Bone and Alveolar Process Handout PDF

Al-Alamein International University Faculty of Dentistry Oral Biology Department Bone and alveolar process Oral Biology Department Composition of bone Bone is a mineralized connective tissue consisting by dry weight of about 67% mineral and 33% o...

Al-Alamein International University Faculty of Dentistry Oral Biology Department Bone and alveolar process Oral Biology Department Composition of bone Bone is a mineralized connective tissue consisting by dry weight of about 67% mineral and 33% organic matrix. The organic matrix contains about 28% type I collagen and 5% noncollagenous matrix protein. CLASSIFICATION OF BONES Bones may be classified according to their shape, mode of development or by their histological appearance. According to Histological appearance Mature lamellar 1. Compact bone 2. Cancellous bone 3. Bundle bone. Immature, non-lamellar Woven bone 2 Organizational components of bone All mature bones have a dense outer sheet of compact bone and a central medullary cavity. The cavity is filled with red or yellow bone marrow in living bone. This cavity shows a network of bone trabeculae. Trabecular, spongy, or cancellous bone is the term used to describe this network. The outer aspect of compact bone is surrounded by a condensed fibrocollagen layer, the periosteum which has two layers. An outer layer which is a dense, irregular connective tissue termed fibrous layer; and an inner osteogenic layer, next to the bone surface consisting of bone cells, their precursors, and a rich vascular supply. The periosteum is active during fetal development. It is also important in the repair of fractures. The inner surface of compact and cancellous bone is covered by a thin cellular layer called endosteum. In resting adult bone, quiescent osteoblasts and osteoprogenitor cells are present on the endosteal surfaces. These cells act as reservoirs of new bone forming cells for remodeling or repair. Bone cells 1.Osteoprogenitor cells (the mother cells) Also Known as osteogenic cells and preosteoblast cells They are the mother cells of osteoblast, osteocyte, and bone lining cells. They are flattened spindle shape cells found in the bone marrow, the periosteum, the endosteum, and the periodontal ligament. They can proliferate and differentiate into osteoblast at sites of rich blood supply and into chondrocyte at sites of poor blood supply. 2. Osteoblast cells Osteoblasts are mononucleated cells that synthesize the organic matrix of bone. They are found along surfaces of growing bones. They are plump, cuboidal cells (when very active) or slightly flattened cells with large nuclei and protein synthesis organelles. They exhibit high levels of alkaline phosphatase activity on the outer surface of their plasma membrane. Functionally, the enzyme is believed to cleave inorganically bound phosphate. 3 The liberated phosphate likely contributes to the initiation and progressive growth of bone mineral crystals. It produces the bone organic matrix include type I collagen, and several non-collagenous proteins this uncalcified matrix called osteoid. 3. Osteocyte cells As osteoblasts form bone, some become trapped in the matrix they secrete, these cells then are called osteocytes. The space in the matrix occupied by an osteocyte is called lacuna. Narrow extensions of these lacunae form enclosed channels, or canaliculi, that house radiating osteocytic processes. Through these channels, osteocytes maintain contact with adjacent osteocytes and with the osteoblasts or lining cells on the bone surfaces. The canaliculi penetrate the bone matrix and permit diffusion of nutrients, gases and waste products between osteocytes and blood vessels. This interconnecting system maintains the bone integrity and bone vitality. 4. Bone lining cells Resting osteoblast cells, they are elongated thin cells with reduced number of organelles that cover the surface of resting bone. They maintain their gap junction with osteocytes creating a network that functions to ensure bone vitality and control mineral homeostasis, removal of bone lining cells is essential in starting osteoclastic bone resorption. 5. Osteoclast cells Osteoclasts are found against the bone-resorbing surfaces occupying irregular concavity, called Howship's lacunae. They are large multinucleated giant cells (more nuclei in one cell). The cytoplasm of the osteoclast shows Acid Phosphatase containing vesicles and vacuoles. The presence of acid phosphatase distinguishes the osteoclast from other multinucleated giant cells. 4  Osteoclastogenesis Osteoblast play a major role in osteoclastogenesis via secreted a protein molecule called the receptor activator nuclear factor-KB ligand (RANKL) that bind to the receptor activator nuclear factorKB (RANK) expressed on the plasma membrane of osteoclast precursor to induce fusion and differentiation of osteoclast precursor into mature osteoclast, also to promote their survival and activity, this interaction is called the RANKL/RANK pathway. Osteoblast also secrete another protein molecule called the receptor osteoprotegerin (OPG) which is a free-floating receptor for RANKL. OPG inhibit the process of resorption by binding to RANKL and preventing it from binding to its receptor RANK thus blocking the interaction between RANKL and RANK and interferes with osteoclast formation. Therefore, the RANKL/RANK/ OPG play an important role in regulation of osteoclastogenesis. The RANKL/RANK/ OPG belongs to tumor necrosis factor /receptor family. Number of hormones and cytokines modulate osteoclastogenesis by enhancing osteoclast differentiation, activation, function, and life span, these include parathyroid hormones. The process of resorption Bone resorption is the removal of mineral and organic components of extracellular matrix of bone under the action of osteolytic cells, of which the most important is the osteoclast. Resorptive event occur on two phases. I. Extracellular phase Attachment of osteoclast to mineralized surface of bone which is mediated by integrins that bind matrix protein with other protein (osteonectin and type I collagen) Creation of a sealed acidic microenvironment. Demineralization of bone, release of crystals and exposure of organic matrix Degradation of the exposed matrix by collagenase and cathepsin 5  II. Intracellular phase Ingestion of the released crystals and degraded organic matrix by endocytosis at the ruffled border. The crystal is then transferred to the cytoplasmic vacuoles to be digested. The degraded organic matrix including collagen fragments is extruded outside the cell by exocytosis to be completely degraded by fibroblasts in the extracellular tissue, the final product is then transported to the blood stream. Resorption Decalcification Both mineral organic Mineral is removed and and matrix are organic matrix remain removed The resorbed bone is replaced by loose connective tissue, after time the new bone is apposed onto old bone. Incremental lines of bone a. Reversal line The scalloped outline of Howship's lacunae turns its convexities toward the old bone. It remains visible as a darkly staining scalloped cementing line between old and new bone and called reversal line. b. Resting line Parallel lines appear separating successive increment of bone. It corresponds to the rest period of osteoblast cells. These lines represent a thin layer of granular organic matrix rich in glycoprotein and proteoglycan. Lamina lamitans and cement lines. All inactive bone surfaces are covered by thin densely stained lamina lamitans. It consists of dense granular matrix rich in osteopontin and similar to that of cement line. 6 Histology of bone Bone exists in two main types (lamellar and embryonic bone) 1. Mature (lamellar bone) It is the bone constructed from microscopic layers or lamellae containing osteocytes. a. Compact or cortical bone b. Cancellous bone c. Bundle or cancellous bone. 2. Immature (embryonic) bone Woven bone I) Compact bone Three distinct types of lamellae are organized, they are Concentric, interstitial, and circumferential lamellae. Main bulk of compact bone is made up of concentric bony layers or concentric lamellae surrounding a central canal containing blood vessels, lymphatics and nerves. This canal is called a Haversian canal which together with the concentric lamellae forms a Haversian system or osteon. Adjacent Haversian canals are connected to each other, to the marrow cavity and to the periosteum by horizontal or oblique channels called Volkmann's canals that ensure a rich vascular network throughout compact bone. Interspersed between intact Haversian systems are the interstitial lamellae which are irregular lamellae, remnants of old Haversian systems created during resorption and reapposition of bone (remodeling). The outer surface of compact bone is formed of parallel lamellae known as the outer circumferential lamellae. At the inner surface, similar but irregular circumferential lamellae merge with the trabeculae of cancellous bone to form the inner circumferential lamella. The osteocytes are arranged in parallel with the lamellae and interconnected by their fine processes. 7 II) Cancellous bone Cancellous bone is built up of many trabeculae which form a network that fills the medullary cavity of the compact bone and surrounds the marrow spaces. Each trabecula consists of one or more lamella containing osteocytes between them the marrow spaces are lined by flattened endosteal cells and are filled with red marrow in childhood in adult most of the red marrow is replaced by fatty marrow. III) Bundle bone It is a type of bone seen when the incremental layers of superficial bone are penetrated by perforating fibers or Sharpey’s fibers e.g. Perforating fibers in external circumferential lamellae and Sharpey's fibers in periodontal ligament. Embryonic bone: Woven bone It is called immature bone or coarse fibered bone. It is the first bone to appear in bone repair. It is called woven bone because the bundles of collagen fibers in the matrix are irregularly arranged in the form of interlacing network with wide spaces between the fibers. If the fibers are parallel, in arrangement it is called embryonic woven bone. Histologically, embryonic bone is characterized by the greater number, size, and irregular arrangement of osteocytes at the expense of the intercellular substance than mature bone. The greater number of cells and the reduced volume of calcified intercellular substance make bone more radiolucent in X- ray than mature bone. This explains why bone callus (embryonic bone formed at the site of bone fracture or fills the extraction wound) cannot be seen in X-ray at a time when the histological examination of a fracture reveals a well- developed union between the fragments and why a socket after an extraction wound appears to be empty at a time when it is almost filled with immature bone. 8 ALVEOLAR BONE The alveolar process is defined as that part of the maxilla and the mandible that forms and supports the crypts of developing teeth and the sockets of the erupted teeth. It is named according to its location as follows: A. Interdental septum: The alveolar bone between adjacent teeth B. Inter-radicular septum: The alveolar bone between the roots of multi-rooted teeth. C. Alveolar margin: The free border of the socket D. Alveolar crest: The coronal portion of the alveolar bone in the interdental space. The alveolar process is called functional bone because it is susceptible to functional changes and depends on the presence of teeth. After tooth extraction the alveolar bone may be resorbed to basal bone. The basal bone is the unalterable part of the jaw which provides attachment to muscle and houses the major nerve and vessels. Structure of the alveolar process Adult alveolar process is composed of 2 parts 1. Alveolar bone proper 2. Supporting bone 1. Alveolar bone prober (Cribriform plate - lamina Dura) Alveolar bone prober is a thin lamella of compact bone that surround root of the tooth and give attachment to the principal fibers of periodontal ligament. It is perforated by numerous minute foramina (Volkmann’s canals) that carry branches of the interalveolar nerve and blood vessels from the marrow spaces to the periodontal ligaments that is why it is called Cribriform plate The term lamina Dura (hard) is given from its dense radiopaque (white) appearance in x-ray. 9 Histology Alveolar bone prober is made up of cortical plate which consists partly of Lamellated and partly of bundle bone. The deeper part of cortical plate is made up of Lamellated bone which consist of Haversian system (osteon). The superficial portion is formed of bundle bone in which the principal fibers of PDL are anchored. The term bundle bone was chosen because the bundles of the principal fibers of the periodontal ligament continue into the bone as Sharper’s fibers. Bundle bone is characterized by the scarcity of the fibrils in the intercellular substance, and by their arrangement parallel to the surface of the root, at right angles to Sharpey's fibers. Since bundle bone contains fewer fibrils than does lamellated bone, it therefore appears much lighter in silver stain sections it also appears more radio- opaque in x-ray because it contains more calcium salts per unit areas than other types. 2. The Supporting bone consists of 2 parts A) Outer and inner cortical plates, which are made of compact bone. B) Spongy or cancellous bone, which fills the area between these plates and the alveolar bone proper. A) The cortical plates The cortical plates of the alveolar processes are continuous with the compact layers of the body of the maxilla and mandible. They are thinner in the maxilla than in the mandible. They are thickest buccally in the premolar and molar regions of the lower jaw. In the region of the anterior teeth of both jaws the cortical plates are very thin and fused directly with the alveolar bone proper with no spongy bone in between except on the palatal side. In the maxilla, the outer cortical plate is perforated by Volkmann's Canals and may show defects in the posterior teeth region: 1- Defect exposing part of the root with the alveolar margin intact is known as fenestration. 2-If the defect includes the alveolar margin, it is called dehiscence. 10 B. The spongy or cancellous bone It passes between the alveolar bone proper and the cortical plates and between the alveolar bone proper of the Interdental and interradicular septa or the sockets. Its degree of development is related to the forces of mastication; It is absent in the sockets of the anterior region the walls of the sockets are very thin except on the palate. Cancellous bone is extremely sensitive to variation in functional forces, whereas increased function leads to formation of new bone. Decreased function leads to a decrease in the volume of bone. This can be observed in the cancellous bone of teeth which have lost their antagonists. Here the cancellous bone around the alveolus is reduced and the trabeculae become less numerous and very thin. The alveolar bone proper is generally not affected because it continues to receive some stimuli from the tension of the periodontal ligament. A. bone trabeculae disappear in tooth has no antagonist B. Normal functioning tooth Around isolated teeth which have antagonists and when the forces of mastication are excessive, the cancellous bone is usually very dense, the trabeculae being numerous and thick. 11 Radiographic appearance of the alveolar bone 1- The alveolar bone proper (lamina Dura) Appear radiopaque (white) line adjacent to the root and separated from it by a narrow radiolucent line (dark) representing periodontal ligament 2- Alveolar process trabeculae According to the arrangements of the alveolar bone trabeculae in X-ray, there are 2 main types Type I Interdental and interradicular trabeculae are arranged horizontal in a ladder-like arrangement and interconnected by shorter and finer trabeculae this type is seen more in mandible. Type II Interdental and interradicular trabeculae are more numerous, delicate, and irregularly arranged this type is more seen in maxilla. 3- Alveolar crest The alveolar crest appears as a radio opaque line interproximally between adjacent teeth. The outline of the crest of the alveolar septa in x-ray is dependent on the position of adjacent teeth. In a healthy mouth the distance between the cement-enamel junction and the free border of the alveolar bone proper is constant apical to the cervical line. In the anterior region the alveolar crests are horizontal, slightly rounded or pyramidal. 12  In the posterior region the alveolar crests are oblique, because the teeth are inclined forward in the majority of individuals. The inclination is most pronounced in the premolar and molar region with the teeth being tipped mesially. Then the cemento-enamel junction of the distal surface of the mesial tooth is situated in a more occlusal plane than that of the mesial surface of the distal tooth, and the alveolar crest slopes distally, resulting in a constant distance between the alveolar crest and the cervical lines. Internal reconstruction of bone Bone remodeling Continuous Bone modeling renewing of bone tissue Process by which overall size and throughout life shape of bone is established from embryonic to pre adult period Remodeling is the replacement of existing bone with new bone without substantially altering the architecture of the skeleton. During remodeling bone formation is usually preceded by bone resorption. They occur in response to certain stimuli in the tooth socket. 1. Tension (pull) Pressure on PDL induce induces bone formation. 2. Pressure on the PDL on the bone induces resorption. The internal structure of bone is adapted to mechanical stress. It changes during growth and function. In the jaw, structural changes are correlated to growth, eruption movements, wear, and loss of teeth. All these processes are made possible only by coordination of destructive and formative activities. Specialized cells, osteoclasts eliminate bone that is no longer adapted to mechanical forces, whereas osteoblasts produce new bone. 13 Aging of alveolar bone  Alveolar sockets appear jagged and uneven.  The marrow spaces have fatty infiltration.  Internal trabecular arrangement is more open, which indicates bone loss. Clinical consideration The biologic plasticity that enables the orthodontist to move teeth without disrupting their relations to the alveolar bone. On the pressure side the fibers of the periodontal ligament are compressed, the vascular flow decreases, cell death may occur and osteoclasts appear along the bone front On the tension side of the ligament, the collagen fibers are stretched. The fibroblasts become more spindle-shaped and appear oriented with their long axis in the direction of the fiber bundles and the osteoblasts align along the bone. With loss of teeth the supporting alveolar bone is resorbed. The controlling factor in the disappearance of the alveolar bone is probably pressure exerted upon the bone through the gums and is most evident when all the teeth are lost and the Upper and lower gums are used as organs of mastication. When artificial denture is worn the pressure exerted during mastication is partly transmitted to the alveolar bone and resorption is less extensive. Good luck 14