 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 five types of functions, physical functions, formative and remodeling 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 acts as a casing and protects the underlying nerves and vessels. The second physical function is it is transmitting the occlusion forces to the bone. The force is happening at the occlusional side and it is transmitted to the bone. So when there is a force, it will be transmitted to the bone and attachment of teeth to the bone as we have seen and maintenance of the gingeral tissue in the proper relationship with the teeth and resistance to the impact of occlusion forces. They are the physical functions. It provides casing, it transmits forces, it attaches the teeth to the bone, it maintains the gingeral tissues and it resist the impact of occlusion 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? Tension theory says when a force is applied to the crown, okay, so principal fibers first unfolds and straightens. So these fibers unfolds and straightens, then transmit the force to the alveolar bone, okay, 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 alveolar bone, then we have a basal bone, that is mandible and maxilla. So that is the tension theory. It is basically says that the principal fibers of periodontal ligament are the major factors in supporting the tooth and transmitting the forces to the bone, okay. So the principal fibers 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, okay. Because the tooth will be compressed, when force is there, tooth will be compressed. So fluid will be escaping to the marrow spaces. So depletion of fluid, so the fibers absorbs and it becomes tightened. So fluid will be into this marrow spaces, the fibers absorb the pressure and it tightens. So there will be 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 fibers, 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 fibers having only secondary role. But tension theory was explaining the primary cause is due to the principal fibers. So there is a big difference between tension 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 gel. Thixotropic gel we have seen in fluorides when it applies pressure, it becomes liquid. When we apply 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. Usually 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 fibers makes this theory unacceptable. So the most accepted one is viscoelastic theory. 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 periodontium to occlusion forces and also in repair of injuries. And remodeling, the three-dimensional organization of fiber mesh work is adapted to accommodate for positional change of tooth. When there is a changes in functional state happens. It relates to the adaptability of periodontal 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 fibers are broken down and replaced by new bone or new ones and mitotic activity can be observed in fibroblasts and other cells. Third function is nutritional. PDL supplies nutrients to cement and bond, ginger by the blood vessels which provide all the anabolites and other substances to cement and bond and ginger and which removes catabolites. 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 the 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. Second we have sensory function though the periodontal ligament is abundantly supplied with sensory nerve fibers 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 and proprioceptive mechanism. So four neural terminations are free nerve endings then roughening like mechanoreceptors which is seen in the apical area and miscellaneous corpuscles which is seen at the middle third 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 corpuscles 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 periodontal 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 periodontal fibres, cellularity and formation of multi-nucleated fibroblasts, decrease in collagen synthesis, the surface of periodontal alveolar bones are jacked 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 with the periodontal 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 periodontal ligament space with age seen with those particular teeth. So that is all about periodontal 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 moments 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 apacal vessels that is a dental artery which supply dental pulp then the trans alveolar vessels, trans alveolar vessels which is penetrating vessels from alveolar bone and third one is intraceptal vessels which is anastomossing vessels from the ginger, okay and now supply we have sensory and autonomic nerves that is basically trigeminal nerve, the nerve endings we have four types raffinis endings, miscellaneous copacils, free nerve endings also we have seen and encapsulated spindle type raffinis endings found near the root apex it appear as dendritic and in terminal expansion among the PDL fibro bundles they are mechanoreceptors, miscellaneous copacils seen at mid root for tactile perception encapsulated spindle type which is a temperature receptor associated with root apex, the lymphatic drainage which just cause which follow the course of blood vessels, okay 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 there will be lots of questions will be asked so if you understood this topic 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, okay so I will come up with cementum and my next session thank you