Mechanics: Unit 2 Skeletal tissue (online) Flashcards
What is bone composed of?
osteocytes
non-cellular component: collagen and ground substance
inorganic component: calcium phosphate crystals
Describe collagen fibres
strong and flexible but don’t allow stretching
Describe ground substance
jelly-like cement
weak
What gives bone its hardness and rigidity?
inorganic component
What makes up
a) 25-30%
b) 65-70%
of bone’s dry weight?
a) collagen
b) inorganic component
Name the 2 types of bone
compact
cancellous
Give another name for
a) compact bone
b) cancellous bone
a) cortical
b) spongy
What does compact bone form?
outer layer of bone as it is a dense structure
What does cancellous bone form?
inner part of short, flat and irregular bones
liner inner part of long bones + metaphyses + epiphyses
What is the basic structural unit of
a) compact bone?
b) cancellous bone?
a) haversion system
b) trabecula
Describe cancellous bone
mesh-like
spaces have red bone marrow
How is a haversion system formed?
laminae form cylinders around haversion canal
Where are osteocytes found?
in lacunae ( the spaces between laminae)
What does a haversion canal contain?
blood vessels and nerve fibres
Which structure links lacunae to the canal?
canaliculi
What connects lamellae?
collagen fibres
What structures
a) do
b) don’t
trabeculae have?
a) lamellae, lacunae, canaliculi
b) canals
Which type of bone tissue contains haversian canals and why are they needed?
cortical bone
contain blood vessels which are needed to supply the bone tissue with nutrients
What is a tensile load? + example
a load that acts to stretch a material like in a rope
eg radius and ulna when lifting an object
What is a compressive load? + example
a load that acts to compress a material
eg vertebrae carrying upper body weight
What is stress?
a measure of the concentration of a load
How do we calculate stress?
force/area
What are the units for stress?
n/m2
What is strain?
a measure of the amount of deformation a material has undergone.
How do we calculate strain?
change in length/ original length
What does a strain-stress curve show?
how a material deforms as it is loaded
What are the 2 regions of the strain stress curve? and what separates them?
elastic region and plastic region
separated by yield point
Describe the elastic region of the stress-strain curve
linear ie. stress is proportional to strain
will return to original shape/size once load is removed
Describe the plastic region of the stress-strain curve
small inc in stress = large deformity
will not return to original
What is Young’s modulus?
ratio of stress to strain tells us how flexible or stiff a material is
What does a small young’s modulus indicate?
material is flexible
What is a shearing force?
when two forces act in the opposite direction
Give an example of a shearing force
screw on fracture fixation plate
bone cement by hip prosthesis
Is cortical bone stronger in shear, compression or tension?
compression then tension then shearing
Give an example of a fracture caused by shearing
intra-articular fracture of the femoral condyle
Give the two types of bending force and an example of each
cantilever ie. one side fixed eg. diving board
3 point eg seesaw
describe the sides of a bent structure
one elongated side due to tension
one compressed side
and a neutral axis with no distortion between the two
Along which surface will a fracture most likely occur when an excessive bending load is applied to a long bone?
elongated side
Describe a torsion load
twist along a longitudial axis ie. one side of bone fixed and one twisted
What type of fracture results from an excessive torsion load?
spiral fracture
When considering torsional loads, do hollow or solid bars have the best strength-to-weight ratio?
hollow
How do muscles affect stress distribution of bones?
attach via tendons and change the loads when they contract
Why is it sometimes desirable for muscles to contract even when it is not to produce movement?
muscles contract when bone subject to large tensile load to make the overall load compressive
this is because the bone is stronger in compression
What is Wolff’s law?
bone is laid down where needed and resorbed where it is not
What affect does
a) exercise
b) inactivity
have on bone?
a) increases stress so bone lays down more collagen and minerals and is stronger
b) bone is resorbed causing bone atrophy
How does fracture fixation affect bone remodelling?
plates cause bone immobility and stress shielding ie they take most of the stress, this makes bone weaker
How do screws affect bone remodelling?
increase the stress causing hypertrophy
What is a fatigue/stress fracture?
fracture caused by a repeated application of a load that is smaller than the ultimate strength of the bone over a short period of time
Why are children prone to greenstick fractures but not adults?
their bones contain a greater proportion of collagen than adult bones which gives their bones a greater amount of flexibility
How would you recognise a greenstick fracture?
an incomplete fracture with one side bent and the other buckled
How is cancellous bone tissue affected by ageing?
affected more so than cortical bone
thinning of the longitudinal trabeculae and the resorption of some transverse trabeculae
What age is bone lost quicker than it is made? What qualities does this change make?
35-40
bone only loses a little strength but becomes a lot more brittle
What does cartilage consist of?
cartilage cells, fibres and ground substance
Name the 3 types of cartilage and give an example of each
hyaline eg articular, tip of nose
elastic eg external ear, epiglottis
fibrocartilage eg pubis symphysis, intervertebral discs
In what type of joints is articular cartilage found?
synovial eg hip and knee
What are the two main roles of articular cartilage?
to cushion the bones and to provide a smooth, lubricated, bearing surface.
What consistency is articular cartilage and what function does this provide?
firm but pliable means shock is absorbed evenly to reduce contact stress
What is articular cartilage composed of?
organic matrix of non-cellular material (collagen)
chondrocytes
interstitial fluid (water)
proteoglycans
What forms the organic matrix of non-cellular material in articular cartilage ?
collagen structures in strong fibrils
Where are proteoglycans found in articular cartilage?
middle zone
Where are chondrocytes most densely packed?
deep layer
what function do chondrocytes have?
they maintain, manufacture and secrete the organic matrix
How many layers (or zones) does articular cartilage have?
Name them
3: superficial tangential, middle and deep
How is articular cartilage attached to bone?
the collagen fibrils that extend from the deep zone into the underlying bone tissue
how are collagen fibrils arranged in
a) superficial zone of articular cartilage?
b) deep zone of articular cartilage?
a) parallel sheets
b) large bundles anchored perpendicular to bone
What shape are chondrocytes in
a) superficial layer?
b) middle layer?
c) deep layer?
a) oblong
b) circular
c) circular in columns
What does the tidemark in articular cartilage divide?
deep zone from calcified cartilage
How does articular cartilage behave mechanically
viscoelastic
What is the difference between creep and stress relaxation?
Creep is an increase in strain under a constant stress whilst stress relaxation is a reduction in stress under a constant strain.
Will viscoelastic material return to its original size and shape when a load is removed?
yes but not immediately
Why does stress relaxation occur in articular cartilage?
initially as fluid is forced out there is a large frictional drag that needs a large force to compress the tissue but then once fluid is all in the surface layer which is the most compacted the stress needed to compress it is decreased
Give the 3 types of lubrication
elastohydrodynamic
boosted
boundary
What is elastohydrodynamic lubrication?
when 2 surfaces, one of which is deformable, are lubricated by a film of fluid so that they do not touch
Summarise the differences between hydrodynamic and squeeze film lubrication in terms of when they occur
hydrodynamic: when surfaces slide over each other
squeeze film: when surfaces are forced together
describe boosted lubrication
as two surfaces are forced together the gap between them decreases and the resistance to lubricant is greater than the resistance of small molecules like water flowing through permeations into articular cartilage
so solvent of synovial fluid is lost and leave behind a viscose gel that can withstand the increased load
Which type of lubrication stops direct contact of articulating surfaces when large loads completely deplete the fluid film?
boundary
describe boundary lubrication
molecules in lubricant attach chemically to the surfaces to stop them touching when the fluid is depleted completely
which molecule allows for boundary lubrication in humans?
lubricin
What are tendons and ligaments?
dense and fibrous connective tissues
tendons connect muscles to bones
ligaments connect bone to bone
What are the cells in tendons and ligaments called?
fibroblasts
How are the collagen fibres arranged in tendons and ligaments? why?
In tendons collagen fibres are aligned in parallel and in ligaments collagen fibres are aligned nearly in parallel.
tendons have one large load in one direction but ligaments have this and also smaller loads in other directions
What is the basic mechanical behaviour of tendons and ligaments?
viscoelastic
What tensile properties must
a) a ligament
b) a tendon
withstand?
a) the forces that could split joints
b) muscle contraction
What happens when a tensile load is applied to move tibia and femur apart?
ACL elongates up to 7mm before it fails
If the compressive load on a bone is doubled, how will the stress and strain be affected?
the stress doubles
the strain doubles if it is in the linear elastic range
What happens to the distribution of pressure in elastohydrodynamic lubrication when we consider the surface as deformable not rigid?
distribution of pressure increases so the magnitude decreases and the film of fluid remains relatively thick
