Dentistry: Cementum Characteristics

Cementum

• Definition: Specialized calcified mesenchymal tissue that covers the root and is part of the hard tissue component of the periodontium.
• Characteristics:
  • Hardness is less than that of dentin.
  • Light yellow in color.
  • Semi-permeable to a variety of materials.
  • Contains inorganic substances, organic material, and water.
  • Highest fluoride content of all mineralized tissues.
• Composition:
  • Organic: 30% (collagen: Type I 90%, Type III 5%)
  • Inorganic: 70% (hydroxyapatite: Ca10(PO4)6(OH)2)
• Structure:
  • Cellular: cementocytes (form cementum during tooth formation) and cementoblasts (found in lacuni, form cementum in resorption foci)
  • Fibrillar: extrinsic and intrinsic fibers
  • Ground substance
• Thickness:
  • Varies: 16-60um (coronal), 120-200um (apical)
  • Thicker in distal surfaces than in mesial surfaces
• Types:
  • Acellular: Sharpey's fibers make up most of its structure, thickness 30-230 μm
  • Cellular: less calcified, formed after tooth reaches occlusal plane, irregular and contains cells (cementocytes)

Cementum Classification

• Acellular Afibrillar: product of cementoblasts, most coronal, thickness 1-15 μm
• Acellular Extrinsic Fibrillar: product of fibroblasts and cementoblasts, composed of densely packed bundles of Sharpey's fibers, cervical third of roots, thickness 30-230 μm
• Cellular Mixed: composed of extrinsic and intrinsic fibers, may contain cells, co-product of cementoblasts and fibroblasts, apical third of roots, thickness 100-1000 μm
• Cellular Intrinsic: contains cells but no extrinsic collagen fibers, formed by cementoblasts, fills resorption lacunae
• Intermediate: poorly defined zone near the cementodentinal junction, contains cellular remnants of Hertwig's sheath embedded in calcified ground substance

Cementoenamel Junction (CEJ)

• Borderline between cementum and enamel
• Three types of relationships:
  • Intact: 30%, edge-to-edge butt joint exists
  • Deficient: 5-10%, enamel and cementum fail to contact
  • Overlapping: 60-65%, cementum overlaps the enamel

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
• Functional adaptation: restores the integrity of the attachment apparatus, excessive formation of cementum on the apical root surface to compensate for occlusal and incisal wear

Cementum Resorption and Repair

• Causes: local or systemic, idiopathic, trauma from occlusion, orthodontic movement, pressure from malaligned erupting teeth, cysts, tumors, etc.
• Abnormalities:
  • Hypercementosis: abnormal thickening of cementum, diffuse or circumscribed
  • Ankylosis: union of tooth and bone with no intervening connective tissue, develops after chronic periapical inflammation, tooth replantation, and occlusal trauma
  • Excementosis: embedded calcified round bodies in localized areas of hyperplastic cementum

Alveolar Bone

• Definition: part of the jaw that forms and supports the teeth socket
• Formation: forms when the teeth erupt to provide the osseous attachment to the forming PDL
• Disappearance: disappears gradually after the tooth is lost
• Components:
  • External plate of cortical bone: formed by Haversian bone and compact bone lamellae
  • Alveolar bone proper: inner socket wall formed of thin, compact bone and bundle bone
  • Cancellous trabeculae: lie between the two compact bone, acts as support for the alveolar bone

Socket Wall

• Consists of dense, lamellated bone, some of which is arranged in haversian systems, and bundle bone
• Bundle bone: bone adjacent to the periodontal ligament that contains a great number of Sharpey's fibers

Functions of Alveolar Bone

• Protection: forms and protects the sockets for the teeth
• Attachment: gives the attachment to the periodontal ligament fibers
• Support: supports the tooth roots on the facial and on the palatal/lingual sides
• Shock-absorber: helps absorb the forces placed upon the tooth by disseminating the force to underlying tissues