Biomech - Mechanical Behavior of Bone Tissue (Lecture 13)

anisotropic means what with respect to bones

having a physical property that has a different value when measured in different directions

bone is (homogeneous//nonhomogeneous)

nonhomogeneous

(which refers to the natural, inherent differences in bone structure and composition)

what’s the % breakdown of bone composition amongst its 3 main components

• 35% organic material

• 20% water

• 45% minerals

what is the organic material in bones?

• collagen

• GAGs

what is the mineral content in bones?

• calcium

• phosphate

the ______ provide the strength and stiffness to the bone

mineral crystals

the _______ provide the flexibility and resilience of bone

organic materials

________ refers to the mechanical work done in deforming an elastic material

resilience

a _____________ returns as much mechanical work in regaining its original shape and dimensions as was done upon it in straining it originally

perfectly resilient material

when you fracture a bone, the callus that forms to heal it back together will be even (stronger//weaker) than it was before the fracture, and the surrounding areas will be (stronger//weaker)

stronger; weaker

stress/strain is a _________ version of a load deformation plot

normalized

_____ is the percent change in length of material

strain (as a normalized value)

___is force per unit area

stress (as a normalized value)

____ is a nonhomogeneous anisotropic composite structure that behaves viscoelastically

bone

bone is surrounded by _____, which covers the entire bone except for joints

periosteum

_____ layer of bone is permeated by blood vessels and nerves

outer

_____ indicates how much you can deform something before it ultimately breaks

strain

(inner//outer), osteogenic layer contains bone cells responsible for growth and repair with osteoblasts

inner

central (in long bones), ______ lines medullary cavity

endosteum

_____ is filled with marrow, osteoblasts, and osteoclasts

endosteum

bone healing timeline

• 6-8 wks is normal

• more like 4-6 wks for kids

• higher than 8 wks for older people

bone composition is a bit like ______, which is a combo of glass (brittle) and resin (makes it bendy)

fiberglass

structural unit of bone

osteon

transverse or diagonal canals that run perpendicular to osteons, connecting blood vessels and nerves from the periosteum to Haversian canals

Volkmann’s canals

path from surface for blood vessels and nerves

Volkmann’s (or Haversian) canals

longitudinal canals that run parallel to the length of the bone, located at the center of each osteon, carrying blood vessels and nerves

Haversian canals

a cylindrical unit of compact bone made of concentric lamellae wrapped around a central Haversian canal

osteon

what is the structural unit of bone?

osteon

____ surround osteons in concentric rings

lamellae

small spaces b/w lamellae that house osteocytes (bone cells)

lacunae

_____ are at boundaries of lamellae, and the cavities contain a single bone cell

lacunae

there seems to be a (universal//conditional) relationship b/w bone density and its mechanical behavior

universal

there is a marked reduction of density in bone with age, especially (cortical//cancellous)

cancellous

(Cancellous bone density declines more rapidly, especially in women after menopause due to accelerated bone turnover and loss of trabecular structure.)

in this load deformation plot, B and C indicate what

• B indicates the point after which bone will deform past a point of no return (it will never be the same)

• C indicates the point at which bone will break (ultimate failure)

a load-deformation graph relates what to what

stress (force/unit area) to strain (change in length)

the normalized version of load-deformation is what

stress-strain

what are the 3 parameters for determining the strength of a structure

1. ultimate failure point (sustained load before failing)

2. deformation before failing

3. energy (defined as area under the curve)

bone is (monophasic//biphasic)

biphasic

(contains minerals + collagen and ground substance)

(cortical//cancellous) bone is stiffer

cortical

(cortical//cancellous) bone can handle more strain

cancellous (75% strain = failure)

(cortical//cancellous) bone can withstand more stress

cortical

(cortical//cancellous) bone can handle less strain til failure

cortical (2% strain = failure)

______ is the property of a material or object having different physical or mechanical properties when measured along different directions. This is in contrast to an isotropic material, which has the same properties in all directions

anisotropy

________ refers to different mechanical properties when loaded along different axis

anisotropy

which is the stiffest, can handle the most stress before its yield point, and has the highest ultimate failure point?

L, the top one

bone can handle the most stress/strain in the direction that is ______ loaded

normally

b/w cortical and cancellous bone, which can handle the most…

• compression?

• tension?

• shear?

cortical bone can handle way more than cancellous bone for all of these!

bone is strongest in… (rank shear, compression, and tension from highest to lowest)

1. compression

2. tension

3. shear

in animal studies, immobilization (increased//decreased) the amount of load to failure, energy storage, and stiffness of bone

decreased

bone is more brittle when loaded at (higher/lower) speeds

higher

bone is stiffer when loaded at a (higher/lower) rate

higher

bone can sustain a higher load to failure when loaded at a (higher/lower) rate

higher

bone can store more energy when loaded at a (higher/lower) rate

higher

bone experiences less deformation at failure when loaded at a (lower/higher) rate

higher

the ultimate failure point shows less deformation (but higher load) prior to failure at (lowest//highest) loading rate

highest

do loading rates influence the fracture pattern?

YES

what are the 2 possible causes of fracture?

1. result of a single load > ultimate material strength

2. result of repeated low level loads

cortical (compact) bone is (inner//outer)

outer

cancellous (trabecular, spongy) bone is (inner//outer)

inner

(cancellous//cortical) bone has a trabeculae oriented lattice pattern

cancellous

what percent porosity is the density of cortical bone?

5-30% porous

what percent porosity is the density of trabecular bone?

30-90% porous

the effect of _______ is a decrease in load to failure, energy storage, and stiffness 

immobilization

not all trauma to bone results in either fully reversible loading or a fracture— instead, what could happen that’s much less common?

bone can permanently bend!

(like in Rickets)

_______ produces micro structural damage to bone tissue

cyclic loading

damage accumulates faster at (higher//lower) intensities of cyclic loading

higher

if bone is bent, which side will break first

the side experiencing tensile forces, NOT the compressed side

torsion load will cause spiral and tibia fractures which heal (faster//slower) than some transverse fractures. why?

faster!

more surface area and less soft tissue damage → low load injury

butterfly fragment is always located on the (tension//compression) side of bone

compression

the following are all types of _____ for bones:

• tension

• compression

• bending

• torsional force

• axial compression and bending

loading

the greater the force ______, the higher its energy content and hence more tissue destruction

magnitude

the more ____ the fracture pattern, the greater the energy needed to produce the fracture

complex

Any sudden change in material shape alters the distribution of stresses within the structure, giving rise to what engineers refer to as…

stress concentrations (or stress risers)

what are some structural considerations of “stress concentrations” in the body?

after a person gets a screw hole created in their bone during surgery, they will experience a 60% decrease in how much energy the bone can handle. When the screw is removed, the person will experience a ___% decrease in how much energy the bone can handle

50

an open section defect of bone means the defect is (greater//less) than the bone diameter

greater

area segments farther from the neutral axis contribute significantly (more//less) to the area moment of inertia

more

a callous following a fracture (increases//decreases) the area and polar moment of inertia during the healing process

increases

(later excess bone is re-absorbed as bone returns to normal strength)

can bone increase due to screw movement?

yes

nice lecture pic of bone healing and polar moment of inertia

through third decade of life, people have (increased//decreased) elastic modulus and (increased//decreased) yield strength

increased; increased

elongation to failure of bone (increases//decreases) throughout life

decreases

(old//young) bone is more brittle with decreased energy storage capacity

old

chart of classification of fractures of indirect injury mechanism

chart of classification of fractures by direct injury mechanisms

review questions