Tissue Mechanics: Bones and Cartilage

What are the 2 principal structural components of bone

collagen and hyrdoxyapatite

what type of collagen is in bone

primarily type 1 is in it

what is collagen

it is organic material found in all the body’s connective tissue

how much of the bone does collagen make up

it makes up 90% of the bone’s organic content and about 40% of the bones dry weight

how much of the bone does hydroxyapatite make up

it makes up 60% of the bone’s dry weight (inorganic content)

what is hydroxyapatite

it is a calcium phosphate based mineral found primarily between collagen fibers and is a ceramic that can be found in crystal form as a mineral

what are woven bones

immature bone with cartilage fibers randomly distributed to give some strength in all directions

what do osteoclasts do

they dig tunnels into the bone as it matures

what do osteoblasts do

they line the tunnels with type 1 collagen

what do osteocytes do

they mineralize collagen with HA

what are osteons

they are the hallow canals which represent the structural units of bone

what is bone modeling determined by

it is determined by gravity, physical activity, hormones and age

what are the functions of bone

protects internal organs

support body weight

provide rigid kinetic links and muscle attachment sites

facilitate muscle action and body movement

acts as storage area for minerals

manufactures blood cells

what are the types of bone

cortical and cancellous

what are the characteristics of cortical bone

hard, dense, compact, 80% of skeletal mass, 33% of bone surface

what are the characteristics of cancellous bone

not as dense, filled with spaces, spongy, 205 of skeletal mass, 67% of bone surface

what is wolff’s law

the shape of the bone is determined only by static loading. physical laws are a major factor influencing bone modeling and remodeling

what does an applied bending moment do

it creates both compression and tension

what does is mean for bones to be anisotropic

it means they have different mechanical properties in different directions

how much does young’s modulus change for typical daily activities

it changes only by 15%

what happens to bone at very high strain rates

at these rates ultimate strain decreases and cortical bone exhibits a ductile to brittle transition

what happens to young’s modulus over time

it decreases 2.3% every 10 years after age 35

what happens to fracture toughness over time

it decreases 4% per 10 years

what happens to bending strength over time

it decreases by 3.75 per 10 years

what are the steps of fracture healing

inflammatory response

repair cells gather at fracture site

in 2 weeks callus begins to form

callus calcifies and becomes woven bone

woven bone undergoes remodeling

what are the 3 types of cartilage

articular cartilage, fibrocartilage and elastic cartilage

where is articular cartilage and what does it do

it is found in synovial joints to permit smooth motion between bony segments

what are characteristics of fibrocartilage

it is inflexible and tough and has considerable tensile and compressive strength

what does elastic cartilage do

it helps to maintain the shape of structures

what is a joint enclosed by

it is enclosed in a fibrous tissue capsule and the inner surface is lined with the synovial membrane that secretes synovial fluid

what is the composition of articular cartilage

70-85% water

30% of dry weight is proteoglycans

60-70% of dry weight is type 2 collagen

lipids is remainder of dry weight

what are proteoglycans

they consist of a protein core with chondroitin sulfate and keratin sulfate attached. their electrical charge makes them spread out

what is the spread of proteoglycan limited by

it is limited by the collagen framework contributing to the compressive stiffness

what does damage to the collagen framework do

it reduces the compressive stiffness

what is the mechanical response of cartilage linked to

it is strongly linked to the flow of fluid through the tissue

what does mechanical failure of cartilage result from

it results from more shear stresses than tensile

what is abnormal loading important in

it is important in the development of osteoarthritis