Classifications of Cementum

Key Concepts:

• Cementum: Mineralized connective tissue covering the tooth root.
• Cemento-enamel junction (CEJ): Where enamel and cementum meet.
• Periodontium: Tissues surrounding and supporting the teeth (including cementum, periodontal ligament, alveolar bone, and gingiva).
• Hertwig's Epithelial Root Sheath: Structure involved in root development.
• Cementoblasts: Cells that form cementum.
• Cementocytes: Cementoblasts trapped within the cementum matrix.
• Extrinsic Fibers: Collagen fibers from periodontal ligament fibroblasts, perpendicular to cementum surface.
• Intrinsic Fibers: Collagen fibers produced by cementoblasts, parallel to cementum surface.
• Cellular Cementum: Cementum containing cementocytes.
• Acellular Cementum: Cementum lacking cementocytes.
• Schroeder Classification: Classification of cementum based on the presence of cementocytes and the nature/origin of organic matter.
• Hyaline Layer of Hopewell-Smith: Narrow homogeneous structure at the dentin-cementum junction in the region of Acellular Extrinsic Fiber Cementum.
• Intermediate Cementum: Narrow homogeneous structure at the dentin-cementum junction in the region of Cellular Mixed Stratified Cementum.

1. Introduction to Cementum

• Cementum is a mineralized connective tissue covering the root of the tooth.
• It extends from the cemento-enamel junction (CEJ), where it's thinnest, to the apex, where it's thickest.
• It may sometimes extend onto the inner dentinal wall of the root canal or coronally onto the enamel surface.
• Functionally, cementum belongs to the periodontium due to its position between radicular dentin and the periodontal ligament.

2. Development of Cementum

• Cementum develops during the Advanced Bell stage of tooth development.
• Hertwig's Epithelial Root Sheath plays a crucial role.
• Cementoblasts are the cells responsible for depositing cementum.

3. Microscopic Structure of Cementum

• Cells:
  • Cementoblasts: Located on the cementum surface towards the periodontal ligament.
  • Cementocytes: Trapped cementoblasts within the cementum, with cytoplasmic extensions directed towards the periodontal ligament.
• Organic Matter:
  • Extrinsic Fibers: Perpendicular to the cementum surface, extensions of the principal fibers of the periodontal ligament, made by fibroblasts, 5-7 microns wide.
  • Intrinsic Fibers: Parallel to the cementum surface, formed by cementoblasts, 1-2 microns wide.

4. Classification of Cementum Based on Presence of Cementocytes

• Cellular Cementum:
  • Contains cementocytes.
  • Rapid deposition rate.
  • Presence of a cementoid layer.
  • Widely spaced incremental lines.
• Acellular Cementum:
  • Lacks cementocytes.
  • Slow deposition rate.
  • Absence of cementoid layer.
  • Closely spaced incremental lines.

5. Classification of Cementum Based on Nature and Origin of Organic Matter

• Extrinsic Fiber Cementum: Contains only extrinsic fibers (perpendicular).
• Intrinsic Fiber Cementum: Contains only intrinsic fibers (parallel).
• Mixed Fiber Cementum: Contains both intrinsic and extrinsic fibers.
• Afibrillar Cementum: Contains no collagen fibers.

6. Schroeder Classification of Cementum (Most Commonly Used)

• Based on the presence of cementocytes and the nature/origin of organic matter.
• Five types:
  • Acellular Extrinsic Fiber Cementum (AEFC):
    • Covers 60-90% of the root in single-rooted teeth, towards the cervical line.
    • Covers the cervical 2/3rd of the root in multi-rooted teeth.
    • Thickness: 50-200 microns.
    • Absent cementocytes.
    • Contains only extrinsic fibers.
    • Major role in tooth anchorage.
  • Cellular Intrinsic Fiber Cementum (CIFC):
    • May be absent or present in the apical portion of single-rooted teeth.
    • Present in the apical third and furcation area of multi-rooted teeth.
    • Contains cementocytes.
    • Contains only intrinsic fibers.
    • Major role in adaptation and repair, not anchorage.
  • Cellular Mixed Stratified Cementum (CMSC):
    • Rare in single-rooted teeth (near apex if present).
    • Seen in the apical third and furcation area of multi-rooted teeth.
    • Thickness: 100-1000 microns.
    • Alternating layers of CIFC (intrinsic fibers, cementocytes) and Cellular Extrinsic Fiber Cementum (extrinsic fibers, no cementocytes).
    • Extrinsic fibers within layers lose contact with periodontal ligament, so anchorage is provided by the peripheral-most layer.
  • Acellular Afibrillar Cementum:
    • Isolated patches on enamel coronal to the CEJ.
    • Mineralized matrix with no cementocytes or collagen fibers.
    • No role in tooth anchorage or repair.
  • Intermediate Cementum:
    • Narrow homogeneous structure (5-8 microns thick) at the dentin-cementum junction.
    • Present on the entire root.
    • Also known as Hyaline Layer of Hopewell-Smith in the region of Acellular Extrinsic Fiber Cementum.

7. Functional Stimuli and Cementum Formation

• The transition from acellular to cellular cementum is not fully understood.
• Some researchers suggested that functional stimuli (force from tooth contact and mastication) might trigger cellular cementum formation in the apical third.
• However, this theory is debated, as cellular cementum is also found in teeth without antagonists or in impacted teeth.

8. Cellular Mixed Fiber Cementum

• Not classified by Schroeder.
• Shows cementocytes, intrinsic fibers, and extrinsic fibers in a non-stratified, mixed manner.

9. Intermediate Cementum Controversy

• Arthur Hopewell-Smith considered this structure as part of dentin.
• Bencze considered it as a part of cementum.

10. Synthesis/Conclusion

Cementum is a complex tissue with varied classifications and functions. The Schroeder classification, based on cellularity and fiber origin, is the most widely used. Acellular Extrinsic Fiber Cementum is crucial for tooth anchorage, while Cellular Intrinsic Fiber Cementum plays a role in repair. The formation and transition between different cementum types, particularly the role of functional stimuli, remain areas of ongoing research. The nature and origin of Intermediate Cementum is still controversial.