Physics of the Skeleton PDF

Islamic University/Najaf College of Medical Technologies Department of Anesthesia Techniques First stage /medical physics Lecture 1 Physics of the skeleton By M.Sc. Ahmed Radhi Sabbar  Skeletal system is one of the important...

Islamic University/Najaf College of Medical Technologies Department of Anesthesia Techniques First stage /medical physics Lecture 1 Physics of the skeleton By M.Sc. Ahmed Radhi Sabbar  Skeletal system is one of the important systems in the human the body. At birth, the human body consists of about 300 bones. Babies have more cartilage than bone tissue. As they grow up, some of the bones fuse together to form a single bone. As a result, the adult human skeleton consists of 206 bones.   The size and shape of a bone varies according to its location in the body. Bones sort into various piles according to their shapes, so there are five piles as shown in figure below: 1- Flat Bones: flat bones are thin, flattened, like bones the shoulder blade (scapula) & some of the bones of the skull. 2- Long Hollow Bones : such as those of arms , legs & fingers. 3- Cylindrical Bones: such as vertebrae. 4- Irregular Bones: such as the bones in the wrist & ankle. 5- Other bones: ribs, etc. Functions of the bone: Bone has at least six functions in the body: 1- Support 2- Locomotion 3- Protection of various organs 4- Storage of chemicals 5- Nourishment 6- -Sound transmission (in the middle ear) 1- supporting: The support function of bone is most obvious in the legs. The muscle are attached to the bones through tendons & ligaments. The system of bones plus muscles supports the body. 2-locomotion: for example, bone joints permit movement of bones. 3-protection: For protection, the skull bones, which protects the brain and several of the most important sensory organs like (eyes and ears). Also, ribs form a protective cage for the heart and lungs 4-storage: bones store chemicals like calcium (Ca) is stored in bones which are released when it is needed. 5- Nourishment: the teeth are specialized bones that can cut food, tear it & grind it.thus the teeth serve in providing nourishment for the body. 6- Sound transmission: the smallest bone of the body are the ossicles in the middle ear. These three small bones act as levers and provide an impedance matching system for converting sound vibration in air to sound vibration in the fluid in the cochlea. Bone remodeling:  Actually, bone is a living tissue and has a blood supply has as well as nerves.  Osteocytes: kind of cells that maintain the bone in a healthy condition. Cells make up about 2% of the volume of bone.  If these cells dies the bone dies and losses some of its strength.  Since bone is a living tissue it under goes change through out life.  A continuous process of destroying old bone & building a new bone is called. " bone remodeling " Bone Remodeling is performed by specialized bone cells they are:- 1- Osteoclast: They are the cells that found in a bone tissue & they destroy the old bone. 2- Osteoblasts: They are the cells that found in a bone tissue & they build a new bone.  Compared to many body processes , bone remodeling is slow work.  We have the equivalent of a new skeleton about every seven years; each day the osteoclasts destroy bone containing about 0.5g of calcium. (The bone have 1000g of Ca), and the osteoblasts build new bone using about the same amount of calcium. What is bone made of?  Note the large percentage of calcium (Ca) in bone; since calcium has a much heavier nucleus than most elements of  the body, it absorbs x- rays much better than the surrounding soft tissue. This is the reason x- ray show bones so well. Bone consists of two quite different materials plus water these two materials are:- 1- Collagen: it is the major organic fraction. It is about 40% of the weight of solid bone & 60% of its volume.  The collagen remainder is quite flexible some what like a chunk of rubber & can even be bent into a loop. While it has a fair amount of tensile strength, it bends easily if it is compressed.  When the collagen is removed from the bone, the bone will be very fragile & can be crushed with fingers.  Collagen is produced by the osteoblasts. 2-Bone mineral = inorganic component:- Bone mineral is about 60% of the weight of the bone &40% of its volume.  The mineral is very fragile without collagen to produce bone.  Bone mineral is made up of calcium hydroxyapatite (Ca10(Po4)6(OH)2(. So the composition of Bone: Collagen, Mineral Ca10(PO4)6(OH) 2, Water Collagen makes bones flexible (elastic) Mineral makes bones rigid Bone = collagen + bone mineral + water  How strong are your bones? Two quite different types of bone: 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 trabecular (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. Longitudinal Stretch and Compression:  Stress S is the internal force per unit area acting on the material; it is defined S =F/A F is the applied force  A is the area on which the force is applied.  The force applied to the bone causes the bone to elongate by an amount ∆L. The fractional change in length ∆L/ L is called the longitudinal strain St; that is, St=∆L/L L is the length of the bone ∆L is the change in the length due to the applied force. Hooke’s law:  Robert Hooke observed that while the body remains elastic, the ratio of stress to strain is constant (Hooke’s law); that is, Y = S/ St  The constant of proportionality Y is called Young’s modulus. Y = (F / A) / (ΔL / L) Y = (L F) / (A ΔL)  If the bone is placed under tension or compression its length will be changed.  When a sample of fresh bone is placed in a special instrument for measuring the elongation under tension, a following curve will be obtained.  The strain (∆L/ L) increases linearly at first , indicating that it is proportional to the stress (F/A)  As the force increases, the length increases more rapidly. The bone breaks at a stress of about (120 N/mm2).  Scheme When A Piece of Bone Placed under Increasing Tension, Its Strain ΔL/L Increases Linearly at First (Hooks Law) And Then More Rapidly Just before It Breaks in two at 120 N/mm2. Young's modulus of elasticity:  The ratio of stress to strain in the initial linear portion is called young's modulus Y. Y = LF/A ΔL Example: Assume leg has a 1.2 m shaft of bone with an average cross-sectional area of 3cm² (3x10¯⁴ m²).What is the amount of shortening when all of the body weight of 700 N is supported on this leg? Young modulus of elasticity of bone (1.8x10¹⁰ N/m²). SOLUTION: Y = Stress / Strain Y = (F / A) / (ΔL / L) ΔL =LF/AY ΔL= (1.2m)(700N) /(3x10¯⁴ m² )(1.8x10¹⁰ N/m²) ∆L =1.5x10¯⁴ m =0.15 mm H.W Q1/ Why x-ray shows bone so well? Q2)If the length of a steel rod 2.5 m long is reduced by 0.5 mm, what is the compression stress in the rod?Y=21 N/m².