 Hello everyone, welcome back to another session in dentistry and more. Today's topic is parodontal ligament. Parodontal ligament is one of the four components of parodontium. So parodontium is a structure which supports the tooth. So it includes two hard tissues and two soft tissues. Hard tissues are cementum and alveolar board. Soft tissues are ginjeva and parodontal ligament. Ginjeva we are already covered. So today's session is about parodontal ligament. So let's get into the details of parodontal ligament. So parodontal ligament the name itself gives an idea. Peri means around and don't means tooth. So the structures which is around the tooth. So the ligament which is surrounding the tooth is parodontal ligament. So as I mentioned it is one of the four components of parodontium. So it is defined as the parodontal ligament is a connective tissue that surrounds the root and connects it to the bone. So this orange color is parodontal ligament. So it is a connective tissue that surrounds the root and connects it to bone. So it starts from root to the bone and it communicates with the marrow spaces through vascular channels in the bone. So it is something or it is a connective tissue which surrounds the root and connects it to the bone. It is given by Caranza and it has got many names. The synonyms include parodontal membrane, alveolar dental ligament, desmodont, perisemum, gum forces or dental periosteum. So this is a very important essay in dental histology. So this is very frequently asked essay and there are lots of short notes will be asked from this chapter. So you might get a question like explain about gum forces and write about its principal fibers or draw a neat picture and explain about its structure function. So you might be knowing the complete answer of parodontal ligament but you might not be knowing that gum forces is parodontal ligament. So always make sure that you remember all these names such as parodontal membrane, alveolar dental ligament because it connects alveolar bone and tooth, desmodont, perisemum because it covers the symptom, gum forces and dental periosteum. So these are the synonyms of parodontal ligament. Now let us see the extension of parodontal membrane. So this is the parodontal membrane I have drawn in orange color lines. So it extends from coronal direction it is continuous with lamina property of gingiva. So you will be knowing what is lamina property of gingiva because in the gingiva session we had well covered the connective tissue part of gingiva. So this is the this pink color is the connective tissue part of gingiva. So it is continuous with or it is associated with lamina property of gingiva at the coronal side. So this is the crown part, this is the root part. So we say this is the coronal side, this is the apical side. So on coronal side it is continuous with lamina property and it is demarcated by the alveolar crest fiber. So here it is alveolar crest fiber because this is a bone, this is a tip of the bone on a alveolar crest. So such fibers are demarcating this coronal extension of parodontal ligament and at the root apex it merges with the dental pulp. So at the root apex, see this is the root apex and this pink color is a pulp. So at the root apex it merges with the dental pulp. And it ranges in width from 0.15 to 0.38 mm. So 0.15 to 0.38 mm is the width of parodontal ligament, this is the width 0.15 to 0.38. So next is the shape of parodontal ligament. So it is thinnest around the middle third of the root. So this is a root portion. So this is middle portion, it is thinnest around the middle portion or middle third of the root with an R glass appearance. This is an R glass appearance. It is broadened at the coronal and apical side but it is thinnest at the middle third of root. Okay, so this is a coronal third, this is a middle third, this is apical third. So it is thinnest at the middle third. So it looks like an R glass with widened coronal and apical third. And we have a radio opaque boundaries of parodontal ligament. So when we take an X-ray, so we can see an empty space. So parodontal ligament will be shown in X-ray as radio loosened area, that means it is completely black. So there is no structure within it. So the X-ray will not produce any image. So it will be very black in color which is known as radio loosened. But it has two white borders which is known as radio opaque borders. So what are those white borders? One is a alveolar bone and other one is a cementum. So it is outlined by alveolar bone and cementum. So in radiograph it looks like a radio loosened area with radio opaque boundaries. So it is between cementum, this is cementum and alveolar bone. So these are the mineralized structures. So it will appear as radio opaque or white areas. So it appears as black area between two white lines, that is radio loosened area between radio opaque lines. So the average width changes based on the age around 10 to 15 years, it is around 0.21 mm and 30 to 50, 30 to 50.21 mm in 10 to 15 years and 0.18 mm around 30 to 50 years and it is around 0.15. So as the age increases the width of this perirondal ligament decreases and also it changes according to time of eruption at function or hyperfunction. Time of eruption it is around 0.1 to 0.5, it is very highest at function it is around 0.2 to 0.3, this one 0.15 to 0.3 but hyperfunction it again reduces. Now let us move on to the development of perirondal ligament. How does it develop? So we have seen develops from dental follicles. So hope you remember our bell stage, advanced bell stage of tooth formation. We have what stage, gap stage, bell stage. We have learned dentine and pulp develops from dental papilla and cementum, perirondal ligament, alveolar bone develops from dental follicle or dental sac. So it is developed from dental follicle. So what happens? It begins with root formation and prior to the tooth eruption. So at later bell stage when amylogenesis and dentinogenesis are well advanced the internal and external lamina so we know that the internal and external lamina when the stratum intermedium collapses the outer enamel epithelium and inner enamel epithelium approximates and it becomes two layer cells, two layer epithelium with outer enamel and inner enamel epithelium. So it forms or at the cervical loop of enamel organ it becomes bent and makes this double layered epithelial root sheath and this root sheath which proliferates epically and forms the future root and forms the future root. So this is how it forms outer enamel epithelium, inner enamel epithelium it bent at root portion or the cervical portion and it proliferates epically so you know what is epically towards the root epically and forms the future root. So what happens? There will be hard wicks epithelial root sheath at advance bell stage which covers the root area. So this root sheath is continuous so this root sheath is continuous. So it loses its structural integrity and forms the remnants which is known as epithelial rest of molasses. So epithelial rest of molasses forms. So once epithelial rest of molasses forms what happens there will be connective tissue. So there will be the connective tissue, okay this is all let it be connective tissue of dandelion follicle. So from dandelion follicle the connective tissue cells from dandelion follicle migrate to the newly formed root of dentine. So it will be like this. So the remnants gives the endurance, gives the pathway of proliferation towards the root dentine, okay. Before it was a hard wicks epithelial root sheath. It was a continuous layer after that it loses its structural integrity. So the dandelion follicle which is present outside continuous proliferation happens and it goes between the epithelial rest of molasses and forms a periodontal ligament. So that is how it forms and dandelion follicle cell basically produces fibroblasts, cementoblasts and osteoblasts. This is collagen, this is cementum and this is bone, okay. Collagen is a principal fibre of peritontal ligament. So ultimately dandelion follicle give rise to peritontal ligament, cementum and alveolar bone. So as the root formation continues cells in the peripholecular meson give rise to, there will be active synthesis of collagen fibres and this collagen fibres assemble and it forms as a bundle on the bone and cemental surface, okay. So it becomes bundles. So from the dental follicle there will be active synthesis and this fibres will be formed and it will be attached to cementum and bone. So this is how it happens. This is alveolar bone proper. This is a peritontal ligament space and this is a root cementum. So as the growth happens this is a root cemental area, this is the alveolar bone area, there will be a continuous proliferation, mitosis happens and ultimately it joins and it become peritontal ligament. So we have a mature peritontal ligament, three areas that is bone, bone related region bone related region and the middle region and the cementum related region. The bone related region is basically very rich in cells and the middle zone is fewer cells with thinner collagen fibres and the cemental related region which is dense and ordered collagen fibres are present. So bone related region associated with alveolar bone proper, middle region it is thinner and thick bundles of fibres are seen in cemental region because this is a tooth area, this is a bone area see this is a tooth, this is a bone. So this is the picture I have showing here just opposite here it is a cementum is a alveolar bone but here it is alveolar bone here is a cementum. So this is just expanded picture. If I draw this picture it will be like this. So this is how it forms, this is enlarged version of this area but only thing you need to think opposite this is alveolar bone this is root but here it is a root this is alveolar bone. So that is how it forms you can see the various formation one the toothy reps where this is covered within the ginger bar. So there is little bit of periodontal ligament formation as the toothy reps there will be more and more periodontal ligament formation when it is completely replete the bundles will be formed and principle fibres will be completely around the root. So what are the functional changes happens in periodontal ligament? So the basically periodontal ligament is always subjected to change when there is increased functional limit the width may increase as much 50% of the present width because more function more functional limit more stress it need to bear so it will increase its width up to 50% it can increase and fibre bundles also increase in thickness when there is a increased functional demand. Well when functional demand decreases the narrowing of periodontal ligament and decrease in number and thickness of fibres happens. So it can modify itself based on the functional changes. So that is the beginning part of periodontal ligament I have explained to you about its extension its shape and the in detail about the development. So you need to study properly about the tooth formation that is but stage cap stage and bell stage then only it is easy to understand if you don't know that chapter properly it's very difficult because you need to what is Hadoic epithelial rest of Hadoic epithelial root treat and epithelial rest of molasses and how this collagion fibres starting from dental follicle to the cemento man bone. So it is a very important chapter because this is commonly asked essay question so the short notes might be Hadoic epithelial root treat, epithelial cell rest of molasses so all these might come as short notes. So next thing is the structural elements that is cells and extracellular elements. So this is the introduction part development extension shapes and how the periodontal ligament changed according to the functional demands. Now we move on to the structure the cells and extracellular elements of periodontal ligament. Thank you. So now we have the structure of periodontal ligament. So we have basically two categories one is cells and the next one is extracellular substance. So under cells we have basically five categories. The first category is synthetic cells the cells which produces other cells that is osteoblasts which produces bone fibroblast, cementoblast. The second one is resoptive cell which distract the cell, osteoclast fibroblast and cementoclast which distracts bone fibres and cementum. Third one is progenitor cell then epithelial rest of molasses and differential. So the basic differential muscle macrophages and snuffles. So synthetic cells we have osteoblast fibroblast cementoblast resoptive cell the destructive cells osteoclast fibroclast and cementoclast this is blast means create, blast means distract. Progenitor cell epithelial rest of molasses we have seen when the sheath was disrupted the hard wicks epithelial root sheath once it loses its continuity the remnants will be on the periodontal ligament as epithelial rest of molasses and the common differential and in extracellular substances that means the main bulk of substances we have fibres and ground substances in fibres we have collagen fibres and oxytalin fibres in ground substances we have glycosaminoglycans and glycoproteins. Now we will move on to the our principle or the chief component of cells sector that is synthetic cell that is fibroblast which is the architect which is a builder which is a caretaker of periodontal ligament which is very predominant in the periodontal ligament. So basically it originates from cemental surface and also from alveolar bone surface. Cemental surface it originates from ectomation kind of investing layer of dental papilla and dental follicle from the cemental surface whereas the alveolar bone side it is originated from peri vascular meson kind. So both sides it originates towards the cemental surface it is originates from the ectomation kind we have seen in dental papilla and dental follicle and in alveolar bone side it is originated from peri vascular meson kind. So these cells which is oriented with their long axis parallel to the direction of collagen fibres. So how the collagen fibres are oriented so it just follows the long axis parallel to the collagen fibres and which is aligned along between collagen fibres and appearance governed by the surrounding matrix. So these fibroblast of periodontal ligament generate an organizational pattern as they have the ability to both synthesize and shape the proteins of extracellular matrix. So it has both the properties that is the ability to synthesize and shape the proteins of extracellular matrix. So these proteins also it can shape and it can synthesize, synthesize other elements also. So the certain fibres or the fibrils form bundles to get inserted into tooth and bone which is known as sharp piece fibres. This is very important sharp piece fibres. Commonly asked short note so what is sharp piece fibres? So certain fibres or fibres in fibroblast it gets bundled or it gets forms in a bundle and get inserted into tooth and bone. So once it is embedded in the wall of alveolus or tooth these fibres calcify to certain degree which fibres sharpies fibres calcify to certain degree and are associated with an abundance of collagenous proteins found in the bone. So these proteins are known as osteopontin and bone silo protein. So these are associated with sharpies fibres. So what are the functions of fibroblast? Okay not osteoblast, functions of fibroblast. So the first function is to synthesize collagen. First is a collagen synthesize then synthesize fibres. Organized fibres network can generate force on tooth eruption, produce extracellular matrix of periodontal ligament which has capacity to give rise to cementoblast and osteoblast. So fibroblast can give rise to osteoblast and also cementoblast. It maintains a normal width of periodontal ligament which is synthesized and shaped the proteins of extracellular matrix I mentioned earlier in which collagen fibres form bundles and insert into the bone or tooth as sharpies fibres. So it can shape the extra cellular matrix proteins. So it regulates collagen turnover by phagocytosis, phagocytosing the old collagen fibres. Now we will move on to the second synthetic cell that is osteoblast. These cells covers the periodontal surface of alveolar bone which line the tooth socket and our cuboidal in shape with prominent round nucleus at the base cell end of the cell which has rough endoplasmic reticulum, mitochondria and vesicles which are very active and abundant in osteoblast. Microfilaments are prominent beneath the cell membrane and the cell contacts one another through dysmosomes and tight junction. What about cementoblast? Cementoblast they line the surface of cementum which are cuboidal with large vesicle nucleus with one or more nuclei and all the organelles required for proteins in the cesan secretion are present in cementoblast. There are two types of cells that is cells with cytoplasmic process and cells without cytoplasmic process. So cells actively depositing cellular cementum exhibit the cytoplasmic processes and basophilic cytoplasm but whereas a cellular cementum producing cells with which it doesn't have prominent cytoplasmic processes. So these are the two types of cells which you seen in cementoblast type cells. Now let's see the resopty cell. Those are osteoclast, fibroclast and cementoclast. So next we have resopty cells. Resopty cells are nothing but which distracts the cells which is doing the function just opposite of synthetic cells. They create this resopty cells. They distract the cells. So the most common one is osteoclast which basically they resorb on and tend to be very large and multi-nucleated but can also be small and mononuclear. So these multi-nucleated osteoclast are formed by fusion of precursor cells similar to circulating monocytes. The part of plasma membrane lying adjacent which is being resorbed is raised in characteristic folds. So you can see the folds over here, the folds. So this is known as ruffled or striated border. So the part of plasma membrane lying adjacent to bone that is being resorbed is raised in characteristic folds which is known as ruffled or striated border. So these are found against the bony surface occupying shallow depression which is known as how ships lacunae. So there are many short notes will be coming from this one. The ruffled or striated border how ships lacunae and one more thing we have that is clear zone. So you can see this area which is devoid of all the organelles. So this ruffled border is separated from the rest of plasma membrane by a zone specialized membrane that is closely applied to the bone. So it is separated from the rest of plasma membrane by a zone of specialized membrane which is very closely applied to the bone and the underlying is a cytoplasm. This is a cytoplasm which tend to be devoid of organelles and that is known as clear zone. So you know still class we have learned what is ruffled or striated border that is this particular appearance that plasma which is plasma cells, plasma membrane which is close to the bone and it is being resorbed which gives that because resorption will not happen in a linear fashion it will be folding in a foldable fashion it happens. And this seen in a particular depression that is this osteoclast seen in particular depression known as how ships lacunae and there will be a clear zone which is a cytoplasm which is devoid of organelles. Whereas a cemento-clast, cemento-clast the peculiar thing about cemento-clast there does not remodel. So these cemento-clast are not usually found in the periodontal argument because it does not remodel. So these cells seen when there is pathological conditions or during resorption of deciduous teeth and when regressive forces are applied because we forcefully apply forces that forces are being applied on orthodontic therapy. So in orthodontic therapy we apply forces that is intentional forces and that cases we can see cemento-clast. So these cemento-clast resemble osteoclast and are located in depression and cementum resembling how ships lacunae. So this is not usually seen in periodontal argument it comes when there is a pathological problem resorption of deciduous bone and the orthodontic forces. And one more thing these cells not only just resorpt cementum but also they can destroy dentine and enamel. So this is also known as odendoclast. So odendome means tooth. So clast is something which is destructing structure. So this is also known as odendoclast because it destruct enamel and dentine. Now we have our third category. So we finished fibroclast is nothing but cell switch destruct fibbus or collagen. So osteoblast creates bone, fibroblast creates collagen, cemento-blast creates cementum, osteoclast destruct bone and fibroclast destruct collagen, cemento-clast destruct cementum. Now we are moving on to our third cell which is progenitor cell. So progenitor cell is all connected tissue including PDL which contains progenitor cells that have the capacity to undergo mitotic division. So mitosis is the basis of replication of cells. So these are undifferentiated mesenchymal cells that have a perivascular location within around 5 micrometers of blood vessels. So when stimulated appropriately these cells undergo mitosis and what are the cells will be formed. So we have fibroblast, osteoblast or cemento-blast that in turn produces collagen, bone and cementum. So progenitor cells are the cells which produces osteoblast, fibroblast and cemento-blast. So this should be here progenitor as a primitive cells that itself give rise to osteoblast, fibroblast, cemento-blast. So that is the synthetic cell of synthetic cell we can say. And we have epithelial rest of molasses. We already discussed it. There will be epithelial ruci that is half weeks epithelial ruci which undergo lysis and there will be epithelial rest of molasses because it loses its continuity and the epithelial rest will be seen along the root surface as like strands or islands or tubular structures which will be parallel to the surface of root which has around 25 micrometre in diameter. So their function is not yet clear but they could be involved in periodontal repair and regeneration. So it is most numerous in apical and cervical areas children it is very numerous as we age the number reduces these cells may proliferate to form am tumors. And one more thing we have the cells that the epithelial rest of cells undergo calcification to become cementicals. So what all we learnt? We learnt ruffle or striated border, how ships lack on it, clear zone, odendoclast and cementicals. These all are shock notes. So osteoclast, osteoplast, fibroblast, fibroclast progenitor cell, epithelial rest of molasses hardwicks, epithelial root sheath cementicals all will be asked for shock notes. So this is a very common SA periodontal recommend so we have not done so we are into our cells. Now we go to the defense cells. So defense cells are common defense cells we have mast cell, e-snow fill and macrophages. Now we need to study the extracellular substances. So we had seen the basic structure the basic origin and its shape after that we moved to the structural cell. Structural cell we have structure and we have cells and extracellular proteins. We finished cells now we have extracellular substances. Now let's see the extracellular substances. So extracellular substances we have fibres and ground substances. In fibres we have collagen elastic reticular, secondary fibres, oxitalin fibres and also in different fibres plexus. So the collagen fibres are the main fibres which is the basic types are type 1 and type 3 the 70% age belongs to type 1 and it is uniformly distributed in the ligament whereas type 3 which accounts for around 20% age found in periphery of sharpies fibres and type 4 and type 7 which are associated with epithelial cell rest and blood vessels. So type 13 which is associated with PDL when the ligament is completely functional and the collagen is gathered to form bundles and approximately 5 micrometer diameter and these bundles are termed as principal fibres so principal fibres are very important the next video I will be doing the principal fibres so within each collagen bundle the subunits are present which is known as collagen fibrills so fibrills combine to form fibres now we have the turnover rate of collagen this is faster than all other connective tissue collagen present in the connective tissue because it is highestly seen in pedodontal ligament the turnover of collagen and the rate appears to be highest towards root apex and the collagen on both side has lower turnover rate than that on the bone side where it shows higher turnover rate so towards the bone side it has having higher turnover rate and now we are moving on to ground substances principal fibres will be dealt in next video the major glycosaminoglycans are contributing sulphate, dermatone sulphate, heparin sulphate, hyaluronic acid and keratin sulphate so glycoproteins we have various type fibronectin these glycoproteins are densely packed proteins with extracellular matrix and they have been localized in the calcified sections of human pedodontal ligament the most common is fibronectin it promotes attachment of cells to substratum especially to collagen it is expressed strongly along attachment sites of collagen fibres to cementum but not bone in addition to its function it is also having functions such as blood coagulation wound healing and chemotaxis so it promotes cell addition to collagen it is associated with collagen fibres to cementum next we have tenacin it is also known as cytotactin it is the other glycoprotein identified in the pedodontal ligament it is found mostly in healing wounds unlike fibronectin it is not uniformly distributed throughout the PDL but is concentrated between the less densely packed collagen fibres near cementum and alveolar bone so laminin is a glycoprotein component of basement membrane of epithelial cell rest of molasses which has properties like cell adhesion, migration and differentiation so these are the basic extracellular substances which are glycosaminoglycans we have many types and also the glycoproteins now let's move on to the principle fibres of pedodontal ligament so pedodontal principle fibres so these fibres are collagenous in nature and it follows a wavy pattern so you can see a wavy pattern when viewed in a longitudinal section this is the cross section this is the longitudinal section so you can see a wavy pattern so they are thought to contribute the regulation of mineralization and tissue cohesion at sites of increased biomechanical strain so when there is a strain so in orthodontic movement when there is strain and stress there will be remodelling and changes in the pedodontal ligaments or the fibres so we have basically five groups principle fibres they are alveolar crest group horizontal group, oblique group apical group and interradicular group it's a very important topic in pedodontal ligament it might be asked as a short note or a long essay so let's see the five principle fibres alveolar crest, horizontal group, oblique group apical group and interradicular group so I have drawn all these the green colour is apical the black colour is oblique the pink colour is horizontal the violet is interradicular the orange is alveolar crest so let's see one by one alveolar crest group is extends obliquely from cementum this is a cementum just beneath the junctional epithelium to the alveolar crest and to the fibres layer of periosteum covering the alveolar bone so this is the alveolar bone and it is a crest so it is starting from the cementum that is oblique you can see this is not horizontal this is not perpendicular this is at an angle ok so this is oblique this is horizontal this is oblique so it extends from cementum just beneath the junctional epithelium and to the alveolar crest so this is alveolar crest and to the fibres layer of periosteum covering the alveolar bone the horizontal fibres they extend from cementum to alveolar bone in horizontal direction at right angle to the long axis of tooth so this is a long axis of tooth and horizontal is at right angle which is starting from cementum to the alveolar bone and it occupy 10-15% of coronal root surface now we have oblique group oblique group it is the largest group of fibres and it is around 80-85% of root surface it extends from cementum in a coronal direction oblique to bone so it is extends from cementum going at oblique direction towards the bone ok so this is the alveolar crest group which is running oblique direction this is the oblique group is the largest periodontal ligament fibres and it is running from cementum to alveolar bone it is around 80-85% of total root surface now we have apical group so this is apical group the apical fibres radiate in a rather irregular fashion from cementum from cementum to the apical region of socket and they do not occur in incompletely formed roots these may occur in incompletely formed roots but apical will not occur until and unless the root formation is complete so this is going in irregular fashion from the cementum to the alveolar to the socket ok so this is apical group and the last one is interradicular group so this is interradicular group this violet one they fan out from cementum to the tooth in the forkation areas of multirooted tooth ok so this is like multirooted tooth they fan out from cementum to the forkation areas now let's see what are the functions of these fibres the alveolar crest are involved in retaining the tooth in the socket which opposes lateral forces forces and it prevents extrusion and intrusion extrusion is moving away from the socket and intrusion is moving towards the socket so it prevents extrusion and intrusion of tooth protect deeper periodontal ligament structures the horizontal group is restraining the lateral tooth movement whereas the oblique group which bear the vertical masturbatory stresses and transform them into tension on alveolar bone and resist intrusive forces so intrusive forces is towards apex so it resists vertical masturbatory forces and the next one is apical group so the basic function of apical group fibres are it prevents tooth tipping and it resists lexation it protect blade, lymph and also place to the tooth since it is at the apical position and the inter radical group which helps in resisting tipping, talking and lexation movements of the tooth so these are the principle fibres of periodontal ligament it's a very important topic so alveolar crest horizontal oblique apical and inter radical you need to draw this picture very neatly and explain the origin and its end and the basic functions now we have various other fibres that is sharpies fibres we already seen these sharpies fibres they are collagen bundles of periodontal ligament embedded into cementum and alveolar bone the orientation is similar to that of adjacent periodontal ligament bundles and they are more numerous but smaller at their attachment into cementum than alveolar bone these fibres in cellular cementum will be fully mineralized but in cellular cementum it is partially mineralized and few sharpies fibres pass uninterruptedly through bone of alveolar process which is known as trans alveolar fibres trans alveolar fibres are nothing but sharpies fibres which pass uninterruptedly through bone of alveolar process and the next one is intermediate plexus so intermediate plexus in the beginning it was believed that principle fibres follow a wavey course from cementum to bone and are joined in the mid region of the periodontal space giving rise to a zone of distinct appearance which is known as intermediate plexus which is starting from cementum to bone in a wavey course at the middle region a peculiar appearance periodontal space giving a zone of distinct appearance which is known as intermediate plexus in recent time the concept has little bit changed so the recent concept is that fibres cross the entire width of periodontal space but branches and join neighboring fibres to form a complex three dimensional network so forming a three dimensional network is the reason for that peculiar appearance of that mid region next is elastic fibres elastic fibres there are three types of elastic fibres which are histochemically and ultrastructurally different they are mature elastic fibres eulene fibres and oxytalene fibres eulene fibres and oxytalene fibres have been described as immature elastic fibres so mature elastic fibres they consist of microfibrillar component which is surrounding an amorphous core of elastin protein whereas oxytalene fibres they are microfibres which run in apicocoronal direction to bend and attach to cervical third of fruit and thus the function of oxytalene fibres basically unknown but it has role in supporting blood vessels of periodontal ligament and there is also tooth supporting function eulene fibres are bundles of microfibres embedded in small amount of amorphous elastin their basic functions regulate vascular flow and role in tooth support and facilitate fibroblast attachment and migration the reticular fibres they are immature collagen fibres with staining properties and are related to basement membrane of blood vessels and epithelial cells which is adjacent to periodontal ligament whereas the secondary fibres they represent the newly formed collagenous element but still it is not incorporated into the principal fibres so located between these principal fibres they are basically non directional and randomly oriented they appear to transverse the periodontal ligament space in corona apical and are often associated with the vasculature and nervous elements but so that is all about principal fibres so we have seen the principal fibres and other fibres so next we have the functions of periodontal ligament so principal fibres are very important question it might be asked as short assay or long assay and even a short knot either any group of principal fibres so I will come up with functions of periodontal ligament so now we are into the last segment of our periodontal ligament so this session is about functions of periodontal ligament so there are basically 5 types of functions physical functions, formative and remodelling function nutritional function, homeostatic function and sensory functions in physical functions the first thing is it provides a soft tissue casing to protect the vessels and nerves from injury by mechanical forces so it protects it because it act as a casing and protects the underlying nerves and vessels the second physical function is the occlusal forces to the bone the bone is transmitted the force is happening at the occlusal side and it is transmitted to the bone so when there is a force so when there is a force it will be transmitted to the bone so it will be transmitted to the bone and attachment of teeth to the bone as we had seen and maintenance of the gingival tissue in the proper relationship with the teeth and resistance to the impact of occlusal forces they are the physical functions it provides casing, it transmit forces, it attach the teeth to the bone it maintain the gingival tissues and it resist the impact of occlusal forces so basically there are various theories which explains this the forces how the force is transmitted to the bones through periodontal ligament the first theory is tension theory then the viscoelastic theory and one more theory we have thixotropic theory so what is tension theory? when a force is applied to the crown principal fibres first unfolds and straightens then transmit the force to the alveolar bone which causes elastic deformation of the bone socket then finally the alveolar bone has reached its limit the load is transmitted to the basal bone so this is the alveolar bone then we have a basal bone that is mandible and maxilla so that is the tension theory basically says that the principal fibres of periodontal ligament are the major factors in supporting the tooth and transmitting the forces to the bone so the principal fibres unfolds it transmits to the alveolar bone alveolar bone elastic formation changes happens and it reaches a limit then it transmit to the basal bone but many investigators find this theory insufficient to explain the experimental evidence then came the viscoelastic theory according to this theory what happens when a force is applied on the tooth there is a change in extracellular fluid so extracellular fluid which fluid from periodontal ligament escapes to this marrow spaces because the tooth will be compressed when force is there the tooth will be compressed so fluid will be escaping to the marrow spaces so depletion of fluid so the fibres absorbs and it becomes titan so fluid will be into this marrow spaces the fibres absorb the pressure and it titans blood vessels stenosis so arterial back pressure created ballooning of vessels and then passage of blood ultra-filtrates into the tissues so the lost fluid replenished so this is a viscoelastic theory when force applied the fluid enters into marrow spaces then there will be tightening of this fibres blood vessels stenosis arterial back pressure created ballooning of vessels then passage of blood ultra-filtrates into the tissues so the lost fluid replenished so according to this theory the displacement of tooth is largely controlled by the fluid moments with fibres having only secondary role so the intentional theory was explaining the primary cause is due to the principal fibres so there is a big difference between tangential theory and viscoelastic theory viscoelastic theory is an accepted one and the next theory we have thixotropic theory so it says that the PDL has rheologic behavior of a thixotropic cell and fluoride when it applies pressure it becomes liquid when applied pressure it becomes gel when there is no pressure it is semi-solid again so this is commonly used technique in fluoride application fluoride gel application mostly they are in semi-solid state but when we put it in the trays and apply pressure it becomes liquid or it becomes gel type and it enters into the internal spaces so the presence of organized collagen fibres makes this theory unacceptable so the most accepted one is viscoelastic theory and the second function is formative and remodeling the cells of PDL which participate in formation and resorption of cement and bond which occurs in physiologic tooth movement, accommodation of pyridontium to occlusion forces and also in repair of injuries and remodeling the three-dimensional organization of fibre mesh work is adapted to accommodate for positional change of tooth when there is changes in functional state happens it relates to the adaptability of pyridontal ligament tissues both these process can occur simultaneously and may therefore be indistinguishable the formation and remodeling so this PDL is constantly undergoing remodeling all cells and fibres are broken down and replaced by new bone or new ones and mitotic activity can be observed in fibroblast and other cells third function is nutritional PDL supplies nutrients to cement and bond ginger by the blood vessels which provide all the anabolides and other substances to cement and bond and ginger and which removes catabolides fourth function is homeostatic which is the adaptability to rapidly changing applied forces and its capacity to maintain its width at constant diameter that is a constant diameter throughout the life it is evident that the cells of PDL have the ability to resolve and synthesize extra cellular substances of connected tissue alveolar bone and cement so that is homeostatic property then we have sensory function though the pyridontal ligament is abundantly supplied with sensory neurofibers which is capable of repair of transmitting tactile pressure and pain sensation by the trigeminal pathway so basically four types of neural terminations are seen most efficient in proprioceptive mechanism so four neural terminations are free nerve endings then roughening like mechanoreceptors which is seen in the apical area and miscellaneous copper cells which is seen at the middle and spindle like pressure and vibration endings which is also seen at epics so which are the four one is free nerve endings which is basically elicit pain and roughening mechanoreceptors which is seen at the apical area miscellaneous copper cells mechanoreceptors seen at the middle third and spindle like pressure and vibration endings which is also seen at epics now we need to study the age changes in pyridontal ligament so what are the changes happening over the age so increase in collagen fibrosis and decreasing in cellularity there will be areas of hyalinization the sporadic mineralization of fibres may also occur decrease in the number of pyridontal fibres cellularity and formation of multi-nucleated fibroblasts decrease in collagen synthesis the surface of pyridontal alveolar bones are dragged and uneven and become irregular in nature replacement of some of the PDL space by fat cells so there are many changes happens as age progresses and width of pyridontal ligament space for non-functioning teeth it is narrower than that of functional teeth and with increasing age less teeth are present the force acting on the remaining teeth may increase and an increasing width of the pyridontal ligament space with age seen with those particular teeth so that is all about pyridontal ligament functions we have five functions physical, formative, remodeling nutritional, homeostatic sensory functions and the tension theory viscoelastic theory and tixotropic theory we have seen and the most accepted one is viscoelastic theory that is saying the fluid movements is cause for transmission of force now last but not the least we need to study the blood supply which is basically inferior and superior alveolar arteries which has three sources like apical vessels that is dental artery which supply dental pulp then the trans alveolar vessels trans alveolar vessels which is penetrating vessels from alveolar bone and the third one is intraceptal vessels which is anastomosing vessels from the ginger in nerve supply we have sensory and autonomic nerves that is basically trigeminal nerve the nerve endings we have four types, roughenies endings miscellaneous corpuscles free nerve endings also we have seen an encapsulated spindle type roughenies endings found near the root apex appear as dendritic and in terminal expansion among the PDL fibro bundles they are mechanoreceptors miscellaneous corpuscles seen at mid root for tactile perception encapsulated spindle type which is a temperature receptor associated with root apex the lymphatic drainage which follow the course of blood vessels ok so that's all about periodontal ligament we had covered in four sessions first session was the basic structure its formation second session was its cells extracellular material third session was the principal fibres and the last session was about the function age changes, blood cell linear supply and lymphatic drainage so we finished periodontal ligament it was a lengthy chapter lots of questions will be asked so we have covered dendriva and periodontal ligament those are the soft tissues of periodontium now we will move on to the cementum and alveoli which are the two hard tissue components of periodontium ok so I will come up with cementum in my next session, thank you