Oral Anatomy & Histology: The Periodontium

Oral Anatomy & Histology: The Periodontium

Course Overview

  • Outcome 8: Focus on understanding the following areas related to the periodontium.
    • 8.1 Describe the characteristics of cementum.
    • 8.2 Explain the clinical importance of cementum.
    • 8.3 Describe the physical and histological structure of the periodontal ligament.
    • 8.4 Discuss the formation of the periodontal ligament.
    • 8.5 Discuss the clinical importance of the periodontal ligament fibers.
    • 8.6 Discuss the histological structure of the alveolar bone.
    • 8.7 Explain the clinical significance of the alveolar process.

Recap of Previous Session

  • Outcome 7: Covered topics including enamel, dentin, and pulp, along with gingival observations.

Understanding the Periodontium

  • Definition: The periodontium comprises the supporting structures of the teeth, crucial for anchorage and function.
  • Key Components:
    • Cementum
    • Periodontal ligaments
    • Alveolar bone
    • Gingiva (minor role in anchorage)
  • Importance of Histology: Understanding a healthy periodontium’s histology is vital for recognizing pathological changes during periodontium diseases (Fehrenbach & Popowics, 2020).

Cementum

Characteristics of Cementum (8.1)

  • Function: Attaches teeth to the alveolar process by anchoring the periodontal ligaments (PDL).
  • Structure: Hard dental tissue covering the root, joining enamel at the Cemento-Enamel Junction (CEJ).
  • Chemical Composition Breakdown:
    • Inorganic Substance: 65% (Cementum), 70% (Dentin), 96% (Enamel)
    • Organic Substance (Proteins): 23% (Cementum), 20% (Dentin), 1% (Enamel)
    • Water: 12% (Cementum), 10% (Dentin), 3% (Enamel)

Physical Characteristics of Cementum

  • Color: Yellowish in appearance, resembling dentin clinically.
  • Thickness:
    • Coronal half: 16-60 microns
    • Apical half: 150-200 microns
  • Resistance:
    • Vulnerable to being worn away by brushing.
    • Prone to decay.

Histological Structure of Cementum

  • Composition: Mineralized fibrous matrix and cells
  • Key Cells:
    • Cementoblasts: Derived from dental sac, found in PDL, form cementum.
    • Some become embedded in cementum, forming cementocytes which are housed in lacunae.
    • Canaliculi: Tiny canals oriented towards the PDL, containing processes that help derive nutrients for the PDL.
  • Sharpey’s Fibers: Ends of PDL fibers that become trapped in cementum, anchoring the PDL firmly to the tooth and suspending it in the socket.

Formation of Cementum

  1. Layered Formation:
    • Cementoblasts begin at CEJ, moving down, secreting ground substance that calcifies.
    • Toward apex, cementoblasts get trapped, forming a thicker layer. (Bird & Robinson, 2018)
  2. Relationship with Enamel and Dentin:
    • Overlaps enamel at CEJ: 15%
    • Meets enamel at CEJ: 52%
    • Does not meet enamel at CEJ: 33%
    • This relationship is critical as it can cause sensitivity, exposing dentinal tubules (Fehrenbach & Popowics, 2020).
  3. Types of Cementum:
    • Acellular Cementum:
      • First layer deposited at the Dentino-Cemental Junction (DCJ), no cementocytes, covers the entire tooth.
      • Thins at the coronal half to one-third of the tooth.
    • Cellular Cementum:
      • Also known as secondary cementum, found at the apical portion, thicker, contains cementocytes allowing more cellular cementum production (Bird & Robinson, 2021; Fehrenbach & Popowics, 2020).

Clinical Importance of Cementum (8.2)

  • Anchorage: Anchors the tooth to the bony socket, connecting cementum to bone through Sharpey's fibers.
  • Compensation for enamel loss: Cementum is produced throughout life to compensate for wear from occlusion/attrition, adding layers at the root apex to maintain occlusion, potentially causing natural gingival recession.
  • Repair Mechanism: Cementum can repair damaged tooth roots by replacing resorbed dentin, particularly after trauma (e.g., impacted third molars).
  • Resorption: Trauma can lead to resorption of cementum at the root apex, affecting mobility.

Clinical Considerations for Cementum

  • Hypercementosis/Cementum Hyperplasia:
    • Thickening of cellular cementum at the root apex, typically not problematic unless extraction is necessary.

Periodontal Ligaments (PDL)

Physical and Histological Structure of Periodontal Ligament (8.3)

  • Description:
    • Connective tissue surrounding the root of the tooth.
    • Main suspensory tissue of the periodontium, connecting cementum to bone through Sharpey’s fibers.
    • At the cervical area, connects the tooth to the gingiva.

Formation of Periodontal Ligament (8.4)

  • Origin: Develops from the dental sac, beginning after the formation of cementum.
  • Key Components:
    • Fibroblasts: Produce the intercellular substance and collagen fibers of PDL.
    • Cementoblasts: Form cementum.
    • Osteoblasts: Form bone.
    • Nerves: Primarily sensory for tactile feedback.
    • Rests of Malassez: Remnants of Hertwig’s root sheath which can play a role in the formation of cysts or tumors.
    • Cementicles: Small calcified bodies that have no clinical significance but occur in the PDL.

Principal Fiber Groups in PDL

  1. Gingival Fibers
    • Location: Cervical part of the root
    • Function: Connects tooth to gingiva, helps maintain gingival tissue relationship.
  2. Transseptal Fibers
    • Location: At level of the alveolar crest
    • Function: Resists horizontal movements, maintains tooth in socket.
  3. Alveolar Crest Fibers
    • Location: Apical to alveolar crest fibers
    • Function: Resists lateral pressures on the tooth.
  4. Horizontal Fibers
    • Location: Below horizontal fibers
    • Function: Resists forces along the tooth's long axis.
  5. Oblique Fibers
    • Location: Around the apex of the tooth
    • Function: Prevent tipping, resist twisting forces, and protect the tooth’s blood and nerve supply.
  6. Apical Fibers
    • Location: Between roots in furcation areas
    • Function: Stabilize tooth against tipping.
  7. Interradicular Fibers
    • Location: In the furcations between roots of multi-rooted teeth.
    • Function: Stabilizes the tooth, preventing tipping and tilting.

Functions of the PDL

  1. Supportive: Transmits occlusal forces from teeth to the bone, allowing slight movement and shock absorption.
  2. Formative: Throughout life, responds to tension, facilitating cementum and bone formation by fibroblasts, cementoblasts, and osteoblasts.
  3. Resorptive: Pressure causes PDL to narrow; severe pressure causes bone and cementum resorption, damaging PDL integrity.
  4. Sensory: Detects pressure and touch, with pain signals originating from the tooth pulp.
  5. Nutritive: Contains blood vessels supplying essential nutrients to the area.

Clinical Importance of PDL Fibers (8.5)

  • Main purpose: Anchor teeth in the socket, maintain gingival tissue positioning, transmit occlusal forces (acting as shock absorbers), and facilitate development/resorption of hard tissues.
  • Periodontal Disease: Can lead to the destruction of PDL fibers, resulting in tooth mobility.
  • Occlusal Trauma: Widening of the PDL space in response to occlusal trauma can lead to eventual mobility of the tooth.
  • Orthodontics: Controlled stress on PDL resembles occlusal trauma effects, manipulating spacing in the dental arch.

Alveolar Bone

Histological Structure of Alveolar Bone (8.6)

  • Definition: The alveolar process refers to the part of the maxilla or mandible that supports and protects teeth, forming part of the periodontium connecting cementum through PDL.
  • Composition: Hard mineralized tissue alongside the components found in other bone types:
    • Osteocytes (bone cells)
    • Organic matrix that mineralizes
Composition Breakdown:
  • Cementum: 65% inorganic, 23% organic, 12% water
  • Dentin: 70% inorganic, 20% organic, 10% water
  • Enamel: 96% inorganic, 1% organic, 3% water
  • Alveolar Bone: 60% inorganic, 25% organic, 15% water
  • Comparison: Elasticity and density differ, with alveolar bone being less hard than dentin or enamel but essential for tooth support.
  • Lamina Dura: Radiopaque part of alveolar bone lining the tooth socket.
  • Crest: Alveolar crest is the most superior part of the bone.

Supporting Structures of Alveolar Bone

  1. Cortical Bone: Compact bone forming the outer plates of the alveolar process.
  2. Trabecular Bone: Cancellous bone located between alveolar bone plates.

Bone Properties and Functions

  • Osteocytes: Bone cells residing in lacunae, interconnected by canaliculi (small channels for nutrient transport).
  • Periosteum: Tough connective tissue covering outer bone surface.
  • Endosteum: Delicate connective tissue on inner surfaces, including trabecular surfaces and Haversian canals.
  • Bone Growth: Continuous formation and resorption throughout life; new bone forms from periosteum or endosteum via osteoblasts, while osteoclasts facilitate resorption.

Clinical Significance of Alveolar Process (8.7)

  • Orthodontics Impact: Mechanical forces create tension and compression zones in PDL, leading to localized bone resorption and deposition as teeth move.
  • Mesial Drift: A natural slight movement of all teeth towards the midline which can lead to crowding over time.
  • Periodontal Disease: Chronic conditions can cause progressive bone loss, starting at the alveolar crest and moving apically.
  • Bone Density: Affects infection routes and anesthesia efficacy.
  • Post-Extraction: After tooth removal, imature bone initially fills the socket, remodeled into mature bone, with loss of vertical height due to resorption if tooth loss occurs.
  • Implants: Placing implants can preserve bone integrity in an edentulous area, preventing further resorption.

Review Questions

  1. Which of the following best describes the periodontal ligament (PDL)?
    • B: A soft tissue structure that attaches the tooth to the alveolar bone.
  2. What type of tissue primarily makes up the periodontal ligament?
    • C: Connective tissue.
  3. Which of the following cells is most directly involved in forming the cementum?
    • C: Cementoblasts.
  4. The primary function of cementum is to:
    • B: Protect the root dentin and anchor the periodontal ligament.
  5. Which of the following is NOT a function of the periodontal ligament?
    • D: Protection against enamel caries.
  6. What is the primary mineral component of alveolar bone?
    • B: Hydroxyapatite.
  7. Cementum is most similar in composition to which other dental tissue?
    • B: Dentin.
  8. The alveolar bone is:
    • B: The portion of the jawbone that supports and anchors teeth.
  9. Which structure provides nutrients to the periodontal ligament?
    • D: Blood vessels in the alveolar bone.
  10. Which of the following is the main function of alveolar bone?
    • B: To support teeth and anchor them in the jaw.
  11. In response to forces like chewing, which structure helps maintain the stability and position of the tooth?
    • D: Periodontal ligament.
  12. Which term describes the type of bone remodeling in alveolar bone due to tooth movement?
    • D: Bone resorption and deposition.
  13. How does cementum differ from enamel?
    • A: Cementum is softer and less mineralized than enamel.
  14. The primary function of cementum is to anchor:
    • C: Periodontal ligament fibers to the tooth.
  15. Alveolar bone can be lost due to which condition?
    • C: Periodontitis.

References

  • Bird, D.L & Robinson, D.S. (2021). Modern dental assisting (13th ed.). Elsevier
  • Fehrenbach, M.J. & Popowics, T. (2020). Illustrated dental embryology, histology and anatomy (5th ed.). Elsevier