Periodontal Ligament (PDL) PDF

PERIODONTAL LIGAMENT (PDL) Periodontium Tissues supporting and investing the tooth. Consists of 1. Cementum 2. Periodontal ligament (PDL) 3. Alveolar bone & 4. gingiva Gingiva Bone  INTRODUCTION Periodontal ligament (PDL) is a soft, specialized connective tissue that occupies the...

PERIODONTAL LIGAMENT (PDL) Periodontium Tissues supporting and investing the tooth. Consists of 1. Cementum 2. Periodontal ligament (PDL) 3. Alveolar bone & 4. gingiva Gingiva Bone  INTRODUCTION Periodontal ligament (PDL) is a soft, specialized connective tissue that occupies the periodontal space between the cementum covering the root of the tooth and the bone forming the socket wall. Peri – Around Dontal – tooth Ligament - band of fibrous tissue binding together 2 skeletal elements.  Collagen fibers of PDL are embedded into cementum & alveolar bone hence it provides soft tissue continuity between the mineralized connective tissues of the periodontium. Other names used are : Periodontal membrane, Alveolo-dental ligament, Desmodont, Pericementum, Term “Periodontal ligament ” is most commonly used.  In the coronal direction it is continuous with the gingiva. At the root apex, merges with the dental pulp. communicates through vascular channels with the marrow spaces of the alveolar bone. With cementum, alveolar bone and the gingiva, the PDL supports the tooth in the jaw.  Shape and Width Shape of hourglass Narrowest in the midroot region. It is usually widest cervically. Teeth in heavy function - wider ligaments than non-functioning teeth. Average width of the PDL usually is 0.21mm. (0.15mm – 0.38mm) Thickness- decreases with age Young adult (11-16yrs) -0.21mm Mature adult (32-52yrs) -0.18mm Older adult (51-67yrs) -0.15mm  Occupies the PDL space between cementum and alveolar bone proper. appears as radiolucent area or line. Narrow in permanent teeth than deciduous teeth.  Structure of PDL A. CELLS B. EXTRACELLULAR SUBSTANCE A) CELLS IN PERIODONTAL LIGAMENT 1. SYNTHETIC CELLS Fibroblasts Osteoblasts Cementoblasts 2. RESORPTIVE CELLS Osteoclasts Fibroblasts Cementoclasts 3. PROGENITOR CELLS 4. EPITHELIAL CELLS- Epithelial rests of Malassez 5. DEFENSE CELLS Mast cells Macrophages Eosinophils B. EXTRA CELLULAR SUBSTANCE OF PDL comprises two major compartments: A. Fibers B. Ground Substance B) EXTRACELLULAR SUBSTANCE 1. FIBERS i. Collagen ii. Elastic fibres iii. Reticular iv. Secondary v. Indifferent fiber plexus 2. GROUND SUBSTANCE Proteoglycans Glycoproteins glycosaminoglycans COLLAGEN Collagen fibrils form bundles of 5µm diameter. These bundles are called the PRINCIPAL FIBERS Collagen fibrils show a characteristic transverse striations at a periodicity of 64 nm Main types of collagen in PDL: Type I (70%)& Type III (20%) Other types-Type V, VI, IV, VII, XIII  Principal fibers of periodontal ligament Also called as Alveolo-dental Ligament. Five fiber groups: a) Alveolar crest fibers b) Horizontal fibers c) Oblique group d) Apical group e) Interradicular group- in multirooted teeth Sharpey's fibers – terminal portion of principal fibers inserted into cementum and alveolar bone. a) Alveolar crest fibers Attached to cementum just below the CEJ They run downward & outward to get inserted into rim of alveolus. Function: Resist tilting, intrusion, extrusive & rotational forces. b) Horizontal group Just apical to alveolar crest group. These fibers run at right angles to long axis of tooth i.e from cementum to bone. Occupies the coronal ¼ th of pdl. Function: Resist Horizontal & tipping forces c) Oblique group Most numerous in PDL Run from cementum obliquelyto insert into bone coronally Occupy nearly 2/3rd of PDL Function: Resist vertical & intrusive forces d) Apical group Radiate from cementum around apex of root to bone forming the base of the socket. Not seen in incompletely formed roots Function: Resist forces of luxation, prevent tipping. Protect delicate blood & lymph vessels & nerves near the root apex. Interradicular group Found only in multirooted teeth. Run from cementum into crest of inter-radicular septum of bone Function: Resists tooth tipping, Torquing, Luxation These fibers are lost in: Age related gingival recession Chronic inflammatory periodontal disease.  Other groups of collagen fibers also maintain the functional integrity of the periodontium. These groups are found in the lamina propria of the gingiva and collectively form the gingival ligament. Composed of 5 groups of fiber bundles: 1. Dentogingival group 2. Alveologingival group 3. Circular group 4. Dentoperiosteal group 5. Transseptal fiber system 1. Dentogingival group: most numerous fibers, extending from cervical cementum to lamina propria of the free and attached gingivae. 2. Alveologingival group: These fibers radiate from the bone of the alveolar crest and extend into the lamina propria of the free and attached gingivae. 3. Circular group: small group of fibers forms a band around the neck of the tooth, interlacing with other groups of fibers in the free gingiva and helping to bind the free gingiva to the tooth. 4. Dentoperiosteal group: Run apically from the cementum over the periosteum of the outer cortical plates of the alveolar process, These fibers insert into the alveolar process. 5. Transseptal fiber system: These fibers run interdentally from the cementum of one tooth over the alveolar crest and insert into a comparable region of the cementum of the adjacent tooth. Together these fibers constitute the transseptal fiber system, collectively forming an interdental ligament connecting all the teeth of the arch. Clinical Implication of Trans-septal fibres- Implicated as a major cause of relapse of orthodontically positioned teeth. Reason for relapse: Inability of the transseptal fiber system to undergo physiologic rearrangement. Retention after orthodontic tooth movement allows reorganization of the transseptal fiber system to ensure the clinical stability of tooth position. Elastic Fibers There are 3 types of elastic fibers, A) Elastin B) Elaunin C) Oxytalan Only oxytalan fibers are present within the PDL; elaunin fibers may be found within fibers of the gingival ligament.  Oxytalan fibers form a three-dimensional branching meshwork that surrounds the root and terminates in the apical complex of arteries, veins, and lymphatic vessels. The fibers also are associated with neural and vascular elements. As they are elastic, they can expand & they are thought to regulate vascular flow in relation to tooth function. Reticular Fibers These are fine immature collagen fibers. They are related to basement membrane of blood vessels and epithelial cells which lie within the PDL. Secondary Fibers These are located between the principal fibers. These fibers are relatively non-directional and randomly oriented. Represent newly formed collagenous elements that have not yet been incorporated into principal fiber bundles. These fibers are often associated with paths of vasculature and nervous elements. Indifferent Fiber Plexus In addition to these fiber types, small collagen fibers associated with the large principal collagen fibers have been described. These fibers run in all directions, forming a plexus called indifferent fiber plexus.  Ground substance All components of the ground substance secreted by fibroblasts. Main type of glycosaminoglycan- is hyaluronan. Dermatan, chondroitin and heparin sulfates may also be found. Proteoglycans- The two main types in the PDL are dermatan sulfate and proteoglycan containing chondroitin sulfate and dermatan sulfate hybrids. The predominant glycoprotein is fibronectin. It promotes attachment of cells to the collagen fibrils. It may also be involved in cell migration and orientation.  The other glycoproteins seen in the PDL are tenascin and vitronectin. Tenascin - found adjacent to alveolar bone and cementum. Role in transfer of the forces of mastication to specific protein structures. The PDL ground substance has 70% water and has significant effect on the ability of the tooth to withstand stress loads.  A) CELLS IN PERIODONTAL LIGAMENT 1. SYNTHETIC CELLS Fibroblasts Osteoblasts Cementoblasts 2. RESORPTIVE CELLS Osteoclasts Fibroblasts Cementoclasts 3. PROGENITOR CELLS 4. EPITHELIAL CELLS- Epithelial rests of Malassez 5. DEFENSE CELLS Mast cells Macrophages Eosinophils SYNTHETIC CELLS Characteristic of synthetic cells are: Large cell Increase in Golgi apparatus, Rough endoplasmic reticulum (rER) covered by ribosomes - hence the cytoplasm of the cell appears Haematoxyphilic/basophilic. The cells consist of large, open faced or vesicular nucleus with prominent nucleoli.  A) Fibroblasts Fibroblasts are predominant cells in periodontal ligament. They constitute about 65% of total cell population Fusiform in shape (spindle Shaped) They produce collagen Origin-dental papilla or dental follicle  Fibroblasts in PDL is different from fibroblasts in other Connective tissue - PDL fibroblast causes rapid turnover of collagen in PDL by rapid degradation of collagen by phagocytosis. Regularly distributed in PDL & oriented with long axis parallel to direction of collagen fibrils. Functions: Fibroblasts are responsible for formation & remodeling of PDL fibers. Act as signaling system –to maintain the width of PDL. B) Osteoblasts Osteoblasts are bone forming cells lining the tooth socket/ any bone The osteoblasts covers the periodontal surface of alveolar bone Cuboidal in shape with prominent round nucleus placed at basal end of cell  Cells appear basophilic due to abundant rER. Golgi complex, mitochondria are seen extensively. They are in contact with osteocytes –through cytoplasmic processes Cementoblasts Cementoblasts are distributed on the surface of cementum-but not regularly as osteoblasts on surface of bone. Origin -The undifferentiated mesenchymal cells of dental follicle Synthesize collagen & protein polysaccharides constitutes the organic matrix.  The cementoblasts that form cellular cementum have abundant basophilic cytoplasm & cytoplasmic processes. nuclei are folded & irregularly shaped The cementoblats that form acellular cementum do not have prominent cytoplasmic processes. RESORPTIVE CELLS A) Osteoclasts Cells that resorb bone These cells are large & multinucleated or small & mononucleated with eosinophilic cytoplasm. When viewed in the light microscope, the cells appear to occupy bays in bone or the Howship’s lacunae. The surface of osteoclast in contact with bone has a ruffled border. Resorption by osteoclasts occurs in two stages: a) The area of bone that is sealed off by ruffled border is exposed to highly acidic pH by active pumping of protons (H+ ions) by osteoclasts. The bone related to the ruffled order undergoes resorption. First the mineral is removed at bone margins. b) The exposed organic matrix disintegrates.  B) Fibroblasts Fibroblasts are capable of both synthesis and degradation of collagen Degradation of collagen-Extracellular & Intracellular events a) Extracellular degradation of collagen involves the enzyme collagenase secreted by fibroblasts, which cleaves the triple helix structure of the fibril. The rest of the molecule undergoes further proteolysis by the enzymes gelatinase & stomelysin. b) Intracellular degradation takes place within the fibroblasts. The fibroblasts are capable of phagocytosing collagen fibrils from extracellular environment & degrade them inside phagolysosomal bodies. Lysosomal cysteine proteinases of lysosomal granules are capable of rapid degradation of these collagen fibrils.  C) Cementoclasts Resemble the osteoclasts but rarely found in normal PDL, this indicates that cementum is not remodeled frequently like bone. Rarely cementum resorption takes place, but when there is resorption - the cementoclasts are seen in Howship’s lacunae on surface of cementum. Progenitor cells These are undifferentiated cells which have the capacity to differentiate into any synthetic cells such as cementoblasts, osteoblasts and fibroblasts. They replace the differentiated cells when they die at the end of their life span or due to trauma. These cells are small in size, with darkly stained small nucleus & very little cytoplasm. highest concentration- perivascular region  Epithelial rests of Malassez Epithelial cells found in PDL close to cementum are the remnants of HERS. Function not clear but may be involved in periodontal repair & regeneration. Distribution: In Older individuals few rests are seen, while in children many rests may be seen.  These cells appear as Clusters or strands of cells cuboidal in shape with scanty cytoplasm Nucleus stains deeply connect to adjacent cell by desmosomes. hemidesmosomes are seen between these cells & basal lamina  DEFENSE CELLS Mast Cells Small, round or oval cells- diameter ~12 to 15 µm Often associated with blood vessels Cytoplasm-consists of numerous granules.  The granules contain heparin and histamine. The granules are stained by dyes e.g. Azure A Role in inflammatory reaction: The mast cells degranulate in response to antigen-antibody reaction. Role in PDL: The release of histamine by the mast cells into the extracellular compartment causes proliferation of the endothelial and mesenchymal cells/fibroblasts. Hence has a role in regulating the endothelial & fibroblast cell population. Macrophages Derived from blood, are involved in the elimination of dead cells. Located adjacent to blood vessels Consists of numerous microvilli, lysosomes, less rER, Golgi complex Nucleus –round, horse shoe or kidney shaped. Role in PDL: Phagocytosis of dead cells Secrets growth factors which regulate the proliferation of fibroblasts. Eosinophils Seen occasionally Role-phagocytosis Blood supply PDL is very well vascularized- reflects the high rate of turnover of its cellular and extracellular constituents. Main blood supply - from the superior & inferior alveolar arteries. Blood supply increases from incisors to molars. Lymphatic vessels follow the path of blood vessels & provide lymph drainage of PDL. NERVE SUPPLY Trigeminal nerve innervate the PDL from either its maxillary nerve or inferior alveolar nerve branches. These nerve fibers provide sense of touch, pressure, pain and proprioception during mastication.  4 types of nerve-termination have been described in PDL. a) Free Nerve endings( Most common type)- tree like ramification - Present at regular intervals along length of root b) Resembles Ruffini’s corpuscles- appear to be dendritic and end in terminal expansions among the PDL fiber bundles root apex. c) Coiled form- mid region of PDL Function- Not clear. d) Spindle like endings (least common) consists of spindle like endings Seen at root apex  FUNCTIONS OF PDL 1. PHYSICAL FUNCTION/ SUPPORTIVE FUNCTION Provision of soft tissue ‘casing” in order to protect the vessels and nerves from injury due to mechanical forces. Transmission of occlusal forces to bone, the load is transmitted through oblique fibers to bone. On release of load it recoils back to the resting position of tooth. Hence PDL is also called Suspensory ligament.  Attachment of teeth to bone Maintenance of gingival tissues in their proper relationship to the teeth “Shock absorption” resists the impact of occlusal surfaces. 2. SENSORY FUNCTION: Capable of transmitting tactile pressure and pain sensations. Lightest pressure on tooth and smallest particles between the contacting surface can be felt by the individual. 3. NUTRITIONAL FUNCTION:- PDL supplies nutrients to the cementum, bone, and gingiva and provides lymphatic drainage. 4. ERUPTIVE FUNCTION PDL provides space & acts as medium for cellular remodeling facilitating continued eruption. 5. HOMEOSTATIC FUNCTION The cells of PDL have the capacity to resorb and synthesize of extracellular substances of the connective tissue of the ligament, cementum, & bone. PDL has a very high turn over rate.  Age changes Decrease in cell density and fibrous component. Mitotic activity or proliferation rate of cells of pdl is decreased Decrease in production of organic matrix Increase in elastic fibers. Functional variations Teeth without antagonists – width of PDL is narrow. PDL is thin in functionless teeth. PDL is wide in teeth with excessive occlusal stresses. b) Changes in PDL following an acute trauma Results in pathological changes with damage to the fibers & on elimination or with time there may be repair. c) Changes in PDL during Orthodontic treatment Orthodontic treatment depends on resorption and formation of bone and remodelling of PDL. force within the physiological limits - the initial compression of PDL on the pressure side is compensated for by bone resorption, whereas on the tension side the bone apposition is seen.  Application of large forces results in necrosis of PDL and alveolar bone on pressure side and movement of tooth will occur only after the necrotic bone has been resorbed.