Bone mechanical properties

Question 1

what are the mechanical/material properties of bone heavily dependent on

Answer 1

density / porosity

Question 2

how does density affect modulus

Answer 2

more dense = higher modulus = more stiff = more brittle

Question 3

what are the 4 characteristics of loading that primarily affect the mechanical properties of bone

Answer 3

sample orientation
sample hydration
strain rate
loading mode

Question 4

how does sample orientation affect the mech properties of bone

Answer 4

bone is stiffer and stronger in the longitudinal direction than the tranverse direction
(anisotropic)

Question 5

how does sample hydration affect the mech properties of bone

Answer 5

both collagen bound and unbound water is important for bone toughness
- dry = brittle, and locked collagen can’t absorb energy

Question 6

how does strain rate affect the mech properties of bone

Answer 6

bone is stiffer and stronger the faster you load it but also becomes more brittle
- bone is only slightly viscoelastic
- not extremely sensitive to changes in strain rate

Question 7

how does loading mode affect the mech properties of bone

Answer 7

cortical bone is strongest in compression then tension then shear
- much less difference in strength between compression and tension in trabecular bone

Question 8

why is there a big difference in strength between compression and tension in cortical bone

Answer 8

due to osteonal organisation

Question 9

what does trabecular bone strength depend on

Answer 9

density (bone volume fraction)
fabric (trabeculae orientation)

Question 10

is density or fabirc more important to trabecular bone strength

Answer 10

90% behaviour determined by density
10% explained by fabric (trabeculae orientation)

Question 11

what are the 3 toughening mechanisms of bone

Answer 11

crack bridging by collagen fibres
crack bridging by uncracked ligaments
microcracks

Question 12

what is the function of toughening mechanisms

Answer 12

allows bones to carry more load for longer amounts of time in the presence of microdamage

Question 13

what is the function of beam theory

Answer 13

used to calc the normal and shear stresses and strains acting on a cross section of bone

Question 14

what are the 3 primary assumptions of beam theory

Answer 14

1. constant cross sectional geometry
2. longitudinal plane of symmetry
3. homogenous material

Question 15

what are the values of compressive and tensile stress

Answer 15

compressive = negative
tensile = positive

Question 16

what occurs to the sample during bending

Answer 16

one side experiences normal compression
one side experiences normal tension
middle experiences no stress

Question 17

what happens to stress experienced when moving away from neutral axis

Answer 17

neutral axis - stress from bending = 0
- farther from neutral = stress increases
- therefore as position x increases - stress increases

Question 18

what is the areal moment of inertia

Answer 18

measure of the distribution of area
- object’s resistance to bending
- higher I = harder to bend