Chapter 3: Physics of the skeleton
Bone has at least six functions in the body
1. Support
2. Locomotion
3. Protection
4. Storage of chemicals
5. Nourishment
6. Sound transmission (in the middle of ear)
• The body's muscles attached to the bone through
tendons and ligaments (the system of bones plus
muscles support the body)
• Bone joints permit movement of one bone with
respect to another.
• The skull protects the brain and several of the
most important sensory organs (eye and ears),
the ribs form protective cage for the heart and
lungs
• Bones acts as chemical bank for storing element
for future use.
Ex. Minimum calcium is needed in the blood if the
level falls too low calcium causes parathyroid glands
to release more para hormone into the blood and this
causes the bones to release the needed Ca.
The teeth are specialized bones can cut food and serve
nourishment.
Three smallest bones of the body are oscillate in
middle ear to provide a matching system for
converting sound vibration in air to sound vibration
in fluid in the cochlea. They are the only bones that
attain full adult size before birth.
What is the bone made of?
Bone consists of two different materials plus water,
collagen(organic fraction, 40% of the bone weight,
60% of its volume and bone mineral (inorganic 60%
of its weight, 40% its volume).
There are a large percentage of calicium in the body,
it has much heavier nucleus than most elements of the
body, it absorbs x-ray much better than surrounding
soft tissue.
This the reason x-ray show bones so well, using x-ray
scattering indicates that bone mineral crystals are rod
shaped of diameter 20-70Aº and length 50-100Aº
• How strong are your bones?
Let us look at how the bones have developed to meet
our needs, the approximately 200 bones sort into
various piles according to their shapes, so there are
five piles:
1. Pile of flat, plate-like bones (shoulder blade,
some of skull bones
2. Long hollow bones (arms, legs, fingers)
3. Cylindrical bones form the spine (vertebrae)
4. Irregular bones the wrist and ankle.
5. Ribs that don't belong to the above piles
Trabecular bone is weaker than compact due to the
reduced amount of bone in a given volume.
• Bone is a living tissue and has a blood supply as
well as nerves. It undergoes change destroying
old bone and building a new one. This
continuous process is called bone remodeling. Is
performed by specialized bone cells. Osteoclasts
destroy bone old cell, osteoblasts build new one.
• Compared to many body processes, bone
remodeling is slow work, we have the equivalent
of the new skeleton every seven years. Each day
osteoclast destroy bone containing of 0.5 g of Ca
and osteoblast build the same amount , while the
body is young and growing, osteoblast do more
than osteoclast, but after 35-40 years old the
activity of osteoclast is greater than osteoblast
resulting in a gradual decrease in bone mass that
continuous until death. This decrease is faster in
women than in men and lead to weak bones in
older women this called osteoporosis result in
spontaneous fractures especially in the spine and
hips.
• All material change in length when placed under
tension or compression.
Stress=F/A N/m2
120
100
80
60
40
20
0
0.005
0.01
0.015
Strain ∆L/L
• When a piece of bone is under tension its strain
increases linearly at first (Hooke's law) and then
rapidly before it break at about 120 N/m2
• The ratio of stress to strain in linear portion is
Young's modulus Y that is :
• Y= FL / A∆L
• Elongation ∆L= FL/AY
The viscosity of synovial fluid decreases under large
shear stress found in the joint. The good lubricating
properties of synovial fluid are thought to be due to
the
presence
of
hyaluronic
acid
and
mucopolysaccharide that deform under load.
Example:
a leg has a 1.2 m shaft of bone with an average crosssectional area of 3 cm2 what is the amount of
shortening when body weight 0f 700 N is supported
on the leg? Y=1.8x1010N/m2
• In running, the forces on the hip bone when the
heel strikes the ground may be four times the
body's weight. In normal walking the force on
the hip are twice the body's weight. Exceeding
maximum compressive strength of bone is not as
dangerous as the same force applied over a
longer period of time this property called
viscoelasticity.
• The local electrical fields may play a role when
bone is bent it generates an electrical charge on
its surface called (piezoelectricity) may be the
physical stimulus for bone growth and repair.
Experiments with animal bone fracture have
shown that bone heals faster if an electrical
potential is applied across the break bone.
• The synovial membrane encases the joint and
retains the lubricating fluid. The surface of joints
is articular cartilage, a smooth, somewhat
rubbery materials that are attached to the bone.
• The lubricating properties depend on fluid
viscosity so it decreases under large shear stress
in the joint.
• Measurement of bone mineral
• A few years ago, osteoporosis was difficult to
detect until a patient appeared with a broken hip
or crushed vertebra. At that time it was too late
to use preventive therapy. The strength of bone
depends on the mass of the bone mineral present.
The physical techniques for studying bones are:
• 1. X-ray image: to measure the bone mineral, its
an old one. There are some problems of using xray, these are: x-ray beam has different energies
and the absorption of x-ray by Ca varies rapidly
with energy, scattered radiation when it reaches
the film, the film is a poor detector for making
quantitative measurements.
The three problems are eliminated by using
Monoenergetic x-ray or gamma ray source, a
narrow beam to minimize scatter, a scintillation
detector that detects all photons
1.photon absorptiometry technique:
the determination of bone mineral mass by using
MB=K Log (Iº/I)
MB: bone mineral:
Iº: initial intensity
I : final intensity
K: constant
2. Activation technique:
Take the fact that nearly all of calcium in the
body is in the bones. The whole body is
irradiated with energetic neutrons that convert a
small amount of calcium and some other
elements into radioactive forms that given off
gamma rays, and the emitted gamma rays then
detected and counted, the gamma rays from
radioactive calcium can be identified by their
unique energy.