bone design & mechanics

what does a composite material receive?

benefits of multiple materials, with few consequences

bone is considered what at both macrostructural & microstructural levels?

composite material

bone macrostructure

• relatively hard, brittle outer shell

• relatively soft, ductile interior

bone microstructure

• relatively hard inorganic minerals

• relatively flexible organic matrix

what does the hollow structure of bone allow for?

greater resistance to bending & twisting

mechanical function of bone

• structural framework for body

• strong to protect vital organs

• rigid to act like a lever for muscles

• light for mobility

storage function of bone

• reservoir for essential organic & inorganic material

• stores nearly all the body’s calcium & most of its phosphorous and magnesium

• filled with marrow → produces blood cells

bone is an optimal tradeoff between what?

strength & mobility

bone structure maximizes what while minimizing what?

strength; weight

mechanical stress modulates what?

growth & remodeling of bone

Wolff’s Law

bones adapt & remodel in response to mechanical stresses

in response to loading, bones can what?

change size, shape, & structure

normal everyday responses to loading bone past yield?

developing & healing microfractures/cracks

periosteum

surrounds bone & plays key role in bone modeling

2 layers of periosteum

• osteogenic (inner) layer

• outer layer

in who is periosteum better developed in & why?

children because of its role in increasing bone diameter

what is periosteum permeated by?

blood vessels & nerves

inner surface of bone has what?

osteogenic endosteum

what is one of the most dynamic & metabolically active tissues in the body?

bone, b/c it is constantly remodeling

why do bones have an exceptional ability to repair?

bones are very vascular

robust routes of blood supply

• nutrient artery (inner)

• periosteal system (outer)

• metaphyseal system (physis)

• epiphyseal system (layers)

osteoclasts

break down damaged bone

osteoblasts

build bone

osteoblasts can become what when they finish building

• osteocytes

• bone lining cells

3 types of bone tissue in bone macrostructure

• cortical bone

• trabecular bone

• woven bone

cortical bone (compact or lamellar bone)

hard outer shell, very organized microstructure; strongest bone tissue, fails at low strain

trabecular bone (cancellous or spongy bone)

thin rods or plates in a loose mesh-like structure connecting inner surfaces of cortical bone

what is new bone made of?

temporary tissue that gets replaced by cortical or trabecular bone (woven bone)

woven bone

weak & disorganized microstructure; found in embryo & newborn, during healing after fracture, in metaphyseal region of growing bone

what is bone continuously undergoing?

resorption followed by deposition of new bone

what do osteoblasts & osteoclasts replace the immature bone with?

cortical & trabecular bone

intramembranous ossification

mesenchymal tissue → immature bone → mature bone

when does intramembranous ossification occur?

• within fibrous connective tissue membranes to form flat or irregular bones

• during recovery of stable fractures

endochondral ossification

mesenchymal tissue → cartilage model/scaffold → immature bone → mature bone

when does endochondral ossification occur?

• most bones - long, short, irregular bones

• during recovery of unstable fractures

subperiosteal deposition

directly laying down new mature bone tissue on the surface of existing bone to increase diamter

when does subperiosteal deposition occur?

during growth & remodeling

endosteal deposition

laying down mature bone tissue inside the cortex or around trabeculae

what is the distinguishing feature of bone tissue?

it has a lot of inorganic minerals

inorganic minerals of bone

mineral salts (calcium, phosphorous, magnesium) - make bones hard & rigid

what do non-collagenous matrix proteins do?

help regulate mineralization

what is bone’s makeup that is the same ingredients as everything else?

• fibrous matrix - type I collagen

• ground substance - proteoglycans & GAGS

• fluid content

• cells

does bone microstructure differ across the different types of bones?

woven bone lacks what?

structured organization

what does woven bone contain rather than osteocalcin?

phosphoproteins

where is woven bone found in adults?

during rapid bone formation; fracture healing, hyperparathyroidism, & Paget’s disease

what is the strongest & densest bone tissue?

cortical bone

how is cortical bone organized?

highly organized, composed of osteons

what is an osteon?

functional unit of cortical bone

what does each osteon have?

a central canal (haversian canal) that contains blood vessels, nerve fibers, & lymphatic tissue

what does the space between osteons contain?

remains of old osteons; dead bone tissue with no vascular supply

what is the outer boundary of each osteon is called?

cement line

what do osteoblasts deposit in thin concentric layers aroundthe central canal?

matrix

where is fiber density the lowest?

at the borders between lamellae

where do a fraction of osteoblasts become trapped?

in pockets (lacunae) between layers (lamellae)

what do the pockets (lacunae) connect to?

central canal via small channels (canaliculi)

why are canaliculi essential for osteocyctes?

to sense mechanical loads & send signals to initiate remodeling

what is secreted by osteoblasts during formation of bone?

uncalcified osteoid

what are cement lines?

outer border of living osteons

what do not cross cement lines?

collagen fibers & canaliculi

weakest portion of the bone microstructure & common place for cracking

cement lines

what are volkmann’s canals

penetrate the bone perpendicular to its surface; route for vessels & nerves to reach central canals

what does the interior of bone contain?

trabecular bone tissue

what branches and interconnects to form a sponge-like network in trabecular bone?

delicate bars & sheets of bone

is trabecular bone very porous?

yes

does trabecular bone contain true osteons?

no

what is the arrangement of trabecular bone?

layered

does trabecular bone have lacunae?

yes

does trabecular bone have canaliculi?

yes

where are osteoblasts derived from?

mesenchymal precursor (stem) cells in marrow

what do osteoblasts do?

make the bone matrix & guide its subsequent mineralization

if an osteoblast is trapped between layers after finishing building, what does it become?

osteocyte

if an osteoblast is on the inner or outer surface of bone after finishing building, what does it become?

bone lining cells

what do osteocytes do?

sense & respond to mechanical loads to initiate bone remodeling; form a network to sense loads and coordinate remodeling

what do osteocytes regulate?

the response of bone to the mechanical environment

where do bone lining cells rest?

on the surface, awaiting signals to convert back into osteoblasts

what do bone lining cells do?

help remove residual matrix after osteoclasts have resorbed bone, preparing the surface for new formation; also contribute to mineral homestasis

what can bone lining cells re-differentiate back into?

osteoblasts

what type of cells are osteoclasts?

large motile multi-nucleated cells located on bone surfaces

what do osteoclasts resemble?

macrophages

what is osteoclast activity regulated by?

parathyroid hormone, Vit D3, calcitonin, glucocorticoids, prostaglandins, interleukin factors, transforming growth factor beta

what bone provides primary mechanical strength & support?

cortical bone (stiffer, stronger, brittle)

what bone has higher turnover & is crucial for shock absorption?

trabecular bone (lower mineral content, less stiff, weaker, ductile)

mechanical properties of cortical bone

• stiff, stronger, but less flexible than trabecular bone

• more homogenous in composition

• strongest along long axis of bone (compression → tension → shear)

stress-strain curve for cortical bone

• steep elastic region, w/out much elongation

• short plastic region, failing ~2% strain

what does failure depend on in cortical bone?

loading type & direction

where do cracks form w/ compression failure in cortical bone?

cracks typically start at interstitial bone and are stopped or rerouted by osteons

where do cracks form w/ tension failure in cortical bone?

cracks typically form & propagate along cement lines around osteons

what do microcracks provide?

a way to dissipate energy and protect the structure from total failure

mechanical properties of trabecular bone

• more flexible & less stiff than cortical bone

• more heterogenous

stress-strain curve in trabecular bone

• elastic & plastic regions allow longer strains at lower stresses than cortical bone

• capable of substantial permanent deformation before total failure

• total compressive failure occurs around 50% strain

damage & repair of individual trabeculae increase with?

age (normal part of everyday remodeling otherwise)

what has a big effect on strength & stiffness?

small changes in density

strength is strongly related to the what of the tissue?

apparent density

what is a major contributor to fragility fractures in osteoporosis?

shear fractures

how do shear fractures typically occur?

bending or torsion, or asymmetric compressive or tensile loads

factors that influence the mechanical properties of bone

• percentage of trabecular bone vs. cortical bone

• loading mode/direction of loading

• frequency of loading

• loading rate

• muscle activity

• geometric characteristics

• age & sex

can cortical & trabecular bone be separated (in vivo)?

no

the whole bone is

strong (especially in compression) & somewhat flexible

creep in bone types is

similar