Category Archives: BONE AND ALVEOLAR PROCESS

Types of bone

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Types of bone

 Bone is classified according to its type into:

1. Lamellar bone (mature or secondary)

a- Compact (cortical) bone.

b- Spongy ( Cancellous, trabecular ) bone. Compact and spongy bones are microscopically similar and are formed in the same way.

2.   Non lamellar bone (woven, primary or immature bone).

3.   Bundle bone

[1] Lamellar bone

Lamellar bone is either dense ivory compact bone or spongy bone . Bone is arranged in layers called lamellae. ( a ) Compact bone It is very dense and does not have marrow spaces. In compact bone, there are three kinds of lamellae.

Circumferential lamellae: These lamellae form the external and internal lamination of the cortical bone and are adjacent to periosteum and endosteum.

-  Haversian ( concentric ) lamellae :

These are in Haversion system and also called osteons. The lamellae are arranged concentrically from (4-20 lamellae) around a central canal ( Haversian canal ) which contains blood vessels, nerves and some connective tissue. The canals are lined by endosteum. Lacunae with osteocytes are arranged concentrically with their canaliculi radiating and connecting the Haversian canal. The Haversian canals are connected to the periosteum, marrow cavity and with each other by transverse canals (Volkman’s canals).

The purpose of the Haversian system is to distribute nutrients to osteocytes in compact bone.

-  Interstitial lamellae

These are parallel lamellae randomly scattered between Haversian and circumferential lamellae. They represent remnants of Haversian and circumferential lamellae left over after bone remodeling.

( b ) Spongy bone

It has a sponge – like appearance due to bone trabeculae and numerous marrow spaces.

Spongy bone is formed of network of bony trabeculae with many intervening thin irregular marrow spaces. These trabeculae are composed of layers of thin irregular bone lamellae with lacunae containing osteocytes. The trabeculae are lined by a delicate layer of connective tissue called endosteum which contains osteoprogenitor cells, osteoblasts and osteoclasts. The marrow spaces contain the blood supply of the spongy bone.

In both types of lamellar bone, the collagen fibers are oriented in parallel way forming an angle of 45° with that of the succeeded and preceded lamellae.

The incremental lines of lamellar bone

These lines can be identified in decalcified sections. 1-Faint lines:

Appear in sections stained with silver impregnation as faint black lines, formed due to change of the direction of the collagen fibers of the matrix from one layer to another. The angle is about 45°.

2-Resting lines :

They appear in sections stained with (H&E) as dark blue lines which are either straight or gently undulated. They demonstrate the incremental pattern of bone formation. The resting lines correspond to the rest period between the successive layers of bone. 3-ReversaI lines :

They appear in sections stained with (H&E) as dark blue scalloped lines. They demonstrate the osteoclastic activity on the

bone undergoing resorption, followed by osteoblastic new bone formation over the old bone. Both old bone and new bone are separated by scalloped line, in which the convexities are towards the old resorbed bone.

[21 Non lamellar bone
: (Woven, primary, immature bone, embryonic bone and bone of emergency). This bone is present in: a- Fetal life.

b- In the extraction socket, c- In the fracture site.

 

It is characterized by the high concentration of osteocytes, irregular arrangement of collagen fibers and the smaller content of minerals. So it appears more radiolucent than mature bone. Woven bone is resorbed and replaced by lamellar bone.

T31 Bundle bone

It is the third type of bone. It is called bone of attachment because it is present at sites of attachment of muscles and ligaments. It lines the socket of the teeth in which the periodontal ligament fibers are anchored (Sharpey’s fibers ). Its histological structure consists of bundles of woven bone with the fibers running parallel to the socket wall.  

The Alveolar process of Maxilla and Mandible

The alveolar process is that bone of the jaws which contains and supports the socket (alveoli) of the erupted teeth. It consists of: (1) Alveolar bone proper

a- Bundle bone

b- Haversian (lamellar) bone

(2) Supporting alveolar bone a- Cortical plates b- Spongy bone

       

Development of the alveolar bone

The alveolar bone starts to develop near the end of the second month of fetal life. Both the maxilla and mandible form a groove at their free surface (towards the oral cavity). The tooth germs of the deciduous teeth are contained in this groove. Gradually, bony septa develop between the adjacent tooth germs.

The alveolar process develops during eruption of the teeth.

In fetal life, the developing bone is a non lamellar type of bone surrounded by a thick periosteum. Areas of secondary cartilages may appear at the growing alveolar margins during the rapid growth of alveolar bone.

After eruption of teeth, the alveolar bone gradually takes its adult form.

Physiological changes of the alveolar process

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Physiological changes of the alveolar process

The internal structure of bone is adapted to mechanical stresses.

It changes continuously during growth and alteration of functional stresses.

In the jaws, structural changes are correlated to the growth, eruption, movements, wear and loss of teeth.

All these processes are made possible only by a coordination of formative and destructive activities.

The osteoblasts produce new bone, they secrete type I collagen as well as the non-collagenous matrix of bone (osteoid tissue). The exact mechanism by which hydroxy apatite crystals are deposited In the bone matrix is unknown.

However, certain enzymes such as alkaline phosphatase, ATPase, pyrophosphatase participate in this process.

Bone resorption occurs by the osteclasts and on rare occasion by the osteocytes.

Alteration in the structure of the alveolar bone in connection with the physiologic eruptive movement of the teeth.

As the teeth erupt, and their roots develop, the crypts in which they lie are replaced by sockets.

Superficially to the developing tooth, the bone between the roof of each crypt and alveolar margin is resorbed. Meanwhile, the floor of the crypt is filled in, from below upward, as the newly deposited bone adapts itself to developing roots of the erupting teeth.

At the alveolar fundus the continual apposition of bone can be recognized by resting lines separating parallel layers of bundle bone.

The new bone is laid down in lamellae which run parallel to the surface of the crypts or sockets, or of bundle bone where periodontal ligament fibers are actually attached.

These parallel lamellae are replaced at a deeper level by Haversian system. Remains of the parallel lamellae, or of bundle bone are often found among the Haversian system and indicates the former position of crypt and socket surface.

Alteration of alveolar bone during Mesial Drift of the teeth

During the mesial drift of the tooth, bone is deposited on the distal and resorbed on the mesial alveolar wall.

On the mesial wall, signs of active resorption and the presence of Howship’s lacuna containing osteoclasts are observed.

Resorption does not involve the entire mesial surface at the same time. Moreover, periods of resorption alternate with periods of rest and repair. It is during these periods of repair that bundle bone is formed and detached periodontal fibers are again secured.

Islands of new bundle bone are separated from the old bone by reversal lines.

 
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On the distal alveolar wall, the apposition of bundle bone layers separated by resting lines occur to maintain the normal thickness of the periodontal ligament. When the bundle bone reaches a certain thickness, it is resorbed from the marrow spaces side and then becomes replaced by lamellated bone. The presence of bundle bone indicates the level at which the socket was situated previously.

Macro and micro anatomy of the alveolar process

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Macro and micro anatomy of the alveolar process

The adult alveolar process is formed of:

1- The alveolar bone proper

The osteoblasts that form the alveolar bone proper are derived from the dental sac.

The alveolar bone proper (ABP) is a thin layer of bone that forms the inner wall of the sockets facing the roots.

Its main function is to furnish a medium for the attachment of the periodontal ligament fibers.

The alveolar bone proper is also called the cribriform plate because it is perforated by many openings that carry branches of the interalveolar nerves and blood vessels into the periodontal ligament. These openings are called Zucker kandle and Hirschfeld canals.

Radiographically, the alveolar bone proper is called lamina dura because it appears radiopaque.

Histologically, the alveolar bone proper consists of two types of bone .

a- Bundle bone :

Which is adjacent to the periodontal ligament. It is formed of bundles of woven bone, but it never matures into lamellar bone.

The principal fiber bundles of periodontal ligament insert into bundle bone as Sharpey’s fibers . Thus, it is very important for tooth support.

A reversal line is usually present separating lamellated and bundle bones.

b- Lamellated bone :

It lies adjacent to the bundle bone layer. It is formed of lamellae that are arranged parallel to the surface of the adjacent marrow spaces, or form Haversion systems.

2- Supporting alveolar bone

It is the bone that surrounds the alveolar bone proper and gives support to the socket of teeth. The supporting” alveolar bone consists of:

a- Cortical plates of compact bone;

They form the outer and inner (labial and lingual) plates of the alveolar process. These are continuous with the compact layers of the maxillary and mandibular jaws.

These are much thinner in the maxilla than in the mandible, and are thickest in the premolar and molar regions of the lower jaw especially on the buccal side.

In the anterior region of both jaws, the supporting bone is usually very thin, no spongy bone is found and the cortical plate is fused with the alveolar bone proper.

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Histologically, the cortical plates consists of:

• Longitudinal lamellae, formed of calcified ground substance and osteocytes.

• Haversian systems.

b-Spongy bone;

It fills the area between the cortical plates and the alveolar bone proper.

Histologically, cancellous bone is usually very dense around the teeth which are subjected to excessive forces of mastication.

Around functionless teeth, the spongy bone shows very wide medullary spaces and few numbers of trabeculae.

The spongiosa of the alveolar process is classified according to roentgenograms study into two main types:

Type I : In which the interdental and interradicular trabeculae are regular and horizontal in a ladder like arrangement. This is seen only in the mandible.

Type II : Shows irregularly arranged, numerous delicate interdental and interradicular trabeculae. This arrangement is more common in the maxilla.

The trabeculae of the spongiosa below the root apices of the lower molars appear as if they are radiating from the fundus of the socket.

The marrow spaces in the alveolar process may contain hematopiotic marrow, but usually they contain fatty marrow.

Structure of bone

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Structure of bone

Periosteum and Endosteum

The Periosteum lines the outer surface of compact bone. It consists of 2 layers:

1. Outer fibrous layer contains dense irregular connective tissue.

2. Inner cellular layer consists of osteoprogenitor cells, osteoblasts, and blood vessels.

The Sharpey’s fibers (also called perforating fibers) which are collagenous fibers from the outer fibrous layer of the periosteum, insert into the bone and tightly attach periosteum to bone.

The Endosteum consists of a layer of osteoprogenitor cells and osteoblasts.

It lines the internal surface of compact bone, surrounds the surfaces of spongy bone, and lines Haversion canals. Functions of periosteum and endosteum :

1. Contains blood vessels for nutrition of bone.

2. Source of osteoblasts for bone growth, repair, and remodeling.

Bone cells

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Bone cells

1- Osteoprogenitor cells

2- Osteoblasts

3- Osteocytes

4- Osteoclasts

1-Osteoprogenitor cells:

They are undifferentiated mesenchymal cells that have the capacity to divide and differentiate to other bone cells ( osteoblast and osteoclast).

These ceils are located in the deepest layer of the periosteum, and lining the vascular canals of the compact bone.

Morphologically, they resemble mesenchymal cells with a pale staining elongated nucleus and little basophilic cytoplasm.

2- Osteoblasts

• They are derived from the osteoprogenitor cells.

• They are responsible for production of the organic bone matrix (osteoid) which includes the collagen fibers and the ground substances.

They are also responsible for the deposition of the inorgnic matrix.

The osteoblasts produce alkaline phosphatase enzyme which is an initiator of mineralization:

Clinically, an increase in the number or activity of osteoblasts in synthesizing bone, causes an increased concentration of alkaline phosphatase enzyme in the blood.

They are also rich in other enzymes concerned with calcification such as ATPase and pyrophosphatase.

The osteoblasts form a cellular layer along the edge of bone. A layer of osteoid is present between the osteoblasts and the mineralized bone.

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Alveolar bone cells. Osteoblasts (A), osteoclasts (B)

osteocytes (C), bone lining cells (D) and osteoid layer (E).

Morphologically, the active secretory osteoblasts are oval or columnar in shape with eccentric nucleus and basophilic cytoplasm. They contain abundant endoplasmic reticulum, Golgi apparatus and secretory granules.

The inactive osteoblasts are flattened and are called lining

cells.

The osteoblasts may be trapped in the bone tissue and become osteocytes.

3- Osteocytes

They are imprisoned osteoblasts. They are located in bony spaces called lacunae.

They possess several cytoplasmic processes that extend in the canaliculi of the lacunae and are joined with neighbouring ones by gap junctions.

Osteocytes pass nutrients and hormones from blood to other osteocytes through these junctions.These cells cannot divide.

They respond to hormones and can release calcium from bone matrix into blood. This prevents hypermineralization and death of bone.

On rare occasions bone resorption by osteocytes has been described (osteocytic osteolysis).

Morphologically, they are flattened cells with dark nucleus, minimal rough endoplasmic reticulum and Golgi appartus.

4- Osteoclasts

The osteoclast may be derived from:

a- Circulating blood monocytes -> macrophages -> osteoclasts

b- Osteoprogenitor cells

c- Osteocytes. It might be that aging osteocytes, in their degeneration and death, liberate the substance that cause the differentiation of osteoclasts.

Osteoclasts lie in bay-like depressions called Howship’s lacunae on the surface of bone undergoing resorption.

They are giant (20-100 microns), multinucleated (up to 30 nuclei and more) cells.

 
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The cell membrane adjacent to bone has folds, forming ruffled border surrounded by a clear zone containing cytoplasm rich in actin and myosin, but devoid of organelles and attaches osteoclast to bone.

The osteoclasts possess eosinophilic cytoplasm rich in lysosomes, numerous mitochondria, abundant Golgi apparatus and little rough endoplasmic reticulum.

The osteoclasts show high activity of acid phosphatase, collagenase and other enzymes that are essential in degradation of osteoid and calcium release during bone resorption. They also liberate carbon dioxide (C02) which is important in bone decalcification.

The parathyroid hormone stimulates osteoclasts to secrete these substances.

The main function of osteoclasts is bone resorption. They are also phagocytic.

Bone Resorption

Bone resorption occurs by three processes:

a- Decalcification. This is acheived by the organic acids secreted

by the osteoclasts (mainly citric and lactic) that chelate bone

and increase hydroxyapatite solubility, b- Degradation of organic matrix by the activity of lysosomal

acid protease and collagenase enzymes, c- Transport of soluble products to the extracellular fluid and

blood stream.

Composition of bone

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Composition of bone

Bone is composed of:

1- Inorganic substances ( 65% by weight)

Which consist mostly of calcium and phosphate in the form of hydroxyapatite crystals. Besides other radicals such as sodium, magnesium, sulfate fluoride and iron are present. The hydroxyapatite crystals are deposited on, and in between the collagen fibrils resulting in the hardness of bone.

2- Organic substances and water (35% by weight)

Which consist of bone cells and intercellular substances.

The intercellular substances. Are composed of:-

1- Collagen fibers primarily type I ( 88% ).

ii- Ground substances consisting of:

a-Glycosaminoglycans ( GAG ) Mostly chondroitin sulfate

and keratan sulfate. b-Glycoproteins: include osteocalcin and sialoprotein; bind calcium and regulate mineralization. c-Proteoglycans.

BONE AND ALVEOLAR PROCESS

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BONE AND ALVEOLAR PROCESS

Bone is a highly specialized calcified type of connective tissue which is rich in blood supply.

Function:

1. It forms the skeleton of the body. The calcified bone gives the body support and strength, (bone matrix has a high degree of structural organization which enable bone to recover from tension and compression.

2. It protects the vital organs (e.g. brain and lung).

3.It acts as a reservoir for minerals, particularly calcium.

4. It serves as attachment media for the muscles and ligaments.