Cementum

( vinculum is the calcified avascular mesenchymal tissue that lorms the outer covering of tin- anatomic root. I he two main types ol cementum are accllular (primary) and cellular (secondary) cementum.r Both consist of a calcified interftbrillar matrix and collagen fibrils.

I he two sources of collagen fibers in cementum are Sharpey's (extrinsic) fibers, which are the embedded portion ol the principal fibers of the periodontal ligament '' and are formed by the fibroblasts, and fibers that belong to the cementum matrix per se (intrinsic) and are produced by the cementoblasts.10' ( ernentoblasts also form the noncollagenous components of the interftbrillar ground substance, such as proteoglycans, glycoproteins, and pliosphoproteins.

\cellular cementum is the first to be formal and covers approximately the cervical third or halt of the root; it does not contain cells (Fig. 2-12). fills cementum is U»lined before the tooth reaches the occlusal plane, and its thickness ranges from \Q to 230 fxm.l<Ml Sharpey's fibers comprise most of the structure ol acellular cementum, which has a principal role in supporting the tooth. Most fibers are inserted at approximately right angles into the root surface and penetrate deep into the cementum, but others enter from several different directions. Their si/e, number, and distribution increase with function.' Sharpey's fibers are completely calcified, with the mineral crystals oriented parallel to the fibrils as in dentin and bone, except in a 10- to SO-jim-wide zone near the cementodentinal junction, where they are only partially calcified. I he peripheral portions of Sharpey's libers in actively mineralizing cementum tend to be more call ified than the interior regions, according to evidence obtained by scanning electron microscopy."1 Acellular cementum also contains intrinsic collagen fibrils that are calcified and irregularly arranged or parallel to the surface."*0

Cellular cementum, formed after the lootli reaches the occlusal plane, is more irregular and contains cells (ce-mentocyles) in individual spaces (lacunae) that communicate with each other through a system of anastomosing canaliculi (Fig. 2-1 C ellular cementum is less calcified than the acellular type.™ Sharpey's fibers oc-cup\ a smaller portion of c cllular cementum and are separated l>\ other fibers that are arranged either parallel to the root surface or «it random. Sharpey's fibers may be completely or partially calcified or have a central, tincal-c ified c ore surrounded by a calcified border.'''

Uoth acellular cementum and cellular cementum are arranged in lamellae separated by incremental lines parallel to the long axis of the root (see Figs. 2-12 and 2-1 *). Ihese lines represent rest periods in cementum formation and are more mineralized than the adjacent cementum.'" In addition, loss of the cervical part of the reduced enamel epithelium at the time of tooth eruption may place portions of mature enamel in contact with the connective tissue, which then will deposit over it an acellular alibrillar type of cementum.n,>

Based on these findings. Schrocdcr"M> has classified cementum as follows:

Acellular afibrillat cementum (AA( ) contains neither cells nor extrinsic or intrinsic collagen libers, apart from a mineralized ground substance. It is a product of cementoblasts and is found as coronal cementum in humans, with a thickness of I to 15 /.on. Aeellultir extrinsic fiber cementum (AEfC) is composed almost entirely of densely packed bundles of Sharpey's fibers and lacks cells. It is a product of fibroblasts and cementoblasts and is found in the cervical third of roots in humans but may extend further a pica I ly. Its thickness is between 30 and 230 fxm. ( ellttlar mixed stratified cementum (( MSi') is composed of extrinsic (Sharpey's) and intrinsic libers and may contain cells. It is a co-product ot fibroblasts and cementoblasts. and in humans it appears primarily in the apical third of the roots and apices and in furcation areas. Its thickness ranges from 100 to 1000 ¿em. ('cllular intrinsic fiber cementum (< //( ) contains cells but no extrinsic collagen fibers. It is formed by cementoblasts. and in humans it fills resorption lacunae.

Intermediate cementum is an ill-defined /one near the cementodentinal junction of certain teeth that appears to contain cellular remnants of llertwig's sheath embedded in calcified ground substance.{{"s

I lie inorganic content of cementum (hydroxvapatite; ('.aJPojUOIII.) is 45% to 50%, which is less than that of bone (65%), enamel (97%), or dentin (70%)., ? Opinions differ about whether the microhardness increases '' or decreases with age.1'' and no relationship has been established between aging and the mineral content of cementum.

Permeability of Cementum

In very young animals, cellular and acellular cementum are very permeable and permit the diffusion of elves from the pulp and external root surface. In cellular cementum, the canaliculi in some areas are contiguous with the dentinal tubuli. Ihe permeability (»1 cementum diminishes with age.1

Cementoenamel Junction

I he cementum at and immediateh subjacent to the cementoenamel junction is ot particular clinical importance in root scaling procedures. Three types of relationships involving the cementum may exist .it the cementoenamel junction.*" In about 60% to 65% of cases, cementum overlaps the enamel (lig. 2-14); in about 30% .in edge-to-edge butt joint exists; and in 5% to 10% the cementum and enamel tail to meet. In the last instance, gingival recession may result in accentuated sensitivity because the dentin is exposed.

Thickness of Cementum

Cementum deposition is a continuous process that proceeds at varying rates throughout life. Omentum

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Responses

  • isabella
    Why cementum is permeable?
    3 years ago
  • JONATHAN WHEAT
    Are incremental lines found in acellular extrinsic cementum?
    3 years ago

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