HMI 102: Medical Imaging - Skeletal System

• HMI 102 is about Foundational Principles and Application of Medical Imaging
• This is lecture 2, semester 1
• Lecture by Dr Greg van Egmond, PhD

Acknowledgement of Country

• Traditional Owners of the lands are acknowledged
• The lecture is presented from the lands of the Wadawurrung people of the Kulin Nation
• Respects are paid to their Elders - past, present, and emerging
• Warm welcome is stated for any Traditional Owners present at this meeting

Skeletal system

• Consists of connective tissues, where bone is a connective tissue
• Connective tissues rely on bones for places to hang onto
• The total system includes bones, cartilages & other connective tissues
• Protects soft internal organs
• Lungs are protected under costal bones
• Movement relies on muscle tissues
• One muscle that movement replies on is the brain
• Blood cell production space is inside bones, where red blood cells are made
• Storage is for Calcium, phosphorus & fatty tissue
• Bones use bone marrow
• Less cells and more non-cellular stuff
• Bones become brittle as they are needed in the Calcium intake

Axial vs. Appendicular skeleton

• The skeleton divides into Axial and Appendicular skeleton
• Axial includes all the middle-centric bones, around the central axis
• Appendicular includes all the peripheral or lateral bones - the Limb bones
• Axial includes Skull bones, Vertebrae, Costal bones, Sternum complex, and Hyoid bone
• The Hyoid bone is the loneliest bone in the body, only Surrounded by soft tissue in the neck
• Appendicular includes Clavicles, Scapulae, Pelvic bones, and Upper & lower limb bones
• The Pelvic bone sides are around our hips

Bone types

• Sutural bones are only in the skull
• Suture joints are like a zip
• Bones grow in suture joints
• Irregular bones include pelvic and scapula bones
• Irregular bones have weird shapes
• Short bones are often Squared
• Short bones have no medullary cavity
• Tarsals are examples of Short bones
• Flat bones are in the brain
• Flat bones include frontal, parietal and temporal bones
• Brain is surrounded by flat bone
• Inside flat bones has trabecular bone
• Solid or compact bone is on external or outside
• Long bones have lots of these bones
• Long bones have a hollow area
• Called the medullary cavity
• Sesamoid Bones tendons grow in
• Sesamoid Bones occur near the thumb, & under big toe
• Muscles connects tendon, which connects to bone
• Ligaments = bone-bone
• Tendon = muscle-bone

Bone types – which bones fit where?

• Sutural bones vary in number
• Sutural are Only in skull joints (sutures)
• Irregular bones includes Vertebrae, facial bones, mandible, base of skull bones, scapulae, pelvic innominate bones (ilium, ischium, pubis), inner ear bones
• Some bones may fit into multiple categories
• Flat bones includes Upper cranial bones (skull), sternum complex, costal bones (ribs), scapulae, iliac bones
• Long bones includes Humerus, radius, ulna, metacarpals, phalanges (both hand & foot), femur, tibia, fibula, metatarsals, clavicles
• Short bones Includes Carpal bones (except pisiform), tarsal bones
• Sesamoid bones includes Patella, pisiform, others at the 1st digit of hand & foot
• Bones across multiple types fit Fit more than one type classification

Anatomy of long bones

• Anatomy of long bones have expanded parts for muscles to hold onto & tendon
• Anatomy of long bones share 3 parts
• In a growing person, a humerous would be both radiolucent and radio paque
• Epiphysis has 2 epiphysial ends
• Epiphysis provides expanded ends of long bones bony region
• Epiphysis has features that take part in joints, which covered in hyaline cartilage
• Metaphysis is between epiphysis & diaphysis where bones grow or change
• There are 2 regions on either side of active growth zone in young bones
• 80 can find cartilage there to slowly grow into bone
• Diaphysis is tubular main shaft / body section
• Tubualr the main shaft or body section
• Composed of Thick compact bone, thin trabecular & a medullary cavity
• Usually filled with blood or fat

Anatomy of short, irregular & flat bones

• There are no diaphysis and epiphysis, with no directional, or medullary cavity
• There are Sheets of periosteum on the outside
• Compact bone is deep to periosteum
• Trabecular bone deep to compact bones
• fibrous connective tissue on is outside of bone
• Can hold blood vessels and nerves
• Endosteum covers inside of trabecular bone
• Marrow (mostly red) is inside
• There may still be growth plates
• Deep to endosteum
• Means further in
• This occurs in the occasion of a bone break

Bone parts

• Head
  • Nice and rounded
• Neck
• Body
• Trochanter

Bony projections / elevations

• The function is rounded boney features
• Tubercle
  • Creates round lumps on surface of bone and is small
• Tuberosity
  • Medium
• Trochanter
  • Large and only located in femur
• Line
  • Its a Little Change on Shaft
• Ridge
  • on tibia
• Crest
  • on illiac bone, called illinc crest

Bone depressions / holes

• Can only see the outline of what is inside
• Depression
• Dip in shape
• Fossa
  • The depression in bone
• Foramen
  • The hole is through the bone
• Notch
  • Bite shape

Basics of bone structure

• Coverings, cells & histology.

Structure of bones

• Bones exists in two major structural forms
• Minimal holes
• Dense and solid
• Compact bone: Dense bone often forming the outer sections that covers bones
• Trabecular/cancellous spongy bone: usually deep to compact bone, network of delicate bars of bone which branch & intersect to form a hard sponge-like mesh
• Medullary cavity has Diaphysis, thicker compact bone

More Bone Structure

• Osteons (haversian systems) and covers bones
• Periosteum houses vessels and nerves
• Also where tendons attach
• Endosteum
  • Covers trubecular -if blood comes in contact, endosteum prevents clotting
• Compact bone
• Cancellous bone with trabeculae
• Medullary cavity

Bone coverings

• Periosteum: 2 layers of connective tissue surrounding the outside of the bone (compact bone)
• Where tendons or ligaments attach to
• Outer fibrous (strength) dense connective tissue layer
• Inner "cellular” layer (containing bone cells, their precursors & blood vessels for the bone)
  • Cells that can build bone or break it down
• Endosteum: 1 layer of cells and connective tissue lining the inner surfaces of the bone (trabecular bone)
  • Its a cellular layer
• Separates the bone from the bone marrow
• Follows the channels through trabecular bone
• Very similar to the Inner periosteum contains bone cells, etc.

Terminology

• DW - Diaphyseal wall
• MC – Medullary cavity
• HC – Haversian canals
• cav –cavities in the cancellous (spongey) bone
• peri- periosteum
• end - endosteum

Bone Cells

• Mesenchymal stem cells
• Osteoprogenitor cells, turn into
• Osteoblasts
• break down bone
• Osteoclasts, a type of bone which resorb the bone
• Originate from haematopoietic stem cells macrophage lineage
• Have siblings with WBC
• So when WBC are activited in an immune responses, Osetoclasts are activiated
• Osteoblasts usually regulate osteoclast activity

Bone Remodeling

• Osteoblasts - build bone
  • Known as builders
  • Originate from osteoprogenitor cells (type of mesenchymal stem cell)
  • Not specialiaed and cant build bone
  • Found in both periosteum & endosteum
  • Osteoblasts become osteocytes if trapped within hard bone matrix
    • Builds bone around itself
• Osteoclasts - resorption
  • Sibling with WBC
  • Originate from haematopoietic stem cells macrophage lineage
  • Also found in periosteum & endosteum

Compact Bone

• Arteries and veins run through the center of the osteon
• Each lamellae is an individual ring
• Bones may show Osteocytes

Histology of Compact Bone

• Osteon (Haversian system) – structural unit of compact bone
• Lamellae – Column-ring of bony matrix (collagen & minerals)
• Central (Haversian) canal - canal that contains blood vessels & nerves
  • Goes across the bone
• Perforating (Volkmann's) canal - canals that allow transverse link up between structures within adjacent central canals & periosteal vessels/nerves
• Lacunae – 'Lake' of softer matrix where osteocytes sit
• Canaliculi - tiny canals between lacunae -> osteocyte links

Trabecular (cancellous/spongy) Bone

• Meaning no marrow or blood
• Meaning not controlled
• Blood is outside them

Trabecular (cancellous/spongy) Bone Details

• An osteon is a true osteon when it has another Osteon next to it
• No osteon units
• Lack adjacent units
• Has got lamellae, but lacks central canals with vessels
• Vessels are usually present between trabeculae and vessels reach out to get blood
• Single lamellae columns form trabecular projections

Extracellular matrix

• Bone cells make up ~2% of weight
• Made by osteonists
• Osteocytes make up ~2% of weight bone
• Non-cellular components extracellular matrix = the rest!
• Osteoid - Collagen & proteins (~33%) – Organic
• Hydroxyapatite (~67%) – Inorganic (minerals / compounds) Calcium (~39%) Phosphate (~17%) Carbonate (~9.8%) Sodium (~0.7%) Magnesium (~0.5%) Potassium (~0.2%)
• Osteoids are added
• What makes bones hard

Summary

• Bone extracellular-matrix
  • Part collagen and part hardened minerals
• Bone cells
  • Osteoblasts are on the outside and osteoclasts are insides
• Bone structure
  • Compact vs. trabecular/cancellous has osteons has trabeculae

Endochondral & Intramembranous bone formation

• (ossification)

Formation of the skeleton starts

• Before week 8 – skeleton
• Consists of fibrous
• Membranes & hyaline
• Cartilage
• 2 processes of ossification begin:
  • Inside cartilage and within membrane

Endochondral ossification

• Is where bone forms by replacing an original hyaline cartilage model
• The bones are made from hyaline cartilage
• Forming long bones of the body
• Metaphysis section of developing long bones has a growth (epiphyseal) plate which remains as cartilage until growth stops
• Shaft or body ossifies first
• Create cartilage model of the bones to build
• Replace that hyaline cartilage with bone
• There is a story to watch and find it

Reminder of the “-physis" parts

• Epiphysis
  • end sections
• Diaphysis
  • shaft/body section
• Metaphysis
  • sections/between, which are changing sections

Stages of Endochondral

1. Cartilage model
2. Bony collar formation
3. Primary ossification centre (POC)
4. Medullary cavity & shaft formation
5. Secondary ossification centres
6. Epiphysis remodeling
7. Further growth – Elongation & appositional growth
8. Growth plate closure

Processes of bone growth

• Bone is made up of a bunch of cells
• Growing bones will have
• cartilage present
• in the metaphasis When bone imaged
• cartilage appears dark
• Due to cartilage being present that is radiolucent
• When bones present that is radio-opaque
• Grows when a bone breaks/fracture and quick tempory fix of fracture site
• soft calleous
• Cartilage tree and will fall over and take a long time

1. Cartilage model

•  Mesenchymal stem cells (MSC's) -> chondroblasts (cartilage making cells)
  

•  Chondroblasts surrounded by cartilage matrix -> chondrocytes
  

•  Cartilage condenses & begins forming cartilage bone model
  

•  Some MSC's -> fibroblasts – make outer fibrous covering around cartilage, plus inner MSC layer = perichondrium
  

•  Perichondrium contains nerves & some blood vessels
  

2. Bony collar formation

• Involves cartilage
• MSC's -> osteoprogenitor cells
• Osteoprogenitor cells along diaphysis -> osteoblasts
• Osteoblasts - build bony collar on diaphysis
• Covering over new bone renamed – perichondrium ->periosteum
• Bone formed prevents nutrient diffusion to chondrocytes in diaphysis
• Lack of nutrient diffusion = hypertrophy of chondrocytes -> some die off in centre of diaphysis

3. Primary ossification centre (POC)

• Dead chondrocytes release Ca2+ – hardens cartilage
• Nutrient artery (NA) enters through nutrient foramen (NF) – takes some cellular periosteum inside – renamed as endosteum
• NA burrows into space left by dead chondrocytes
• MSC's -> Osteoprogenitor cells in endosteum -> osteoblasts build POC
• Haematopoietic SC's -> osteoclasts – make NF, clear path for NA, help clear dead chondrocytes, pass Ca2+ to osteoblasts.

4. Medullary cavity (MC) & shaft formation

•  Bony collar thickens growing inwardly (osteoblasts)
  

•  Inside POC - osteoclasts begin hollowing out, as osteoblasts build bone outwardly
  

•  MC forms as bone building (osteoblasts) & resorption (osteoclast) activity is equal
  

•  Bone from POC eventually meets bony collar mostly compact
  

•  Nutrient artery branches & grows inside MC
  

•  Haematopoietic SC population grows in MC known as bone marrow produces blood cells
  

5. Secondary ossification centres (SOC)

•  Bony collars begin forming on each epiphysis
  

•  Perichondrium there renamed periosteum
  

•  Cartilage cells stressed die in centre of epiphyses
  

• NA's enter via new NF's – take cell layer of periosteum inside renamed as endosteum
  

• MSC's - Osteoprogenitor cells osteoblasts form trabecular bone in SOC's eventually filling up epiphysis ends NA's branch fill gaps between trabeculae with bone marrow (6) Epiphysis remodeling Bone remodeled in epiphyses develop the bony features needed for joint formation

•   Some epiphyses develop more than one SOC build bone features
  

•  Only remaining hyaline cartilage is now on articular surfaces & in the epiphyseal growth plate (metaphysis)
  

7. Further growth of bone Growth from here involves (steps 7a & 7b): a) Elongation/ expansion of metaphysis (growth plate); or b) Appositional bone growth

Bone lengthening – Metaphysis expansion and zones include:

• Resting zone small, inactive cartilage cells
• Proliferation zone
  • Chondroblasts quickly divide and push the epiphysis away from the diaphysis, lengthening the bone. 2 Hypertrophic zone
  • Older chondrocytes enlarge and signal the surrounding matrix to calcify. (3) Calcification zone
  • Matrix becomes calcified; chondrocytes die,
  • leaving behind trabeculae-shaped calcified cartilage. -THIS IS NOT YET BONE! (4)Ossification zone Osteoclasts digest the calcified cartilage, and osteoblasts replace it with actual bone tissue in the shape of the calcified cartilage resulting in bone trabeculae.

More Bone Growth

1 Cartilage

• grows here 2 Cartilage replaced by bone here 3 Cartilage. grows here 4 Cartilage replaced by bone here

• Bone is an alive & active tissue!

• Bone remodeling

• Continuous process balanced between Osteoclast & Osteoblast activity

• May be increased or decreased during lifespan

• What factors would affect bone remodeling?

  • Shapes reflect applied loads -Mineral turnover

8. • Closure of growth plates

•  Epiphyseal growth plates are in the metaphysis regions of the developing bone
  

•  Bones grow in length until 12-25yrs old (up to 30yrs)
  

•  Growth of cartilage in metaphysis slows
  

•  Ossification overtakes cartilage Closed 'epiphyseal growth plates' become 'epiphyseal lines'  usually seen as a white line of compact bone.
  

Bone Remodeling Phases

1. Activation of osteoclasts
2. Resorption of bone (cylindrical holes)
3. Reversal phase (slow down of osteoclasts)
4. Formation of bone (osteoblast activation)
5. Resting phase

Bone remodeling

• Activity in bone
• Small changes occur continually – repair micro-fractures
• 5-7% of bone mass is recycled weekly Trabecular bone is fully replaced every 3-4 years
• Compact bone is fully replaced approx. every 10 years

Intramembranous ossification

• Bone growth without cartilage model
• Bone grows within fibrous connective tissue sheets Trabecular bone first, then compact Forms flat bones of skull, clavicles (1) MS"s within fibrous membrane osteoprogenitor cells osteoblasts multiple ossification centres 4 Osteoblasts create trabecular bone, by secreting osteoid 8 matrix) Random network of bony trabeculae laid & endosteum/periosteum forms (4 Compact bone is created on the outside of trabecular bone 3 Remodeling of 'woven' (random) trabeculae replaced with more typical trabecular patterned bone formations Red marrow fills spaces inside