KNES 363 MIDTERM

What is biomechanics?

science that studies the forces acting upon and within a biological structure, and effects produced by such forces

What occurs in bone ash fraction?

Burn bone to get rid of water and only be left with dry mass

How does the porosity for trabecular bone compare to cortical?

Trabecular > 0.5, cortical <0.5

What is meant by resorbing bone?

To remove/breakdown bone

What is the typical value for ash fraction?

0.65

What can stress be defined as? Formula

F/A = elastic/Young’s modulus x strain = P/Acossquaredø

About how much of your cardiac output does bone receives?

5-10%

When does max stress occur?

When the x components is at an angle of 0

When does minimum stress occur?

When the y component is at an angle of 90

What is the formula for normal shear stress?

-P/Asinøcosø

What is the formula for strain?

Change in length/original length = stress/elastic modulus

why study biomechanics?

improve performance, preventing or treating injury

what are the reactions of biological structures to force stimuli?

mechanical (acceleration and deformation) and biological

what is the formula that represents acceleration?

F=ma

what law relates to an accelerating force?

newton’s second law of motion

what is the formula which represents deformation?

F=K*x

what are the biological consequences of a force stimuli applied to biological structure?

adaptation, repair, damage, rupture of tissue

what does k represent in the deformation formula?

stiffness constant, amount the object deforms based on the magnitude of force applied

what does x represent in the formula for deformation?

displacement of an object

what are the newtonian mechanics for forces?

sum of all forces equal the product of mass and acceleration of the COM

what are the newtonian mechanics for moments?

sum of all moments equals the product of moment of inertia and the angular acceleration of the COM

what is inertia?

the amount and distribution of mass/matter in an object

what does the moment of inertia determine?

the resistance of an object to rotational acceleration

what comparison can be made between what the moment of inertia determines and what mass determines?

mass determines the resistance of an object to linear acceleration

explain the concept of ‘moment of inertia’

the more close a mass is distributed about the axis of rotation the easier it is to produce angular acceleration so resistance changes in angular motion is dependent on the mass and distribution of mass

how is the moment of inertia defined for a rigid body?

it is fixed

how is the moment of inertia of linked rigid body segments defined?

can be manipulated through changes in limb position

what is an example of linked rigid body segments?

humans

in static equilibrium are we concerned with moment of inertia?

no

what are the mechanical purposes of bone?

protects our internal organs, provides the body its basic shape, facilitates movement, provides a framework for support

what are the types of bones?

long, short, flat, irregular

what are the components of long bone?

periosteum (membrane lining bone), cortical bone, trabecular bone, articular cartilage, epiphyseal plate, medullary cavity, marrow, blood vessels

where is marrow found?

in nonmineralized tissue

what is marrow composed of?

blood vessels, nerves, and various types of cells

what is the purpose of bone marrow?

generates principal cells found in blood, nearly universal feature of bones (not in ossicles of inner ear), highly osteogenic material that can stimulate bone formation at extraskeletal regions

what is another term for the trabecular bone?

cancellous/spongy bone

what is the trabecular bone?

high porosity bone found in cuboidal bones, flat bones, and ends of long bones that have trabeculae sometimes organized orthogonally but often randomly

what is the porosity of trabecular bone?

75-95%

What are trabeculae formed from?

bone matrix plates and rods

what is another term for cortical bone?

hard/compact bone

what is cortical bone?

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

what is the porosity of cortical bone?

5-10%

what are haversian canals?

aligned longitudinally, contain capillaries and nerves, about the size of human hair, found in cortical bone

what is vokmann’s canal?

short transverse aligned canals connecting haversian canals, contain blood vessels/nerves, found in cortical bone

what are resorption cavities?

temporary spaces created by osteoclasts during initial stage of bone remodeling, seen in cortical bone, where we see Haversian canals from bone remodeling 

what is lamellar bone?

Cortical bone, slowly formed, highly organized, parallel layers of anisotropic matrix of mineral crystals and collagen fibers

what is woven bone?

quickly formed, poorly organized, randomly arranged mineral and collagen fibers, sites of fracture healing, tendon/ligament attachments

what is primary bone?

tissue laid down de novo on existing surface (periosteum on surface in growth), circumferential lamellar bone and plexiform bone

what is secondary bone?

bone resulting from remodeling of primary bone, in compact bone consists of secondary osteons, signalled by haversiann canals

what type of bone is most adult bone?

secondary bone

what are the components of bone composition?

water (25%), organic matrix (32%), apatite mineral (43%)

what is the site or specific molecule where water is found in bone composition and what is the associated volume?

bonded to collagen (60%), other (40%)

what is the site or specific molecule where organix matrix is found in bone composition and what is the associated volume?

collagen (89%), proteoglycan (1%), other organic molecules (10%)

what is the site or specific molecule where apatite mineral is found in bone composition and what is the associated volume?

between collagen ends (28%), intrafibrillar (58%), interfibrillar (14%)

what can total bone volume be divided into?

hard bony matrix and voids with soft tissue

what is bone apparent density?

mass of a volume of bone divided by its volume, both hard and soft tissue mass inside total bone volume

what is bone ash fraction?

degree of mineralization of bone tissue, independent of porosity

what are the two categories of bone cells?

those that resorb bone or those that form or have formed bone

what are the bone resorbers closely related to?

macrophages, cells that migrate throughout all tissues and remove debris and pathological material

what are the bone formers closely related to?

cells like fibroblasts, which produce structural molecules in other tissue

are osteoclasts resorbers or formers?

resorber

what is a osteoclast?

multinucleated cells formed by the fusion of monocytes originating in bone marrow

what are the characteristics of osteoclasts?

move at a rate of tens of microns per day by first demineralizing the bone with acids then dissolving the collagen with enzymes

are osteoblasts resorbers or formers?

former

what are osteoblasts?

mononuclear cells that produce osteoids (the organic portion of the bone matrix),

what are the characteristics of osteoblasts?

differentiated from mesenchymal cells, osteoblasts lay down osteoid at a rate of about 1 micro/day, lay down collagen, more present than osteoclasts

what are osteocytes?

osteoblasts that have become buried in bone/mineralized tissue

where are osteocytes found?

they sit in cavities called lacunae and communicate with each other via dendrinic processes called canaliculi

Explain the distribution of osteocytes in bone

there about 15000 lacunae per cubic mm of bone, occupy only 1% of BV but surface area is large

what are bone lining cells?

like osteocytes that have escaped being buried in the bone and become quiescent on the bone surface

what is bone modeling?

independent action of osteoclasts and osteoblasts on different surfaces

what is the purpose and characteristics of bone modeling?

produces changes in bone size and shape, highly active during growth and development and greatly decreases after skeletal maturity, hihgly influenced by PA during childhood depending on mechanical stimulus

what is bone remodeling?

sequential, coupled action of osteoclasts and osteoblasts

does bone remodeling influence bone size and shape?

not usually

what is the purpose/mechanism of bone remodeling?

removes a portion of old bone and replaces it with newly formed bone, occurs throughout life but decreases after growth

What is bone remodeling hypothesized to do?

remove bone damage and mechanically fine tune the skeleton to best resist load and maintain tissue quality

what does mechanically efficient mean in bone remodeling?

to extract calcium and flow into the circulatory system

what does ARF stand for in BMU remodeling?

activation, resorption, formation

what is activation in ARF sequence of BMU remodeling?

differentiation of precursor cells to produce osteoclasts, requires 3 days

what is resorption in ARF sequence of BMU remodeling?

osteoclasts start to resorb bone at a rate around 40 um/day, requires 30 days

what is formation in ARF sequence of BMU remodeling?

osteoblasts appear and begin to refill, but leave a vascular loop (haversian canal) to support metabolism of the BMU and carry nutrients, takes about 3 months

how does BMU activation rate change with age for human ribs?

it reduces

is stress scalar or vector?

it is a tensor, has magnitude and two directions (stress/force component and area)

what can axial or normal stress be?

compressive or tensile

what are the characteristics of principal stress?

oriented 90 degrees apart

what are the characteristics of max shear?

oriented 45 degrees between principal stresses

are normal and shear stresses always the same?

they vary continuously as the orientation of the axis is changed

what is the unit for strain?

unitless, sometimes expressed as % or units of microstrain

what is poisson’s ratio?

a material loaded in one direction will undergo strains both parallel and perpendicular to the direction of load

what is the relationship between stress and strain determined by?

mechanical properties

what are the two important material properties of the stress strain relationship?

elastic and shear modulus

what does Hooke’s law define?

defines the linear relationship between stress and strain

what does the elastic modulus depend on?

bonding types between atoms and molecules, somewhat on underlying density of bone as well

what is yield strength?

the stress at which the material will no longer go back to its original shape when unloaded, can be defined in terms of strain

what is ultimate strength?

the max stress a material can withstand before failure, can be defined in terms of strain

what is failure?

the point where the material fractures or ruptures (unable to withstand load), can be defined in terms of stress or strain

what is toughness?

the total energy a material can absorb prior to failure, can be broken up into elastic and plastic (post yield) components

can elastic or strength modulus be defined in terms of loading mode

yes

what is elastic deformation?

if the stress applied is equal or less than the elastic limit (yield strength), the deformation will be completely recovered when the load is removed

what is plastic deformation?

if the stress applied is beyond the elastic limit, the deformation will be permanent (the material has been damaged)