Cementum

Definition

Cementum is a calcified avascular mesenchymal tissue that forms the outer covering of the anatomic root. It provides anchorage mainly to the principal fibers of periodontal ligament.

The cementum is the surface layer of the tooth root, covering the dentine (which is labeled B). Rather than being a passive entity like paint on a wall, cementum is a dynamic entity within the periodontium. It is attached to the alveolar bone (C) by the fibers of the periodontal ligament and to the soft tissue of the gingiva by the gingival fibers (H).

Fibers in Cementum

Two sources of collagen fibers can be found in the cementum:

1. Sharpey’s (extrinsic) fibers which are formed by the fibroblasts.
2. Fibers belonging to the cementum matrix per se (intrinsic) produced by cementoblasts.

Types of Cementum

Acellular cementum

• Forms during root formation.
• Does not contain any cells.
• Seen at the coronal portion of root.
• Formation is slow.
• Arrangement of collagen fibers are more organized.

Cellular cementum

• Forms after the eruption of the tooth and in response to functional demands.
• Contains cementocytes.
• Seen at a more apical portion of root.
• Deposition is more rapid.
• Collagen fibers are irregularly arranged.

Classification

Depending on location, morphology and histological appearance, Shroeder and Page have classified cementum as:

Acellular afibrillar cementum (AAC)

It contains only the mineralized ground substance. It does not contain collagen fibers nor does it exhibit entrapped Cemento-cytes. It is a product of cementoblasts and is found almost exclusively on the enamel near the cementoenamel junction with a thickness of 1 to 15 μm.

Acellular extrinsic fiber cementum (AEFC)

By definition it is composed primarily of Sharpey’s fibers of periodontal ligament but does not contain cementocytes. Developmentally they come to occupy the coronal one half of the root surface. Its thickness is between 30 and 230 μm.

Cellular mixed stratified cementum (CMSC)

It harbors both intrinsic (cementoblasts derived) and extrinsic (fibroblast derived) fibers and may contain cells. In humans it is seen in the apical third of the roots, apices and furcation areas. Its thickness varies from 100 to 1000 μm.

Cellular intrinsic fiber cementum (CIFC)

It contains only intrinsic fibers secreted by cementoblasts and not by the periodontal ligament fibroblasts. In humans it fills the resorption lacunae.

Intermediate cementum (or) the hyaline layer of Hope Well Smith

It is an ill-defined zone extending from precementoenamel junction to the apical 1/3rd of the root. It appears to contain cellular remnants of Hertwig’s Sheath embedded in calcified ground substance. The significance of this layer is that, it contains enamel like proteins, which helps in attachment of cementum to dentin. It has been observed by many that, when this layer is removed during root planing procedure, the resultant reparative cementum that is formed will not be attached firmly on the dentin.

Functions

• Primary function of cementum is to provide anchorage to the tooth in its alveolus. This is achieved through the collagen fiber bundles of the periodontal ligament, whose ends are embedded in cementum.
• Cementum also plays an important role in maintaining occlusal relationships, whenever the incisal and occlusal surfaces are abraded due to attrition, the tooth supra erupts in order to compensate for the loss and deposition of new cementum occurs at the apical root area.

Composition

The cementum is composed of both inorganic (46%) and organic matter. The organic matrix is chiefly composed of 90% Type I collagen, 5% Type III collagen and non-collagenous proteins like enamel proteins, adhesion molecules like tenascin and fibronectin, glycosaminoglycans like chondroitin sulfate, dermatan sulfate and heparan sulfate which constitute the remaining organic matrix.

Thickness of Cementum

Formation of cementum is a continuous process, the formative rate of which varies throughout life. It is most rapid at the apical regions. At the coronal half the thickness varies from 16 to 60 μm (almost the thickness of hair) and at the apical third it varies from 150 to 200 μm. It is thicker in the distal surfaces as compared to the mesial surfaces and this can be explained by functional stimulation following mesial migration.

Hypercementosis or cemental hyperplasia is a prominent thickening of the cementum. It can be localized or generalized. It may appear as a generalized thickening of the cementum, with nodular enlargement at the apex or as spike like projections (cemental spikes). The etiology of hypercementosis is not very well understood. The spike like projections could be as a result of excessive tension from orthodontic appliance or occlusal forces. The generalized type may be associated with a variety of situations, like teeth without antagonists, in teeth with chronic pulpal and periapical infections. Hypercementosis of the entire dentition may be seen in patients with Paget’s disease.

Cementoenamel Junction

At the cementoenamel junction three types of relationships may exist. In about 60 to 65% of cases, the cementum overlaps the enamel, in about 30% of cases, end-to end relationship of enamel and cementum is seen and in 5 to 10%, the cementum and enamel fail to meet.

Cemental Resorption and Repair

Cemental resorption may be caused by local, systemic or idiopathic factors. Local conditions that contribute to cemental resorption are trauma from occlusion, orthodontic tooth movement, pressure from erupting teeth, cysts and tumors, teeth without functional antagonist, periapical disease and periodontal disease. Systemic conditions that may predispose to cemental resorption are calcium deficiency, hypothyroidism and Paget’s disease.

The resorptive process may not necessarily be a continuous process; it may alternate with periods of repair and deposition, which can be demarcated by formation of reversal line. Remodelling of cementum requires the presence of viable connective tissue and can occur even in non-vital teeth.

Cementum is not exposed to the oral environment because it is covered by alveolar bone and gingiva. In cases of gingival recession and as a consequence of loss of attachment in pocket formation, cementum can become exposed to the oral environment. Once exposed, organic substances, inorganic ions and bacteria penetrate the sufficiently permeable cementum. Caries of the cementum may also develop.

Cementum Resources
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