Biomechanics Unit 2 Flashcards Preview

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Flashcards in Biomechanics Unit 2 Deck (89):

what is the most abundant tissue in the body and what is its function

connective tissue

specialised to protect and support the body and its organs, connect and hold them together and to transport substances throughout the body


what are the 4 types of connective tissue

bone tissue
articular cartilage


what is the function of bones and what are the 4 groups

protect delicate structures such as the heart and lungs, and act as lever arms for movement

long bones, short bones, flat bones and irregular bones


what is bone tissue composed of

osteocytes [bone forming cells]

non-cellular component
- very strong collagen fibres embedded in jelly-like matrix called ground substance
- fibres are flexible but resist stretching
- 25-30% dry weight of bone

inorganic component
- calcium phosphate crystals within the matrix
- give the bone its characteristic hardness and rigidity
- 65-70% dry weigh of bone


what are the 2 types of bone

cortical [a.k.a compact]
- forms the outer layer of bones
- has a dense structure

- inner part of short, flat and irregular bones
- lines the inner surfaces and makes up the greater part of the metaphyses and epiphyses in long bones


what is the other name for cancellous bone and why is it called this

spongy bone

due to its mesh like structure, the spaces of which contain red bone marrow


how do the cells in cancellous bone align themselves

align themselves in the direction that will best support the load


what is the basic structural unit of compact bone and how is this arranged

haversian system
- arranged longitudinally in columns
- in those units, the bone tissue is arranged in layers called LAMELLAE forming cylinders around a canal


what is maintained within the haversian canal

blood vessels and nerve fibres


what is located betweens lamellae, and what do they contain, how are they linked?

small cavities called LACUNAE

these contain OSTEOCYTES

osteocytes is linked to canal and other lacunae's by channels called CANALICULI
[carries nutrients to blood vessel]


what inter-connects the layers of lamellae within the haversian system

collagen fibres


what is the weakest part of the haversian system

the cement-like ground substance that surrounds the haversian system

[as it contains no collagen fibres]


what is the basic structural unit of cancellous bone

- are arranged in a latticework of branching sheets and coloumns


trabeculae are similar in structure to haversian system = consisting of layers of lamellae with lacunae containing osteocytes connected by canaliculi

what is the main difference

NO haversian canals

not needed as blood vessels pass though the marrow filled spaces - supplying nutrients to the osteocytes through the canaliculi.


what is the load acting to do in tension and compression

in tension - acting to stretch the material like in a rope.

in compression - load is acting to compress the material like in the supporting column of a building


what is definition, equation and unit for stress

Stress is defined as the force per cross-sectional area

Stress = Force/Area

unit = newtons per metre squared (N m-2)

[force increases, stress increases]


what is definition, equation and unit for strain

Strain is defined as the change in length divided by the original length

[measure of amount of deformation a material has undergone]

Strain = change in length/original length

No units as it is a ratio


what can be used to show the relationship between stress and strain

stress-strain curve


what are the 2 regions of the stress-strain curve and what divides these two regions

elastic region
plastic region

the yield point

[yield strain = strain at the yield point
yield stress = stress at the yield point]


what happens in the elastic region

curve is linear

stress is directly proportional to the strain
[stress doubles, strain doubles]

if the load is removed BEFORE the material hits the yield point, then it will return to its original size and shape once the load is removed


what happens in the plastic region

curve is not linear

bone yields to the applied load - for a small increase in stress the bone deforms by a large amount

once the material is deformed PAST the yield point, even after the load is removed, it WILL NOT return to its original shape and size


at the point of the graph where is shows that the bone has fracture, what is the name given to the strength and strain at this point

Ultimate strength [or stress]

Ultimate strain


what is young's modulus, the equation and the units

the ratio of stress to strain, describes how flexible or stiff a material is

Young’s modulus = stress/strain

unit = newtons per metre squared (N m-2) [same as stress, as strain has no units]


what would a small young's modulus mean and a large one

small = material is flexible, requires small amount of stress to produce a large strain

large = material is stiff, arge amount of stress to produce a small strain


what are the types of loading



what is shear loading

two forces acting in opposite directions tend to cause layers within the material to slip or shear.


what is the ultimate strength of bone in compression, tension and shear

Compression - 200 MN m-2

Tension - 130 MN m-2

Shear - 70 MN m-2

i.e. bone is strongest in compression and weakest in shear


what is an example of a bone fracture due to shear force

intra-articular shearing fracture of the femoral condyles

[fractures caused by shear alone are rare]


what is bending loading and what are types of it

loads are applied to a structure that tend to cause the structure to bend.

2 types
- cantilever
- 3 point bending


what is cantilever bending

one end of the object is fixed and a load is applied to the other end causing the object to bend

i.e. a diving board


what is 3 point bending

three forces are applied to the object

i.e. a seesaw


what happens when a structure is bent

one side is elongated
one side is in compression

between the 2 sides, there is a neutral axis along which no bending occurs


what happens when the femur is loaded vertically

the medial side being compressed and the lateral side elongated

neutral axis runs approximately along the centre of the femur


when a bone is bending what side is more likely to fracture and why

the side being elongated

as bone is stronger in compression than tension


what is a common example of a fracture due to a bending force

"boot top" fracture seen in skiers

due to 3 point bending

As the skier falls forward over the top of the ski boot a force is exerted on the proximal end of the tibia

As the distal end of the tibia is fixed in the boot, the tibia is bent over the top of the rigid ski boot

Tibia #


when does torsion loads occur in bone

when bone is twisted about its longitudinal axis

occur when one end of the bone is fixed and the other end is twisted.


what is the characteristic appear of a fracture due to a torsion load and who commonly gets them

spiral fracture

common in many sports [football, rugby, skiing] occurring when the foot is held in a fixed position and the rest of the body is twisted.


what happens in a structure when a torsional load is applied in relation to stress and strain

stress and strain within the structure are NOT evenly distributed.


how are long bones designed to resist torsional loads efficiently

hollow with strong cortical bone forming the outer layer

hollow structure maximises bone strength-to-weight ratio


what bone is most prone to fractures from torsional loads and why

the tibia
- fracture often found distally

the distal cross-sectional area is smaller than the proximal cross-sectional area and although the amount of bone tissue is the same, the distal part is less able to resist torsional loads and therefore is most liable to fracture


what is combined loading

the presence of more than 1 type of loading

[most fractures occur due to combined loading]


what is the role of muscles in prevention of fractions and give an example

muscle contract to alter the stress distribution within a bone

work to put the bone under compressive load rather than tensile [bone stronger in compression]

the soleus muscle can contract to produce a compressive load on the tibia by pulling downwards on the proximal end of the tibia


why are tired athletes more likely to fracture a bone

their muscles are fatigued and they are therefore unable to control the distribution of stress within their bones.


what is Wolff's law

Bone is laid down where needed and resorbed where not needed.


give examples of how Wolff's law works

[one in bone being laid down, one in bone being reabsorbed]

1) if a person jogs, the bones are subjected to increased levels of stress, bone response by laying down more collagen fibres and mineral salts to strengthen bones

2) inactivity and lack of exercise leads to the resorption of bone tissue = bone atrophy. Seen in bed bound patients.


what is an issue related to Wolff's law seen in ortho

[fracture fixation plate]

a # fixation plate immobilises a broken bone during healing

the plate carries most of the load of the limb

If the plate is not removed soon after the # has healed then the bone will weaken as unstressed bone tissue is resorbed



what is an issue related to Wolff's law seen in ortho


where screws are inserted, bone will strengthen as the bone tissue at these sites will be carrying a greater load than normally



what causes fatigue fractures

repeated application of a load that is smaller than the ultimate strength of the bone

a.k.a stress fracture and march fracture


why is a fatigue fracture also called a march fracture

fatigue # of the 2nd metatarsal of the foot

often suffered by young army recruits after long marches


what can also influence if a bone gets a fatigue fracture

frequency of repetition (the number of repetitions in a given time)

If the repetitions are well spaced then the bone will have time to remodel itself and repair any damage

[fatigue # occur when freq of repetition is too fast for the remodelling process]


what is the relationship of bone formation and bone reabsorption in young adults, children and 35-40 y/o

young adults
- bone formation = bone reabsorption

- bone formation > bone reabsorption

35-40 y/o
- bone reabsorption > bone formation


how do children's bones differ from adult bones

contain a greater proportion of collagen than adult’s bones

this makes them more flexible i.e. less brittle than adult bones

why they get greenstick fractures


what are greenstick fractures

incomplete fracture whereby one side of the bone is bent and the other side is buckled

caused by excessive bending or torsional loads


what is the state of bones in adults between 35-40 y/o

bone tissue begins to be lost as resorption exceeds formation

some thinning of the compact bone tissue

larger reduction in the amount of cancellous bone tissue
- due to the thinning of the longitudinal trabeculae
- and the resorption of some transverse trabeculae

i.e. bones slightly weaker but significantly more brittle


what does it mean when elderly bones become slightly weaker but significantly more brittle

elderly person will be more likely to break their bones then a young adult when subjected to the same loading

[the actual strength of the elderly bone is one slightly reduced]


what happens to out cancellous bone as we age

amount of cancellous bone reduces with ageing


what are the 3 types of articular cartilage

hyaline cartilage
- covers surfaces of bones in synovial joints

elastic cartilage
- more elastic that other cartilage
- forms external ear and epiglottis

- forms symphysis pubis and the intervertebral disc


what is the anatomy of a synovial joint

hyaline cartilage on the articulating ends of bones in synovial joints

held together by muscles and ligaments

enclosed in a joint capsules lined with synovial membrane containing synovial fluid


what is the function of articular cartilage

cushions the bone

provides a smooth, lubricated, bearing surface

shock absorbing
- allows applied loads to be evenly distributed over a large surface area, thus reducing contact stress (stress = force/area) and wear.


what is the composition of articular cartilage

[makes 50-80% dry weight, 10-20% wet weight]

concentrated solution of PROTEOGLYCANS
[ 3-10% of the wet weight of articular cartilage]

- manufacture, secrete and maintain the organic matrix.
- less than 10% of weight

- mainly made up of water
- 68-85% of the wet weight
- 0% of the dry weight


where is PROTEOGLYCANS mainly concentrated

middle portion of the articular cartilage

[less concentrated in the deeper layers adjacent to the bone]


what are the 3 zones of articular cartilage

superficial tangential, middle and deep zone


what are the features of the superficial tangential zone

collagen fibrils are tightly woven into sheets arranged parallel to the articular surface

chondrocytes are oblong with their longitudinal axes aligned parallel to the articular surface.


what are the features of the middle zone

collagen fibrils are arranged more randomly but still broadly parallel to the articular surface

less densely packed to accommodate the high concentration of proteoglycans

chondrocytes are circular and randomly distributed.


what are the features of the deep zone

collagen fibrils are arranged in larger fibre bundles, anchored in the underlying bone tissue
- attach articular cartilage to the bone

chondrocytes are arranged in loose columns


what is below the deep zone

thin layer of calcified cartilage which gradually merges into the underlying subchondral bone


what is the tidemark

interface between the articular cartilage and calcified cartilage beneath it


what is the mechanical behaviour of articular cartilage called and what is features of it

- is time dependant
- response of the material varies according to the length of time that a load is applied and the rate at which a load is applied
- when load is removed it will return to original size and shape [but will take a period of time]


what are the 2 properties of articular cartilage

- increase in strain under a constant stress

stress relaxation
- reduction in stress under a constant strain

[look at the graphs in SD mechanics unit 2 hand in Q's]


what does the viscoelastic properties of articular cartilage provide

gives it its ability to cushion the high loads that occur between the bones in joints


what is the difference between synovial joints and artificial joints in terms of friction

coefficient of friction is very low for synovial joints compared to that in artificial joints.

higher coefficient for artificial joints mean they wear out quicker than synovial joints


what are the 3 main types of lubrication in synovial joints

elastohydrodynamic lubrication, 􏰀 boosted lubrication
boundary lubrication

[at any time, 1 mechanism will be dominant, but all will be present]


when does elastohydrodynamic lubrication occur

when two surfaces, one of which is deformable, are lubricated by a film of fluid as they move relative to one another

fluid of film completely separates the two surfaces so that they do not actually touch

amount of friction is largely dependent upon the fluid and the shape of the gap between the two surfaces.


what are the 2 ways in which 2 surfaces can move relative to each other

hydrodynamic lubrication
- they slide over each other and form a WEDGE OF FLUID

squeeze film lubrication
- move closer together


what happens in hydrodynamic lubrication

two surfaces slide over one another, forming a wedge of fluid

As the surfaces slide, a lifting pressure is generated as the motion drags the viscous lubricant into the narrowing gap between the surfaces.

[what causes a car to skid on a wet road]


what happens in squeeze film lubrication

occurs when two surfaces are forced together

viscous lubricant will NOT instantaneously be squeezed out from the gap between the two surfaces

lubrication therefore acts to cushion and so protect the surfaces

However, if the high loads are maintained the lubricant will eventually be depleted and the two surfaces will come into contact.


what happens in synovial joints in relation to elastohydrodynamic lubrication

2 articular surfaces deform to increase the area over which the load is distributed
- deform as they move over one another as the joint if flexed/extended or forced together if joint is fixed

magnitude of the pressure is decreased and the film remains relatively thick


what happens in boosted lubrication

occurs when 2 lubricated surfaces are forced together for so long that the lubricant is squeezed out

- The permeability of articular cartilage means that water and other small molecules can move freely in and out of the cartilage.

- this makea up the solvent component of the synovial fluid and once the water has been depleted a thick gel is left behind.

- This gel is the basis of boosted lubrication as it can support large loads as well as keeping the two auricular surfaces of the joint apart.


what is boundary lubrication

- If the joint continues to be under a load for a long enough time that boosted lubrication is no longer enough, then boundary lubrication is needed to prevent the two articular surfaces from touching.

- Articular cartilage is coated with LUBRICIN [protein from synovial fluid] which gives it a low shear strength and creates a lower friction co-efficient reducing joint friction.


what is the function of tendons and ligaments

tendons - connect muscle to bones

ligament - connects bone to bone


what is embedded in tendons and ligaments

- they synthesis extracellular matrix and collagen


what is the arrangement of collagen fibres in tendons

arranged completely in parallel
- as they need to withstand large loads in 1 direction only


what is the arrangement of collagen fibres in ligaments

branched and interwoven, are running nearly parallel
- as they need to withstand large loads mainly in one direction, they also need to withstand smaller loads in other directions


how are fibroblasts arranged in tendons and ligaments

elongated along the direction of the collagen fibre.


what properties do tendons and ligaments have


able to withstand large tensile forces and are very flexible.


why do tendons need to be able to withstand large forces and be flexible

need to withstand the large tensile forces exerted by muscles during contraction

need to be flexible enough to bend around the surfaces of the bones as joints move.


why do ligaments need to be able to withstand large forces and be flexible

need to be strong enough to resist the forces that could wrench joints apart

need to be flexible enough to allow joints to move normally


what length can the ACL be elongated to until it fails

7 mm


what is the rule of thumb, for the length a ligament can be elongated to until it fails

Up to a joint displacement of around 4 mm is within the normal physiological range and the ligament remains undamaged.

Beyond 4mm, collagen fibres are progressively ruptured resulting in progressively more pain and joint instability.