Cementum: Introduction to Periodontium

Cementum

• Part of the periodontium, which is the investing and supporting attachment system of the teeth
• Consists of two soft tissues (gingiva and periodontal ligament) and two hard tissues (cementum and alveolar bone)
• A calcified, hard connective tissue derived from ectomesenchyme of the dental follicle, forming the outer covering of the anatomic root
• Begins at the cervical portion of the tooth at the cemento-enamel junction and continues to the apex
• Avascular and non-innervated

Physical Characteristics

• Color: Yellow, lighter than dentin but darker than enamel
• Hardness: Lesser than dentin
• Permeability: Permeable to inorganic ions and bacteria, which diminishes with age; cellular cementum is more permeable than acellular cementum
• Thickness: Thinnest at the cemento-enamel junction (20-50μm) and gradually thicker apically, ranging from 150-200μm or more

Chemical Composition

• 45-50% inorganic substances (calcium phosphate in the form of hydroxyapatite crystals)
• 50-55% organic substances (collagen fibers embedded in a ground substance, polysaccharides, and proteins)
• Contains the greatest amount of fluoride in mineralized tissues

Histological Structure of Cementum

• Two forms of collagen: collagen fibrils and collagen bundles
• Collagen fibrils constitute the main bulk of the organic material of cementum
• Collagen bundles represent the embedded portion of the principal fibers of the periodontal ligament, known as Sharpey's fibers
• Two types of cells functionally concerned with cementum: cementoblasts and cementocytes
• Cementoblasts line the root surface and contain numerous mitochondria and a well-developed Golgi complex; they become entrapped in the cementum matrix and form cementocytes
• Cementocytes are seen in lacunae in the cementum matrix and have numerous processes that may branch and anastomose with those of adjacent cementocytes

Types of Cementum

• Based on cellular content:
  • Cellular cementum: contains cells called cementocytes in lacunae, more frequently found on the apical half of the root
  • Intermediate cementum: a thin, amorphous layer of hard tissue approximately 10 microns thick, intermediate between cementum and dentin
  • Acellular cementum: doesn't contain cells, present on the cervical third or half of the root
• Based on fibrillar content:
  • Intrinsic fibers cementum: fibers derived from the synthetic and secretory activity of cementoblasts
  • Extrinsic fibers cementum: fibers originate from the principal fibers of the periodontal ligament (Sharpey's fibers)
  • Mixed fibers cementum: contains both intrinsic and extrinsic fibers
  • Afibrillar cementum: lacks fibers, formed when the reduced enamel epithelium retracts from the cervical region of enamel, allowing mesenchymal cells to invade and differentiate into cementoblasts

Cementoenamel Junction (CEJ)

• In approximately 60% of teeth, cementum overlaps the cervical end of enamel for a very narrow area at the CEJ
• In 30% of teeth, cementum meets the cervical end of enamel in a knife-edge-to-edge pattern
• In approximately 10% of teeth, cementum does not meet enamel, resulting in a zone of root dentin appearing devoid of cementum

Cementodentinal Junction

• The dentin surface upon which cementum is deposited is relatively smooth in permanent teeth
• The cementodentinal junction in deciduous teeth is sometimes scalloped

Cementogenesis

• Takes place in two phases: matrix formation and mineralization
• Three cell types responsible for cementogenesis: cementoblasts, cementocytes, and fibroblasts, all derived from ectomesenchymal cells
• Growth of cementum is a rhythmic process, with new cementoid formed and old one calcified
• Mineralization begins after forming the first layer of matrix, with mineral crystals deposited within and between collagen fibers

Clinical Considerations

• Hypercementosis: abnormal thickening of cementum, may occur in one tooth or all teeth, in local areas of one root or generalized to the whole root
• Cementum is more resistant to resorption from bone, making orthodontic therapy possible
• Fusion of cementum to alveolar bone with obliteration of the periodontal ligament is termed ankylosis
• Decrease in permeability of cementum with age
• Cementicles: calcified oval or round nodules found in the periodontal ligament, single or multiple, near the cemental surface

Functions of Cementum

• Furnishes a medium for the incorporation of the principal periodontal fibers, securing the binding of the tooth root to the alveolar bone
• Serves as a reparative tissue in case of root fracture or resorption
• Allows for functional adaptation of the tooth by cementum deposition at the apices, compensating for tooth loss occlusally by attrition
• Protects dentin by blocking the dentinal tubules