Cementum - Classification

III. Cementum

B. CLASSIFICATION


  1. Classification

Fig. 81: Distribution of cementum on the tooth surface (see classification below).

Cementum may be found both on the root as well as the crowns of teeth. It may also vary in its structure. Some forms of cementum may be cellular, while others are not. Some have a fibrillar collagenous matrix, while others do not.
 
Cementum may be classified in the following ways:
 
By location:
 
  • Radicular cementum: The cementum that is found on the root surface.
  • Coronal cementum: The cementum that forms on the enamel covering the crown.
 
By cellularity:
 
  • Cellular cementum: Cementum containing cementocytes in lacunae within the cementum matrix.
  • Acellular cementum: Cementum without any cells in its matrix.
 
By the presence of collagen fibrils in the matrix:
 
  • Fibrillar cementum: Cementum with a matrix that contains well-defined fibrils of type I collagen.
  • Afibrillar cementum: Cementum that has a matrix devoid of detectable type I collagen fibrils. Instead, the matrix tends to have a fine, granular consistency.
 
By the origin of the matrix fibers (applies only to fibrillar forms of collagen):
 
  • Extrinsic fiber cementum: Cementum that contains primarily extrinsic fibers, i.e. Sharpey's fibers that are continuous with the principal fibers of the periodontal ligament. Since the fibers were originally produced by periodontal ligament fibroblasts, they are considered "extrinsic" to the cementum. These fibers are orientated more or less perpendicularly to the cementum surface and play a major role in tooth anchorage.
  • Intrinsic fiber cementum: Cementum that contains primarily intrinsic fibers, i.e. fibers produced by cementoblasts and that are orientated more or less parallel to the cementum surface. This form of cementum is located predominantly at sites undergoing repair, following surface resorption. It plays no role in tooth anchorage.
  • Mixed fiber cementum: Cementum that contains a mixture of extrinsic and intrinsic fiber cementum.
 
The descriptors for the classes of cementum described above can be used in various combinations to more precisely describe a specific type of cementum. Examples follow:
 
  1. Acellular, afibrillar cementum

    This cementum is mostly composed of mineralized matrix, without detectable collagen fibrils or cementocytes. It is produced exclusively by cementoblasts. It is typically found as coronal cementum on human teeth.

 
  1. Acellular, extrinsic fiber cementum

    This type of cementum has a matrix of well-defined, type I collagen fibrils. The fibrils are part of the, densely packed Sharpey's fibers, that are continuous with the principal fibers of the periodontal ligament. Because of their dense packing, the individual Sharpey's fibers that form the bulk of the matrix may no longer be identifiable as individual fibers within the cementum layer. This cementum, which is acellular, is located in the cervical two-thirds of the root of human teeth. It plays a major role in tooth anchorage.

 
  1. Cellular, intrinsic fiber cementum

    This cementum contains cementocytes in a matrix composed almost exclusively of intrinsic fiber cementum. It is located almost exclusively at sites of cementum repair. It plays no part in tooth anchorage. However, it may be covered over by extrinsic or mixed fiber cementum, both of which are able to provide new anchorage.

 
  1. Cellular, mixed fiber cementum

    It is found on the apical third of the root and in furcations (i.e. between roots). In these locations, the rate of cementum formation is usually more rapid than in the cervical region. The mineralized, extrinsic collagen fibers (Sharpey's fibers) run a more irregular course than in acellular, extrinsic fiber cementum. Intrinsic fibers are found interspersed among the extrinsic fibers of the cementum matrix, so that individual Sharpey’s fibers are more readily identifiable than in extrinsic fiber cementum. Cementoblasts are trapped in hollow chambers (or lacunae) where they become cementocytes. (For additional details see Schroeder, 1986, 1991).

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