Bone Structure and Elastic Properties Flashcards
6 functions of bones
support locomotions - joints allow motion protection of organs storage of chemicals e.g. calcium nourishment - teeth break food sound transmission - ossicles in middle ear
osteocytes
cells which keep the bone in a healthy condition
aseptic necrosis
If these cells die (perhaps due to a poor blood supply), the bone will die and lose strength
bone remodelling
a continuous process of destroying old bone and building new bone
cells involved in bone remodelling
osteoclasts
osteoblasts
osteoclast function
destroy bone containing about 0.5 grams of calcium per day.
osteoblast function
build new bone using about the same amount of calcium.
below 35-40 yrs old osteoblast and osteoclast ratio
Below the age of 35-40 years old, osteoblast activity dominates over osteoclast activity
Skeleton grows
osteoclast and osteoblast ratio after 40
more osteoclast than osteoblast activity
bone mass decreases gradually until death
collagen
Organic material. Constitutes 40% of the weight of bone and 60% of the volume.
bone material
Inorganic material. Constitutes 60% of the weight of the bone and 40% of the volume.
bone mineral crystals have what kind of shape
rod shaped
5 categories of bones by size
flat long cylindrical irregular others
trabecular bone location
at the end of long bones in the centre
compact bone location
around the edge/ around the centre of long bones
elastic properties of biological materials
all materials undergo deformation (change in shape) when stress is added
what does deformation depend on
the FORCE PER UNIT AREA, not on the total force
stress
in a material with cross-sectional area A subjected to a force F is given by
force /area (N/m^2)
how is the stress in an object resisted
The stress in any object is resisted by intermolecular forces within the object
3 types of stresses applied
compression
tension
shear stress
strain/ fractional change in length
when a material of length (L) is subjected to a compressional or tensional stress - a change of length (omegaL) occurs
=omegaL/L
how are stress and strain related
when low volumes of stress is added the strain is directly proportional - has a linear limit
after limit elastic limit - it is no longer linear stretch elastic permanently
after maximum stress before it breaks or deforms
elastic deformation
from 0 to point B
plastic deformation
point B to D
youngs modulus
(E) is defined as the ratio of Stress to Strain, i.e.
stress/strain
E = s/e (N m^-2)
youngs modulus is a measure of …
stiffness or rigidity
slope of stress/strain
What does Young’s Modulus tell us?
lower slope is more elastic deforms easily
the smaller the value of young’s modulus the more elastic a material is
Is Young’s Modulus the same for different types of Stress??
no
anisotropy
if a stress applied in a particular direction will the bone act the same if that same stress is applied in a different direction
materials can be
ductile - more elastic
brittle
Bones and joints which are heavily used for prolonged periods may suffer from …
fatigue
ultimate tensile strength
12 x 10^7 Nm^-2
ultimate compressive strength
17x107 Nm-2
bone tension
1.6 x 10^10 N m-2
bone compression
0.9 x 1010 N m-2
greenstick fracture results from
direct trauma
why do greenstick fracture arises
fact that bone is more elastic and ‘stronger’ under compression than under tension (smaller Young’s modulus value, greater compressive strength).
how do intermolecular forces try to restore its original shape under tension
is under tension and the intermolecular forces are trying to ‘pull’ the surface back into its original shape
how do intermolecular forces try to restore its original shape under compression
under compression and the intermolecular forces are trying to ‘push’ the surface back into its original shape.
neutral surface
a surface in between which undergoes NO change in length
where does the greatest intermolecular resisting forces occur
occur at the surfaces which undergo greatest tension or compression
further you from neutral surface
the bigger contribution you will get from restoring torque
how do the resisting forces change the further they are from the neutral surface
Since the resisting forces increase the further they are from the neutral surface,
neutral surface of long cylindrical bone
centre
what kind of bone will have more resistance to bending a solid bone or hollow
hollow will have more resistance
goal of bones in terms of physical properties
maximum resistance and minimum weight
why do other connective tissue e.g. tendon, skin no display a linear behaviour at stress below their elastic limit
presence of collagen in these tissues
what does the spine support
offering maximum load-bearing support.
what does the spine support
the head, upper limbs and thoracic cage during movement and weight bearing activities
what type of stress can the vertebrae resist
compressional, tensional and torsional stresses associated with various movements.
how come the vertebrae is able to resist certain stresses
is due to the orientation of the trabeculae within the vertebrae.
when does crushing of vertebral disc occur
common when the disc receives excessive loading that the ‘intervertebral disc’ can bulge, causing a ‘squeeze’ on the spinal cord, or in severe cases rupture.
what type of stress is vertical trabeculae resistant to
compressional load
what type of stress is horizontal trabeculae resistant to
tensional tress
