What is tooth cement, its thickness and function

Being a layer in the tooth wall, medium in location, cement by its properties is also a substance, average between enamel and dentin for a variety of values: density, hardness, heat and electrical conductivity.

The earlier notion that cement not under the enamel in the coronal part of the tooth, now it is refuted by studies using the light-optical method: its layer is not is a substrate for enamel, while in 60-70% the coronal cement layer serves as the basis for this most durable in the body substances.

In the area not protected by enamel (necks and roots of teeth), the cementitious layer is the most superficial, protecting the softer dentin.

In terms of physicochemical properties, the cement layer is a tissue similar to bone, but calcified, more mineralized than she, devoid of blood vessels and less active in terms of rearrangements structures.

The thickness of the cementum of the tooth is minimal under the enamel (a thin layer-gap), but in the zone of the apex of the dental roots it reaches from 100 to 1500 microns (on molars). For comparison: in the area of ​​the necks, this is just a layer with a thickness of 20 to 50 microns.

The deposition of the cement composition on the surfaces of the dental roots continues throughout a person's life, due to which at the age of 60-70 years the layer in this area is three times thicker than in 20.

The structure and types of dental cement

Depending on the structure (structure), there are 2 types of dental cement layer:

• primary;
• secondary.

Features of the structure of primary cement

The primary, in contrast to the secondary, is also called acellular, because it is formed by calcified a matrix of tightly fitting collagen fibers and a basic substance, but cells in its composition does not have. Its surface is covered with a layer of pre-cement (cementoid) 0.25-5 microns thick from collagen fibers without calcifying impregnation.

The primary cement layer is characterized by striation and stratification. The first (radial direction) is due to the interweaving of the periodontal ligament fibrils into the layer of fibrils, the second (concentric, forming a close growth lines located to each other, similar to tree rings on a wood cut) - episodes of activity of the deposition of the main substances.

Cement is called primary because of the primacy of its formation, it serves to cover the surfaces of dental necks and roots with a layer of 30 to 230 microns (with a minimum in the zone of the enamel-cement border and a maximum at tops).

Features of the structure of secondary cement

As is clear from the designation, the secondary (cellular) cement layer, in addition to the matrix (a framework of calcified collagen fibers) and the main substance, also contains two types of cells.

Collagen fibers can be both internal and external. The first - with an orientation parallel to the surfaces of the dental roots, the second (Sharpey structures of the periodontal ligament) - are oriented radially.

Bodies of cells of the first type (cementocytes), separated from their enclosing cavities - lacunae with calcified walls pericementocytic space, have a flattened body with a large nucleus inside and many densely branched processes long up to 15 microns. The processes fill the system of tubules connecting the lacunae and penetrating the entire thickness of the layer.

In addition to the tight junction, the neighboring cells are interconnected by nexuses - slit-like contacts between the processes of adjacent cementocytes. Nutrition of cells is also carried out at the expense of processes, oriented mostly in the direction of the periodontal ligament.

Cells of the second type (cementoblasts) have two options for further evolution. Either (when they form a primary cement substance), they, by producing intercellular substance, are pushed outward, or (in the process formation of secondary cement) are embedded in the layer, so that, after undergoing a decrease in volume and transformation of the internal structure of the cell, become cementocytes.

Cementoblasts are located on the periphery of the periodontal ligament, covering dental roots.

The purpose of the secondary cement layer is to protect the apical (apical) third of the roots and the zone of their bifurcation (branching) in the variant of multirooted teeth. It can be applied both on top of the primary cement, and (in its absence) in direct contact with dentin with the formation of a clear boundary between them.

The layer reaches its greatest thickness (up to 1500 microns) on molars, on other teeth - from 100 and above.

Tooth cement functions

From the structure and structure, the tasks of tissue-forming substances and cell formations follow:

• protection of the dentin layer of the roots from mechanical and chemical damage;
• maintaining the position of the tooth in the hole in a state of dynamic equilibrium (tight, but not rigid, fixing it in the hole with the possibility of a certain freedom of movement);
• the implementation of reparative processes in tissues.

Maintaining the proper condition of the ligamentous apparatus holding the tooth occurs in two ways:

• fixation of already existing periodontal fibers to the necks and roots of the teeth;
• the deposition of cement in the zone of newly formed fibers of the periodontal ligament recovering from damage, facilitating its attachment to the tooth root.

In addition, the deposition of cement in the zone of the apex (apex) of the root compensates for the loss of the total height of the tooth due to the abrasion of the layer. enamels.

How does dental cement form

Dental cement production is possible due to the existence of cementoblast cells. The process has 2 stages.

First step

At the first, cementoblasts are created over the root dentin and around the filaments that have begun to form periodontal ligament matrix-cementoid - an organic basis consisting of collagen fibers and the main substances. Further, the calcification of the matrix occurs simultaneously with dentin. This creates the basis of the initial layer of the primary cement layer, which subsequently thickens due to the repetition of the stages of the first phase of the cementogenesis process.

Second phase

At the second stage, the matrix mineralization mechanism is activated - hydroxyapatite crystallizes into it.

Cementogenesis proceeds in a certain rhythm, combining the formation of a new layer of the matrix with the calcification of the existing one.

The formation of the primary cement layer begins before the eruption of teeth, the secondary - after the eruption and continues throughout the entire period of a person's active life.

Resorption of dental cement

The mechanism of resorption of tooth root structures (including the cementum) is called resorption.

Resorption can be:

• physiological or
• pathological.

The first type is the resorption of the roots of a milk tooth, preceding its replacement with a permanent one.

The second includes tissue loss due to:

• trauma;
• chronic pulpitis;
• tooth reimplantation;
• pressure exerted by unerupted neighbors;
• tumors of odontogenic or other nature.

The focus of pathological resorption occurs in the zone of pressure or other effect on the tooth, resulting in a shortening tooth roots and the total height of the tooth, the degeneration of collagen fibers of the periodontal ligament into hyaline or its aseptic necrosis.

In addition to internal resorption of the root, there is also external resorption, where options are distinguished:

• surface;
• inflammatory:
• substitutional (or ankylosis).

Giant multinucleated cells - dentoclasts (or odontoclasts) - are responsible for the loss of root tissues. Calcifying the root surface, as well as phagocytizing its constituent structures, they cause formation of resorption lacunae with the possibility of their subsequent filling using secondary cement layer.

The dynamic balance of resorption and deposition of cement layers allows to achieve restoration of the structure and vital activity of the roots in a natural way, while the predominance of the resorptive component in the system requires intervention dentist.

Role of cement in periodontal regeneration

Regeneration in the periodontium is responsible not only for the substances of the bone alveolar tissue, but also for the presence of certain chemical compounds contained in the intercellular substance of the cement layer of the tooth:

• CGF (cement growth factor);
• CAP-protein (responsible for the adhesion of periodontal fibroblasts - cells that synthesize proteins that maintain the state of the structure and functioning of cells in the dental bone environment).

Equally important is the presence of growth factors in it, which appear in huge volumes in the damaged area, favoring tissue regeneration:

• IGF-1;
• IGF-P;
• TFR-31;
• TRFR.

Artificially created dental cement

In the arsenal of the dentist there are many of the newest means of filling teeth - artificial dental cements.

These include composition groups:

• zinc phosphate (unifas class, visphate and phosphate cement);
• silicophosphate (lactodont, silidont class);
• silicate (class of silicin, alumodent, velacin);
• zinc oxide eugenol (category of cariosan).

There are also compositions with bactericidal properties (such as phosphate cement with a silver content or compositions of the class dioxivisthata, unicem, argyl), polycarboxylate and glass ionomer cements, as well as amalgams (including silver, with 75% silver).