The periodontal ligament (PDL) is a soft connective tissue located between the cementum on the root of a tooth and the alveolar bone. It consists of collagen fibers, cells like fibroblasts and cementoblasts, blood vessels, and nerves. The principal fibers of the PDL are arranged in groups to help support the tooth, resist movement, and absorb forces during chewing. The PDL transmits occlusal forces to the bone, attaches the tooth, and maintains the gingiva.

1. PERIODONTAL LIGAMENT

2. Periodontium is an attachment apparatus of the teeth. Teeth are attached to the bones of the jaws by periodontium. It consists of four connective tissues. Periodontal Ligament is the integral part of the periodontium

3. (a) Cementum. (b) Periodontal Ligment. (c) Bone that lines the alveolus. (d) Deeper part of the gingiva. The cementum & the alveolar bone are the mineralized tissues while the periodontal ligament & the part of the gingiva are fibrous tissues. " The PDL is that soft, specialized connective tissue situated between the cementum covering the root of the tooth & the bone forming the socket wall.

5. * Width of PDL = 0.15 to 0.38 mm  PDL width decreases with age.  The PDL extends coronally up to the most apical of the connective tissue of the gingiva.  The PDL is a cellular connective tissue.  It consists of the blood vessels, various cells & extracellular matrix.

7. Other terms previously used for PDL are "Desmodont, Gomphosis, Pericementum, Dental periosteum, Alveolo dental ligament & Periodontal membrane.

8. EVOLUTION:-  There is a replacement of the ankylosis of the tooth & bone to a ligamentous suspension of the tooth. Because of this, movement of the mammalian teeth is made possible, resulting in continual repositioning as required by the jaw growth & also toothwear . DEVELOPMENT:-  Shortly after the beginning of root formation & the formation of the outer dentinal layer of root, the PDL is formed.  The external & internal dental epithelia proliferate from the cervical loop of dental organ to form a Hertwigs epithelial root sheath, which then fragments to form the discrete clusters of the epithelial cells called as " epithelial cell rests of Malassez"  The enamel organ & root sheath are surrounded by dental sac which is formed by condensed cells.

10.  A thin layer of cells lying adjacent to the dental organ, known as dental follicle, divide & differentiate into cementoblasts, fibroblasts & osteoblasts.  The fibroblasts synthesize the fibers & ground substance of the PDL.  These fibers initially occupy the periodontal space without any specific orientation, but just before tooth eruption acquire an orientation oblique to the tooth towards the coronal portion.

11.  Similar fibers appear on the side of bone also.  They fibers elongate, meet & fuse in the middle. They mature progressively towards the root apex as the eruption progresses & with the establishment of occlusal contact the proper arrangement of the principle fibers of the PDL occurs.  The damaged periodontal fibers are replaced & remodeled by newly formed fibers.

13. CELLS: Synthetic Cells - contains a large open faced nucleus, with prominent nucleoli & an abundant hematoxylophilic cytoplasm . The cells are follows:- 1. Osteoblasts:  Derived from the multipotent mesenchymal cells.  They cover the periodontal surface of the alveolar bone.  The collagen fibers of ligament that penetrate the alveolar bone are present b/w the cells. 2. Fibroblasts:-  Principal cells of PDL  Ovoid or elongated cells oriented along the principal fibers & exhibiting pseudopodia like processes.  The collagen turnover is regulated by fibroblasts.  Phenotypically distinct & functionally different subpopulations of fibroblasts exists in adult PDL. They look identical at both light & electron microscopic levels but may have different functions such as secretion of collagen of different types or production of collagenase.

15. 3- Cementoblasts:- found on the surface of the cementum. help in cementum formation. produce cementoid layer which undergoes mineralization. some of these cells get embeded in the cementum as cementocytes. Resorptive Cells: 1. Osteoclasts:-  multinucleated cells which resorb bone.  helps in remodeling, which allows functional changes in the position of tooth. Fibroblasts:-  help in both synthesis & resorption of collagen.  the collagen fibers can be resorbed by the mononuclear fibroblasts.  help in remodeling process of PDL.  they exhibit lysosomes, which contain collagen fragments undergoing digestion.

17. 3. Cementoclasts :-  found on surface of cementum.  cementum is not remodeled like alveolar bone & PDL Progenitor Cells  are the undifferentiated mesenchymal cells.  have the capacity to undergo mitotic division & replace the differentiated cells dying at the end of their lifespan.  located in perivascular region. Epithelial cell Rests of Malassez: -  remnants of the epithelium of Hertwig's epithelial root sheath.  found close to the cementum.  arranged parallel to the root surface.  formed at the time of cementum formation. when stimulated these cells undergo rapid proliferation & produce a variety of cysts & tumours.

19. Mast Cells :  contain numerous cytoplasmic granules which contains heparin & histamine.  role of heparin not clear.  the release of histamine into the extracellular compartment causes proliferation of the endothelial & mesenchymal cells. Macrophages :  help in phagocytosing dead cells & secreting growth factor, which helps to regulate the proliferation of adjacent fibroblasts.  they can be differentiated from fibroblasts by the phagocytosed material present in its cytoplasm. EXTRA CELLULAR SUBSTANCE:- Fibers Ground Substance (a) Collagen (a) Glycosaminoglycans (b) Elastic fibers (b) Glycoproteins

20. FIBERS:- (a) Collagen:-  Collagen fibers are the principal fibers of the PDL.  PDL is basically made up of type I & type III collagen.  Collagen is a high molecular weight protein composed of different amino acids.  Collagen biosynthesis occurs inside the fibroblasts to form tropocollagen molecules.  These aggragate into microfibrils that are packed together to form fibrils.  Fibrils are packed side by side to form bundles or fibers. These fibers are the smallest order of collagen that can be seen by light microscope. Many collagen fibers are arranged in larger bundles & are termed principal fibers.

21.  The principal fibers of the PDL are arranged in five groups. (i) Transseptal group: - - extend interproximally over the alveolar bone crest & are embedded in the cementum of adjacent teeth. - reconstructed even after destruction of the alveolar bone has occured in periodontal disease. (ii) Alveolar crest group:- - extend obliquely from the cementum just beneath the junctional epithelium to the alveolar crest. - prevent extrusion & resist lateral tooth movement. (iii) Horizontal group:- - extend at right angles to the long axis of the tooth from the cementum to the alveolar bone. - prevents lateral movement of teeth. (iv) Oblique group:- - most predominant ones in PDL. - extend from the cementum in a coronal direction obliquely to the bone. - resist apical movement of teeth during mastication. -during function they get extended, thus producing a tensile force on the alveolar bone rather than pressure. (v) Apical group:- -radiate in a irregular fashion from the cementum to the bone at the apical region of the socket. - protect the vessels & nerves at the root apex from injury. - resist the forces of luxation & tipping.

23.  Interradicular group arise from the furcation area of the multirooted teeth & are inserted into crest of the inter-radicular septum.  The terminal portion of the principal fibers that insert into cementum & bone are termed S harpey's Fibers.

24. (b) Elastic fibers: - * Oxytalan & eluanin - run parallel to the root surface in a vertical direction & bend to attach to cementum in the cervical third of the root. * They are thought to regulate vascular flow. (c) Indifferent Fiber Plexus:- GROUND SUBSTANCE:-  Glycosaminoglycans - such as hyaluronic acid & proteoglycans.  Glycoproteins - such as fibronectin & laminin.

25. STRUCTURES PRESENT IN CONNECTIVE TISSUE - Blood Vessels:-  the main blood supply to PDL is from superior & inferior alveolar arteries.  the blood vessels are derived from the following. Branches from apical vessels. Branches from intra-alveolar vessels. Branches from gingival vessels. Lymphatics : The lymphatic drainage is from the ligament to the alveolar bone. follows the path of blood vessels.

26. Nerve Supply: - - branches of second & third division of fifth cranial nerve (Trigeminal nerve) - provide sense of touch, pressure, pain & proprioception (mechano receptors). Cementicles:- - cementicles are the small calcified bodies present in PDL, they are not the cells of cementum. - may be fused with cementum or may be free.

27. FUNCTIONS OF PDL:- 1. Physical function:-  Provision of a soft tissue "casing" to protect the vessels & nerves from injury by mechanical forces.  Transmission of occlusal forces to the bone.  Attachment of the teeth to the bone.  Maintenance of the gingival tissues in their proper relationship to the teeth.  Resistance to the impact of occlusal. forces (shock absorption). Three theories to explain the mechanism of tooth support – 1. Tensional theory- acc. to this , when a force is applied to the crown the principal fibers first unfold & straighten & then transmit forces to the alveolar bone ,causing an elastic deformation of the bony socket . Finally when the alveolar bone has reached its limit,the load is transmitted to the basal bone. 2. Viscoelastic system theory- based on the concept that when forces are applied to the tooth ,movement of the fluid content of PDL occurs.The ECF is pushed from PDL into the marrow spaces through the cribriform plate.The periodontal fibers may only have a minor role in the transmission of forces. 3. Acc.to third concept ,the PDL acts like a thixotropic gel .When the gel is stirred or shaken ,as when forces are applied to the tooth,it becomes fluid,but it return to gel state when the forces are removed. _

28. 2. Formative & Remodeling function  Cells of the PDL participate in the formation & resorption of cementum & bone, which occur in-  the rate of collagen turnover in PDL is believed to be twice as that of gingiva. 3.Sensory function: -  The PDL nerve fibers transmit sensation of touch, pressure & pain to higher centers.  also provide the proprioceptive mechanism to detect an application of small force to the teeth & also very small displacement of teeth.  physiologic tooth movement accommodation of periodontium to occlusal forces. r epair of injuries

29. 5. Homeostatic Mechanism. AGING OF LIGAMENT:- - The number of cells & their activity decrease, scalloping occurs in cementum & alveolar bone. - Some fibers are attached at the peaks of those scallops only & not in the depressions .  Thus, it helps in protecting the supporting structures of tooth & substance of crown from effects of the masticatory force.  Pain sensation is transmitted through small diameter nerves, temperature by intermediate type & pressure by large myelinated fibers 4. Nutritive function :-  PDL has rich vascular supply  It provides nutrition to the cementum, bone & gingiva.

30. CLINICAL CONSIDERATIONS:- The main function of PDL is to support the tooth in its socket. It is thick in the teeth that are exposed to excessive occlusal stress & thin in functionless & embedded teeth. - If a tooth is long out of function, the supporting tissues of this tooth are not fully adapted to carry the load suddenly placed on the tooth by a restoration like full cast crown. 1. The width of PDL varies from 0.15 to 0.38 mm. It decreases as the age advances. The thickness varies in different teeth in the same person & in different locations on the same tooth. 2. Aging results in more no. of elastic fibers & decrease in the vascularity, mitotic activity, fibroplasia & in the no. of collagen fibers & mucopolysaccharides. 3. If the gingivitis is not cured & supporting structures becomes involved, the disease is termed as periodontitis. 4. Resorption & formation of both bone & PDL play an important role in the orthodontic teeth movement. 5. Trauma to PDL may produce changes such as fracture or resorption of the cementum, tears of fiber bundles, hemorrhage & necrosis. These result in resorption of bone & PDL is widened, so teeth become loose. If trauma eliminated, repair will take place. 6. Chronic periodontal disease can lead to infusion of micro-organism into the blood stream. 7. Inflammation of pulp reaches to the apical PDL & replaces its fiber bundles with granulation tissue, called as a granuloma. It progresses into apical cyst - most common pathologic lesion of the jaws.

31. REFERENCES CARRANZA’S CLINICAL PERIODONTOLOGY – ninth edition PERIODONTICS – MEDICINE ,SURGERY & IMPLANTS – Louis F.Rose , Brain L.Mealey. ORAL HISTOLOGY- A.R.Ten Cate. DENTAL & ORAL HISTOLOGY WITH EMBRYOLOGY – Satish Chandra , Shaleen Chandra.