Biomechanics Unit 2 Flashcards
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
tendon
ligament
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
cancellous
- 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
- i.e. HAVERSIAN 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
TRABECULA
- 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
i.e. ELASTIC BEHAVIOUR
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
i.e. PLASTIC BEHAVIOUR
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
tension compression bending shear torsion
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
called = STRESS SHIELDING
what is an issue related to Wolff’s law seen in ortho
[screws]
where screws are inserted, bone will strengthen as the bone tissue at these sites will be carrying a greater load than normally
= BONE HYPERTROPHY
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
children
- 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
fibrocartilage
- 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
strong, fine COLLAGEN FIBRILS
[makes 50-80% dry weight, 10-20% wet weight]
concentrated solution of PROTEOGLYCANS
[ 3-10% of the wet weight of articular cartilage]
CHONDROCYTES
- manufacture, secrete and maintain the organic matrix.
- less than 10% of weight
INTERSTITSAL FLUID
- 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
viscoelastic
- 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
creep
- 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
fibroblasts
- 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
viscoelastic
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.
