Mechanical Properties of Bones
The bone is a solid object, and its main properties are durability and elasticity. Durability is the ability to resist to the external destroying force. It depends on the macro- and microscopic structure, and on the osseous tissue composition. As for the macroscopic structure, each bone has its specific form which enables withstanding the maximal strain in a certain part of the skeleton.
The internal structure of the bone is also complicated. As already stated, the osteon is a hollow cylinder tube the walls of which are built of numerous lamellae. It is known that in architectural constructions, hollow (tubular) columns have greater durability per a unit of mass as compared to solid columns. Therefore, the osteon-based structure of the bone itself predetermines a high level of its durability. Groups of osteal lamellae, being arranged along the axes of maximal strains, form osteal trabeculae of spongy bone and terminal lamellae of compact bone. It should be noted that osteal trabeculae are archshaped in places of maximal strains. As well as tubular systems, arch-shaped systems are most durable. The arch principle in the structure of spongy bone trabeculae is typical of the proximal epiphysis of the femur, as well as of the calcaneus spongy tissue etc.
The bone composition significantly influences its durability. Decalcification causes a considerable decrease in the level of compression, tension and torsion strength. As a result, it is easy to bend, compress and twist the bone. If the calcium content increases, the bone becomes fragile.
Bone durability in a healthy adult is higher than the durability of some construction materials – it is like a cast iron. The first examinations of bone durability were conducted in XIX century. According to Lesgaft`s researches, the human bone withstood tensile strain of 5500 N/cm2, compressive strain – 7787 N/cm2. The tibia withstood compressive strain of 1650 N/cm2, whichis comparable to the weight of more than 20 men. These data show a high level of reserve capabilities of bones against various strains. Changes in the tubular structure of a bone (both macro- and microscopic) reduces its mechanical durability. For example, the tubular structure of bones is disrupted after fracture healing, and the durability of such bones significantly decreases.
Elasticity is the ability to regain the initial shape after cessation of an external impact. Bone elasticity is equal to that of hard tree species. Like durability, it depends on the macro- and misroscopic structure and the chemical composition of the bone.
Thus, the mechanical properties of bones – durability and elasticity – are predetermined by the optimal combination of organic and inorganic substances contained in them.
1.7. Functions of Skeleton
1. The bones serve as support for soft tissues (muscles, ligaments, fasciae, visceral organs).
2. Most of bones are leverages which are moved by attached muscles. According to these two functions, the skeleton may be considered to be the passive part of the musculoskeletal system.
3. The human skeleton is an antigravitational structure which counteracts the force of gravity. It prevents any changes in the body shape under the impact of gravitation pressing the human body to the ground.
4. Protective function: the skull, trunk and pelvis bones prevent any potential damage to the vital organs, major vessels and nerve trunks. For example, the skull encloses the brain, organs of vision, hearing and equilibrium. In the vertebral canal there is the spinal cord. The chest protects the heart, lungs, major vessels and nerve trunks. The pelvic bones protect the rectum, urinary bladder and internal genital organs against injuries.
5. Hematopoietic function: most bones contain red bone marrow which is the hematopoietic organ, as well asthe immune system organ. The bones protect the red bone marrow against damages, and provide favorable conditions for its trophism and for maturation of blood elements.
6. Involvement in mineral metabolism: bones deposit numerous chemical elements, predominantly calcium and phosphorus salts.
According to V. S. Speransky, the human skeleton is a perfect dynamic structure adapted to the motor function and human way of life; it is responsive to various changes which occur both in the body itself and in the environment.
1.8. Development of Bones
The osseous tissue appears in the human embryo in the middle of the second month of fetal development, when all other tissues have been already formed. The development of bones may proceed in two ways: on the basis of connective tissue and on the basis of cartilage. It should be noted that connective tissue never turns directly into cartilaginous tissue or osseous tissue. Osseous tissue is capable of developing by the way of growth along the surface of connective tissue or cartilage (appositional bone growth), or it develops to replace a resorbed cartilage.
Bones developing on the basis of connective tissue are termed primary. They are calvarial bones, and viscerocranial bones. Ossification of the primary bones is termed endesmal. It occurs in the following way: within the anlage of connective tissue, an ossification centre, punctum ossificationis (centrum ossificationis), appears and then expands into the depth and across the surface. From the ossification centre, osteal trabeculae start to form along the radii. They are interconnected with bone rods. In these spaces between the rods there are red bone marrow and blood vessels. In most of primary (membrane) bones, not one but several ossification centres are formed. They gradually grow and merge with each other. Eventually, only the most superficial layer of the initial connective tissue stratum remains unchanged. Then this layer turns into the periosteum.
Bones developing on the basis of cartilages are termed secondary. They pass through connective, cartilaginous and, in the last turn, osseous stages. Secondary bones are the skull base bones, trunk bones and bones of extremities. Let`s study the development of a secondary bone on the example of the long tubular bone. By the end of the second month of the fetal period, the cartilaginous anlage appears; it resembles a definite bone by shape. The cartilaginous anlage is covered by perichondrium. In the area of the future diaphysis of the bone, the perichondrium transforms into the periosteum. Lime salts are accumulated in the cartilaginous tissue under the periosteum, and cartilaginous cells die away. The osteal cells – osteoblasts – come from the periosteum to replace the dead cells. They start to produce an organic matrix of osseous tissue which endures calcification. The osteoblasts enclosed in the intercellular substance transform into osteosytes. Thus, the osteal cylinder termed periosteal or perichondral bone is formed in the diaphysis area. This stage of ossification of secondary bones is termed perichondral. Subsequently, new bone layers overgrow from the periosteum. Bone lamellae evolve, i.e. Haversian systems (osteons) start to develop around the vessels growing from the periosteum. The vessels sprouting from the periosteum are directed to the midst of the cartilaginous anlage. The cartilage located in the center of the diaphysis accumulates lime salts, dissolves and is substituted by spongy bone. This process is termed enchondral ossification of the diaphysis. The medullary canal is absent at first. It is formed in the process of transformation of spongy tissue of the enchondral bone inside the diaphysis and during red bone marrow development inside it.
In the epiphyses, ossification starts later, some bones are ossified even after the birth. Ossification begins from the ossification centre which appears within the cartilaginous anlage of the epiphysis. Such ossification process is called enchondral. It occurs in the following way: firstly, from the periosteum, blood vessels sprout into depth of the cartilage along the radii. In the midst of the epiphysis, the cartilage accumulates lime salts, dissolves and is substituted by osseous tissue. Later, the periosteal (perichondral) bone develops from the periosteum along the edge of the cartilaginous anlage of the epiphysis. The periosteal bone is comprised by a thin layer of compact tissue. The perichondral lamina is absent only in the areas of future articular surfaces – a quite thick layer of cartilage remains there. The cartilaginous layer also remains between the epiphysis and diaphysis – this is a metaepiphysial cartilage. It is the area of bone
