alveolarbone.pdf

ALVEOLAR BONE It is the portion of maxilla and mandible that forms and supports the tooth sockets (alveoli). Alveolar bone is a specialized connective tissue that is mainly characterized by its mineralized organic matrix. Together with the root...

ALVEOLAR BONE It is the portion of maxilla and mandible that forms and supports the tooth sockets (alveoli). Alveolar bone is a specialized connective tissue that is mainly characterized by its mineralized organic matrix. Together with the root cementum and periodontal ligament, the alveolar bone constitutes the attachment apparatus of the teeth. The main function is to distribute and reabsorb forces generated by mastication and other tooth contact. In addition, the jaw bones consist of the basal bone, which is the portion of the jaw located apically but unrelated to the teeth DEVELOPMENT The alveolar bone begins to first form by an intramembranous ossification within the ectomesenchyme surrounding the developing tooth. This first bone formed called as woven bone is less organized and is replaced with a more organized lamellar bone. When a deciduous tooth is shed, its alveolar bone is resorbed. The succedaneous permanent tooth moves into place, developing its own alveolar bone from its own dental follicle. As the tooth root forms and the surrounding tissues develop and mature, alveolar bone merges with the separately developing basal bone and the two become one continuous structure. Although alveolar bone and basal bone have different intermediate origins, both are ultimately derived from neural crest ectomesenchyme. Mandibular basal bone begins mineralization at the exit of the mental nerve from the mental foramen, whereas the maxillary basal bone begins at the exit of the infraorbital nerve from the infraorbital foramen. PARTS The alveolar process is divisible into separate areas on an anatomic basis, but it functions as a unit, with all parts interrelated in the support of the teeth. 1. Alveolar bone proper: Thin lamella of bone that surrounds the root of the tooth and gives attachment to principal fibers of the periodontal ligament is called as alveolar bone proper. The alveolar bone proper forms the inner wall of the socket. It is perforated by many openings that carry branches of the interalveolar blood vessels and nerves into the periodontal ligament and is called as Cribriform plate.The bone in which principal fibers called Sharpey’s fibers of periodontal ligament are anchored is known as bundle bone. The bundle bone contains few fibrils in the intercellular substances. This bone is not unique to the jaws, it occurs throughout the skeletal system wherever ligament and muscles are attached. Radiographically, this bundle bone appears as a thin radiopaque line surrounding the roots of teeth and is called as lamina dura. Lamina dura appears denser than the adjacent supporting bone, but this radiographic density may be due to the mineral orientation around the fiber bundles and the apparent lack of nutrient canals. But there is no difference in mineral content between lamina dura and the supporting bone. The lamina dura is evaluated clinically for periapical or periodontal pathology. 2. Supporting alveolar bone: Bone that surrounds the alveolar bone proper and gives support to the socket is called as supporting alveolar bone. It consists of 2 parts: a. Cortical plates: Which consists of compact bone and form the outer and inner plates of the alveolar bone. b. Spongy bone: Which fills the area between these plates and alveolar bone proper. It is also called as trabecular bone or cancellous bone. Most of the facial and lingual portions of the sockets are formed by compact bone alone; cancellous bone surrounds the lamina dura in apical, apicolingual, and interradicular areas. INTERDENTAL SEPTUM The interdental septum consists of cancellous bone bordered by the socket wall cribriform plates (lamina dura or alveolar bone proper) of approximating teeth and the facial and lingual cortical plates. If the interdental space is less than 0.5 mm, i.e. in kissing roots, cancellous bone is lacking. The septum may consist of only the cribriform plate leading to diminished blood supply. If roots are too close together, an irregular “window” can appear in the bone between adjacent roots. Determining root proximity radiographically is important. The mesiodistal angulation of the crest of the interdental septum usually parallels a line drawn between the cementoenamel junctions of the approximating teeth. The distance between the crest of the alveolar bone and the cementoenamel junction in young adults varies between 0.75 and 1.49 mm (average 1.08 mm). The mesiodistal and faciolingual dimensions and shape of the interdental septum are governed by the size and convexity of the crowns of the two approximating teeth, as well as by the position of the teeth in the jaw and their degree of eruption. Kissing roots where cancellous bone is lacking Boneless window between adjoining close roots of molars COMPOSITION 1. Inorganic: 67% Hydroxyapatite 2. Organic: 33% Collagen – 28% Type I – (Mainly), Type III, V, XII and XIV Non-collagenous protein – 5%. The various noncollagenous proteins are osteonectin, osteopontin, bone sialoprotein, osteocalcin, bone proteoglycan, biglycan, bone proteoglycan II decorin, thrombospondin and bone morphogenetic proteins (BMPs). Osteonectin functions primarily to link collagen to the mineralized matrix of bone. Osteopontin is an important glycoprotein of bone and is vital for chemotaxis of bone forming cells. Osteopontin is synthesized by a variety of mesenchymal cells especially fibroblasts, osteoblasts, osteoclasts and the differentiating mesenchymal cells. Osteopontin has been shown to respond to mechanical stimulus and its upregulation is thought to play an important role in the turnover of bone following mechanical stress. Osteopontin through its chemotactic ability is able to bring in osteoblasts and osteoclasts into area in which it is expressed. It has been detected in GCF and its potential as a marker for periodontal disease has been explored.  Bone sialoprotein is more specific to bone forming cells than osteopontin. Bone sialoprotein plays a role in chemotaxis of osteoblasts. However, its primary role is to act as an initial nucleator to hydroxyapatite crystal formation and is thus essential to the mineralization process. Gla proteins are important for the regulation of mineralization in the extracellular matrix. The important members of the gla protein family are Bone gla protein (osteocalcin) and Matrix gla protein (MGP). These proteins are called so due to the presence of amino acids that are γ - carboxylated, called the gla residues. Bone gla protein regulates crystal growth and limits the size of the hydroxyapatite crystals so that they form a three dimensional integrated mineralized mass. Chondroitin 4-sulphate is especially found in greater proportion in alveolar bone, it has been used as a marker of periodontal disease activity. This ability to promote mineralization has been utilized in regenerativeperiodontal therapy. CELLULAR COMPONENTS Osteoblast: These are generally cuboidal or slightly elongated cells that line a large percentage of bone surfaces. Osteoblasts are uninucleated cells that synthesize both collagenous and non-collagenous bone proteins and are thought to be derived from multipotent mesenchymal cells. These cells exhibit high level of alkaline phosphatase on the outer surface of their plasma membrane. This enzyme is used as a cytochemical marker to distinguish preo- steoblasts from fibroblasts. Functions of osteoblast are bone formation by synthesizing organic matrix of bone, cell to cell communication and maintenance of bone matrix and bone resorption by producing proteases which are involved in matrix degradation and maturation Osteoblasts produce type I collagen, non-collagenous proteins (osteocalcin, osteopontin, osteonectin) and various proteoglycans. These also produce cytokines and growth factors like BMPs (BMP-2 and BMP-7), TGF-β, IGF and PDGF. Osteocyte: As osteoblasts secretes bone matrix, some of them become entrapped in lacunae and are then called osteocytes. The number of osteoblasts that become osteocytes varies depending on their rapidity of bone formation, the more rapid the formation; the more osteocytes are present per unit volume. The osteocyte extends processes called canaliculi that radiate from the lacunae. These canaliculi bring oxygen and nutrients to the osteocytes through blood and remove metabolic waste products. They have decreased quantity of synthetic and secretory organelles and indeed are smaller cells than osteoblasts. Bone lining cells: These cells cover most, but not all quiescent bone surfaces in the adult skeleton. Together with osteocytes, bone forming cells and their connecting cell processes appear to form an extensive homeostatic network of cells capable of regulating plasma calcium concentration. Osteoclast: These are multinucleated giant cells of 50 to 100 μm size. These are irregular oval or club shaped having branching processes. They are derived from circulating blood cell monocytes. Osteoclasts are found in baylike depressions in the bone called Howship’s lacunae. The part of the cell in contact with bone shows convoluted surface, the ruffled border which is the site of great activity due to ion transport and protein secretion. Ruffled border is surrounded by a clear zone that has no organelles other than microfilaments. Peripheral region of apical membrane is tightly juxtaposed to matrix which is called as sealing zone. Clear zone and sealing zone are responsible for attachment of osteoclast to bone matrix. Basolateral membrane is the major site for receipt and integration of regulatory signals. The enzymes released by Osteoclast are acid phosphatase, aryl sulfatase, β- glucuronidase, several cysteine proteinases such as cathepsin B and L, tissue plasminogen activator (TPA), MMP- 1 and lysosymes. Osteoprogenitor cells: They are long, thin stem cell population to generate osteoblast. Periosteum consists of an inner layer of osteoblasts surrounded by osteoprogenitor cells, which have the potential to ifferentiate into osteoblasts, and an outer layer rich in blood vessels and nerves and composed of collagen fibers which penetrate the bone, binding the periosteum to the bone. Periostin is a recently identified protein which is termed so because it was initially identified in the periosteum. It is secreted cell adhesion protein that is 90Kda in size. Structurally, it is a disulfide linked protein that favors osteoblast attachment and spreading. Osteoblast adherence is mediated through the presence of αvβ3 and αvβ5 integrins that are upregulated in the presence of periostin. Endosteum is composed of a single layer of osteoblasts and a small amount of connective tissue. The inner layer is the osteogenic layer and the outer is the fibrous layer. FENESTRATION AND DEHISCENCE The anatomy of the alveolar processes depends upon the alignment and position of the teeth. When the teeth are in extreme buccal or lingual version the alveolar process is extremely thin or missing on that side of the teeth. Fenestrations are the isolated areas in which root is denuded of bone and marginal bone is intact. Dehiscences are the denuded areas that extend through the marginal bone. Dehiscence and fenestration are both associated with extreme buccal or lingual version of teeth. It occurs in 20% of all teeth. The defects are very important clinically because where they occur the root is covered only by the periosteum and overlying gingiva, which may atrophy under irritation and expose the root. COUPLING It is interdependency of osteoblasts and osteoclasts in remodeling of the bone. Osteoblast–osteoclast Coupling The development of osteoclasts is controlled by the stromal cells through the RANK/ RANKL/ OPG axis. RANK (Receptor activator of nuclear factor kappa) is activated through its ligand RANKL. RANKL is produced from the osteoblasts/stromal cells. Upon RANK/RANKL binding, activation of osteoclasts occurs which subsequently leads to bone resorption. On the other hand, osteoprotegrin (OPG) is also produced by various mesenchymal cells and acts as a soluble decoy receptor of RANKL. OPG binds to RANKL and prevents downstream activation. As downstream signaling does not occur, there is no activation of transcription factors and therefore no osteoclastogenesis. BONE MODELING AND REMODELING Process by which the overall size and shape of bone is established is called as bone modeling. It extends from embryonic bone development to the pre-adult period of human growth, which is continuous and covers a large surface. It represents a change that occurs within the mineralized bone without a concomitant alteration of the architecture of the tissues. Thus, there is change in the initial bone architecture. Bone remodeling or bone turnover occur in order to allow the replacement of old bone by new bone. It does not stop when adulthood is reached, although its rate slows down, which is cyclical and usually covers a small area. It involves two processes - bone resorption and bone apposition. Thus, modeling and remodeling occur throughout life to allow bone to adapt to the external and internal demands. As the osteoclasts move through the bone, the leading edge of resorption is termed the cutting cone and is characterized in cross section by a scalloped array of Howship’s lacunae, each housing osteoclasts. When portion of an earlier osteon is left unresorbed, it becomes an interstitial lamella. Behind the cutting cone is the migration of mononucleated cells onto the roughened cylinder. As these cells differentiate into osteoblasts, they produce a coating termed the cement or reversal line. It is a thin layer of glycoproteins comprising atleast bone sialoprotein and osteopontin that acts as a cohesive, mineralized layer between the old bone and the new bone to be secreted. On top of the cement line osteoblasts begin to lay down new bone matrix, mineralizing it from the outer to inner side. The entire area of the osteon where active formation occurs is termed the filling cone. As formation proceeds, some osteoblasts become osteocytes. Once formation is complete, the Haversian canal contains a central blood vessel and a layer of inactive osteoblasts, the lining cells that communicate by means of cell processes with the embedded osteocytes.  The repeated deposition and removal of bone tissue accommodates the growth of a bone without losing function or its relationship to neighboring structures during the remodeling phase. The various bone resorbing factors are: Systemic factors (parathyroid hormone, parathyroid related peptide, Vitamin D3 and thyroid hormone); Local factors (prostanoids, lipoxygenase metabolites, IL – 1, TNF – α, TNF – β, IL – 6); Growth factors (EGF, TGF – α, TGF – β, PDGF) and Bacterial factors lipopolysaccharides, capsular material, peptidoglycans, lipoteichoic acids). Ten Cate described the sequence of events in the resorptive process as below: 1. Attachment of osteoclasts to the mineralized surface of bone. 2. Creation of a sealed acidic environment through action of the proton pump, which demineralizes bone and exposes the organic matrix 3. Degradation of the exposed organic matrix to its constituents amino acids by the action of released enzymes, such as acid phosphatase and cathepsine 4. Sequestering of mineral ions and amino acids within the osteoclast. Rest is explained in chapter no. 24 Bone Defects. VASCULAR SUPPLY The vascular supply to the bone enters the interdental septa through nutrient canals together with veins, nerves and lymphatics. Dental arterioles, which also branch off the alveolar arteries, send tributaries through the periodontal ligament and some small branches enter the marrow spaces of the bone through the perforations in the cribriform plate. Small vessels emanating from the facial and lingual compact bone also enter the marrow and spongy bone.