Periodontal ligament


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Periodontal ligament

  1. 1. INTRODUCTION Periodontium Is defined as the connective tissue organ that attaches the teeth to bone of jaws and provides a continually adapting apparatus for the support of teeth during function. Periodontium comprises of four connective tissue Two of which are mineralized - Cementum - A.B. Two are fibrous - Periodontal ligament - L.P of gingiva Periodontal ligament periodontal ligament is the connective tissue that surrounds the roots and connects the tooth to the bone. - It is continuous with the connective tissue of the gingiva and communicates with the marrow and space this vascular channels in the bone. - In a healthy tooth it covers the root up to the cemento-enamel junction. - In radiograph periodontal ligament appears as a radiolucent line paralleling the root surface. 1
  2. 2. - Lamina dura in radiograph appear as a radio-dense line which represent the alveolar bone fibres. - The average width of periodontal ligament is about 0.15 to 0.38mm. - periodontal ligament is thinnest in middle portion of root and its width decrease with age. At age of 11-16 years  0.21mm. 32-52 years  0.18mm 52-67 years  0.15mm Periodontal space: This space lies between two hard tissue i.e. cementum and bone within this space periodontal ligament is present. Its rot measures approximately 30mm3 – 100mm3  single root 65-150mm3  multirooted teeth - The width of periodontal space varies from end to end and from tooth to tooth and on different location on same tooth. - With increase in function space widens and with loss of function thinning and atrophy of fibres takes place. 2
  3. 3. Synonyms: - The other name used for periodontal ligament are Desmodont Ghomphosis Pericementum, Alveodental ligament; periodontal membrane, Dental periosteum. - Most commonly used are periodontal ligament and periodontal membrane, both neither of them/well describes structure and function. - Periodontal ligament is most appropriate term as it provides continuously between two mineralized tissues  bone and cementum. Development  The periodontal ligament forms from dental follicle soon after root deviation begins.  Formation of periodontal ligament starts, after Hertwig’s epithelial rest sheath cells are separated forming strands known as epithelial cell rest’s of malassez.  This separation permits the cells of dental follicle to migrate to the external surface of newly formed root dentine. 3
  4. 4.  These cells show high degree of mitotic activity, thus differentiating into different types of cells which give rise to cells of periodontal ligament namely: Cementoblast  deposit cementum on deviated root. Osteoblast  of the developing alveolar bone. Fibroblast  to synthesize collagen and ground substance of periodontal ligament.  The fibroblasts cells are oriented obliquely at early stages and thus fibres secreted by them have the same orientation.  These obliquely oriented fibre bundles get entrapped in bone and cementum as they are deposited and this give rise to principal fibres of periodontal ligament.  Before eruption  Alveolar bone crest is above cemento- enamel junction thus fibres are laid down obliquely in a coronal direction giving rise to oblique fibres.  As eruption begins  Alveolar crest coincides with the cemento-enamel junction and fibres are aligned horizontally.  When the tooth eruption is completed and it is in full function, alveolar crest below cemento-enamel junction near apex  thus fibres are again aligned obliquely but in an 4
  5. 5. opposite direction to first laid fibre bundles, thus forming alveolar crest fibres. As the tooth is in function, fibres begin to thicken and are remodeled constantly over a period of time. Evolution:  There is fundamental difference between Repitilian and Mammalian teeth attachment.  In reptiles teeth are ankylosed to bone.  In mammals teeth are suspended in sockets by ligament.  The central point of these changes is “reconstruction of mandible”.  In reptiles mandible consisting of several bones united by sutures.  In mammals the newly acquired cartilage of condyle is the most important growth site for mandible.  In reptiles the mandibular teeth more with the bones to which they are fused but in mammals they move as independent units and this movement is made possible by remodeling of periodontium.  The evolutionary change from reptiles to mammals replaces the ankylosis of tooth and bone to a ligamentous suspension of the tooth. 5
  6. 6. Structure - Cellular - Extracellular - connective tissue Components Cellular Periodontal ligament comprise of three groups of cells. These are: - Synthetic cells. - Resorptive cells. - Progenitor cells. Apart from these periodontal ligament comprises of : - Epithelial cells – Epithelial cell rests of malassez. - Defence cells derived from hemopoietic line. Synthetic cells: These include: - Fibroblast. - Osteoblast. - Cementoblast. Fibroblast  These cells are the most numerous and densely packed in periodontal ligament. 6
  7. 7.  They appear as ovoid or flattened with numerous cytoplasmic processes.  Shape of these cells vary according to the orientation of fibres, where active fibres and fibroblasts are densely packed  cells appear elongated along the fibre length.  In the loose connective tissue cells are ovoid, or spindle shaped.  These cells are surrounded by fibres and ground substance.  periodontal ligament fibroblasts are attached to one another by  a. gap junction.  b. Simplified desmosomes.  They are involved in both synthesis and resorption of collagen fibres. Osteoblasts:  These cells are found on surface of alveolar bone in periodontal ligament.  They constitute modified endosteum instead of periosteum as an outer layer i.e. outer layer is cellular instead of fibrous as seen in osteoblasts found elsewhere in body.  A prominent nucleus lies towards the base of these cells. 7
  8. 8.  Collagen fibres of periodontal ligament penetrating the bone intervene between these cells. Cementoblasts  These cells resemble osteoblasts.  These cells are found in portion of dental follicle adjacent to root and at the boundary between the periodontal ligament and tooth surface.  These cells have a diameter of about 8-12µm and display several cytoplasmic processes. Resorptive cells - Osteoclasts. - Fibroblasts / fibroclasts. - Cementoclasts. Osteoclasts  These are the bone resorbing cells and tend to be large and multinucleating but may be small and mononuclear.  Derived from circulating monocytes.  Under light microscope  These are present in Howship’s lacunae. 8
  9. 9.  Under electron microscope  exhibit numerous mitochondria, lysosomes, Golgi sacules, few ribosomes and few rough endoplasmic reticulum.  The part of plasma membrane of these cells adjacent to bone is continually resorbed and has characteristic folds giving rise to ruffled or striated border.  This border is separated from the rest of plasma membrane by a zone of specialized membrane devoid of organelles known as clear zone. Resorption of bone margin occurs in two stages: 1. Remineralization of bone margin. 2. Disintegration of orgenic matrix.  Osteoclasts are rich in acid phosphatase (present in lysosomes).  The presence of osteoclasts in periodontal ligament indicates active resorption or previous resorption in that area. Fibroclasts Collagen fibres of the mammalian periodontal ligament are resorbed under normal physiologic conditions by mononuclear fibroblasts. 9
  10. 10.  These cells contain lysosomes – which bring about resorption.  There presence indicate active resorption occurring during physiologic turnover or remodeling of periodontal ligament. Cementoblasts:  Resembles osteoclasts and are occasionally found in periodontal ligament.  Origin is unknown.  Resorption of cementum may take place under certain circumstances and in such instances these cells are present in Howships lacunae on the surface of cementum. Progenitor cells:  Like all connective tissues periodontal ligament also contain progenitor cells or undifferentiated mesenchymal cells that undergo mitotic division.  But very little is known about progenitor cells of periodontal ligament.  Such as it is not known whether a single population of progenitor cells give rise to all the synthetic cells of periodontal ligament or there are a number of population. 10
  11. 11.  There presence is evident from mitosis occurring after the application of a pressure such as in orthodontic treatment and after wounding.  These cells lie close to blood vessels.  These cells have a close-faced nucleus and very less cytoplasm. Epithelial Cell Rests of Malassez  Periodontal ligament contains epithelial cells that lie close to cementum.  Described by malassez in 1884.  They are remnants of Hertwig’s epithelial rest sheath that are embedded in periodontal ligament during deviation.  Under electron microscope these cells exhibit tonofilaments and are attached to each other by desmosomes.  Role of these cells under normal physiologic conditions is unknown, but under pathologic condition. These cells may proliferate to give rise to cysts such as periapical and lateral root cysts and tumours of jaws.  Seen in form of islands / clumps / tubules. 11
  12. 12. Defence cells - Mast cells. - Macrophages. - Eosinophils. Mast cells  These cells are sometimes seen in periodontal ligament.  These are small round / oval in shape about 12-15 µm in dia.  Contain numerous cytoplasmic granules such as heparin, serotonin, histamine  Granules have a dia-0.5-1µm and are membrane bound.  These granules play a role in inflammation reaction especially histamine.  Cells degenerate in response to Ag-Ab reaction. Macrophages  These cells may also be present in the ligament.  These are delivered from blood monocytes.  These cells need to be differentiation from fibroblasts.  Fibroblasts contains phagocyted mat in their cytoplasm.  Macrophages have a nucleus with regular contour. 12
  13. 13.  Cell surface is raised because of microvilli.  Cytoplasm has numerous free ribosomes, lysosomes.  Less of RER, and Golgi apparatus not well developed. Relationship between cells  Cells of periodontal ligament form a three dimensional network and there processes surround collagen fibres and these are attached to one another and to cells of bone, cementum by gap junction. Extracellular substances: Fibres Ground substance - Collagen - Glycosaminoglycans - Oxytalan - Glycoproteins - Elavanin Fibres Collagen  These comprise the majority of fibres of periodontal ligament.  Collagen is a specific high molecular weight protein. 13
  14. 14.  Protein macromolecules are clustered to form fibrils which are packed to form fibres. These fibres are gathered into bundles having clear orientation relative to periodontal space and are known as principal fibres of periodontal ligament.  Collagen are of five types periodontal ligament consists of Type I collagen predominantly and a little of type III collagen. Principal fibres  These comprise of five groups of fibres. 1. Alveolar crest fibres: Alveolar crest fibres extend obliquely from cementum below the functional epithelium to the alveolar crest. Function – these fibres prevent extrusion and lateral tooth movement of tooth. There incision does not significantly increase tooth mobility. 2. Horizontal group  Restricted to coronal ½ of periodontal ligament.  These fibres run horizontally at right angles to long axis of tooth from cementum to alveolar bone. Function: prevent lateral tooth movement 14
  15. 15. 3. Oblique group  Occupy middle 2/3rd of periodontal ligament.  These fibre bundles account for the main attachment of tooth.  These fibres extend in a coronal direction obliquely from cementum to bone. Function: These fibres bear the forces of vertical masticatory stresses and transform them into tension on the alveolar bone. 4. Apical group These fibres are irregularly arranged and radiate from cementum to bone at the fundus of socket. Function: Resist forces of luxation, prevent tooth tipping. Provide for protection of blood vessels and nerves of periodontal ligament. 5. Inter-radicular group: From the crest of inter-radicular septum, bundles extend to furcation of multi-rooted teeth. Function:  Resist tooth tipping, torque and luxation of tooth. 15
  16. 16. Transeptal group – These fibres extend interproximally over the alveolar crest to be embedded into the cementum of adjacent teeth.  Collagen fibres are embedded on one side into cementum and on the other into bone. These embedded fibres are known as “Sharpey’s fibres”.  The principal fibres run a wavy course from cementum to bone. These fibres appear to join in mid-region of ligament giving rise to a zone of distinct appearance known as “Intermediate plexus”.  Previously considered to be zone of remodeling this zone is an artifact as seen under electron microscope which arises due to plane of sectioning. Oxytalan:  This is an immature elastic fibre restricted to walls of blood vessels.  Their orientation differ from that of principal fibres.  These fibres run axially i.e. one end is in either bone/cementum and other in blood vessels.  They from a complex network close to apex.  Function is unknown but may play a role in supporting blood vessels of periodontal ligament. Eluanin: 16
  17. 17.  This is the other form of immature elastic fibres and along with collagen fibres it forms a meshwork extending from cementum to bone and sheathing the collagen fibre bundles. Ground substance The space between cells, fibres, blood vessels, nerves in periodontal ligament is occupied by ground substance. Function  Exchange of metabolites between microcirculation and cells. Enables tooth to withstand stress along with fibres. Comprises of 70% water  Two major components are:  Glycosaminoglycans  Proteoglycans and Hyaluronic acid.  Glycoproteins  Fibronectin and laminin.  Both components comprise of proteins and polysaccharides but of different nature and arrangement.  They can be demonstrated by histochemical method under electron microscope and light microscope. Chemicals used for Electron microscope  Ruthenium Red Light microscope  Alcian blue, 8GX and toulidiene blue. Glycoproteins contain a group 1,2 glycol which can be demonstrated in light microscope by use of periodic Acid Schiff method. 17
  18. 18. In electron microscope by  Periodic acid silver methanamine technique. periodontal ligament contains a glycoprotein “fibronectin” which is present in filamentous form. It contains chemical groups that attach to surface of fibroblasts, collagen proteoglycans and fibrin. Clinical and physiologic aspect of ground substance  Proteoglycans play a role in transmitting pressure and thus dispersing mechanical forces to protect periodontal ligament from damage.  Changes in ground substance are seen during tooth eruption.  Ground substance are also found to play a role in tooth mobility as studied by Picton in 1984. Interestitial tissue Some of blood vessels, lymphatics, and nerves of periodontal ligament are surrounded by loose connective tissue known as interstitial tissue. 18
  19. 19. Structures in connective tissue Contains: - Blood vessels - Lymphatics - Nerves - Cementicles Blood vessels The arterial vessels of periodontal ligament are delivered from three sources: 1. Branches from apical vessels that supply dental pulp. 2. Branches from intra-alveolar vessels that penetrate the alveolar bone horizontally to enter periodontal ligament. 3. Branches of gingival vessels. They enter periodontal ligament from coronal direction. The arterioles and cappaliries form a rich network adjacent to bone. There is a rich vascular plexus also at the apex and cervical part of ligament. Venous plexus run axially to drain to the apex. 19
  20. 20. Lymphatics A network of lymphatic vessels along the blood vessels provides the lymph drainage of periodontal ligament. The flow is from periodontal ligament toward and into adjacent alveolar bone. Nerves Associated with blood vessels and nerves pass through the foramina in the alveolar bone including apical foramen to enter the ligament. In region of apex, they run toward the cervix, whereas along the length of root run both coronally and apically. Nerves are of both large (myelinated) diameter as well as small (may or may not myelinated) diameter. Large fibres are concerned with sense of touch and end in different type of endings such as knob like, spindle-like etc. small are responsible for pain. Cementicles Calcified bodies cementicles are sometimes found in periodontal ligament seen commonly in old people. These may join to form large calcified masses. As cementum thickens, it envelops these masses. Origin is not known, but it is thought that degenerated epithelial cells form nidus for their calcification. 20
  21. 21. Functions of periodontal ligament The functions of periodontal ligament are: 1. Physical 2. Nutritive 3. Sensory 4. Homeostatic (formative and remodeling). 1) Physical functions of periodontal ligament include the following: a. Transmission of occlusal forces to bone. b. Attachment of teeth to bone. c. Maintenance of gingival tissue in the proper relationship to the teeth. d. Resistance to impact of occlusal forces. e. Provision of a soft tissue cassing to protect the vessels and nerves from injury by mechanical forces. As a result of forces, (on tooth) during mastication or due to orthodontic forces, part of PD space is narrowed and periodontal ligament gets compressed. Other parts are widened. Compressed part provides support for the loaded tooth. Collagen fibres acts as a cushion for the displaced tooth. 21
  22. 22. 2) Sensory The periodontal ligament is abundantly supplied with sensory nerve fibres capable of transmitting tactile pressure and pain sensation by the trigeminal pathways. This proprioceptive mechanism, allows the organism to detect the application of most delicate forces to the teeth and very slight displacement of the teeth. E.g., Bitings on a small hard object such as stone while eating. 3) Nutritive : The periodontal ligament supplies nutrients to the cementum (Cementocytes) bone (Superficial osteocyctes) gingiva and metabolites to other cells of ligament via blood vessels. 4) Homeostatic Cells of periodontal ligament participate in the formation and resorption of cementum and bone and occur in physiologic tooth movement in the accommodation of periodontium to the occlusal forces and in the repair of injuries. Periodontal ligament is constantly undergoning remodeling. Old cells are replaced by new ones. The rate of formation and the differentiation of fibroblasts affect the rate of formation of collagen, cementum and bone. 22
  23. 23. Rate of collagen synthesis is faster than other any connective tissue of the body. When homeostatic mechanism, is upset derangement of periodontium occurs. If periodontal ligament is irrepairably damged ankylosis may occur due to bone deposition in periodontal space. Theories in periodontal ligament Three theories relative to the mechanism of tooth support have been considered: 1) Tensional theory It states that principal fibres of periodontal ligament have major responsibility in supporting the tooth and transmitting forces to the bone. When a force is applied to the crown, principal fibres first unfold and straighten and then transmit forces to the alveolar bone, causing an elastic deformation of the bony socket finally when alveolar bone has reached its limit, load is transmitted to basal bone. Many investigators do not agree to this theory. 2) Viscoelastic system theory This theory states that fluid movement is mainly responsible for displacement of tooth; fibres have the secondary role. 23
  24. 24. When forces are applied on tooth extracellular fluid from periodontal ligament passes into marrow spaces of bone through foramina in the cortical layer. After the depletion of tissue fluids, the fibre bundles absorb the slack and lighten, causing blood vessel stenosis, arterial back pressure cause ballooning of the vessels and passage of blood ultrafiltrates into the tissues, thereby replenishing the tissue fluids. 3) Thixotropic theory It states that periodontal ligament behaves like a thixotrophic gel i.e. property of becoming fluid when shaken or stirred and then becoming semi-solid again. Physiologic response of periodontal ligament is explained by changes in the viscosity of biologic system. According to Schroeder, presence of organized collagen fibres makes the theory untenable. Age Changes in periodontal ligament The principal fibres of the periodontal ligament are thicker in aging human than in younger individuals. Interfibrillar areas are reduced in size. There is a decrease in ratio of ground substance to collagen. Fewer cells are seen. Periodontal ligament show hyalinization and chondroid degeneration related to reduced vascular supply. 24
  25. 25. - Calcified bodies are common in periodontal ligament of elderly humans. - Occassionally they increase in number and appear to calcify a complete fibre bundle producing ankylosis. - Epithelial cell rests may undergo calcification with age. Clinical Consideration: - Primary role of periodontal ligament is to support tooth in socket. - Thickness of periodontal ligament varies and decrease with age. - Periodontal ligament is wide in tooth under function and is thin in functionless teeth. Thus the tooth long out of function is poorly adapted to carry the load suddenly placed on tooth by a restoration and thus may account for inability of a patient to use the restoration immediately after its placement. Acute trauma to periodontal ligament due to accidental blow condensing of a foil, rapid tooth separation may produce pathologic changes such as: - Fracture and resorption of cementum and bone. - Tear of fibre bundles. - Hemorrhage and necrosis. - periodontal ligament widening. 25
  26. 26. - Tooth becomes loose. - Repair occurs on elimination of trauma. Orthodontic movement depends on resorption and formation of both periodontal ligament and bone. Pressure causes bone resorption. Tension causes bone formation.  The periodontal ligament in the periapical area of tooth is often the site of pathologic lesion.  Infectious disease pulp progress to apical periodontal ligament and replace its fibres by granulation tissue giving rise to dental granuloma  which may develop into cyst later on. • Acute alveolar abscess. • Acute PD abscess. Diseases of periodontal ligament are often irreversible From gingiva it progresses to periodontal ligament, it is a slow process. Once destroyed PDL is difficult to regenerate. Summary and Conclusion  Periodontal ligament is a connective tissue structure that surrounds the roots of teeth and attaches them to bony alveolus. 26
  27. 27.  It is composed of a group of dento-alveolar fibres called principal fibres which help to support the tooth.  Periodontal ligament has rich vascular supply (nourishes) and is well innervated these nerve help prevent trauma to teeth. Periodontal ligament thus forms an important structure of dento- alveolar unit.  The periodontal disease has rapidly taken over dental caries which was considered as the primary disease of oral cavity.  Damage of periodontal ligament results in loss of tooth. Thus a thorough understanding of periodontal ligament is necessary for all health care personnel. References 1. Periodontal Ligament – Oral Histology and Embryology by Orban’s. 2. Tencate A.R. 3. Tooth supporting structures Page 30, Feemin A.Caranza Jr. and Angela MUBIOS, Clinical Periodontology, 8th Edition. 27