Physics of bone

ByMashael Saud

Bone consists of two quite different materials plus water:

1 .Collagen, the major organic fraction, which is about 40% of the weight of solid bone and

60% of its volume .

2 .Bone mineral, the so-called “inorganic” component of bone, which is about 60% of the

weight of the bone and 40% of its volume .

Some properties of the organic component Collagen

is quite flexible material that has a fair amount of tensile strength, but it bends easily

if it is compressed .

Some properties of the inorganic component

•Bone minerals are obtained by removing collagen from bone .

•The collagen is removed by putting the bone in a furnace and “ash” it .

The obtained bone mineral is found to have the following properties :

1)Very fragile and can be crushed with the fingers !

2)Bone mineral is made up of calcium hydroxyapatite-Ca10(PO4)6 (OH)2 .

3)Studies using X-ray scattering have indicated that the bone mineral crystals are rod shaped with diameters of 20 to 70 Ao and length of from 50 to

100Ao .

Because of the small size of the crystals, bone mineral has a very large surface area. In a typical adult, it has a surface area of over 4

x 105 m2 .

•Around each crystal is a layer of water containing in solution many chemicals needed by the body. The large area of exposed bone mineral crystal permits the bones to interact rapidly with chemicals in the blood and other

body fluids .

Bones are composed of one or a combination of two quite different types of bones :

1)Solid, or compact bone, and

2)Spongy, or cancellous ,bone made up of thread-like trabiculae (trabecular

bone) .

Question In an adult femur, trabecular bone is predominately found in the ends of the long bones, while most of the compact bone is in the

central shaft. Why?

Answer Trabecular bone is considerably weaker than compact bone due to the reduced amount of bone in a given volume .

•Osteoporotic bone is even weaker .

Bone is anisotropic material as its mechanical response depends upon the direction of the applied load. For example

•Compressive strength is more than tensile strength and

•Tensile load capacity is more than transverse load capacity of the bone .

The stress and strain diagram for the cortical bone under tensile loading is

shown in the figure (3) .

•The stress and strain diagram has three distinct regions .

The stresses inside the beam (shown by arrows) are pulling it apart at the bottom (tension) and pushing

it together at the top (compression) .

•Use an I-beam, which has a thick top and bottom joined by a thin web, as a support beam in a building

(Fig. 3.8b) .

•When the force may come from any direction, a hollow cylinder is used to get the maximum strength

with a minimum amount of material (Fig. 3.8c) .

It is almost as strong as a solid cylinder of the same diameter .

•Since the force on femur may come from any direction, the hollow cylinder structure of the bone is well suited for support.

*The compact bone of the shaft of the

femur is thickest in the center and thinnest at the ends; note again the quality of the design .

Question: What are the advantages of trabecular

bone over compact bone ?

Answer: There are at least two :

1 .Where a bone is subjected primarily to compressive forces, such as at the ends of the bones and in the spine ,