BONEChapter 10

A narrative report

Submitted by:Bautista, Louis Clyde C.Gatdula, Dave Joseph B.Ong, Charles Adrian P.

Santos, Pauleen Ashley R.Torres, Jhoana Marie O.

Tria Tirona, Rafaelle Jeanna E.

Submitted to:Dr. Marie Antoniette R. Veluz

CHAPTER 10: BONE

What is a bone?A bone is a specialized connective tissue, which consists of intercellular substances and

osteocytes. Systematically, it is finally controlled by hormonal factors. Locally it is controlled by mechanical forces including tooth movement, growth factors, cytokines, and piezoelectric conditions. It consists of 67% of inorganic matrix which is a poorly crystallized calcium-deficient Hydroxyapatite crystals (Ca10(PO4)6(OH)2), while 33% is made up of Organic matrix which contains 28% collagen and 5% noncollagenous protein (Osteonectin, osteocalcin, bone morphogenetic protein, bone proteoglycan, bone sialoprotein). The ratio between hard and soft components is sufficient to ensure a degree of elasticity. Bone resists compressive forces best and tensile forces least.

There are plenty of functions a bone can perform, and these are the following:Mechanical function includes protection of the internal organs of the body, provides

structural framework to keep the body supported, and it provides movement of the body. Synthetic function includes blood production in the bone marrow. This process is called hematopoiesis. Metabolic functions of bone include the storage of important minerals in the body like calcium and phosphorus, storage of important growth factors, and for the storage reserve of fatty acids.

The Structural Elements of the bone are the bone cells, bone matrix, sharpey’s fibers, blood vessels, nerves, lymphatic vessels. Bone cells are primarily responsible for the formation, resorption, and maintenance of osteoarchitechture.

There are 3 types of bone cells are described with each specific function:

Osteoblasts are uninucleated cells that synthesize both collagenous and noncollagenous bone proteins. They are located on the surface of bone or osteoid. Osteoblasts also synthesize the enzyme alkaline phosphatase, which is needed locally for the mineralization of osteoid. When the bone is no longer forming, the surfaces of the osteoblasts become inactive and are called Lining cells. These lining cells retain their gap junctions with osteocytes, creating a syncytium that functions to control mineral hemostasis and ensure bone vitality. Osteoblasts do not divide. They give rise to osteocytes, remain as osteoblasts, or return to the state of osteoprogenitor cells from which they derived. They secrete type I and type V collagen and small amounts of several noncollagenous proteins, and a variety of cytokines. Parathyroid hormone & vitamin D enhance bone resorption at high concentrations but supporting bone formation at lower concentrations, while

Calcitonin & estrogen inhibit bone resorption. On the other hand, Glucocorticoids inhibit both resorption and formation of the bone, but primarily formation. Osteoblasts also synthesize a variety of cytokines and growth factors such as Bone morphogenetic protein (BMP), Transforming growth factor beta (TGF-BETA), Insulin-like growth factor, Platelet-derived growth factor (PDGF-AH) and Fibroblastic growth factor beta (FGF-BETA) that help in regulating cell metabolism.

Osteocytes are osteoblasts secreted in the bone matrixes that are entrapped in lacunae. An osteocyte lies in its own lacuna and contacts its neighboring osteocytes cytoplasmically through canaliculi. The processes of adjacent cells make contact via gap junctions, maintaining the vitality of osteocytes by passing nutrients and metabolites between blood vessels and distant osteocytes, regulating ion homeostasis, and transmitting electrical signals in bone. Osteocytes are responsible for osteolysis or limited resorption of bone materials at the walls of the osteolytic lacunae and canals, and osteoplasia, the secondary rebuilding of perilacunar bone mineral. They are known to be as the “housekeepers” of the bone since they are actively involved in the maintenance of the bony matrix.

Osteoclasts are probably derived from a monocytic-macrophage system, which are responsible for bone resorption. They are large, multinucleated cells with fine, fingerlike cytoplasmic processes and are rich in lysosomes that contain tartrate-resistant acid phosphatase (TRAP). Osteoclasts lie in resorption craters known as Howship’s lacunae on bone surfaces or in deep resorption cavities called cutting cones. They possess an organelle-poor, brush-like cytoplasmic border known as ruffled border which demarcates the zone of resorption. The osteoclasts resorbs the bone by first attaching themselves to the mineralized tissue and create a sealed environment that is acidified to demineralize the hard tissue. After the exposure to the acidic environment, the organic matrix is broken down by the secretion of proteolytic enzymes.

Bone MatrixBone matrix is the intercellular substances of bone and consists of organic and inorganic

components. The association of these substances gives bone its hardness and resistance.The organic component is composed of:

collagen fibers with predominately type I collagen (95%) which provides tensile strength

proteoglycans that are responsible for compressive strength matrix proteins

osteocalcin that functions to promote mineralization and bone formation osteonectin that plays a role in regulating collagen attachment, and osteopontin, a cell binding protein that is similar to an integrin

Cytokine and growth factors that aid in bone cell differentiation, activation, growth, and turnover.

The inorganic component is made up of Hydroxyapatite crystals (Ca10(PO4)6(OH)2) which provides the compressive strength of the bone.Sharpey’s Fibers are lateral fibrous elements extended into the bone matrix.

Blood Vessels, Nerves, Lymphatic vessels (Haversian canals)

Structure of a boneBone tissue of which bones are composed of may be

described as compact bone or trabecular bone. The compact bone forms the outer layer of the bone itself. It is ivory-like and dense in structure and has no cavities. It is the shell of many bones and surrounds the trabecular bone in the center. The trabecular bone may also be reffered to as the spongy or cancellous bone. It has numerous cavities and contains the bone marrow. Complete osteons are usually absent here due to the thinness of the trabeculae.

It consists of three layers namely: circumferential lamella (subperiosteal bone), concentric lamella and the interstitial lamella. The circumferential lamella makes up the outside surface of the bones. It is not made up of small concentric circles and follows the surface of circumference of the bone. The next one is the concentric lamella which contains the basic unit of the bone called the osteon. The osteon contains the Haversian canals which provide a pathway so that nutrients from the blood vessels may reach the osteocytes. The Volkmann’s canals interconnect the Haversian canals forming a network

of blood vessels. The third one is the interstitial lamella which is said to be the incomplete or fragmented osteons that are located between the secondary

osteons. They represent the remnant osteons left from partial resorption of old osteons during bone remodeling.

circumferential lamella concentric lamella

Growth of boneIt is also known as ossification or the formation of the bone. It includes both bone formation and

bone resorption or the removal of mineral materials and organic matrix of bone. There are three types of ossification. These are the endochondral formation, intramembranous formation and the sutural bone growth.

Endochondral formation is the formation of bone tissue that is preceded by the formation of cartilage model that resembles the shape of the bone that is to be formed. The cartilage predecessor of the bone mineralizes and is gradually removed by resorption. The bone tissue formed replaces it. The examples of bones formed through this method are the long bones of the arms and legs.

The second one is the intramembranous formation wherein the bone tissue is formed without preceding cartilage pattern. It is formed by fibrous connective tissue. Osteoblasts secrete bone matrix called the osteoid and the matrix then mineralizes to form the bone proper. Some of the osteoblasts become trapped in the forming bone and become osteocytes. Examples of bones formed through this method are the mandible and maxilla.

Lastly, the sutural bone growth. Sutures are fibrous joints between the bones which permit the skull and face to accommodate growing organs. It has the same osteogenic potential as the periosteum and it connects 2 periosteal surfaces, namely: the cambium which is the osteogenic layer and the capsule which is the inner layer.

Alveolar process

As such develops in the conection with the growth of the jaw and erruption of the of teeth. These are parts of the maxilla and mandible that are especially designed to provide sockets and support of teeth. It is called processus alveolaris in maxila and pars alveolaris in the mandible bone.

FunctionsIt supports the tooth roots on the facial and on the palatal/lingual sides. It is the one responsible

for the separation of teeth from mesial to distal. And also contributes to absorption and distribution of oclussal pressure produced in tooth to tooth contact.

Structures of the alveolar boneCortical plate

It provides strength and protection for the supporting bone (maxilla and mandible also acts as a site for attachment for skeletal muscles. It is covered by periosteum. In labial sections cortical plate is attached directly to the alveolar bone proper. This arrangement causes the bone overlying the roots of the anterior teeth brittle in nature. Cortical plate in mandible is more dense and has fewer perforations for passage of vessels and nerves than in the maxilla.

Alveolar CrestThe alveolar crest is the highest point of the alveolar ridge and joins the facial and lingual

cortical plates.

Trabecular BoneTrabecular or spongy bone lies within the central portion of the alveolar process, and is the less

dense, cancellous bone. When viewed by a radiograph, trabecular bone has a web-like appearance.

Alveolar bone properThe alveolar bone proper is a thin layer of compact bone, which is a specialized continuation of

the cortical plate and forms the tooth socket. The lamina dura is a horseshoe shape white line on a dental radiograph that roughly corresponds to the alveolar bone proper.

Development of the alveolar processThe 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.

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. The alveolar process starts developing strictly during tooth eruption.

During the bell stage, the dental follicle migrates away from the tooth germ in preparation for the formation of periodontium. Histodifferentiation happens. Fibers from outside of the dental follicle will form a membrane containing network of fibers which contain cells. This develops into osteogenic tissue where cells differentiate into osteoblasts.

As the tooth erupts, the membranous bone in the body of mandible and maxilla extends occlusally. It serves as an attachment of the periodontal ligament to hold the tooth in place.

Reorganization of the spongiosa or the cancellous bone also determines the development of the alveolar process. In non-functional arches, the traberculae becomes thinner and therefore lessens the size of the alveolar bone. in functional arches, the traberculae of the alveolar bone thickens to function well in mastication and therefore makes the alveolar process longer or larger.

Vascular Supply of Alveolar BoneThe alveolar processes of the maxilla are supplied with oxyhemoglobinated blood from the

posterior superor alveolar artery, middle superior alveolar artery and anterior superior alveolar artery which are all branches of maxillary artery. The Alveolar processes of the mandible are supplied with oxyhemoglobinated blood by the inferior alveolar nerve which is also a branch of maxillary artery. The maxillary artery is a branch of the external carotid artery.

Age ChangesMesial drifting

It is a gradual movement of all the posterior teeth in a mesial direction. It occurs only if there has been interproximal wear between the teeth. The drift is not a passive one however, as it has been shown that during chewing, the bite force has a mesial component. Bone will be resorbed in the tense area of the periodontal ligament and bone formation in the pressured area.

Masticatory ForcesThe alveolar bone will adapt and bone marrow spaces will become smaller and the trabecula

becomes thicker for increase in masticatory function.

Loss of functionAs a result of loss of function, the bone marrow spaces become wider and the trabecula

becomes thinner.

Tooth extraction/exfoliationAlveolar process disappears because of bone resorption by osteoclasts. There will be an

apposition of embryonic bone. There will be a formation of residual or alveolar ridge. The residual ridge will appear more radiolucent in radiographs because of its lesser calcification.

The MandibleThe mandible is the largest and strongest bone of the face, serves for the reception of the lower teeth. It consists of a curved, horizontal portion, the body, and two perpendicular portions, the rami, which unite with the ends of the body nearly at right angles.

Development of the Mandible: The Body of the Mandible

1. The mandible is ossified in the fibrous membrane covering the outer surfaces of Meckel's cartilages. These cartilages form the cartilaginous bar of the mandibular arch and are two in number, a right and a left.

2. Their proximal or cranial ends are connected with the ear capsules, and their distal extremities are joined to one another at the symphysis by mesodermal tissue.

3. Meckel’s cartilage has a close, relationship to the mandibular nerve, at the junction between posterior and middle thirds, where the mandibular nerve divides into the lingual and inferior dental nerve. The lingual nerve passes forward, on the medial side of the cartilage, while the inferior dental lies lateral to its upper margins & runs forward parallel to it and terminates by dividing into the mental and incisive branches. From the proximal end of each cartilage the malleus and incus, two of the bones of the middle ear, are developed; the next succeeding portion, as far as the lingula, is replaced by fibrous tissue, which persists to form the sphenomandibular ligament & the perichondrium of the cartilage persist as sphenomallular ligament.

4. Between the lingula and the canine tooth the cartilage disappears, while the portion of it below and behind the incisor teeth becomes ossified and incorporated with this part of the mandible. The mandible first appears as a band of dense fibrocellular tissue which lies on the lateral side of the inferior dental and incisive nerves. For each half of the mandible,

5. Ossification takes place in the membrane covering the outer surface of Meckel's cartilage and each half of the bone is formed from a single center which appears, in the region of the bifurcation of the mental and incisive branches, about the sixth week of fetal life.

6. REMNANT’S OF MECKEL’S CARTILAGE

a. Ossification grows medially below the incisive nerve and then spread upwards between this nerve and Meckel’s cartilage and so the incisive nerve is contained in a trough or a groove of bone formed by the lateral and medial plates which are united beneath the nerve. At the same stage the notch containing the incisive nerve extends ventrally around the mental nerve to form the mental foramen. Also the bony trough grow rapidly forwards towards the middle line where it comes into close relationship with the similar bone of the opposite side, but from which it is separated by connective tissue.

b. A similar spread of ossification in the backward direction produces at first a trough of bone in which lies the inferior dental nerve and much later the mandibular canal is formed. The ossification stops at the site of future lingula. By these processes of growth the original primary center ossification produces the body of the mandible.

Development of the Mandible: The Ramus of the Mandible1. The ramus of the mandible develops by a rapid spread of ossification backwards into the

mesenchyme of the first branchial arch diverging away from Meckel’s cartilage. This point of divergence is marked by the mandibular foramen.

2. Somewhat later, accessory nuclei of cartilage make their appearance:a. a wedge-shaped nucleus in the condyloid process and extending downward through the

ramus. b. a small strip along the anterior border of the coronoid process.

3. The condylar cartilage:a. Carrot shaped cartilage appears in the region of the condyle and occupies most of the

developing ramus. It is rapidly converted to bone by endochondral ossification (14th. WIU) it gives rise to:

b. Condyle head and neck of the mandible.c. The posterior half of the ramus to the level of inferior dental foramen

4. The coronoid cartilage:a. It is relatively transient growth cartilage center ( 4th. - 6th. MIU). it gives rise to:

i. Coronoid process.ii. The anterior half of the ramus to the level of inferior dental foramen

5. These accessory nuclei possess no separate ossific centers, but are invaded by the surrounding membrane bone and undergo absorption.

The MaxillaThe maxillæ are the largest bones of the face, excepting the mandible, and form, by their union,

the whole of the upper jaw. Each assists in forming the boundaries of three cavities, the roof of the mouth, the floor and lateral wall of the nose and the floor of the orbit; it also enters into the formation of two fossæ, the infratemporal and pterygopalatine, and two fissures, the inferior orbital and pterygomaxillary. Each bone consists of a body and four processes—zygomatic, frontal, alveolar, and palatine.

Development of the Maxilla: The Maxilla Proper

1. It develops in the mesenchyme of the maxillary process of the mandibular arch as intramembranous ossification. It has one center of ossification which appears in a band of fibrocellular tissue immediately lateral to and slightly below the infra orbital where it gives off its anterior superior dental branch. The ossification center lies above that part of the dental lamina from which develop the enamel organ of the canine.

2. The ossified tissue appears as a thin strip of bone. It spread in different directions as:a. Backward: Below the orbit toward the developing zygomatic bone.b. Forward: Toward the future incisor regionc. Upward: To form the frontal process of the maxilla.

3. As a result of this pattren of bone deposition, a bony trough is formed (infraorbital groove) where the infraorbital nerves lies. The inner and outer edges of this groove grow up, meet and fuse forming a canal that encloses the nerve & open anteriorly at the infraorbital foramen

4. The ossified tissue appears as a thin strip of bone. It spread in different directions as: a. downward: To form the outer alveolar plate for the maxillary tooth germsb. Toward the midline: Ossification spreads with the development of the palatal process in

the substance of the united palatal folds to form the hard palate. At the union between the palatal process and the main part of the developing maxilla, a large mass of bone produced. From this region & on the inner side of the dental lamina & tooth germs, the inner alveolar plate of deciduous canines and molars develops.

5. Development of the maxillary sinus: At 4 MIU as a small depression of the mucosa of the lateral wall of the nasal cavity. In its gradual extension the sinus comes into relation with the maxilla above the level of the palatal process & hallows out the interior of the bone, so separating its upper or orbital surface from its lower or dental region.

Development of the Maxilla: The PremaxillaTwo centers of ossification for the premaxilla:

A. The palato-ficial center:

Appear at the end of 6 WIU. It starts close to the external surface of the nasal capsule, in front of the anterior superior dental nerve and above the germ of the lateral deciduous incisor.

From this center bone formation spreads: 1. Above the teeth germ of the incisors.2. Then downward behind them.

To form the inner wall of their alveoli & palatal part of the premaxilla. B. The prevomerine center (paraseptal center):

It begins at about 8-9 WIU along the outer alveolar wall. It is situated beneath the anterior part of the vomer bone and it forms that part of the bone lies mesial to the nasal paraseptal cartilage.

Accessory Cartilages1. Accessory cartilagenous center appears in the region of the future zygomatic or molar process

and this undergoes rapid ossification & adds considerable thickness to the bulk of this part.2. Also small areas of secondary cartilagenous center appears along the growing margin of the

alveolar plate.3. In the middle line of the developing hard palate between the two palatine processes.

Lines in BonesThere are four lines that can be seen in bone tissues: reversal,

cementing, aplastic and resting lines. Reversal line shows the evidence of previous remodeling activity and it is formed by filling of new bone in a previously resorbed cavity. The relative amount of reversal lines indicates the amount of remodeling that has occurred. Cementing lines separates adjacent lamellae of bone from each other. It is also refered as the incremental lines in bone . Aplastic line is a layer of basophilic substance which laid down on the surface of the bone that has been inactive for a long period of time. While resting line is a line which separates the new layer of bone from the old bone which has been inactive.

Clinical ConsiderationAlthough bone is one of the hardest tissues in the human body, bone is also is biologically a

highly plastic tissue. It is also exceedingly sensitive to pressure. Bone resorbs on the side of pressure and apposes on the side of tension. On sites where bone receives pressure, high amounts of cyclic adenosine monophosphate can also be observed. Bone also gives response to its functionality. A highly functional bone is denser than a bone that does not receive any functional forces at all. When bones are fractured or a tooth was extracted from it, embryonic type of bone or coarse fibrillar bone is formed on the site.

Bone is continuously remodels and is being replaced by a newer bone tissue from embryonic period until death is termed as bone turnover. Bone turnover rate of 30% to 100% per year is common to rapidly growing children. In adults, it is decreased to 5% per year. Periodontal diseases gives the most frequent and harmful change in the alveolar process. Progressive loss of alveolar bone in periodontal disease is difficult to control and even more difficult to regenerate or repair when damaged. This situation is one of the greatest challenges to periodontics. Studies and experiments on implanting artificial roots on the alveolar bone gave promising results in decreasing the speed of bone resorption. Acromegaly is an overgrowth of the jaw bone.

RLAL

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Periodontitis

Acromegaly

BIBLIOGRAPHY

Oral Histology – Development, Structure and Function, 4th Edition by Ten Cate, A. R.

Orban’s Oral Histology and Embryology, 11th Edition by Bhaskar, S. N.

Permar’s Oral Histology and Microscopic Anatomy, 10th Edition by Melfy, R. C.

Northern Illinois University:Department of Biological Sciences Website

Medscap Website

Oral Biology by Berkovitz, Moxham, Linden, and Sloan