PERIODONTAL LIGAMENT

Guided by - Submitted by - Dr. Prashant Bhusari Dr .Hitesh Mankad

CONTENTS INTRODUCTION DEVELOPMENT OF PDL CELLS OF PDL EXTRACELLULAR SUBSTANCE Fibers Ground substance

STRUCTURES PRESENT IN CONNECTIVE TISSUE Blood vessels Nerves Lymphatics Cementicles

FUNCTIONS OF PDL PERIODONTAL LIGAMENT HOMEOSTASIS NORMAL CELL BIOLOGY AGE CHANGES IN PDL UNIQUE FEATURES OF PDL CLINICAL CONSIDERATIONS CONCLUSION

INTRODUCTION The PERIODONTIUM is a connective tissue organ, covered by epithelium that attaches the teeth to the bone of the jaws and provides a continually adapting apparatus for support of the teeth during function.

It comprises of cementum ,periodontal ligament , alveolar bone .It is attached to the dentin of the root of the tooth by cementum and to the bone of the jaws by alveolar bone.

PDL occupies the periodontal space which is located between the cementum and the periodontal space of the alveolar bone , and extends coronally to the most apical part of lamina propria of gingiva . At the apical part foramen it is continuous with the dental pulp.

The average width of periodontal ligament space is documented to be about 0.2 mm, though considerable variation exists.

DEVELOPMENT Immediately before tooth eruption and for sometimes thereafter, active fibroblasts adjacent to cementum of the coronal third of root, appear to become aligned in an oblique direction to long axis of the tooth .

Later the first collagen fiber bundles of the ligament becomes discernible. These are the precursors of the alveolar crest fibre bundle group.

Cemental and alveolar fibers continue to elongate towards each , to meet and fuse .

As the first occlusal contact of the tooth occurs with its antagonist the principal fibres around the coronal third of root, horizontal group are almost completely developed .

Oblique fibres are still being developed in middle third of root. With the formation of apical fiber group, the definitive periodontal ligament architecture is established.

CELLSThe principal cells of healthy, functioning periodontal ligament are concerned with the synthesis and resorption of alveolar bone and fibrous connective of the ligament and cementum . The cells of the PDL may be divided as -

Synthetic cells

Resorptive cells

Progenitor cells

Defence cells

Cells rests of

malassez

FIBROBLASTS

The fibroblasts is the predominant cell in the pdl . These fibroblasts origin in part of from the ectomesenchyme of investing layer of dental papilla and from the dental follicle . Pdl contains a fibroblasts cell populations with different functional characteristics .

These fibroblasts are regularly distributed throughout the ligament and are oriented with their long axis parallel to the direction of collagen fibrils .

Fibroblasts of pdl generate an organizational pattern as they have ability to both synthesize and shape the proteins of the extracellular matrix in which collagen fibrils form bundles that insert into tooth and bone as SHARPEY’S fibers .

Once embedded in the wall of alveolus or tooth , these fibers calcify to a certain degree and are associated with an abundance of non collagenous proteins found in the bone i.e. osteopontin and bone sialoprotein .

SYNTHETIC CELLS

OSTEOBLASTS These cells covering the the periodontal surface of the alveolar bone constitute a modified endosteum and not a periosteum .

A cellular layer but not an fibrous layer is present on the periodontal surface of the alveolar bone . The surface of the bone is covered largely by osteoblasts in various stages of differentiation . As well as by occasional osteoclasts .

These are the cells lining the tooth socket . These cells are cuboidal in shape with a prominent round nucleus at the basal end of the cell . Rough endoplasmic reticulum , mitochondria , and vesicles are abundant in active cells .

These cells appear basophilic due to the presence of abundant rough endoplasmic reticulum . The cells contact one another through desmosomes and tight junctions .

Its distribution is similar to that of osteoblasts on the bone surface . These cells line the surface of cementum . They are cuboidal with a large vesicular nucleus , with one ore more nucleoli and abundant cytoplasm.

All the organelles are required for protein synthesis and secretion are present . Cells actively depositing cellular cementum exhibit abundant basophilic cytoplasm and cytoplasmic processes .

CEMENTOBLASTS

OSTEOCLASTS : - These resorb bone and tend to be large and multinucleated but can also be small and mononuclear . Multinucleated osteoclasts are formed by fusion of precursor cells similar to circulating monocytes.

These when viewed in light microscope are cells occupy bays in bone or surround end of bone spicule .

The part of plasma membrane lying adjacent to bone that is being resorbed is raised in characteristic folds and is termed the ruffled or striated border.

RESORPTIVE CELLS

The ruffled border is separated from the rest of plasma membrane by a zone of specialized membrane that is closely applied to the bone the underlying cytoplasm of which tends to be devoid of organelles and has been called the clear zone .

The area of bone that is sealed off by virtue of active pumping of protons by the osteoclast into this environment .

FIBROBLASTS: - These cells show rapid degradation of collagen by fibroblast phagocytosis and is the basis for fast turnover in periodontal ligament . Collagen degradation was an extracellular event involving the activity of the enzyme collagenase .

Intracellular collagen profiles are organelles present . These are associated with the degradation of collagen that has been ingested from extracellular environment . Some studies suggested that collagen degradation is intracellular .

The extracellular elements in degradation of collagenase involve Collagenase which cleaves the triple helical portion of molecules within the fibrils .

INTRACELLULAR DEGRADATION - Fibroblasts are capable of phagocytosing collagen fibrils from extracellular environment and degrading them inside phagolysomal bodies . Collagenase is not involved in the intracellular phase of degradation of collagen fibrils .

CEMENTOCLASTS: - These resemble osteoclasts and sometimes found in normally functioning periodontal ligament . Cementum is not remodeled in the fashion of alveolar bone and periodontal ligament . Its origin is unknown bit it is conceivable that they arise in the same manner as osteoclasts .

PROGENITOR CELLS : - All connective tissues including periodontal ligament contain progenitors for synthetic cells that have the capacity to undergo mitotic division .

If they were not present there would be no cells available to replace differentiated cells lying at the end of their life span or as a result of trauma.

These cell populations within the ligament appear to be in highest concentrations in locations adjacent to blood vessel and exhibit some of the classical cytological features .

Epithelial rests of malassezThe ligament contains epithelial cells that are found close to the cementum . At the time of cementum formation the continuous layer of epithelium that covers the surface of newly formed dentin breaks into lacelike stands . The epithelial rests persist as a network stands islands or tubelike structures near and parallel to the surface of the root . Their function is not clear but they could be involved in periodontal repair and generation . These cells rests can be distinguished from fibroblasts in pdl by the close packing of their cuboidal cells and their nucleus stains more deeply . They are more numerous in older individuals and more numerous in children . These cells may proliferate to form cysts and tumors. These cells may undergo calcification to become CEMENTICLES.

MAST CELLS – These are relatively small round or oval cell having a diameter of about 12 to 15 um . Mast cells are often associated with blood vessels . These cells are characterized by numerous cytoplasmic granules which frequently obscure the small , round nucleus .

Mast cells histamine plays a role in the inflammatory reaction and have been shown to de granulate in response to antigen – antibody reaction on their surface .

The release of histamine into the extracellular environment causes proliferation of endothelial cells and mesenchymal cells .

DEFENCE CELLS

MACROPHAGES- These are found in the ligament and are predominantly located adjacent to blood vessels . The wandering type are derived from blood monocytes has a characteristic ultrastructure that permits it to be readily distinguished from fibroblasts .

EOSINOPHILLS – These are seen in the periodontal ligament . They posses granules that consist of one or more crystalloid structures . These are capable of phagocytosis .

EOSINOPHILLS

EXTRACELLULAR SUBSTANCE

Collagen Proteoglycans

Elastic Glycoproteins

Reticular Indifferent fiber plexus Oxytalan

GROUND SUBSTANCE FIBERS

FIBERS The connective tissue fibers are mainly collagenous, but there may be small amounts of oxytalan and reticulin fibers and in some species, elastin fibers.

The collagen is gathered to form bundles approximately 5 um in diameter. These bundles are termed as principal fibers. Within each collagen bundle , subunits are present called collagen fibrils. These fibrils are formed by packing togather of individual tropocollagen molecules . The diameter of collagen fibril is 45-55 nm .

COLLAGEN

The main types of collagen in the periodontal ligament are type I and type III. More than 70 % of pdl is type I . Type I is uniformly distributed in the ligament .

Type III collagen accounts for about 20 % of collagen fibers . The function of type III May be involved with collagen turnover , tooth mobility and collagen fibril diameter . Type IV and VII are associated with epithelial cell rests and blood vessels . Type XIII collagen is believed to occur within the pdl only when ligament is fully functional .

The principal fiber group is alveolodental ligament which consists of 5 fiber groups -

Inter radicular

Alveolar crest

Apical

Oblique

1. Alveolar crest : - These fibers extend obliquely from cementum just beneath the junctional epithelium to the alveolar crest .These fibers resist tilting, intrusive , extrusive and rotational forces .

2. Horizontal group : - These fibers run at right angles to the long axis of the tooth from cementum to bone and are roughly parallel to the occlusal plane of the arch. These are apical to the alveolar crest group .They resist horizontal and tipping forces.

3. Oblique group : - These are most numerous and occupy nearby 2/3 rd of ligament . These are inserted into alveolar bone at a position coronal to their attachment to cementum . These fibers resist vertical and intrusive forces .

4. Apical group : - From the cementum at the root tip , fibers radiate through the periodontal space to become anchored into the fundus on bony socket. These fibers resist the forces of luxation may prevent tooth tipping and probably protect delicate blood and lymph vessels and nerves traversing and periodontal ligament space at the root apex .

5. Interradicular group : - The principal fibers of this group are inserted into the cementum from the crest of interradicular septum in multirooted teeth . These fibers resist tooth tipping, torquing and luxation . These fibers are lost, if age related gingival recession proceeds to the extent that the furcation area is exposed.

SHARPEY’S FIBERS

Collagen fibers are embedded into cementum on one side of the periodontal space and into the alveolar bone on other . These embedded fibers are termed Sharpey’s fibers . These fiber’s are more numerous but smaller at their attachment into the alveolar bone .

The mineralized parts of sharpey’s fibers in alveolar bone appear as projecting stubs covered with mineral clusters. These fibers in primary acellular cementum are mineralized fully , those in cellular cementum and bone are generally mineralized.

Few fibers pass through the bone uninterruptedly through the bone of alveolar process to continue as principal fibers of the adjacent periodontal ligament or they may mingle bucally and lingually with fibers of periosteum that covers the outer cortical plates of alveolar process . These fibers pass through the alveolar process only when the process contains entirely of compact bone and contains no haversian system.

ELASTIC FIBERS There are three types of elastic fibers which are histochemically and ultrastructurally different . And are mature elastic fibers , eulanin fibers and the oxytalan fibers

Eulanin fibers and oxytalan fibers have been described as immature elastic fibers . Mature elastic fibers consist of microfibrillar component surrounding an amorphous core of elastin protein .

Elastin proteins contains a high percentage of glycine, proline, and hydrophobic residues with little hydroxyproline . These fibers are observed only in walls of afferent of blood vessels , where they constitute the elastic laminae of larger arterioles .

Eulanin fibers are seen as bundles of microfibrils embedded in a relatively small amounts of amorphous elastin .

These fibers may be found with the fibers of gingival ligament . Oxytalan fibers appear to consist of microfibrillar component only .

The orientation of these fibers is different from that of collagen fibers. Instead of running from bone to tooth they tend to run in axial direction one end being embedded in cementum or possibly bone and the other end in wall of blood vessel.

Within the pdl they run longitudinally oriented crossing the fibers perpendicularly . Its function is unknown.

INTERMEDIATE PLEXUS Earlier it was believed that principal fibers frequently followed a wavy course from cementum to bone and are joined in the mid region of the periodontal space giving rise to a zone of district appearance the so called Intermediate plexus . The plexus was considered to be an area of high metabolic activity .

But recent research suggests cemental fibers meet and fuse with osseous fibers no such plexus remains .

Secondly the entire pdl is metabolically active , not just the middle or intermediate zone .

The recent concept is that, fibers cross the entire width of periodontal space but branch en route and join neighboring fibers to from a complex three dimensional network .

RETICULAR FIBERSThese are immature collagen fibers with argyrophilic staining properties and are related to basement membrane of blood vessels and epithelial cells which lie within the periodontal ligament .

SECONDARY FIBERS These are located between and among the principal fibers . These fibers are relatively non – directional and randomly oriented . These are associated with paths of vasculature and nervous elements .

INDIFFERENT FIBER PLEXUS These fibers run in all directions forming a plexus .

GROUND SUBSTANCE

Ground substance composed of glycoproteins and proteoglycans . Ground substance has been estimated to contain 70 % water and is thought to have a significant effect on the tooth ‘s ability to withstand stress loads .

Ground substance is a gel like matrix in which are embedded the cellular components such as collagen . Berkovitz et al estimated that ground substance accounted for 65 % of the volume in the pdl .

All anabolites reaching the cells from the microcirculation in the ligament and all catabolites passing in the opposite direction must pass through the ground substance . Its integrity is essential if the cells of ligament are to function properly .

The ground substance consists of mainly of hyaluronate , glycosaminoglycans , proteoglycans and glycoproteins . All components are presumed to be secreted by fibroblasts .

Proteoglycans are compounds containing anionic polysaccharides covalently attached to a protein .

Glycosaminoglycans are linear polymers of disaccharide repeat sequence which contains a hexosamine,,heparin sulfate and hexuronic acid .

Substrate adhesion molecules such as tenascin , osteonectin , laminin , undulin , and fibronectin have been identified in pdl .

INTERSTITIAL TISSUE Some of blood vessels , lymphatics , and nerves of the pdl are surrounded by loose connective tissue and can be readily recognized in light microscope .

STRUCTURES PRESENT IN CONNECTIVE TISSUEThe following discrete structures are present in connective tissue of pdl

Blood vessels Lymphatics Nerves Cementicles

BLOOD VESSELS – The blood supply is derived from inferior and superior alveolar arteries to the mandible and maxilla respectively and reach the pdl from these sources – 1. Branches in the pdl from apical vessels that supply the dental pulp . 2. Branches from the intraalveolar vessels – These run horizontally penetrating the alveolar bone to enter the pdl . 3 . Branches from gingival vessels – These enter the ligament from the coronal direction . The arterioles and capillaries of the microcirculation ramify in the pdl forming a rich network of arcades that is more evident in the half of the periodontal space adjacent to bone than that adjacent to cementum . There is a particularly rich vascular plexus at the apex and in the cervical part of the ligament .

The interradicular arteries branch into vessels of lesser caliber to emerge from the cribiform plate as perforating arteries and supply the pdl along most of the coronoapical extent including the bifurcation and trifurcation arteries .

The interdental artery also exit the bone to supply the middle three fifth of the pdl though most of the interdental arteries emerge from the crest of the alveolar process and supply the coronal aspect of pdl .

The pdl has some specialized features in the vasculature namely the presence of large number of fenestrations in the capillaries and a cervical plexus of capillary loops .

VENOUS DRAINAGE- The venous channels accompanying their arterial counterparts . The channels are larger in diameter with mean average of 28 um . These channels receive blood from the capillary network and also specialized shunts called glomera in the pdl . These shunts provides an arteriovenous anastomosis .

LYMPHATIC DRAINAGE - A network of lymphatic vessels following the path of the blood vessels , provides the lymph drainage of the pdl . The flow is from the ligament toward and into the adjacent alveolar bone .

It may course apically through the substance of pdl to arise and pass through the fundus of the socket or may through the cribiform plate . They finally enter into larger channels after pursuing intraosseous path .

The flow is via the alveolar lymph channels which are joined by the dental and interrradicular lymph channels .

NERVES – The pdl has functionally two types of nerve fibers sensory and autonomic . The sensory fibers are associated with nociception and of mechanoception , with touch , pressure , pain and proprioceptive sensations . The autonomic fibers are associated with pdl vessels .

All pdl innervations are mediated by the dental branches of alveolar nerves which enter through apical perforation of the tooth socket and perforating branches of interalveolar nerves traversing the bone . Nerves which usually are associated with blood vessels pass through foramina in the alveolar bone including the apical foramen to enter the pdl . In the region of apex apex they run toward the cervix whereas along the length of root they branch and run both coronally and apically .

Nerve fibers are either of large diameter and myelinated or small diameter in which case they may or not be myelinated .

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The pdl is abundantly supplied with sensory nerve fibers capable of transmitting tactile pressure and pain sensations by the trigeminal pathways . Nerve bundles pass into pdl from the periapical area and through channels from the alveolar bone that follow the course of the blood vessels .

The bundles divide into single myelinated fibers which ultimately loose their myelin sheath and end in one of four types of neural termination

1. Free endings which have a tree like configuration and carry pain sensation

2. Ruffini – like mechanoreceptor located primarily in the apical area

3. Coiled Meissner’s corpuscle also mechanoreceptor found mainly in the midroot region

4. Spindle like pressure and vibration endings which are surrounded by a fibrous capsule and located primarily in the apex .

CEMENTICLES - Calcified bodies called cementicles , sometimes found in the pdl . These bodies are seen in older individuals and they may remain free in the connective tissue and may fuse into large calcified masses or they may be joined with the cementum . As the cementum thickens with advancing age it may envelop these bodies . When they are adherent to the cementum they form excementoses. The origin of these calcified bodies is not established . It is possible that degenerated epithelial cells form the nidus for their calcification .

FUNCTIONS

The Periodontal ligament has the following functions : -

Supportive Sensory Nutritive Homeostatic Eruptive

SUPPORTIVE : - When a tooth is moved in its socket as a result of forces acting on it during mastication or through application of an orthodontic force part of periodontal space will be narrowed and the periodontal ligament in these areas will be compressed . Other parts will be widened .The compressed ligament provides support for the loaded teeth .

The collagen fibers in the ligament in concert with water molecules and other molecules bound to collagen act as a cushion for the displaced tooth . The pressure of blood in the numerous vessels also provides a hydraulic cushion for the support of the teeth . Thus PDL behaves as suspensory ligament . Accordingly load on the PDL is dissipated to alveolar bone thorough the principal fibers of PDL primarily , which is placed in tension and on release of load ,an elastic recoil of tissue enables the tooth recovery to its resting position .

SENSORY : - The PDL provides a most efficient proprioreceptive mechanism , allowing the organism to detect the application of most delicate forces to the teeth and very slight displacement of the teeth . Mechanoreception protects both supporting structures of the tooth and the substances of the crown from excessive masticatory forces .

The responsive elements of stromal cells and actin dependent sensory system are involved in the mechanical signal transduction . To survive a mechanically active environment cells adapt to variations of applied membrane tension . This involves sensing increase in intracellular tension maintaining contact with extracellular matrix ligands and preventing irreversible membrane disruptions .

NUTRITIVE : - The ligament contains blood vessels , which provide anabolites and other substances required by the cells of the ligament , by the cementocytes . The blood vessels are also concerned with the removal of catabolties . Occlusion of blood vessels leads to necrosis of cells in the affected part of ligament this occurs when too heavy a force is applied to a tooth in orthodontic therapy .

HOMEOSTATIC : - Its is evident that the cells of pdl have the ability to resorb and synthesize the extracellular substance of the connective tissue of the ligament , alveolar bone and cementum .

Alveolar bone appears to be resorbed and replaced at a rate higher than other tissue in jaws . Furthermore the collagen of pdl is turned over at a rate that may be the fastest of all connective tissues in the body and the cells in the bone half of ligament may be more active than those on the cementum side.

The mechanism whereby cells responsible for these processes of synthesis and resorption are controlled are largely unknown .

PERIODONTAL LIGAMENT HOMEOSTASIS

A remarkable capacity of pdl is that it maintains its width more or less overtime despite the fact that it is squeezed between two hard tissues . Studies indicate that the population of cells within the pdl both during development and regeneration secreate molecules that can regulate the extent of mineralization and prevent fusion of tooth root with surrounding bone . Various molecules have been proposed which plays a role in maintaining an unmineralized pdl .

PERIODONTAL CELLS can inhibit mineralized bone nodule formation by bone stromal cells and studies have reported that inhibition may be dependent on prostaglandin production .

Msx2 prevents the osteogenic differentiation of pdl fibroblasts by repressing Runx2 also known as cbfa 1 transcriptional activity . The balance between the activities of bone sialoprotein and osteopontin may also contribute in maintaining pdl region .

Matrix Gla protein an inhibitor of minerlization is also present in the pdl tissues .

Glycosaminoglycans or RGD – cementum attachment protein a collagen associated may play a role in maintaining the unmineralised state of pdl .

Matrix gla protein an inhibitor of mineralization is also present in periodontal tissues . Studies suggest that it may play a role in preserving width of the ligament .

When the functional demand increases the width of pdl can increase by as much as 50 % and fiber bundles also increase in thickness . Conversely reduction in function leads to narrowing of ligament and decrease in number and thickness of fiber bundles .

NORMAL CELL BIOLOGY The production and destruction of tissue matix ( turnover ) in a healthy state , involves interaction among a myriad of effector molecules that are synthesized and secreted by resident cell of periodontal ligament . Growth factors and cytokines that are believed to play a role in the pathogenesis of gingivitis and periodontitis .

Cytokines are a series of multifunctional polypeptides and glycoproteins that are secreted by one or several cell types and act locally or systemically . These includes Interleukins , cytotoxic factors , interferons , growth factors , colony stimulating factors and intercrines .

Growth factors have been defined as substances capable of re – initiating proliferation of cells that are in a quiescent state .

In vivo cytokines play an important role in numerous biological events , including development , homeostasis , regeneration , repair , inflammation and neoplasia .

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1 . Fibroblast growth factors (FGF) - Two of seven isoforms of fibroblast growth factors have been described in particular one is acidic and other basic .

Acidic fibroblast growth factors has effects on endothelial cell replication and neovascularisation . It stimulates dna synthesis and cell replication , in bone tissue cultures which results in increased protein synthesis especially type 1 collagen .

Basic fibroblast growth factors has angiogenic properties has highly chemotactic and mitogenic for a variety of cell types . It stimulates bone cell replication and increases the number of cells of osteoblastic lineage .

Fibroblast growth factor bind to heparin sulfate heparin and fibronectin in the extracellular matrix . Its is potent stimulator of periodontal cell migration and mitogenesis but its effect but is effect on matrix production is not clear .

2 . Platelet derived growth factor ( PGDF ) This factor is potent growth factor for various connective tissue cells and is released from the a – granules in platelets in conjunction with blood coagulation . PGDF is a promoter of cell migration and a potent mitogen for cells bearing PGDF receptors . It acts synergistically with other growth factors as a competence factor .PGDF stimulated type v collagen formation and a drop in type III production in gingival fibroblasts .

3 . Transforming Growth factor ( TGF ) : - These factors are polypeptides isolated from normal and neoplastic tissues which are known to cause a change in normal cell growth . TGF is of 2 types α and b according to relationship to EGF . TGF – α similar biological effects acting through EGF receptor . TGF – β was originally purified from human placenta , platelets and bovine kidney . It stimulates the synthesis of connective tissue matrix components such as collagen , fibronectin proteoglycan and glycosaminoglycans .

4 . Interleukin- 1 ( IL – 1 ) : - Interleukin – 1 is a polypeptide with a great number of roles in immunity , inflammation , tissue breakdown and tissue homeostasis . It is synthesized by various cell types including macrophages , monocytes , lymphocytes vascular cells brain cells skin cells and fibroblasts following cellular activation . 2 types of IL are known interleukin – 1 α and 1β .

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5 . Interferon – Y : - It posses important immunomodulatory effect and thus is a lymphokine as much as an interferon . Its production is modulated by other cytokines such as interleukin – 1 . Many biological activities have been ascribed to interferon like action on B and T lymphocytes , antibody production , natural killer cells , macrophages and tumour cells .

6 . Matrix metalloproteinases and their tissue inhibitors : - Connective tissue cells participate in both the formation and breakdown of connective tissue matrix . Such cells are found to synthesize and secrete a family of enzymes known as MMP’s . MMP gene family encodes a total 24 homologous proteinases classified into collagenases , gelatinases , stromelysins , membrane type MMP ‘s depending on their susbstrate specificity and molecular structures .

AGE CHANGES IN PDL

The cell number and cell activity decreases with aging . One of prominent changes seen in the the calcified tissues of periodontium , the bone and the cementum is scalloping and the pdl fibers are attached to the peaks of these scallops than over the entire surface as seen in a younger periodontium . This remarkable changes affect the supporting structures of the teeth .

With aging the activity of the pdl tissue decreases because of restricted diets and therefore normal functional stimulation of the tissue is diminished . Any loss of gingival height and periodontal disease promotes destructive changes in the PDL .

UNIQUE FEATURES OF PERIODONTAL LIGAMENT

The periodontal ligament is made up of collagen fibers in a proteoglycans stroma and many types of connective tissue cells as in any other soft fibrous connective tissue elsewhere in the body . But it has cells that form and resorb cementum and bone and the collagen fibers in a specific orientation connecting the two mineralized tissues make it unique .

The tissue hydrostatic pressure is high . The tissue is extremely cellular with fibroblast showing many intercellular contacts well innervated with many mechanoreceptors and highly vascular unlike any other connective tissue in the adult . The features being high cellularity, very high rates of turnover and with significant amount of type III collagen.

The collagen fibers are also sharp with unimodal size and frequency . The ground substance of pdl occupies large volume with high content of glucornate rich proteoglycans and glycoprotein- tenascin and fibronectin .

Thus the pdl has structural , ultrastructural , and biochemical features like fetal tissue . This has helped us to undertsand periodontal inflammatory diseases and for evolving newer treatment modalities .

CLINICAL CONSIDERATIONSThe primary role of the periodontal socket is to support the tooth in the bony socket . Its thickness varies in different individuals in different teeth in the same person and in different locations on the same tooth .

1. Acute trauma to the periodontal ligament, accidental blows or rapid mechanical destruction may produce pathologic changes such as fractures or resorption of the cementum tears of fiber bundles , hemorrhage and necrosis .

2. The adjacent alveolar bone is resorbed the pdl is widened and tooth becomes loose . When trauma is eliminated repair usually takes place .

3. Orthodontic tooth movement depends on resorption and formation of tooth bone and periodontal ligament . These activities can be stimulated by properly regulated pressure and tension . If the movement of teeth is within phsysiologic limits the initial compression of pdl on the pressure side is compensated for by bone resorption whereas on the tension side bone apposition is seen .

4. Application of large forces results in necrosis of pdl and alveolar bone on the pressure side and movement of the tooth will occur after the necrotic bone has been resorbed by osteoclasts located on its endosteal surface .

5 . Inflammatory diseases of the pulp progress to the apical periodontal ligament and replace its fiber bundles with granulation tissue . This lesion is called a periapical granuloma may contain epithelial cells that undergo proliferation and produce a cyst .

6 . Chronic inflammatory disease is common pathology related to pdl . The toxins released from the bacteria in the dental plaque and metabolites of the host’s defence mechanism destroy the pdl and the adjacent bone very frequently . This leads to tooth mobility and further loss of tooth .

7 . To repair the existing destruction of pdl can be quite challenging . It involves limiting the disease process and to regenerate the host tissues to their original form in such a way that reattachment of pdl to bone becomes possible .

8. Various surgical techniques like Guided Tissue regeneration are being used for correction of Periodontal destruction . Guided Tissue regeneration is based on principle that specific cells contribute to formation of specific tissues .Important cells responsible for periodontal regeneration are derived from PDL.

Exclusion of the faster growing epithelium and connective tissue from a periodontal wound for 6 to 8 weeks allows the slower growing tissues to occupy the space adjacent to the tooth .

Osteoblasts , cementoblasts ,and periodontal ligament cells are then afforded the new opportunity to regenerate a new periodontal attachment on the previously diseased root surface .

Non resorbable materials have been used as barrier membranes including latex and teflon . Resorbable materials include include polyglycolic acid , trimethlene carbonate , bilayer porcine- derived collagen .

9. Fusion of alveolar bone and cementum with obliteration of the periodontal ligament is termed Ankylosis . Ankylosis occurs in teeth with cemental resorption which suggests that it may represent a form of abnormal repair . Ankylosis also may develop after chronic periapical inflammation , tooth implantaion and occlusal trauma and around embedded teeth . Clinically ankylosed tooth sounds DULL or WOODY on percussion. Before extraction such tooth require X-ray to fascilitate surgical extraction.

10 . Osseointegration is an intimate bone to implant contact without presence of PDL in between. So this is not the ideal substitute for natural tooth replacement. Future studies will be directed to regenerate PDL fibers as an interface between bone and implant. .Some fiber bundles are present which have a cuff like circular orientation . The role of these fibers remains unknown but it appears that there presence helps to create a soft – tissue seal around the implant .

CONCLUSION The periodontal ligament is a fibrous connective tissue forming important part of the periodontium . Without it tooth is support less .Cell of the periodontal ligament are pluri-potential and helps in the regeneration of all the components of periodontium lost in the periodontal disease. A better understanding of cell and molecular biology of developing and regenerating periodontium offers newer avenues to regenerate the pdl . Newer options of treatment are made available from time to time yet safeguarding the integrity of the pdl and alveolar bone is still one of the most important challenge .

REFRENCES 1. Orban ‘s Oral histology and embryology - 12 th edition2 . Tencate ‘s Oral histology – 6 th edition3. Carranza’s clinical periodontology – 10 th edition4. Satish chandra ‘s oral histology 5.. dental.pit / pdl6.. wikipedia.com/ pdl 7. . Google.com/images / pdl

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