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Session 1 Lec 2 Physics of the Skeleton

الكلية كلية طب حمورابي     القسم الكلية ذات القسم الواحد     المرحلة 1
أستاذ المادة نادية حسين صاحب طاهر       11/06/2018 07:47:59

Session 1

Lecture 2: Physics of the Skeleton

Lecture Objectives:
Understand the physics of skeleton, (the function of the bone).
Measurement of bone mineral.

The function of the bones or Skeleton
Bone has at least six functions in the body:
Support 2- Locomotion 3- Protection of various organs.
4- Storage of chemicals 5- Nourishment 6- Sound transmission
The body’s muscles attached to the bones through tendons and ligaments, and the system of bones plus muscles support the body.
Bone joints permit movement of one bone with respect to another.
The destruction of joints in arthritis can seriously limit locomotion.
The skull protects the brain and several of most important sensory organs (eye and ears).
The bones act as chemical bank for storing elements for future use by the body.
For example: constant level of calcium is needed in the blood to maintain normal body functions, if the calcium level falls too low, it activate parathyroid glands to release more parathormone hormone into the blood, and this causes bones to release the needed calcium to the blood and restore it to normal value.
These three bones acts as levers, and provide matching system for converting sound vibration in air to sound vibration in fluid in cochlea. They are the only bones that attain full adult size before birth.
What is bone made of?
There are large percentage of calcium (Ca+) in the body Ca+ has much heavier nucleus than most other elements of the body, it absorbs x-ray much better than surrounding soft tissue. This is the reason that x-rays show bones so well.
Using x-rays scattering indicates that bone mineral crystals are rod shaped of diameter 20–70 A? and length 50–100 A?.
How strong are your bones?
The strain is: (?L /L), Hook’s Law is: F/A
F/A ? ?L/L , F/A=Y ?L/L
Y=FL/A?L Y = Young modulus
When the force increase the bone breaks at stress of about 120 N/mm2
? L=LF/AY (Elongation)



Figure 1: When a piece of bone is placed under increasing tension, it’s
strain increase linearly at first (Hook’s law) and then more rapidly just
before it breaks.

The viscosity of synovial fluid decreases under large sheer stress found in the joint. The good lubricating properties of synovial fluid are thought to be due to the presence hyaluronic acid and mucopolysaccharide of molecular weight 500000 which deform under load.
In running the force 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 about twice the body’s weight. Exceeding maximum compressive strength of bone is not as dangerous as the same force applied over a long period of time.
This property called viscoelasticity.
The local electrical fields may also play a rule. When bone is bent it generates an electrical charge on its surfaces called piezoelectricity, may act as physical stimulus for bone growth and repair.
Experiments with animal bone fractures have shown that bone heals faster if an electrical potential is applied across the broken bone.
The synovial membrane encases the joints and retains the lubricating synovial fluid. The surfaces of joint are articular cartilages.
The lubricating properties of fluid depend on its viscosity, the viscosity of synovial fluid decreases under large shear stress found in the joint.
Measurement of bone minerals in the body:
There are many physical techniques for studying bones in the body. A few years ago, osteoporosis (decrease bone density) was difficult to detect, until the patient got broken hip or crushed vertebra, where it’s too late to use preventive therapy.
The strength of bone depends of the mass of bone mineral present.
The available techniques for studying bones are:
X-Ray image to measure bone mineral, it’s an old technique with several problems like:
X-Ray beam has much different energy, and the absorption of X-Ray by calcium varies rapidly with energy.
There are scattered radiations when it reaches the film.
The film is a poor detector for making quantitative measurements.
Photon absorbiometric technique by (JRC ) author ,1960
The three problems of X-Ray use are eliminated here by using:
Mono energetic X-Ray or gamma ray source.
A narrow beam to minimize scatter.
A scintillation detector that detects all photons.
The determination of bone mineral mass by using:
MB ? log (I?/ I)
Where I? is the initial intensity, MB bone mineral mass
MB=K log (I?/I)
This instrument is a modern clinical bone scanner that uses photon absorption technique.
Since bone is living tissue, it undergoes changes throughout life. A continuous process destroying old bone and building new bone, called bone remodeling is slow work.
Bone remodeling is performed by specialized bone cells, the osteoblast that destroy the bone, and osteoblast build it, we have the equivalent of a new skeleton about every 7 years. Each day osteoblast destroys bone containing about 0.5 gm of calcium (the bone has about 1000 gm of calcium, and osteoblast build new bone using about the same amount of calcium).
While the body is young and growing the osteoblast do more than osteoclasts, but after the body is 35 to 40 years old the activity of osteoclasts is greater than of the osteoblast, resulting in a gradual decrease in bone mass that continues until death.


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