Physics Of Skeleton Lecture 3 PDF

The third Lecture Physics of Skeleton Function of bones:- 1. Supporting: The system of bones and muscles support the body. The muscles are attached to the bones through tendons and ligaments Eg. Muscles and bones of legs and vertebrae  2. Locomotion: Eg....

The third Lecture Physics of Skeleton Function of bones:- 1. Supporting: The system of bones and muscles support the body. The muscles are attached to the bones through tendons and ligaments Eg. Muscles and bones of legs and vertebrae  2. Locomotion: Eg. Bone joints which permit the movement of one bone with respect to another. These hinges or articulations are very important for walking as well as for many other motions of the body. 3- Protection: protection of delicate body parts is an important function of some of the bones. The skull, which protects the brain and several of the most important sensory organs (eye and ears), is extremely strong container while the ribs form a protective cage for the heart and lungs. 4-.Storage of chemicals: The bones act as chemical bank for storing elements for future use of the body, then the bones can withdraw these chemicals as needed, for example if the level of Ca fall too low in the blood, a calcium sensor causes the  parathyroid glands to release more parathormone into the blood, and this in turn causes the bones to release the needed Ca. 5- Nourishment: The teeth are specialized bones that can cut food. Eg. Incisors, canines 6-. Sound Transmission: The three smallest bones in the body are the Ossicles. -They act as levers which provide an impedance matching. -They form a system for converting sound vibration in the air into sound vibration in the fluid of the cochlea 7- Red blood cells generation: The stem cells in the bone marrow generate the RBCs Bone Composition: The Bone is a living tissue which has blood supply as well as nerves with a special kind of cells distributed through the bone tissue; these cells are called “Osteocytes” The Octeocytes cells maintain the bone in a healthy condition. These cells make up about 2% of the volume of the bone, if these cells die (e.g. due to poor blood supply), the bone dies and it loses some of its strength. A series hip problem is caused by a condition called aseptic necrosis in which the bone cells in the hip die due to the lack of blood. Bones consist of two quit different materials plus water: 1- Collagen: it is the major organic fraction of the solid bone. It is quite flexible, it has a fair amount of tensile strength so that it can bend easily if it’s compressed. It forms about 40% of the weight of the solid bone and 60% of its volume 2- Bone Minerals: it is the inorganic part of the solid bone. It is believed to be made up of caliciumhydroxyapatite [Ca10(Po4)6(OH)2]. It is very fragile and forms about 60% of the weight of the bone and 40% of its volume. The bone minerals crystals are rod shaped with diameters of 20 to 70 Aₒ. Because of the small size of the crystals, bone minerals have a very large surface area. Around each crystal is a layer of water containing in a solution many chemicals needed by the body. The large area exposed bone minerals permit the bone to interact rapidly with chemicals in the blood and other body fluids. As described above bone is composed of small hard bone mineral crystals attached to a soft flexible collagen matrix. These components have vastly different mechanical properties. This combination provides a material that is strong as granite in compression and 25 times stronger than granite under tension. Bone remodeling: Since the bone is a living tissue it undergoes a change through the life, a continuous process of destroying an old bone and building a new one is called “bone remodeling” Bone remodeling is performed by specialized bone cells, they are: 1- Osteoclasts: They are specialized cells which responsible for destroying the bone. 2- Osteoblasts: They are specialized cells which responsible for building a new bone We have the equivalent of a new skeleton about every seven years, each day the osteoclasts destroy bone containing about (0.5 gm) of calcium, osteoblasts build a new bone by using about the same amount of calcium. While the body is growing and young the osteoblasts do more than osteoclasts, but after the body is 35 to 40 years old the activity of osteoclasts is greater than that of osteoblasts, resulting in a gradual decrease in the bone mass that continuous until death. This decrease is apparently faster in women than in men and leads to serious problems of weak bones in older women. This condition called Osteoporosis. It is one of the bone diseases which is produced by the reduction of the amount of Ca as a result: - The mass of the bone decrease - The volume decrease - The density of the bone = mass /volume= 1.9 kg/cm3 it s remain constant. Bone Strength:  If you cut the bones apart, you would find that it composed of two quite different types of bones: 1- Solid (compact) bone: This is found in the central shaft of the bone. 2 -Spongy (cancellous) bone: This is made up of thin thread-like trabeculae (trabecular bone). It is predominantly found at the ends of the long bones. Trabecular bone is considerably weaker than compact bone due to the reduced amount of bone in a given volume. Therefore the trabecular bone has two advantages over the compact bone: Where the bone is subjected primarily to compressive forces, such as at the ends of the bones and the spine, trabecular bone gives the strength necessary with less material than the compact bone. Because the trabecular is relatively flexible, it can absorb more energy when large forces are involved such as in walking, running and jumbing. Note: Trabecular bone cannot withstand very well the bending stress that occurs mostly in the central portion of long bones. Hook’s Law: The strain (∆L/L) increases linearly, at first indicating that it s proportional to the stress (F/A), as the force increase the length increase more rapidly, and the bone breaks at a stress of about 120 N/mm2. The ratio of stress to strain in the initial linear portion is the Young’s modulus, that is: Y= LF/ A L Where L= the length of the bone (mm) F= the force exerting on the bone (N) A= the cross section area of the bone (mm2) The compressive breaking stress of the compact bone is 170 (N/mm2), while the tensile breaking stress is 120 (N/mm2), Young’s modulus of the compact is about 179 x 102 (N/mm2). Bone Fracture: There are many types of fractures, but the main categories are: 1- Simple Fracture: It is also called a closed fracture, the bone breaks but there is no open wound in the skin. 2- - Compound Fracture: It is also called an open fracture, the bone breaks through the skin; it may then recede back into the wound and not be visible through the skin The bones don’t normally break due to compression; they usually break due to shear or under tension. A common cause of shear is catching the foot and then twisting the leg while bone is apt to puncture the skin, this type of fracture is compound. Physical factors affecting the degree of bone fracture: 1- The amount of the exerting force during collision or falling Force= The change in body momentum/ duration of impact Therefore, the shorter the duration of impact, the greater the force. To reduce the force and thereby the likelihood of fracture, it is necessary to increase the impact time. In both falling down or jumping from elevation, the impact time can be increased significantly by simply rolling with the fall or jumping, thereby spreading the change of momentum of the body over longer time. 2- The direction and the type of the exerting force (Tension, Compression, and Twisting). 3- The age, Broken bones are very common in childhood, though children's fractures are generally less complicated than fractures in adults. Older people, whose bones are more brittle, are more likely to suffer fractures from falls that would not affect younger people Measurement of bone mineral in the body  Osteoporosis is the most common disease in elderly women and the most common feature of it is lower bone mineral mass ,bone mineral mass decrease 1-2% per year, thus precise technique was needed to show the changes. The first tried is using an ordinary X-ray but it forced many problems X-ray beam has many different energies and the absorption of x-ray by calcium varies with energy. X-ray beam contain much scattered radiation when reach the film. The film is a poor detector for quantitative measurement. Then ordinary x-ray can't detect the change in bone mineral until its lost 30-50% of density, at this time it was too late use preventive therapy Assistant Lecturer SAJJAD JASSIM