There are three types of osteal lamellae in the diaphyses of tubular bones: Haversian, intermediate and general (external and internal). The lamellae lie close to each other, they are located parallelly to the axis of the bone and form quite a thick layer of only compact bone. It is 1,5 – 5 mm thick. Thus the diahysis of a long tubular bone is a hollow cylinder with walls formed of compact bone. The cavity of this cylinder is termed medullary canal. The latter is connected with spaces of the spongy tissue in the epiphyses of the bone. The Haversian lamellae form the basic mass of compact bone, thus making up osteons. The intermediate lamellae fill in the gaps between osteons. External and internal general (circumferential) lamellae form the most outer and the most inner layers of the compact bone, being located parallelly to the bone surface under the periosteum and endosteum respectively.
The epyphises of tubular bones consist of spongy tissue which is also formed of osteal lamellae. In structure, spongy bone may have large and small spaces. There are red bone marrow and vessels in these spaces. Compact bone covers epiphyses only on the outside with a comparatively thin layer. Flat and volumetric bones have a similar structure. Lamellae of spongy substance are strictly arranged in each bone. Their direction coincides with that of the maximum compression and stretching forces. The environment of each bone determines its structure. Trabeculae form an integral system in several adjacent bones, which characterizes the trabeculae’s architectonics. Such structure of bones preconditions their maximum solidity. In vertebrae, the stretching and compression forces are perpendicular to the superior and inferior surfaces of the vertebral bodies. This corresponds to the fact that the trabeculae have mainly vertical direction in spongy substance (fig. 1.2). In the proximal epiphysis of the femoral bone there are arch-shaped systems of trabeculae which transfer pressure from the surface of the bone head to the walls of the diaphysis. Besides, there are trabeculae transfering the traction force of muscles attached to the greater trochanter (fig. 1.3).
Fig. 1.2. Orientation of trabeculae in the vertebral body (saggital section)
Fig. 1.3. Orientation of trabeculae in proximal epiphyses of tubular bones: a – in femur; b – in tibia
Trabeculae running in the radial direction are typical of the calcaneus. They distribute loads equally over the surface of the calcaneal tuberosity which serves as a foot support (fig. 1.4).
Fig. 1.4. Orientaton of trabeculae in calcaneus
Compact bone is formed in places of the highest concentration of force trajectories. It is clearly visible on the section of the femoral, tibial and calcaneal bones where the compact tissue is thickened in the areas of crossing between force lines and the bone surface. Thus we can say that compact bone is the result of compression of spongy bone, and vice versa, it is possible to consider spongy bone as sparse compact bone. It should be noted that if static and dynamic conditions are changed (increase or decrease in functional loads), the spongy bone architectonics changes too, a part of trabeculae disappear, or new systems of osteal trabeculae develop. The spongy bone structure changes in a special visible manner after fractures.
1.4. External Structure of Bones
While describing the external structure of bones, we should pay attention to the surfaces, facies, of the bones, which may be flat, concave or convex, smooth or rough. Articular surfaces facies articularis, involved in formation of joints, are the most smoothly polished ones. In some bones the end is rounded, forming a head – caput; at the same time, the end of other bones has concavity, called articular fossa, or fossa articularis. The head may be separated from the bone body with a constricted part – neck, collum. If the articular end is extensive but slightly curved surface, it is termed condyle, condilus. The processes located near the condyle are named epicondyles, epicondyli, they serve for attachment of tendons and ligaments (they may also be called apophyses).
The following surfaces are distinguished in bones (depending on theirlocation in the human body): internal or external, medial or lateral etc. The surfaces are separated by borders, margo. The borders, in turn, are known as superior or inferior, medial or lateral etc. They may be smooth or serrated, blunt or sharp, sometimes they have notches, incisurae, of different sizes.
On the surfaces of bones, there may be such formations as: processes, eminences, depressions, openings etc. (bone process, processus; elevation, eminence, eminentia; large rounded elevation or tuberosity, tuberositas; hillock, tuber; bulge, protuberance, protuberantia; tubercle, tuberculum; sharp process – spine, spina; crest, crista; hollow in the bone, fossa; pit, foveola; groove, sulcus, opening, foramen; canal, canalis; small canal, canaliculus; fissure, fissura; cavity, cavitas).
1.5. Chemical Сomposition of Bone and its Properties
The chemical composition of a bone depends on the condition of the bone under examination, its age and individual characteristics. In a grown-up, a fresh bone which is not treated contains: water – 50 %; fat – 16 %; other organic substances – 12 % and inorganic substances – 22 %. A dehydrated and defatted bone contains approximately two-thirds of inorganic substances and one third of organic substances.
The inorganic substances are mainly represented by calcium salts in the form of submicroscopic crystals of hydroxyapatite. The microscopic examination shows that the axes of crystals are oriented parallelly to osteal fibers. The crystals of hydroxyapatite form mineral fibers.
The organic substance of the bone is called ossein. This protein is the type of collagen. It forms the basic substance of the bone. Ossein is contained in osteal cells – osteocytes. There are osteal fibers containing protein – collagen – in the intercellular matrix of the bone. When bones are boiled, the proteins (collagen and ossein) form glutinous mass. It should be noted that the bony matrix contains mineral fibers, apart from collagen ones. The interlacement of organic and inorganic fibers determines the specific features of osseous tissue: durability and elasticity.
If a bone is treated by acid (decalcification), the mineral salts are removed. Such bone, containing only organic substance keeps its shape in all details, but becomes much more flexible and elastic. If the organic substance is removed from the bone through burning, the elasticity is lost. Such bone is very fragile.
The proportion of organic and inorganic substances in bones primarily depends on age, and it may change under the influence of various reasons (climatic conditions, nutrition, diseases). Thus in children, bones contain much less mineral (inorganic) substances, therefore they are more flexible and less solid. In elderly persons, vise versa, the amount of organic substances decreases. In such age, bones become more fragile and susceptible to fractures.
1.6. 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