KNES 363 - bone composition and structure

4 primary mechanical functions of bone

-protects our internal organs

-provides the body its basic shape

-facilitates movement

-provides a framework for support

4 types of bone

-long bones

-flat bone

-irregular bone

-short bone

2 different skeletons

-axial skeleton (skull, ribs, spinal cord)

-appendicular skeleton (limbs)

porosity of a bone

the porosity of bone can vary from 0 -100% but most bone is either of vary high porosity or low porosity

0 % means all bone tissue, 100% means no bone

primary role of trabecular bone

to distribute the mechanical load from the articular joint surfaces - to get it out to the dense cortical bone in order to carry out that load

marrow

-found in nonmineralized tissue

-composed of blood vessels, nerves, and various types of cells (like mesenchymal stem cells, RBCs, WBCs)

-generates the principle cells found in blood

-nearly universal feature of bones (not in the ossicles of the inner ear)

-highly osteogenic material that can stimulate bone formation at extra skeletal regions

Mesenchymal stem cells (MSCs)

Develop into osteoclasts, fibroblasts, and adipocytes

-cells that develop into connective tissue, blood vessels and lymphatic tissue

marrow - red or yellow?

-predominantly called red marrow when young, as we age it gets replaced with yellow marrow which is comprised more of fat. So the marrow gets more fatty as you age and changes colour

marrow -> osteogenic

-its cool because you can put it in other tissues and it will just make bone. This is what we call oseteogenic.

-you can take marrow and stick it into your thigh muscle and it will actually start to grow a bone.

trabecular bone

-sometimes called cancellous or spongy bone

-high porosity bone found in cuboidal bones, flat bones, and ends of long bones

-bone matrix forms plates and rods called trabeculae 200micrometere thick

-sometimes organized orthogonally, but often randomly arranged

how is trabecular bone arranged

In healthy bone for the most part, the plates sort of orient themselves along the vertical axis and where we see a lot of the rods is actually connecting these plates like transverse

-new research says the rods play a big role in fatigue strength

-before people thought it was all about the plates doing the strength and loading

If you took a trabeculae and cut it in half what you see are like these concentric rings that we call lamellae - kinda looks like how wood grows in rings

what is the porosity of trabecular bone (%)

75%-95%

cortical bone

-sometimes called compact bone

-dense bone found in shafts of long bones and forming shell around vertebral bodies and other spongy bone

-contain haversian canals and vokmanns canals

-contain resorption cavities

haversian canals

aligned longitudinally, contain capillaries and nerves, about the size of a human hair (50micrometers)

blood supply - without them we would have no way to keep the bone healthy and to allow blood to flow into the bone to get nutrients

vokmann's canal

the small channels that allow haversian canals to communicate with one another.

-short transverse canals connecting haversian canals, contain blood vessels/nerves

resorption cavities

temporary spaces created by osteoclasts in the initial stage of remodeling (200 micrometers in diameter)

-created in the transient period of time in bone reabsorption when nothing is formed

how much of your cardiac output does bone receive

5-10%

what are the two main aspects that affect the porosity of bone

cells and the vasculature

2 types of bone at the tissue level

lamellar and woven bone

lamellar bone

-slowly formed, highly organized

-parallel layers of anisotropic matrix of mineral crystal and collagen fibers

Nice homogeneous, clean structure. STRONGER THAN WOVEN but it takes a while to regenerate

woven bone

-quickly formed, poorly organized

-randomly arranged mineral and collagen fibers

-sites of fracture healing, tendon/ligament attachments

what bone is laid down after a fracture?

After a fracture : you just want to stabilize the area so you just throw down as much woven bone as you can. Not very strong but does the trick because it can go down fats. Then over time you can reorganize that woven bone into a well organized lamellar bone. So woven can become lamellar but it just takes a while

two types of lamellar bone

primary and secondary

primary bone

-tissue laid down on existing surface

-circumferential lamellar bone and plexiform bone

-as you grow, the cross-sectional area of the bone grows (like tree rings)

secondary bone

-bone resulting from remodeling

-in compact bone consists of secondary osteons (D=200micrometers)

-most adult bone is secondary bone

what marks secondary bone

osteons!

-no osteons = primary. osteons = secondary

what are the two main things that really define bones

structure and composition

bone composition - percentages

water 25%

organic matrix 32%

apatite mineral (inorganic matrix) 43%

the more water there is, the less minerals there are and vice versa

site or specific molecule of the water component of bone

bounded to collagen 60%

other 40%

site or specific molecule of the organic matrix component of bone

collagen 89%

proteoglycan 1%

other organic molecules 10%

site or specific molecule of the inorganic matrix component of bone

between collagen ends 28%

intrafibrillar 58%

interfibrillar 14%

total bone volume

Vt can be divided into hardy bone matrix Vm and voids with soft tissue Vv

Vt = Vm + Vv

Bone volume fraction

Bv = Vm/Vt

If you can spray away all the marrow and its just the bone matrix itself would be the bone volume fraction. So simply the matrix volume divided by the total volume. The higher bone volume fraction, the higher percentage of bone matrix

porosity relationship to bone matrix fraction

-inversely correlation to the bone matrix fraction and the porosity of bone. Less fraction, more porous

porosity gives you the bone volume fraction and vice versa. 1-porosity = bone volume fraction

Porosity is how much negative space you have in a volume / none bone

Bone volume fraction is how much bone you have in that volume

-unitless because they are a ratio

Porosity Calculation

pv = Vv/Vt

porosity of trabecular bone

pv > 0.5

greater than 0.5

porosity of cortical bone

pv < 0.5

less than 0.5

bone apparent density, p

-mass of a volume of bone divided by its volume

-both hard and soft tissue mass inside total volume

Density measurement that includes void space, water, mineral, marrow, all that stuff

-look at slides for equation

what is heavier, more or less mineralized bone?

If the bone is more mineralized its heavier. Cortical bone is more mineralized so the apparent density is a bit higher, it's a bit heavier

ash fraction

-the degree of mineralization of bone tissue, independent of porosity

Ash fraction is dry mass - take this cube of bone, put it in an oven and get rid of all the water In the void space and all the water in the collagen. Then you put it back In the heat to get the ash mass. This is where you have burned away all of the collagen and just have the mineral. SO THE ASH FRACTION TELLS YOU ABOUT THE DEGREE OF MINERALIZATION

dry mass

Dry mass is the mineral - organic and inorganic

-ash you get rid of all the organic components by heating it for a while. Brun that shit to ashes

Bone is comprised of water, organic and inorganic components

-the degree of mineralization - remove all aspects of porosity

-so density of mineral is the density of organic component times the volume of the organic component