bone structure and elastic properties of biological material

Question 1

what are the 6 functions of bone?

Answer 1

-support
-locomotion - joints
-protection of organs eg skull, ribs, vertebral column
-storage of minerals eg calcium
-nourishment - eg teeth breaking food
-sound transmission - eg ossicles in the middle ear

Question 2

what are osteocytes?

Answer 2

bone cells that are responsible for the maintenance of the bony matrix

Question 3

what is bone remodelling?

Answer 3

a continuous process of destroying old bone and building new bone
-carried out by osteoclasts (bone destroyers) and osteoblasts (bone builders)

Question 4

what 2 important materials are in bone?

Answer 4

-collagen - gives bone elastic properties
-bone mineral- inorganic material

Question 5

compare how easily ‘bendable’ collagen vs bone mineral is

Answer 5

-collagen - like rubber and very pliable - easily bent
-bone mineral is very fragile and can easily be crushed with finger

Question 6

what are 5 different categories of bone according to their shape & give examples ?

Answer 6

-flat - plate like bones - eg scapula
-long - humerus , femur etc
-cylindrical bones eg vertebrae
-irregular shaped function specific joint bones eg wrist, ankle
-others eg ribs

Question 7

what are the 2 different types off bone ?

Answer 7

-compact or solid bone
-spongy or trabecular bone

Question 8

where is trabecular bone found?

Answer 8

-commonly found at the end of the long bones eg femur

Question 9

why is there trabecular bone at the end of bones?

Answer 9

• can reduce impact force as the trabecular bone are resistant to compression
  -light and not as dense and heavy

Question 10

why is the femur hollow?

Answer 10

as it provides maximum strength for minimal weight

Question 11

what do all materials undergo after a stress is applied to them?

Answer 11

they will undergo some deformation (ie change of shape)

Question 12

what does the deformation of the material depend on?

Answer 12

the force per unit area

Question 13

what is the formula related to stress in a material with cross sectional area A subjected to a force F?

Answer 13

stress (theta symbol) = F/A

Question 14

what are the units of stress?

Answer 14

newtons per square meter
N/m^2

Question 15

what are the 3 common types of stress applied to bone and tissues within the human body?

Answer 15

-compression
-tension
-shear stress

Question 16

what is compression force ?

Answer 16

compression force is when something is being pressed or squeezed together

Question 17

what is a tension force ?

Answer 17

-tension is the opposite of compression force
-it occurs when something is being stretched or pulled apart
-eg rubber band being pulled apart

Question 18

what is a shearing force ?

Answer 18

-shearing force happens when 2 surfaces slide past one another in opposite directions
-eg like a scissor movement

Question 19

what is a rotational stress?

Answer 19

• occurs when a force is applied to the bone in a twisting or rotating motion
  eg common in activities that involve sudden changes in direction eg basketball

Question 20

what happens when a material of length L is subjected to a compressional or tensional stress?

Answer 20

a change of length occurs - called STRAIN

Question 21

what is star defined as?

Answer 21

the fractional change in length of the material

Question 22

does strain have units?

Answer 22

no

Question 23

how are stress & strain related - on graph?

Answer 23

-graph - y axis is stress and x axis is strain
- graph is a straight line up until point A- ie if you double the stress, you will double the strain
- A= the linear limit
-between the point A and B - increasing the stress still increases the strain more, but not linearly related
-point C = maximum strength
-point D = fracture point

Question 24

what is the elastic region on the stress / strain graph?

Answer 24

in this area of the graph, if stress is applied to the material, the material will still be able to go back to its OG shape and size once the stress is removed

Question 25

what law does the elastic region of the stress . strain curve follow?

Answer 25

Hookes law (stress = young modulus x strain)

Question 26

what is the plastic deformation area of the stress / strain curve ?

Answer 26

• region of the curve where the material undergoes non linear deformation that is permanent ie it docent return to OG shape or size

Question 27

what is young Modulus (E) defined as?

Answer 27

-ratio of stress to strain
-it is a measure of stiffness or rigidity of a material
-it is the slope of the stress / strain graph in the linear region

Question 28

what does a smaller vs larger value of young modulus mean?

Answer 28

-smaller value = more elastic material
-larger value = more stiff material

Question 29

what is an important point to note about bone and its elastic properties under compression vs tension stress?

Answer 29

-bone is more elastic under compression than it is under tension - ie slope is smaller for compression than tension
-ie lower youngs modulus

Question 30

what is anisotropy?

Answer 30

-a material behaves differently depending on which direction you’re looking at or measuring it from. It’s like having a material that’s stronger or weaker depending on which way you try to stretch, squash, or do anything to it.
-this happens due to its make up

Question 31

what is a greenstick fracture?

Answer 31

-fracture resulting from direct trauma
-incomplete fracture of a long bone eg radius or ulna with one side of the bone intact and the other is bent

Question 32

why do greenstick fractures happen?

Answer 32

as bone is more elastic and stronger under compression than under tension

Question 33

what factors determine how a material can resist bending?

Answer 33

-material composition -youngs modulus - measure of how stiff a material is
-crosss sectional shape and size

Question 34

take a simple shaped object eg a cylinder… what happens when we bend it - think of top and bottom surfaces and forces applied to it?

Answer 34

-top surface is under tension and the intermolecular forces are tying to pull the surface back to its OG shape
-bottom surface is under compression and the intermolecular forces are trying to push the surface back into its OG shape

Question 35

what is the surface in between the top and bottom surfaces of th cylinder that is being bent?

Answer 35

the neutral surface - which undergoes no change in length

Question 36

where do the resisting intermolecular forces increase?

Answer 36

the further they are from the neutral surface

Question 37

why do tendons not have the same stress strain curve as bone?

Answer 37

• they exhibit non linear behaviour at stresses well below their elastic limit
  -due to presence of collagen fibres

Question 38

describe the type of bone in a vertebrae

Answer 38

• body consists of a cylinder of cancellous bone w/ trabeuculae surrounded by a thin layer of cortical bone

Question 39

what forces are vertebrae able to resist & why ?

Answer 39

compressional, tensional and torsional
-due to the orientation of the trabecular within the vertebrae

Question 40

the femur is a long, hollow cylindrical shaped bone with compact bone around the centre of the shaft and trabecular bone at the ends… why?

Answer 40

• heads of the bone - trabecular - will act as shock absorbers
  -trabecular bone is also resistant to compressional and tensional stresses
  -also compact bone in centre to provide max strength to bone