PERIODONTAL LIGAMENT

• Periodontium


  Connective tissue that attaches the teeth to the bones of the jaws and continually adapting apparatus for support of teeth during function
• Components of the periodontium


  • Cementum
  • Periodontal ligament
  • Alveolar bone
  • Gingiva facing the bone
• Periodontal ligament


  Fibrous connective tissue that is noticeably cellular and contains numerous blood vessels, occupying the periodontal space
• Terms synonymous to periodontal ligament

  • Periodontal membrane
  • Pericementum
  • Dental periosteum
  • Alveolo-dental ligament
  • Desmodont
  • Gomphosis
• Development of the periodontal ligament

  1. Elongation of HERS

  2. Separation of HERS

  3. Migration of cells to the external surface of the newly formed root dentin

  4. Differentiation of migrant dental follicle cells into cementoblasts, fibroblasts, and osteoblasts
• Cells in the periodontal ligament

  • Synthetic cells (osteoblasts, fibroblasts, cementoblasts)
  • Resorptive cells (osteoclasts, fibroblasts, cementoclasts)
  • Progenitor cells
  • Epithelial cells (epithelial rests of Malassez)
  • Cells derived from hematopoietic line (mast cells, macrophages)
• Synthetic cells


  • Secrete proteins, have active RER and Golgi apparatus, and a large number of mitochondria
• Osteoblasts


  • Found on the periodontal surface of alveolar bone
• Fibroblasts


  • Lie in the body of connective tissue in close proximity to the periodontal ligament, run parallel to the collagen fibers
• Cementoblasts


  • Lie in the periphery of the cementum tissue
• Resorptive cells


  • Undergo two stages of resorption: removal of mineral from bone (demineralization) and disintegration of organic matrix
• Osteoclasts


  • Multinucleated cells that resorb bone, occupy bays in bone (Howship's lacuna), have numerous mitochondria and lysosomes but little RER
• Fibroblasts (resorptive)


  • Have lysozymes
• Cementoclasts


  • May resemble osteoblasts, multinucleated, cause resorption of cementum under certain conditions like trauma, occupy Howship's lacuna
• Progenitor cells


  • Cells in connective tissue that have the capacity to undergo mitotic division in response to repair, death of cells, trauma, or normal biological requirements, differentiate into resorptive or synthetic cells as needed, have small nucleus and very little cytoplasm
• Epithelial rests of Malassez


  • Epithelial cells that are remnants of HERS, appear as network, strands, islands or tube-like structures near the parallel surface of the root, can undergo rapid proliferation and produce cysts and tumors of the jaw under certain pathologic conditions
• Mast cells


  • Small round, oval cells with numerous cytoplasmic granules and small round nucleus, play a role in inflammatory reactions, regulate endothelial and fibroblast cell production
• Macrophages


  • Located adjacent to blood vessels, appear similar to fibroblasts but have prominent phagocytosed material in the cytoplasm and sparse organelles, function to phagocytose dead cells and secrete growth factors that regulate fibroblast proliferation
• Extracellular substances in the periodontal ligament

  • Fibers (collagen, oxytalan, elastic)
  • Ground substance (proteoglycans, glycoproteins)
• Collagen fibers


  • Type I and III, fibrils gather to form fibers, when gathered into bundles with clear orientation they become principal fibers that exit the cementum or alveolar bone
• Types of principal collagen fibers

  • Dento-alveolar (alveolar crest, horizontal, oblique, apical, interradicular)
  • Gingival (dentogingival, alveologingival, circular/circumferential, transseptal)
• Sharpey's fibers


  • Part of the collagen fibers embedded into cementum on one side of the periodontal space and into alveolar bone on the other
• Oxytalan fibers


  • Immature elastic fibers that run in the axial direction, one end embedded in cementum or bone and the other in the wall of a blood vessel, support blood vessels in the periodontal ligament
• Elastic fibers


  • Restricted to the walls of blood vessels
• Structures present in the connective tissue of the periodontal ligament

  • Blood vessels
  • Lymphatics
  • Nerves
  • Cementicles (calcified bodies)
• Functions of the periodontal ligament

  • Supportive (attach cementum to alveolar bone, provide cushion for displaced tooth, hydraulic cushion from blood vessels)
  • Sensory (proprioceptive mechanism)
  • Nutritive (blood vessels provide anabolites and other substances)
  • Homeostatic (cells can resorb and synthesize periodontal tissues)
• Thickness of the periodontal ligament


  • Varies in different individuals, teeth, and locations (0.15-0.38 mm, thinnest at middle region of root, thin in functionless/embedded teeth, wide in teeth under excessive occlusal stress, absent in ankylosed teeth)
• In restorative dentistry, the supporting tissues of a tooth long out of function are poorly adapted to carry the load suddenly placed on the tooth by a restoration, and an adjustment period must be permitted after orthodontic treatment
• Trauma to the periodontal ligament may produce pathologic changes such as fractures or resorption of cementum, tears of fiber bundles, hemorrhage and necrosis, resulting in bone resorption, widened periodontal ligament, and loose tooth
• Orthodontic tooth movement depends on resorption and formation of both bone and periodontal ligament, with pressure side showing compression/shortening and tension side showing elongation/widening of the periodontal ligament
• The periodontal ligament in the periapical area of the tooth is often the site of pathologic lesions such as periapical abscesses, dental granulomas, and apical cysts
• Safeguarding the integrity of the periodontal ligament is one of the most important challenges for a clinician, as it is affected by conditions like gingivitis and periodontitis