Structure & function of the periodontium: I

Structure & function of the periodontium: I

1. GINGIVA

   • Epithelium
     • Oral epithelium
       • Lines the oral side of the gingiva
       • Parakeratinised
       • Like epidermis - corneal, granular, spinous etc layers
       • Rete's ridges etc
     • Sulcular epithelium
       • Non-keratinised
       • Links the JE to the FGM
       • Semi-permeable - permitting GCF passage (and bacterial entry)
     • Junctional epithelium
       • Non-keratinised
       • Joins the PDL to the sucular epithelium
       • Hemidesmosomal attachment is present which attaches the JE to enamel, dentine and cementum
       • Length ~ 0.25 - 1.35 mm
   • Gingival fibres - NOT = PDL
     • Circular group - surrounding the neck of the tooth, binding the FG to the tooth
     • Dento-gingival group - attaching from the cementum to the lamina propria of the free + attached gingiva
     • Dento-periosteal group - attaching from the cementum and run apically over the outer plate of alveolar processes to insert onto it
     • Alveolar-Gingival group - Attaching from the top of the alveolar crest to the free + attached gingiva

2. Periodontal ligament

   • Made of collagen fibres (Type 1 - 70%, Type 3 - 20%)
     • Developmentally logical - DF -> Fibroblasts -> Secrete these fibres
     • Sharpey's fibres when in alveolar bone (bundle) + cementum
   • Structure
     • CEMENTUM - ALVEOLAR BONE connection
     • Very well vascularised (high turnover rate) + also innervated
     • Present from the apical foramen to the CEJ
     • 0.15-0.38 microns thick (thinnest in mid root zone)
     • Fibre groups of the PDL
       • A = Transseptal - running from the cementum of one tooth to cementum of another (briefly over alveolar crest)
       • B = Alveolar crest - attached to cementum below CEJ running downwards to rim of alveolus
       • C = Horizontal - More apical than alveolar crest group - oriented at right angles to long axis of tooth
       • D = Oblique - Running from the cementum in an oblique direction and inserting into the alveolar bone more coronally
       • E = Apical - Run from the cementum to the bone which forms the base of the tooth socket
       • F = Interradicular - Run from the cementum to the interradicular septum of the alveolar bone
       • Major group is the dentoalveolar group (2/3 of PDL volume)
     • Non-collagen fibre groups (immature elastin)
       • Elaunin
         • Also microfibril bundles
         • Meshwork connecting cementum to bone
         • Similar proposed functions to oxytalan
       • Oxytalan
         • Microfibril bundles
         • Bend and attach to the apical 1/3 of the root
         • Function may be to regulate vascular flow as fibres latch onto periodontal blood vessels
     • GS of = Hyaluronic acid, GP's, PG's, sulfated GAG's (can bind water to recover from compressive loads)
   • Function
     • Transmission of occlusal forces from tooth to alveolar bone - Shock absorber
       • Ankylosis = tooth attached directly to bone = brittle !
     • Can carry nutrients
     • High turnover rate therefore can adapt to any changes in tooth socket location
   • Cell types
     • Osteoblasts - line bone surface
     • Cementoblasts - line cementum surface
     • Fibroblasts - synth + degrade collagen = REMODELLING function
     • SOMETIMES osteoclasts
     • UDMC - can differentiate
     • Defence cells - macrophages etc
   • Epithelial cell rests of malassez
     • Highest number in root area
     • Remnants of HERS
     • Inflammation = periapical cyst | Calcify = cementicle
   • Bloods, Nerves, Lymphatics
     • Blood supply
       • Rich and complex
       • Entrance mostly via alveolar bone
       • May provide hydraulic support
     • Nerve supply
       • Travels with blood supply
       • Pressure receptors found in collagen fibres
     • Lymphatics - Present just below the junctional epithelium and pass into the periodontal ligament to the periapical region. Then pass via alveolar bone to mandibular canal / infraorbital canal to submandibular/submaxillary lymph nodes.

3. Cementum

   • Two types
     • Acellular
       • First to be formed - before the tooth reaches the occlusal plane
       • Sharpey's fibres make up most of its structure
       • Does not contain cells - hence the name
     • Cellular
       • Formed after the tooth reaches the occlusal plane
       • Less calcified than acellular
       • Contains cells in lacunae
       • Inc age = inc cellular in apical half of root
   • Thicker apically (16-60 mic. coronally, 150-200 mic. apically)
   • Deposition continues even after eruption
   • Cementogenesis
     • Collagen fibrils in GS (cementoid) deposited
       • Continues throughout life - especially rapid apically to compensate for tooth eruption/attrition forces
   • CEJ - three configurations
     • Cementum overlaps enamel
     • Space between enamel and dentine
     • Meet perfectly (end to end)
   • Resorption occurs in ~90% of the population - variety of causes such as periapical inflammation - leads to bone replacing root tissue

4. Alveolar bone

   • Alveoli = tooth socket
   • Cribriform plate = inner socket wall
   • Bundle bone = provides attachment for PDL fibre bundles (S Fib's)
   • Periosteum = osteogenic connective tissue layer covering outer surface
   • Interdental septa = cancellous bone
   • Cells of the alveolar bone
     • Osteoblasts deposit osteoid which is unmineralised. As new osteoid is secreted, the old osteoid becomes mineralised.
     • Osteoclasts found in How's or on the surface aid in bone resorption
     • Osteocytes trapped in lacunae
   • Least stable of all periodontal structures as it is constantly changing

Images and files in the map