PERIODONTAL LIGAMENT

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PERIODONTAL LIGAMENT
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INTRODUCTION

• Periodontal ligament is an integral part of
  periodontium.
• The periodontium is an attachment apparatus of
  the teeth. It is a connective tissue organ, which
  is covered by epithelium on top surface.
• Teeth are attached to the bone of the jaws by
  periodontium.

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• The periodontal ligament is a soft, fibrous
  specialized connective tissue which is present in
  the periodontal space, which is situated between
  the cementum of root of the tooth and the bone
  forming the socket wall.
• The periodontal ligament extends coronally up to
  the most apical part of connective tissue of
  gingiva.
• Because the collagen fibers are attached to the
  cementum and alveolar bone, the ligament provides
  soft tissue continuity between the mineralized
  connective tissues of periodontium.

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Other terms which were previously used for
periodontal ligament are-

• Desmondont
• Gomphosis
• Pericementum
• Dental Periosteum
• Alveolodental ligament
• Periodontal membrane

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STRUCTURE

• The periodontal ligament has the shape of an
  HOUR GLASS and is narrowest at the midroot
  level.
• The width of periodontal ligament is
  approximately 0.15-0.38mm.

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CELLULAR COMPOSITION

• The cells of periodontal ligament are categorized
  as
• 1. Synthetic Cells
• a) Osteoblasts
• b) Fibroblasts
• c) Cementoblasts
• 2. Resorptive Cells
• a) Osteoclasts
• b) Cementoclasts
• c) Fibroblasts
• 3. Progenitor Cells
• 4. Epithelial Cell rests of malassez
• 5. Connective Tissue cells
• a) Mast cells
• b) Macrophages

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SYNTHETIC CELLS

• The characteristic of synthetic cells are
• Should be actively synthesizing ribosomes.
• Increase in the complement rough endoplasmic
  reticulum and golgi apparatus.
• Large open faced or vesicular nucleus with
  prominent nucleoli.

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OSTEOBLASTS

• The osteoblasts covers the periodontal surface of
  alveolar bone. Alveolar bone constitute a
  modified endosteum and not a periosteum. A
  periosteum comprises at least two distinct
  layers
• 1. Inner CELLULAR LAYER
• 2. Outer FIBROUS LAYER
• A cellular, but not an outer fibrous layer is
  present on the periodontal surface of alveolar
  bone.

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FunctionOsteoblasts help in the synthesis of
alveolar bone.
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FIBROBLASTS

• Fibroblasts are the most common cells in
  periodontal ligament. They constitute about 65
  of total population.
• They appear as ovoid or elongated cells with
  pseudopodia like process.
• They consist of subtypes with distinct phenotypes
  and found to synthesize higher quantities of
  chondroitin sulphate and lesser quantities of
  heparin sulphate and hyaluronic acid.

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FUNCTIONPRODUCTION OF VARIOUS TYPES OF FIBERS
IS ALSO INSTRUMENTAL IN THE SYNTHESIS OF
CONNECTIVE TISSUE MATRIX.
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• The fibroblast is stellate shaped cell which
  produces
• 1. COLLAGEN FIBERS
• 2. RETICULIN FIBERS
• 3. OXYTALAN FIBERS
• Various stages in the production of collagen
  fibers are as follows
• The first molecule released by fibroblasts is
  tropocollagen which contains three polypeptide
  chains intertwined to form helix. Tropocollagen
  molecules are aggregated longitudinally to form
  protofibrils, which are subsequently laterally
  arranged parallel to form collagen fibrils.

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• Importantly in inflammatory situations such as
  those associated with periodontal diseases, an
  increased expression of matrix metalloproteinase's
  occurs that aggressively destroys collagen.
• Thus attractive therapies for controlling tissue
  destruction may include host-modulators that have
  the capacity to inhibit metalloproteinases.

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• The damaged periodontal fibers are replaced and
  remodeled by newly formed fibers.
• The RENEWAL CAPABILITY is an important
  characteristic of periodontal ligament.

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CEMENTOBLASTS

• Cementoblasts synthesize collagen and protein
  polysaccharides, which make up the organic
  matrix of cementum.
• After some cementum has been laid down, its
  mineralization begins with the help of calcium
  and phosphate ions.
• Sharpeys fibers

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RESORPTIVE CELLSOSTEOCLASTS

• Resorb bone.
• The surface of an osteoclasts which is in contact
  with bone has a ruffled border.
• Resorption occurs in two stages
• The mineral is removed at bone margins and then
  exposed organic matrix disintegrates. The
  osteoclasts demineralise the inorganic part as
  well as disintegrates the organic matrix.

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FIBROBLASTS

• Fibroblasts are capable of both synthesis and
  resorption.
• They exhibit lysosomes, which contain collagen
  fragments undergoing digestion.
• The presence of collagen resorbing fibroblasts in
  a normal functioning periodontal ligament
  indicates resorption of fibers occurring during
  remodeling of periodontal ligament.

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CEMENTOCLASTS

• Cementoclasts are found in periodontal ligament
  but not remodeled like alveolar bone and
  periodontal ligament.
• These are found on the surface of cementum.

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PROGENITOR CELLS

• Progenitor cells are the undifferentiated
  mesenchymal cells, which have the capacity to
  undergo mitotic division and replace the
  differentiated cells dying at the end of their
  life span.
• These cells are located in perivascular region
  and have a small close faced nucleus and little
  cytoplasm.
• When cell division occurs, one of the daughter
  cells differentiate into functional type of
  connective tissue cells. The other remaining
  cells retain their capacity to divide.

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EPITHELIAL CELL RESTS OF MALASSEZ

• These cells are the remnants of the epithelium of
  Hertwigs Epithelial Root Sheath and are found
  close to cementum.
• These cells exhibit monofilaments and are
  attached to each other by desmosomes.
• The epithelia cells are isolated from connective
  tissue by a basal lamina.

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Periodontal ligament showing epithelial cell
rests of malassez, indicated by arrows.
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MAST CELLS

• Mast cells are small round or oval. These cells
  are characterized by numerous cytoplasm, which
  mask its small, indistinct nucleus.
• The diameter of mast cells is about 12 to 15
  microns.
• The granules contain heparin and histamine. The
  release of histamine into the extracellular
  compartment causes proliferation of the
  endothelial and mesenchymal cells.
• Degranulate in response to antigen- antibody
  formation on their surface.

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MACROPHAGES

• Macrophages are derived from blood monocytes and
  are present near the blood vessels.
• These cells have a horse-shoe shaped or kidney
  shaped nucleus with peripheral chromatin and
  cytoplasm contain phagocytosed material.
• Macrophages help in phagocytosing dead cells and
  secreting growth factor, which help to regulate
  the proliferation of adjacent fibroblasts.

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EXTRACELLULAR SUBSTANCE

• Extra cellular substance comprises the following
• 1. Fibers a) Collagen b) Oxytalan
• 2. Ground Substance a) Proteoglycans b)
  Glycoproteins

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PERIODONTAL FIBERS

• The most important element of periodontal
  ligament has principal fibers, the principal
  fibers are collagenous in nature and a arranged
  in bundles and follow a wavy course.
• Collagen is a high molecular weight protein.
• Collagen macromolecules are rod like and are
  arranged in form of fibrils. Fibrils are packed
  side by side to form fibers.
• Vitamin C help in formation and repair of
  collagen.

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• Half life of collagen fibers is between 3 to 23
  days and collagen imparts a unique combination of
  flexibility and strength to tissue.

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TYPES OF PERIODONTAL LIGAMENT FIBERS1.
TRANSEPTAL GROUP

• These fibers extend interproximally over alveolar
  bone crest and are embedded in the cementum of
  adjacent teeth.
• They are reconstructed even after the destruction
  of alveolar bone resulting from periodontal
  disease.
• These fibers may be considered as belonging to
  the gingiva because they do not have osseous
  attachment.

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DIAGRAM OF PRINCIPAL FIBER GROUPS
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ALVEOLAR CREST GROUP

• These fibers extend obliquely from the cementum
  just beneath the junctional epithelium to
  alveolar crest.
• Fibers also run from the cementum over the
  alveolar crest and to fibrous layer of periosteum
  covering alveolar bone.
• The alveolar crest fibers prevent extrusion of
  tooth and resist lateral tooth movements.
• The incision of these fibers during periodontal
  surgery does not increase tooth mobility unless
  significant attachment loss has occurred.

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3. HORIZONTAL GROUP
Horizontal fibers extend at right angles to long
axis of tooth from the cementum to alveolar bone.

4. OBLIQUE GROUP
Oblique fibers, the largest group in periodontal
ligament, extend from cementum in a coronal
direction obliquely to bone. They bear the brunt
of vertical masticatory stresses and transfer
them into tension on the alveolar bone.
5. APICAL GROUP
The apical fibers radiate in a rather irregular
manner from the cementum to bone at apical region
of the socket. They do not occur on incompletely
formed roots.
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INTER-RADICULAR FIBERS

• The interradicular fibers fan out from the
  cementum to the tooth in furcation areas of
  multirooted teeth.
• The remodeling of fibers take place in
  intermediate plexus. This allows adjustments in
  the ligament, which accommodate small movements
  of tooth.

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OXYTALAN FIBERS

• These are immature elastic fibers restricted to
  walls of blood vessels and are oriented in an
  axial direction.
• The function is to support the bloods vessels in
  the periodontal ligament.

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STRUCTURES PRESENT IN THE CONNECTIVE TISSUE

• BLOOD VESSELS
• Main blood supply is from superior and
  inferior alveolar arteries. The blood vessels are
  derived from the following
• 1. BRANCHES FROM APICAL VESSELSVessels
  supplying the pulp.
• 2. BRANCHES FROM INTRA-ALVEOLAR VESSELS-
  Vessels run horizontally and penetrate the
  alveolar bone to enter into the periodontal
  ligament.

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• 3. BRANCHES FROM GINGIVAL VESSELS- The
  arterioles and capillaries ramify and form a rich
  network. Rich vascular plexus is found at the
  apex and in cervical part of ligament.

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NERVE SUPPLY

• Nerves found in ligament pass through foramina in
  alveolar bone.
• The nerves are the branches of second and third
  division of fifth cranial nerve (trigeminal
  nerve) and follow same path as blood vessels.
• These nerve fibers provide sense of touch,
  pressure, pain and proprioception during
  mastication.

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CEMENTICLES

• Cementicles are small calcified bodies present in
  the periodontal ligament.
• They may form into large calcified bodies and
  fuse within cementum or remain free.
• These are found in old age.
• The degenerated epithelial cells form a nidus for
  calcification.

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FUNCTIONS

• PHYSICAL 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.
• Depending on type of force applied, axial force
  when applied causes stretching of oblique fibers
  of periodontal ligament.

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• Transmission of this tensional force to alveolar
  bone encourages bone formation rather then bone
  resorption.
• But when horizontal or tipping force is applied,
  the tooth rotates around the axis.
• When a greater force is applied, displacement of
  facial and lingual plates may occur.
• The axis of rotation in a single rooted tooth is
  located in area between the apical and middle
  third of root.
• In multirooted tooth, axis of rotations is
  located at furcation area.

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• C) Attaches the teeth to the bone.
• D) Maintains the gingival tissues in their
  proper relationship to the teeth.
• E) Shock absorption resists the impact of
  occlusal surfaces. Due to its property of shock
  absorption the teeth are slightly more mobile in
  early morning than in evening.

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• Two theories have been explained for mechanism of
  tooth support.
• TENSIONAL THEORY
• VISCOELASTIC THEORY

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A. TENSIONAL THEORY

• According to it, principal fibers play a major
  role in supporting tooth and transmitting forces
  to bone.
• When forces are applied to tooth, principal
  fibers unfold and straighten and then transmit
  the forces to alveolar bone, causing elastic
  deformation of socket.

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A. Tooth in a resting stateB. The periodontal
ligament fibers are compressed in areas of
pressure and stretched in area of tension.
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VISCOELASTIC THEORY

• According to it, the fluid movement largely
  controls the displacement of the tooth, with
  fibers playing a secondary role.
• When forces are transmitted to the tooth, the
  extracellular fluid is pushed from periodontal
  ligament into marrow spaces through the
  cribriform plate.
• After depletion of tissue fluids, the bundle
  fibers absorb the shock and tighten.
• This leads to blood vessel stenosis ? arterial
  lack pressure ? ballooning of vessels? tissue
  replenishes with fluids.

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2. FORMATIVE REMODELLING
FUNCTION

• Cells of the periodontal ligament have the
  capacity to control the synthesis and resorption
  of cementum, ligament and alveolar bone.
• Periodontal ligament undergoes constant
  remodeling, old cells and fibers are broken down
  and replaced by new ones.

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3. NUTRITIONAL FUNCTION

• Blood vessels of periodontal ligament provide
  nutrition to the cells of periodontium, because
  they contain various anabolites and other
  substances, which are required by cells of
  ligament.
• Compression of blood vessels (due to heavy forces
  applied on tooth) leads to necrosis of cells.
• Blood vessels also remove catabolites.

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4. SENSORY FUNCTION

• The nerve bundles found in periodontal
  ligament, divide into single myelinated nerve,
  which later on lose their myelin sheath and
  end in one of the four types of nerve
  termination1. Free endings, carry pain
  sensations. 2. Ruffini like mechanoreceptors
  located in the apical area. 3. Meisseners
  corpuscles are also mechanoreceptors located
  primarily in mid-root region. 4. Spindle like
  pressure endings, located mainly in apex.
• Pain sensation is transmitted by small
  diameter nerves, temperature by intermediate
  type pressure by large myelinated fibers.

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HOMEOSTATIC MECHANISM

• The resorption and synthesis are controlled
  procedures.
• If there is a long term damage of periodontal
  ligament, which is not repaired, the bone is
  deposited in the periodontal space.
• This results in obliteration of space and
  ankylosis between bone and the tooth.
• The quality of tissue changes if balance between
  synthesis and resorption is disturbed.

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• If there is deprivation of Vit. C which are
  essential for collagen synthesis, resorption of
  collagen will continue.
• So there is progressive destruction and loss of
  extra cellular substance of ligament.
• This occurs more on bone side of ligament.
• Hence, loss of attachment between bone and tooth
  and at last, loss of tooth.

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CLINICAL CONSIDERATIONS

• The primary role of periodontal ligament is to
  support the tooth in the bony socket.
• The width of periodontal ligament varies from
  0.15 to 0.38mm. The average width is - 0.21mm
  at 11 to 16 years of age. - 0.18mm at 32 to 50
  years of age. - 0.15mm at 51 to 67
  years of age.
• So, the width of periodontal ligament decreases
  as age advances.

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• In the periodontal ligament, aging results in
  more number of elastic fibers and decrease in
  vascularity, mitotic activity, fibroplasia and in
  the number of collagen fibers and
  mucopolysaccharides.
• If gingivitis is not cured and supporting
  structure become involved, the disease is termed
  as periodontitis.
• There are few coccal cells and more motile rods
  and spirochetes in the diseased site than in the
  healthy site. The bacteria consists of
  gram-positive facultative rods and cocci in
  healthy site while in diseased site,
  gram-negative rods and anaerobes are more in
  number.

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• Resorption and formation of both bone and
  periodontal ligament play an important role in
  orthodontic tooth movement. If tooth movement
  takes place, the compression of PDL is
  compensated by bone resorption whereas on tension
  side, apposition takes place.
• Periapical area of the tooth is the main
  pathologic site. Inflammation of the pulp reached
  to the apical periodontal ligament and replaces
  its fiber bundles with granulation tissue called
  as granuloma, which then progresses into apical
  cyst.

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• Chronic periodontal disease can lead to infusion
  of microorganisms into the blood stream.
• The pressure receptors in ligament have a
  protective role. Apical blood vessels are
  protected from excessive compression by sensory
  apparatus of the teeth.
• The rate of mesial drift of tooth is related to
  health, dietary factor and age. It varies from
  0.05 to 0.7mm per year.

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