits osteoclasts (directly) → lowers blood Ca2+

More significant in children/pregnancy than in healthy adults 

When blood calcium level decrease ____ gland releases ______, which acts to ______ blood calcium by stimulating _____ activity and enhancing calcium reabsorption in the kidneys and intestines.

Parathyroid, PTH, Rise Ca2+, Osteoclast

When blood calcium levels increase, the ____ gland releases ____, which acts to ______ blood calcium by inhibiting _____ activity

Parathyroid, calcitonin, dump, osteoblast

What is a Joint?

A Joint (articulation) = where two or more bones meet

Functions: allow movement and provide stability

Mobility vs Stability tradeoff:

• High mobility → low stability (ex: Shoulder)

• Low Mobility: high stability (ex: Skull sutures)

Structural Classification of Joints

Fibrous Joints

Sutures: Interlocking bones of skull

Syndesmoses: Connected by ligaments (connect bone to bone) (ex: tibia-fibula)

Gomphoses: Teeth anchored in socket

Cartilaginous Joints

Synchondroses: Bones joined by hyaline cartilage (ex: epiphyseal plate)

Symphyses: Fibrocartilage pad (ex: intervertebral disc, pubic symphysis)

Synovial Joints: General Geatures

Articular Cartilage: Covers bone ends

Joint cavity filled with synovial fluid

Joint capsule (fibrous + synovial membrane)

Ligaments and Tendons reinforce stability

Bursae/Menisci reduce friction and absorb shock

What feature make synovial joints more vulnerable to injury but allow greater motion?

A: Fibrous connective tissue

B: Joint cavity

C: Articular cartilage

D : Menisci

B: Joint cavity

Functional classification of Joints

Types of Synovial Joints

Plane (gliding)

Hinge

Pivot

Condylar (ellipsoid)

Saddle

Ball-and-Socket

Plane Joints

Plane (Gliding): Intercarpal joints → Sliding movement

Hinge Joints

Hinge: Elbow, Knee → Flexion/Extension only 

Pivot Joints

Pivot: Rotation around a single axis (atlas/axis, proximal radioulnar)

Condylar Joints

Condylar: Oval-shaped condyle of one bone fits into the elliptical cavity of another bone (wrist, knuckle joints)

Movement in two planes

Saddle Joints

Saddle: Thumb (1st carpometacarpal) - allow opposition

Ball-and-Socket Joints

Ball-and-Socket: shoulder, hip-widest range of motion

Concept check

Hinge

Pivot

Ball-and-Socket

Saddle

Fractures

Joints and bones work together

When bones are stressed beyond capacity → Fractures

Closed Fracture

Definition: Bone breaks but does not penetrate the skin

Mechanism: Usually caused by a direct blow or bending stress

Clinical note: Lower infection risk; easier to manage if aligned

Key Concept: Skin remains intact → internal healing environment protected

Open Fracture

Definition: Bone breaks through the skin (or skin is broken, exposing bone)

Mechanism: High-energy trauma (ex: motor vehicle accident, gunshot)

Clinical note: High infection risk and delayed healing.

Key concept: Requires surgical cleaning and fixation

Transverse Fracture

Definition: Straight horizontal break across the bone shaft

Mechanism: Perpendicular force to long axis (ex: direct impact)

Clinical note: Often stable if bone ends align well: heals predictably with immobilizaton.

Key Concept: compression or bending directly across bone.

Spiral Fracture

Definition: Twisting or rotational fracture line around the bone

Mechanism: Rotational force (ex: skiing, twisting fall, child abuse case)

Clinical note: Jagged edges → difficult reduction; may damage surrounding soft tissue.

Key Concept: Indicates torque forces; look for rotational injury pattern

Comminuted Fracture

Definition: Bone is shattered into three or more fragments.

Mechanism: high impact trauma (ex: car accidents, falls from height)

Clinical note: Complex repair; may require surgical plates/pins; slow healing.

Key concept: represents severe energy transfer → bone fragmentation

Impacted fracture

Definition: One fragment is driven into another within the same bone

Mechanism: Axial loading (ex: landing hard on extended arm/leg)

Clinical note: Bone ends jammed together; limb may apper shortened

Key concept: common in falls - the bone compresses itself

Greenstick Fracture

Definition: Incomplete break - bone bends and cracks on one side

Mechanism: common in children (softer, more flexible bone)

Clinical note: Often associated with forearm fractures in pediatrics.

Key concept: compare to bending a green twig - it splinters but doesnt snap

Oblique Fracture

Definition: Diagonal fracture line across the bone shaft.

Mechanism: angles force applied along the long axis.

Clinical note: Sharp ends may lead to displacement; requires stabilization.

Key concept: often confused with spiral fractures - but lacks twist pattern

One fracture but Many types

Bone Healing

Hematoma Formation - Clot stabilizes site

Fibrocartilaginous callus - connective tissue bridge

Bony callus formation - spongy bone replaces cartilage

Remodeling - Compact bone restores original shape

Factors Affecting Healing

Adequate blood supply & stabilization are critical

Age, nutrition, and hormones affect repair rate

Possible complications

• Malunion / nonunion

• Infection (especially open fractures)

• Epiphyseal damage in children