BM unit 2

Question 1

4 types of tissue

Answer 1

epithelial tissue
connective tissue
muscle tissue
nervous tissue

MEN C

Question 2

what is the most abundant and widely distributed tissue in the body and what does it do

Answer 2

connective tissue - it supports and protects the body and its organs, connects and holds them together and also transports substances around the body

Question 3

list 4 types of connective tissue

Answer 3

bone tissue, articular cartilage, tendon and ligament

Question 4

4 diff types of bones

Answer 4

long bones, short bones, flat bones and irregular bones

Question 5

what is bone tissue composed of

Answer 5

osteocytes, a non-cellular component and an inorganic component

Question 6

what is the non cellular organic component of bone tissue

Answer 6

consists of strong collagen fibres (95%) embedded in a jelly like matrix called ground substance. collagen fibres are flexible but resist stretching
non cellular component of bone makes up 25-30% dry weight of bone

Question 7

what is the inorganic component of bone tissue

Answer 7

consists mainly of the minerals calcium and phosphate in the form of crystals of calcium phosphate which are deposited in the matrix. this component gives bone its hardness and rigidity. it makes up 65-70% dry weight of bone

Question 8

2 types of bone tissue

Answer 8

compact and cancellous

Question 9

what is compact bone

Answer 9

it forms the outer layer of bones and has a dense structure

Question 10

what is cancellous bone

Answer 10

forms the inner part of short, flat and irregular bones
in long bones it lines the surfaces and makes up the greater part of the metaphyses and epiphyses. Cancellous bone has a mesh like structure (also has the name spongy bone). the spaces between the mesh contain red bone marrow.
the cells in cancellous bone tend to align themselves in the directions that will best support the load

Question 11

describe the pattern of cancellous bone on the femur

Answer 11

thin layer over the greater trochanter, but much thicker down the shaft - reflects the functional requirements

Question 12

describe compact bone

Answer 12

the basic structural unit is the haversian system. these are arranged longitudinally in columns. in these units the bone tissue is arranged in layers called lamellae forming concentric cylinders around a central canal. the small central canal, the haversian canal, contains blood vessels and nerve fibres. between each lamellae there are small cavities, called lacunae, that contain osteocytes. each osteocyte is linked to haversian canal and other lacunae by minute channels, called canaliculi, along which nutrients are carried from the blood vessels. collagen fibres interconnect the layers of lamellae within the harversian system. each haversian system is surrounded by a cement like ground substance. this is the weakest part of the bones microstructure, probably because it contains no collagen fibres

(basically loads of haversian cylinders, within these cylinders are loads of lamellae cylinders - in middle of all these lamellae cylinders is a haversian canal. collagen fibres run in seperate directions between the lamellae layers - gives torsional strength. all the haversian cylinders make up the compact bone (all run parallel to the long bone). spongy bone usually runs in the part closest to the middle part of the bone

Question 13

describe cancellous bone

Answer 13

basic structural unit is the trabecula which are arranged in a latticework of branching sheets and columns. similar to haversian system where layers of lamellae have lacunae containing osteocytes connected by canaliculi. main difference is that trabeculae don’t contain haversian canals as instead blood vessels pass through the marrow filled spaces between latticework of trabeculae supplying nutrients to the osteocytes through canaliculi.

Question 14

what are mechanical properties

Answer 14

the way a material reacts when its loaded

Question 15

what is tension

Answer 15

a load acting to stretch

Question 16

what is compression

Answer 16

a load acting to compress

Question 17

what do stress and strain help us do

Answer 17

describe the behaviour of a material under tensile and compressive loads

Question 18

what is stress

Answer 18

the force per cross sectional area
stress = force/area
SI units = N m-2

Question 19

what is strain

Answer 19

the change in length divided by the original length
strain = change in length/original length
SI units = none as its a length/length

Question 20

describe a typical stress strain curve

Answer 20

as stress increases so does strain - as a material eg bone is increasingly deformed it becomes increasingly harder to deform it further
divided into 2 regions - elastic and plastic region. the division between the two regions is marked as the yield point - at this point the amount of strain is called the yield strain and the amount of stress is called the yield stress

in the elastic region - the curve is linear - stress is directly proportional to strain. provided the specimen isn’t deformed past its yield point by a load then it will return to its original size and shape once the load is removed. this is termed elastic behaviour

in the plastic region - the curve is not linear. the bone yields to the applied load - for a small increase in stress the bone deforms a lot. when bone is deformed past its yield point it wont completely recover to its original size and shape when the load is removed - it is permanently deformed. this is termed plastic behaviour.

at the point of ultimate strain the bone will fracture - at this point the stress and strain are called ultimate stress and ultimate strain

Question 21

equation to link stress and strain in the linear part of teh curve

Answer 21

stress = strain x constant
this constant is youngs modulus 
rearranging:
youngs modulus = stress/strain 
SI unit = N

Question 22

what does youngs modulus describe

Answer 22

how flexible or stiff a material is.

a small youngs modulus means only a small amount of stress is needed to produce a large strain ie it is flexible eg rubber

large YM requires a lot of stress to produce a small strain ie the material is stiff eg diamond

Question 23

what is shear loading

Answer 23

two forces acting in opposite directions - cause layers of a material to slip/shear eh screw being sheared by a fracture fixation plate and bone cement being sheared by hip prosthesis and bone

Question 24

is human cortical bone weaker in compression or tension or shear

Answer 24

shear
strongest in compression
but despite the low strength of bone in shear, fractures caused by shearing alone are rare

Question 25

two common types of bending

Answer 25

cantilever bending (one end of the object are fixed and a load is applied to the other end) and three point bending (three forces applied to the object)

Question 26

what happens when a structure is bent and describe how this relates to the femur during standing

Answer 26

one side elongates and the other is in compression
between the two sides there is a neutral axis - no compression or elongation - no deformation (in a symmetrical structure this is at the geometric centre)

the femur when standing experiences bending - medial side is compressed and the lateral side is elongated.

remember that bone is stronger in compression than tension so when a bone is subject to a large load it will tend to fracture on the elongated surface which is under tension

Question 27

give an example of a bending fracture

Answer 27

ski boot fracture - three point bending
force is exerted on prox end of tibia, the distal end of tibia is fixed in a boot so the top of the tibia is forced over the boot causing the tibia to bend. if bending is large enough then tibia will fracture

Question 28

what is a torsional load on a bone

how are bones structured to resist torsional loads

Answer 28

when a bone is twisted about its longitudinal axis. often occur when one end is fixed and the other is twisted. if a fracture results it will have a characteristic spiral appearance. torsional fractures of the tibia are v common

when a structure is subject to torsional load the stress and strain within a structure are not evenly distributed. the centre is not distorted - there is a neutral axis running through the centre - and the outer surface is the most distorted. stress and strain are greatest at the outer surface - so if the torsional load is increased then a fracture will start at the outer surface.

long bones are designed to resist torsional loads - they are hollow with strong cortical bone forming the outer layer. if the same quantity of bone made a solid one it would break easier as smaller diameter so less able to resist torsion. and if more quantity of bone was used to make same diameter bone but solid - it would be too heavy and only have a small increase in resistance to torsional loads. the hollow structure of bones maximises their strength to weight ratio. (frames of mountain bikes are also constructed like this)

Question 29

what are fractures of the tibia often cause by

Answer 29

torsional loads - usually occur distally. this is because the distal cross section is smaller than the proximal cross section. although the amount of bone tissue present is the same the distal part is less able to resist it so more liable to a fracture.

Question 30

what is the presence of more than one type of load called and give an example of a bone that this happens to

Answer 30

combined loading
it results from irregular geometry of bones, and the combined actions of gravitational forces, muscle forces and ligament forces. eg irregular geometry of femur means that when a compressive load is applied to the head of femur combined loading occurs, resulting in compressive and bending loads. more fractures usually result from combined loads. only rarely can a fracture be attributed to one type of loading

Question 31

in terms of loading what happens when a muscle contracts and give an example

Answer 31

it will load the bone in addition to any external loadings such as those acting at the joints. the load applied by the muscle alters the distribution of stress in the bone. muscles will often contract not to produce movement but to alter the stress distribution. as bones are stronger in compression than tension, if a muscle contracts producing a compressive load it can eliminate any tensile loading and produce an overall compressive load eg soleus muscle pulls down on the prox end of the tibia in the three point bending ski boot scenario - causes more compression on anterior surface and eliminates tensile stress which is good as a fracture is less likely in compression than in tension.

Question 32

why are tired athletes more likely to fracture a bone than a fresh athlete

Answer 32

muscles tired so unable to control the stress distribution in their bones

Question 33

what is Wolff’s law

give example of loss and gain

Answer 33

bone is laid down and reabsorbed where not needed (compact and cancellous bone are continually lost and gained in response to the amount of stress placed on a bone) - bone remodelling

eg in jogging - bones are subject to increased levels of stress and the bones respond to this by laying down more collagen fibres and mineral salts to strengthen the bones
lack of exercise - leads to resorption of bone tissue (bone atrophy). once someone begins to reuse their legs again they are liable to fracture their lower limb bones easily

Question 34

how is bone remodelling a problem in orthopaedics

Answer 34

in fracture fixation a plate is fixed to the broken bone to immobilise it during healing. During healing the plate will carry most of the load on the limb. if the plate isnt removed soon after the fracture has healed then bone will weaken as unstressed bone is resorbed. this is termed stress shielding.

At points in the bone where screws are placed the opposite will happen - bone will strengthen as these sites are carrying a greater load than normal - this is bone hypertrophy

Question 35

what are fatigue fractures

Answer 35

fracture caused by a small load that is applied repeatedly. A fracture from the repeated application load that is smaller than the ultimate strength of the bone is called a fatigue fracture (stress fracture and march fractures - second metatarsal of the foot suffered by young army recruits after long marches)
obvs the smaller the load the more repetitions needed

also if the repetitions are well spaced then bone has time to remodel and repair itself and any damage. therefore a fatigue fracture only results when the frequency of repetitions is too fast for the remodelling process. for this reason they are only sustained during a continuous period of strenuous physical activity as muscles become fatigued and are unable to neutralise the stresses exerted on the bones eg long distance runners can suffer from fractures of metatarsals, tibia, fem neck, pubis. gymnasts can suffer from fractures of the vertebrae

Question 36

outline the changes in bone as we age

Answer 36

in young adults bone formation and resorption is balanced so total amount of bone doesn’t change.

in kids bone formation outweighs resorption as they grow and develop. kids bones also have a greater proportion of collagen than adult bones which gives then greater flexibility - so they are less brittle than adult bones. for this reason they can suffer from greenstick fractures (an incomplete fracture where one side of the bone is bend and the other side is buckled. they are usually caused by excessive bending or torsional loads)

between 35-40 bone tissue begins to be lost as resorption exceeds formation. there is thinning of compact bone tissue and a larger reduction in the amount of cancellous bone tissue due to thinning of the longitudinal trabeculae and the resorption of the transverse trabeculae. this process produces bones that are slightly weaker but significantly more brittle. so the elderly are more likely to suffer from fractures than a young adult subject to the same loading. the stress strain curve for the elderly terminates at about half the strain of the young bone, but the overall strength of the elderly bone is only slightly reduced

Question 37

what is cartilage

Answer 37

a connective tissue containing cartilage cells, fibres and ground substance. divided into hyaline, elastic and fibrocartilage

Question 38

what is hyaline cartilage

Answer 38

covers the articular surfaces of bones in synovial joints and forms the tip of the nose.

Question 39

what is elastic cartilage

Answer 39

more elastic than other types, forms the external ear and the epiglottis which covers the opening to the resp tract when swallowing

Question 40

what is fibrocartilage

Answer 40

forms the symphysis pubis and the intervertebral discs

Question 41

what is articular cartilage

Answer 41

form of hyaline cartilage that is found on articulating ends of bones in synovial joints. synovial joints are v mobile joints eg hip, knee and are help together by muscles and ligaments, and are enclosed in a capsule lined with synovial membrane containing synovial fluid. it is adapted to withstand large loads. it cushions the bones while also providing a smooth, lubricated bearing surface with minimal wear. it is not hard and had a film pliable consistency - this gives it its shock absorbing properties and allows loads to be evenly distributed over a large SA, thus reducing contact stress and wear.

Question 42

composition of artic cartilage

Answer 42

made up of an organic matrix of non cellular material interspersed with cells and fluid . this matrix is mainly made up of collagen - in artic cartilage it is structured into strong, fine collagen fibrils. makes up 50-80% dry weight and 10-20% wet weight.

collagen fibrils are enmeshed in a concentrated solution of proteoglycans - large protein based molecules. important to the mechanical properties. make up 3-10% wet weight. they are most concentrated in the middle portion

the sparsely distributed cells in artic cartilage are called condrocytes. account for <10% of the tissues volume. they are more densely packed in deeper layers adjacent to the bone. chondrocytes manufacture, secrete and maintain the organic matrix.

the interstitial fluid that occupies the spaces in the matrix is mainly made up of water - 65-85% wet weight.

structurally artic cartilage can be split into 3 zones - superficial tangential, middle and deep

superficial tangential - collagen fibres are tightly woven into sheets arranged parallel to the articular surface and the chondrocytes are oblong with their longitudinal axes parallel to the artic surface.

middle zone - collagen fibrils are arranged more randomly but still broadly parallel to artic surface. less densely packed to accommodate the high conc of proeoglycans, and then chondrocytes are circular and randomly distributed.

deep zone - collagen fibrils are arranged in larger fibre bundles that are anchored in the underlying bone tissue, attaching the artic cartilage to the bone. chondrocytes are arranged in loose columns aligned perpendicular to the line dividing the artic cartilage and the underlying bone.

below the deep zone there is a thin layer of calcified cartilage which gradually merges into the underlying subchondral bone. the interface between the artic cartilage and the calcified cartilage beneath it is called the tidemark.

thickness of artic cartilage varies within joints.

Question 43

describe the basic mechanical behaviour of articular cartilage

Answer 43

viscoelastic - time dependent. the 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 a load is removed from a viscoelastic material it will return to its original size and shape, however the response is not immediate

two characteristics of viscoelastic material are creep and stress relaxation.

Question 44

what is creep

Answer 44

creep occurs when viscoelastic material is subject to a constant load. when the load is first applied to the material it will deform quite rapidly, followed by a slowly (creeping) increasing deformation

thinking about artic cartilage - during rapid deformation fluid is rapidly forced out of the artic cartilage. fluid diminishes so the rate of expulsion decreases until at equilibrium fluid flow ceases completely and the applied load is borne entirely on the solid matrix. at equilibrium the majority of the fluid still remains in the artic cartilage.

Question 45

what is stress relaxation

Answer 45

occurs in viscoelastic material when it is kept at constant deformation. the stress is reduced over time as the material is maintained at constant strain. in other words if a material is deformed to a certain degree the load required to maintain that deformation decreases with time.

in artic cartilage - during initial deformation fluid is forced out as the surface layers are compacted. due to large frictional drag associated with the flow of fluid through the solid matrix, large loads are needed to compress the material. during the stress relaxation phase the stress required to maintain the deformation is reduced as fluid no longer is being forced out and the fluid within the tissue is redistributed from the least compacted deeper layers to the most compacted surface layers.
once the load is removed the artic cartilage will regain original size and shape as fluid seeps back in.

its the viscoelastic properties of artic cartilage that gives it its ability to cushion the high loads that occur between the bones in the joints, effectively smoothing out any high peaks thus reducing the potential damage that they may cause.

Question 46

describe synovial joints and their lubrication

Answer 46

they are virtually friction free. the coefficient of friction is v low for synovial joints compared to that in artificial joints. high coefficient in artic prevents them from moving as freely as healthy synovial joints but also means they wear more quickly. low in synovial due to articular cartilage and synovial fluid fluid lubricates surface of the artic cartilage reducing contact between the two surfaces and reducing the amount of friction and wear. the way in which this lubrication is brought about is dependent on a number of factors but the two most important are the magnitude of the load and the length of time the load is maintained.

3 types of lubrication - elastohydrodynamic lubrication, boosted lubrication and boundary lubrication

Question 47

elastohydrodynaic lubrication in synovial joints

Answer 47

occurs when 2 surfaces, one that is deformable, are lubricated by a film of fluid as they move relative to one another. the fluid completely separates the 2 surfaces so they dont touch. the amount of friction is therefore largely dependent on the fluid and the gap between the 2 surfaces

the surfaces can either slide over each other - hydrodynamic lubrication, or move closer together - squeeze film lubrication

hydrodynamic lubrication - two surfaces slide over each other causing 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 surfaces.

squeeze film - two surfaces are forced together. the viscus lubricant will not instantaneously be squeezed out from the gap between . it therefore acts to cushion and protect the surfaces. but if the high loads are maintained the lubricant will eventually be depleted and the two surfaces will come into contact. when one or both of the surfaces are relatively soft then deformation os the surfaces will occur which will increase the area over which the load is distributed. this occurs in synovial joints. the soft articular surfaces deform as they are moved over one another as the joint is flexed/extended or as they are forced together as the joint is fixed.

for the rigid surfaces the pressure distribution is small and high pressures are present. for softer surfaces the pressure distribution is increased over the deformed surfaces. as a consequence the magnitude of the pressure is decreased and the film remains relatively thick - this is elastohydrodynamic lubrication

Question 48

boosted lubrication

Answer 48

2 lubricated surfaces forced together over a period of time eventually lubricant will be completely depleted, it will be squeezed out. this problem is reduced in synovial joints by boosted lubrication. it relies on the permeability of articular cartilage. the surface of artic cartilage is only permeable to molecules below a certain size, such as water or other small molecules. as the size of the gap between the two surfaces marrows the resistance to sideways flow of lubricant becomes greater than the resistance of flow of the small molecules in artic cartilage. these small molecules include water molecules that make up the solvent part of synovial fluid. with the solvent part removed a thick viscous gel is left which acts as an enriched lubricant which is capable of supporting large loads and thus acts to keep the two articular surfaces apart.

Question 49

boundary lubrication

Answer 49

if the loads are large enough and sustained for long enough to deplete the fluid completely then he articular surfaces are still prevented from direct contact by boundary lubrication. the fluid between the 2 surfaces isnt thick enough to prevent contact between the two surfaces - so lubricant molecules attach themselves chemically onto the surfaces creating a boundary layer. this layer has a low shear strength and therefore offers a lower friction than the bare surfaces. in synovial joints the surface of artic cartilage is coated with protein from the synovial fluid called lubricin. this is v effective in reducing joint friction when loads are sustained for long enough to deplete the fluid film

Question 50

what are tendons and ligaments

Answer 50

connective tissues
tendons connect muscle to bone
ligaments connect bone to bone

Question 51

composition and structure of tendons and ligaments

Answer 51

dense fibrous connective tissues. they contain relatively few cells called fibroblasts which are embedded in a matrix of collagen fibres.

in tendons - collagen fibres are arranged completely in parallel as they need to withstand loads in one direction only

in ligaments - collagen fibres are not arranged completely in parallel and in some ligaments eg cruciate ligaments the fibres are branched and interwoven. this is because need to withstand loads mainly in one direction but also smaller loads in other directions. the fibroblasts are elongated along the direction of the collagen fibres.

Question 52

basic mechanical properties of ligs and tendons

Answer 52

both are viscoelastic
both can withstand large tensile forces and are v flexible.

tendons need to withstand large tensile forces exerted by muscles during contractions and be flexible enough to bend around surfaces of the bones as joints move.

ligaments must be strong enough to resist forces that could wrench joints apart and flexible enough to allow joints to move normally.

in contrast artic cartilage must withstand high compressive loads as the bones in the joints are forced together

Question 53

use ACL to describe the properties of ligaments

Answer 53

positioned in centre of the knee with one end joined to the femur an done to the tibia. if a tensile load is applied to pull the femur and tibia apart, the ACL may elongate by around 7mm before it fails completely.

up to a joint displacement of 4mm is within the normal range and the ligament remains undamaged. beyond 4mm the collagen fibres are progressively ruptured resulting in progressively ruptured resulting in progressively more pain and joint instability