Dental Hard Tissues and Enamel

Dental Hard Tissues

• Comprise enamel, dentine, cementum, and alveolar bone.

• These tissues are specialized, mineralized structures vital for tooth function.

• They provide protection against mechanical forces, facilitate mastication and speech.

• They also play a role in sensory perception.

Enamel

• Composition:

  • 96% Inorganic: Primarily hydroxyapatite crystals.
  • 1% Organic Matrix: Enamel proteins.
  • 3% Water.

• Characteristics:

  • Hardest tissue in the human body.
  • Acellular and avascular; cannot regenerate.
  • Translucent.

Enamel Formation (Amelogenesis)

• Ameloblasts: Derived from the inner enamel epithelium of the enamel organ. Responsible for enamel matrix secretion and mineralization.

• Stages:

  • Pre-secretory Stage: Ameloblast differentiation.
  • Secretory Stage: Enamel matrix protein secretion.
  • Maturation Stage: Removal of organic components and water; influx of minerals.
  • Protective Stage: Formation of the reduced enamel epithelium.

Enamel Structure

• Enamel Rods (Prisms): Basic structural units, extending from the dentin-enamel junction (DEJ) to the tooth surface. Arranged in a complex, interwoven pattern for strength.

• Interrod Enamel: Surrounds enamel rods; similar material but differing crystal orientation.

• Key Features:

  • Hunter-Schreger Bands: Alternating light and dark bands due to rod orientation.
  • Striae of Retzius: Incremental growth lines.
  • Perikymata: External manifestations of Striae of Retzius.

Clinical Relevance of Enamel

• Caries Susceptibility: Enamel demineralizes when pH drops below ~5.5.

• Enamel Hypoplasia: Defective enamel formation leads to thin or pitted enamel. Causes include nutritional deficiencies and systemic diseases during tooth development.

• Fluorosis: Excess fluoride intake disrupts enamel mineralization. Leads to mottling or discoloration of enamel.

Dentine

• Composition:

  • 70% Inorganic: Hydroxyapatite.
  • 20% Organic: Mainly Type I collagen and non-collagenous proteins.
  • 10% Water.

• Properties:

  • Less mineralized than enamel but more flexible.
  • Supports enamel and absorbs occlusal forces.
  • Capable of regeneration due to odontoblast activity.

Dentinogenesis

• Odontoblasts: Derived from dental papilla mesenchymal cells. Line the periphery of the dental pulp.

• Formation Process:

  • Predentine: Unmineralised matrix secreted by odontoblasts.
  • Mineralisation: Deposition of hydroxyapatite crystals on collagen fibers.
  • Incremental Lines: Reflect daily deposition (Lines of von Ebner, Contour Lines of Owen).

Dentine Structure

• Dentinal Tubules: Microscopic channels running from the pulp to the DEJ. Contains odontoblastic processes and dentinal fluid.

• Peritubular Dentine: Highly mineralized dentine surrounding each tubule.

• Intertubular Dentine: Less mineralized dentine between tubules.

• Types of Dentine:

  • Primary Dentine: Formed during tooth development.
  • Secondary Dentine: Formed after root completion.
  • Tertiary Dentine: Formed in response to stimuli.

Clinical Implications of Dentine

• Tooth Sensitivity: Exposure of dentinal tubules leads to sensitivity (Hydrodynamic theory).
• Caries Progression: Dentine demineralizes at a higher pH than enamel.
• Restorative Dentistry: Importance of preserving dentine during cavity preparation.

Cementum

• Function: Anchors periodontal ligament fibers to the tooth; protects root dentine; compensates for occlusal wear.

• Composition:

  • 45-50% Inorganic: Hydroxyapatite.
  • 50-55% Organic: Collagen (Type I, III) and non-collagenous proteins.

• Types:

  • Acellular Cementum: First formed; covers cervical third; crucial for attachment.
  • Cellular Cementum: Contains cementocytes; found on the apical third and furcations.

Cementogenesis

• Cementoblasts: Derived from dental follicle cells. Deposit cementum on the root dentine surface.

• Formation Process:

  • Initial Layer: Acellular afibrillar cementum at the cervical region.
  • Subsequent Layers: Alternating acellular and cellular cementum.

Distinction Between Primary & Secondary Cementum

• Primary Cementum:

  • Acellular.
  • Formed before tooth eruption.
  • Lacks cells
  • Fibres mainly extrinsic (Sharpey's fibers).
  • Essential for tooth attachment.

• Secondary Cementum:

  • Cellular.
  • Formed after tooth eruption.
  • Contains cementocytes in lacunae.
  • Adaptable to functional stress. Aids in repair.

Clinical Considerations of Cementum

• Hypercementosis: Excessive cementum deposition; complicates extractions.
• Cementum Resorption: Can occur due to trauma or orthodontic movement; usually repaired by new cementum deposition.
• Cemental Caries: Occurs in exposed root surfaces; Prevention via oral hygiene and periodontal disease management.

Alveolar Bone

• Components:

  • Alveolar Bone Proper: Thin layer lining the tooth socket.
  • Supporting Alveolar Bone: Comprised of cortical and trabecular bone.

• Histology:

  • Compact Bone: Dense outer layer (strength).
  • Cancellous Bone: Spongy inner layer.

• Cells:

  • Osteoblasts: Bone-forming cells.
  • Osteocytes: Mature bone cells.
  • Osteoclasts: Bone-resorbing cells.

Alveolar Bone Remodelling

• Bone Turnover: Continuous process of resorption and formation; influenced by mechanical forces and systemic factors.

• Orthodontic Movement: Application of force results in bone resorption on the pressure side and formation on the tension side.

• Periodontal Disease: Inflammation leads to alveolar bone loss. Detection is via radiographs showing decreased bone height.

Periodontal Ligament (PDL)

• Anatomy: Connective tissue between cementum and alveolar bone. Contains collagen fibers, cells, and ground substance.

• Functions:

  • Supportive: Anchors tooth in alveolus.
  • Sensory: Transmits tactile and pain sensations.
  • Nutritive: Supplies nutrients via blood vessels.
  • Formative and Resorptive: Involved in repair and regeneration.

Components of the PDL

• Principal Fiber Groups:

  • Alveolar Crest Fibres: Resist lateral movements.
  • Horizontal Fibres: Resist horizontal pressure.
  • Oblique Fibres: Resist vertical forces.
  • Apical Fibres: Prevent tooth extrusion.
  • Interradicular Fibres: Stabilise multirooted teeth.

• Cell Types: Fibroblasts, cementoblasts, osteoblasts, epithelial cell rests of Malassez.

Histological Features of the PDL

• Blood Supply: Rich vascular network from superior and inferior alveolar arteries.

• Nerve Supply: Sensory fibers for pain and proprioception.

• Ground Substance: Amorphous material containing proteoglycans and glycoproteins.

• Cellular Elements: Defense cells (e.g. macrophages, mast cells). Stem Cells for regeneration.

Clinical Relevance of the PDL

• Tooth Mobility: Increased mobility indicates PDL or bone loss.
• Orthodontic Treatment: PDL remodels under forces; allows tooth movement.
• Periodontal Disease: Inflammation leads to PDL fiber destruction.
• Trauma from Occlusion: Excessive force can damage the PDL.

Cementum-PDL-Alveolar Bone Interface

• Sharpey's Fibres: Collagen fibres embedded in cementum and bone; essential for tooth stability.

• Continuity: Integration of these three tissues provides a functional unit.

• Clinical Importance: Damage to one component potentially affects the entire periodontium; regeneration requires coordinated healing.

Histological Changes in Disease

• Periodontitis: Characterized by loss of attachment and bone; inflammatory infiltrate; collagen breakdown (Histology).
• Cementum Exposure: Leads to root sensitivity and caries.
• Ankylosis: Fusion of cementum to bone, loss of PDL space; tooth becomes immobile.

Regenerative Potential & Clinical Applications

• Guided Tissue Regeneration (GTR): Barrier membranes to direct new bone and PDL growth.
• Stem Cell Therapy: Research on using stem cells for periodontal regeneration.
• Bone Grafting: Used to restore alveolar bone height and volume.
• Biomimetic Materials: Development of materials mimicking natural tissues for repair.

Summary

• Enamel: Highly mineralized, acellular, unable to regenerate.
• Dentine: Vital tissue, capable of repair, sensitive due to tubules.
• Cementum: Anchors the teeth; two types.
• Alveolar Bone: Dynamic tissue; remodels in response to forces.
• PDL: Essential for support and sensory function.

MCQs

• Question 1: Hydroxyapatite is the main inorganic component of enamel.
• Question 2: Enamel cannot repair itself due to being acellular and avascular.
• Question 3: Reactionary dentine forms in response to external stimuli.
• Question 4: Odontoblasts are responsible for dentine formation.
• Question 5: Alveolar bone remodels with resorption on the pressure side.

This quiz covers the essential aspects of dental hard tissues, focusing on enamel, its composition, characteristics, and formation process (amelogenesis). Understand the roles and stages involved in enamel development and the significance of these tissues in dental health.