Periodontium:

Anatomy and Histology Review

CEMENTUM

Mineralized connective tissue resembling bone which covers the roots of teeth and serves to anchor gingival and periodontal fibers. Some forms of cementum nay also cover varying areas of the crown. In contrast to bone it is not vascular and exhibits little turnover. Grows by surface apposition.

• Classification
• Cells
• Development
• Development Anomalies
• Cemento-Enamel Junction Realtionship
• Clinical Considerations

CLASSIFICATION

1. By location

1. Radicular cementum
2. Coronal cementum

2. By cellularity

1. Cellular cementum
2. Acellular cementum

3. By the presence of collagen fibrils in the matrix

1. Fibrillar cementum
2. Afibrillar cementum

4. By the origin of the matrix fibers

1. Extrinsic fiber (from PDL fibroblasts)
2. Intrinsic fiber (from cementoblasts)
3. Mixed fiber (contains both of the above)  

• Examples:

  • Acellular, afibrillar cementum (coronal cementum of human teeth)
  • Mostly composed of mineralized ground substance.
  • Free of cells or fibers.
  • Exclusively produced by cementoblasts .
  • Acellular, extrinsic fiber cementum (cervical portion of radicular cementum on human teeth)
    • Has a well defined matrix of densely packed fibers (Sharpey’s fibers) that originated as principal fibers of the PDL.
    • Contains no cells .
  • Cellular, mixed fiber cementum.
    • Contains cementocytes in a matrix of extrinsic and extrinsic fibers.
    • Apical portion of radicular cementum of human teeth.
  • Cellular, intrinsic fiber cementum.
    • Contains cementocytes in a matrix composed almost exclusively of instrinsic fiber cementum, without inserting fibers.
    • Only found at sites of cementum repair

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CELLS

1. Cementoblasts

Initially, originate from mesenchyme of follicular connective tissue. Later originate from undifferentiated mesenchymal cells of periodontal ligament (PDL). Generally line cemental surface.

2. Cementocytes

Produced by cementoblasts becoming trapped in cementocytic lacunae. Cytoplasmic organelles become reduced in number. Cytoplasmic processes extend toward cementum surface through canaliculi in the cementum. Metabolically, these cells are relatively inactive.

3. Fibroblasts

Although technically part of the PDL, they produce collagen fibers which become mineralized as they become incorporated into cementum.

4. Odontoclasts (Cementoclasts)

Multinucleated giant cells active in cemental resorption. Indistinguishable from osteoclasts.

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  DEVELOPMENT

1. Coronal cementum

In human teeth it is formed on the cervical portion of the crown, in localized areas of REE degeneration, by cementoblasts. It serves no anchoring function. In histological sections it may appear as an "island" of cementum on the cervical enamel surface or as a "spur" of cementum continuous with radicular cementum and overlapping the cervical enamel. In other mammalian species (e.g. horses, cows, sheep, rabbits) coronal cementum may be fibrillar and/or cellular, and also serve an anchoring function. In these species, most of the enamel of the crown is covered by a well developed layer of coronal cementum.

2. Radicular cementum

Root formation is dependent on the orderly growth of Hertwig’s epithelial rooth sheath (HERS) which ends as the epithelial diaphragm. The latter also governs the formation of multiple roots. As root formation proceeds, HERS becomes perforated by mesenchymal cells of the dental follicle  which traverse the sheath to reach the dentin surface. These cells lay down a collagenous matrix which enlarges the perforations of the sheath and gradually displaces HERS from the dentin surface. HERS gradually breaks up into a network of more or less interconnected epithelial strands, located within the future PDL, that are known as the epithelial cell rests of Malassez.

Just prior to tooth eruption the collagen at the dentin surface becomes remodeled into thin fibers, perpendicular to the dentin surface. These fibers slowly mineralize from the dentin surface toward the developing PDL. The non-mineralized end of the fibers extending into the future PDL space contribute to the formation of the principal fibers of the PDL.

• Thickness of radicular cementum
• It increases with age.
• It is thicker apically than cervically.
• Thickness may range from 0.05 to 0.6mm.

3. Acellular extrinsic fiber cementum

The collagen fibers of the cementum matrix originate primarily as principal fibers of the PDL produced by fibroblasts (extrinsic fibers). These fibers are oriented more or less perpendicularly to the cementum surface. The ground substance embedding these fibers is laid down by cementoblasts. As new cementum is deposited on the surface, the cementoblasts are displaced toward the PDL so as to avoid entrapment. This gives rise to the acellular, fibrillar cementum, most commonly found in the cervical half of the root.

This cementum is laid down comparatively slowly. The collagen fibrils are densely and regularly packed, frequently with adjacent periodic striations in register. With the light microscope appositional lines are readily distinguishable.

4. Cellular mixed fiber cementum

Found on the apical half of te root and in furcations (i.e. interradicularly). Formation usually occurs more rapidly than in the cervical region. The mineralized collagen fibers (Sharpey’s fibers) run a more irregular course and, near the PDL often contain a non=mineralized core continuous with the principal fiber in the PDL. Cementoblasts are trapped in lacunae where they become cementocytes. Unlike around the canaliculi of bone, which radiate evenly around the osseous lacunae, those of cementum tend to extend primarily toward the PDL.

Enamel projections : If amelogenesis is not turned off, thereby allowing the enamel organ to give rise to HERS, continued amelogenesis may produce enamel  projections on the root surface, most commonly extending into molar bifurcations.

 Hypercementosis : Abnormally thick cemental layer. May be localized (affecting one tooth) or generalized (affecting multiple teeth).

Cementicles : Free or sessile. May be composed of intrinsic fibrillar or afibrillar cementum, or a mixture of both.

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CEMENTO-ENAMEL JUNCTION REALTIONSHIP

Cemento-enamel junction relationship : IT varies between tooth types and loations along the cervix. In most cases, the cementum overlaps the cervical enamel. Somewhat less frequently, the cementum and enamel form a butt joint. Occasionally, exposed dentin appears between the cervical enamel and the radicular cementum.  

Cementum is the least mineralized of the 3 calcified dental tissues:

CLINICAL CONSIDERATIONS

Anchoring function (extrinsic fiber or mixed fiber cementum only).

Compensates for wear by apical deposition of cementum

Protects the root surface from resorption during tooth movement

Reparative function. Following initial resorption, intrinsic fiber cementum followed by extrinsic fibercementum deposition mediates a new connective tissue attachement.

If PDL is severely injured, cementum and adjacent bone may become ankylosed, a situation which may interfere with normal eruption and normal function.

Enamel pearls may become exposed and act like calculus to favor plaque retention and promote periodontal disease.

Enamel projections may favor the development of periodontal disease in affected bifurcations.

Hypercementosis may complicate the extraction of affected teeth.

When a gap exists between enamel and cementum, cervical hypersensitivity and cervical caries are more likely to develop.

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