Dental diseases

Root caries

Root caries is a pathological condition that develops in the area of dental roots.

Some authors define root caries as a carious process that develops below the CEJ, while on the other hand, there are authors who believe that root caries affects the cementoenamel junction (CEJ) and part of the enamel above this border.

In order to better understand the process of root caries formation, it is necessary to describe the morphological characteristics of the tooth and its tissues.

The tooth consists of two basic parts:

1. The crown - which is responsible for masticatory function.
2. The root - which anchors the tooth in the alveolar bone.

The crown itself is classified in two parts:

  • Anatomical crown - the part of the crown that covers the enamel.
  • Clinical crown - the part of the tooth that is seen in the oral cavity.

The dental root is covered with cement.

The development and progression of root caries is different from crown caries.
These differences originate from differences in the structure and composition of dental tissues: enamel, dentin and cementum. In order to understand these differences in the carious process, it is essential to describe the chemical structure and mechanical properties of dental tissues.


Structure of dental tissues

Enamel

Enamel is a solid, protective substance that covers the area of the tooth crown. It is the hardest biological tissue in the body, capable of resisting masticatory forces. The enamel provides shape, gives a contour to the tooth, and covers that part of the tooth that is exposed to the oral milieu.

In the area of the molar and premolar cusps, the enamel reaches its greatest thickness, about 2-2.5mm. The thickness gradually decreases, going towards the cervical and is thinnest in the area of the neck of the tooth.

The enamel consists of enamel prisms, which have the task of resisting masticatory forces.
Enamel prisms are produced by formative ameloblast cells. As they produce enamel prisms, a group of ameloblasts migrate peripherally from the enamel dentin junction. Ameloblasts move in different directions along this path, causing the bending of enamel prisms. The configuration of the enamel prisms, observed under a direct light, leaves the impression of dark and light bands of enamel prisms, called Hunter- Schreger stripes. Daily apposition of enamel takes place in increments, about 4 micrometers thick. These increments are noticeable in cross section, in the form of darker stripes, and are called Retzius stripes or stretch marks.

Enamel prisms are built of minerals that make up the crystal, the basic formative unit of the enamel prism: the hydroxyapatite crystal.
This crystal is a crystalline form of calcium phosphate and is also still found in bone, dentin and cement.
Enamel consists of 96% anorganic matter in the form of hydroxyapatite and 4% water and organic matter. The organic component of enamel is the protein called enamelin, similar to keratin, which is found in the skin. The distribution of enamelins between hydroxyapatite crystals determines the permeability of the enamel. The enamel is grayish-white in color, but impresses slightly yellowish, because it is translucent and allows the penetration of the color of underlying dentin.


Dentin

Dentin is a hard tooth tissue that gives the tooth its basic shape. It is a living, biologically active, sensitive tissue, which under normal conditions is not exposed to the conditions of the oral cavity. In the area of the crown of the tooth it is covered with enamel, and in the area of the root it is covered with cement.

Since it begins to form just before the enamel, it also determines the shape of the tooth crown, including cusps, marginal ridges, the number, shape, and size of the roots. As living tissue, it contains extensions of odontoblasts, cells specializing in the production of dentin, inside its tubules. Although the cell bodies of odontoblasts are located on the very periphery of the dental pulp, they morphologically belong to dentin.
Dentin consists of 35% organic matrix and 65% anorganic matter, primarily minerals, organized in the form of hydroxyapatite crystals.
The organic matrix is dominantly composed of collagen fibrils immersed in the mucopolysaccharide base substance.
Type I collagen is the dominant type of collagen that builds collagen dentin fibrils. The mucopolysaccharides that make up the basic substance are different types of proteoglycans and glycoproteins, phosphoproteins and phospholipids: chondroitin sulfate, decorine and biglican, dentin sialoprotein, osteonectin, osteopontin and dentin phosphoprotein.
Based on the time of creation and histological characteristics, dentin is classified as primary, secondary and tertiary dentin.

Primary dentin is a significant structural component of the dental crown and root and consists of mantle dentin, globular dentin and circumpulpal dentin.

Covering or mantle dentin is deposited first, along the enamel-dentin border. It represents a strip of dentinal tissue, about 150 micrometers wide. The covering dentin does not contain dentin sialoprotein and dentin phosphoprotein in the composition of the organic matrix. This dentin is thought to be synthesized by immature odontoblasts. The collagen fibers in the composition of this dentin are larger in size than the fibers of the circumpulpal dentin, which is formed later. The mantle dentin is separated from the circumpulpal dentin by an irregular dentinal formation, called globular dentin. This dentin consists of dentinal globules that are separated from each other by interglobular spaces. Globular dentin is thought to be the result of insufficient mineralization that occurred during the final maturation of the odontoblast. Primary dentin continues to form, collagen fibrils are smaller in size, until the tooth erupts and reaches the occlusal plane.

Once a tooth reaches the occlusal plane and begins its function, the dentin that is synthesized from that point on is called secondary dentin.
This dentin is deposited more slowly than the primary one, and the distance between the incremental lines is 1-1.5 micrometers. Scientists believe that as soon as the tooth begins its function, signals from the cerebral cortex reach the dentin. These signals cause the slowing down of the deposition process. It is believed that this regulation prevents excessive dentin production and consequent obliteration of the pulp chamber.
The tubules of primary and secondary dentin are usually continuous, except in cases of unequal production of secondary dentin. In molars, more secondary dentin is deposited on the floor and roof of the pulp chamber than on the lateral walls. It is the protective mechanism of the pulp horns against the mastication process.

Tertiary dentin is also known as reactive or reparatory dentin.
The production of this dentin is a consequence of pulpal stimulation by a certain factor, such as caries, all forms of tooth wear, restorative procedures, etc. This dentin is produced exactly at the places of stimulation. In the case of stimuli of stronger intensity, tertiary dentin forms rapidly and results in the formation of dentinal tissue of irregular shape, with the presence of dentinal tubules that are irregular, twisted and possibly with cell inclusions. The cells found in tertiary dentin are: odontoblasts, fibroblasts, and blood cells.

In the case of stimuli of lower intensity, tertiary dentin with a more regular structure is formed, similar to the primary and secondary dentin.
In certain cases, reparative dentin looks more like bone than dentin, and it is called osteodentin. This type of dentin usually develops when using a hardening form of calcium hydroxide in direct coating of the pulp.
Recent terminology suggests that the term reactive dentin is used in cases where the original odontoblasts retain function and produce this type of dentin, while the term reparative dentin is reserved for those cases of harmful stimuli that led to interruption of regional odontoblasts, and the newly formed dentin is produced by replacement odontoblasts.


Cement

Cement is the dental tissue that covers the roots of the tooth. It consists of 45-50% anorganic matter. The rest consists of water and an organic component.
There are two types of cement:

Acellular cement - it is characterized by uniform mineralization and the absence of cells. It covers the dentin in the area of the cervical and middle third of the root. It is composed of a fibrous matrix and well-oriented collagen fibrous bundles (Sharpey s fibers) that promote anchor points for the periodontal ligament.

Cellular cement consists of collagen matrix and cement cells. It covers the apical third of the root and the area of the root furcations.

The cementoenamel junction (CEJ)

Represents the demarcation line between the enamel and the dentin.
Under normal conditions, it is located below the gingival margin.

It can be presented in 3 forms:
1. Edge to edge contact between enamel and cement (most often in the premolar area.
2. Cement covers enamel (most often in molars).
3. Exposed area of dentin that is not covered with enamel or cement.


The differences in the anorganic compound between enamel and dentin

The high mineral content in the enamel is responsible for the physical characteristics of the enamel itself, such as its hardness.
The crystals in the enamel composition have a hexagonal shape, with an average length of 1000 nm, a width of 70 nm and a thickness of 25 nm. The crystals are densely packed and form enamel prisms, the basic formative units of the enamel itself. Their length is 9 micrometers and width is 5 micrometers. Enamel prisms make up 75% of the enamel volume. The number of enamel prisms ranges from 5 million in the lower lateral incisors to 12 million in the upper first molars.
Very densely arranged crystals in the composition of enamel prisms cause very small spaces between the crystals we call enamel pores. The largest pores are located at the boundaries of enamel prisms, but they make up only 0.3% of the total enamel porosity. The reason for this is the sudden reorientation of the crystals in the area. Slightly smaller pores are located in the space between the enamel prisms, and the smallest pores are between the previously mentioned, densely arranged crystals in the composition of the enamel prisms.
Considering the known orientation of the crystals in the enamel, we conclude that the pores are oriented at an angle of 70-90% in relation to the tooth surface.

The crystals in the dentin composition are different, more plate-like shape compared to the stripe -like shape of enamel crystals.
Dentine crystals are are significantly smaller and thinner. Their average width is 30 nm and thickness is only 3nm. During dentinogenesis, about 25% of the crystals are deposited in the collagen fibers in the dentin composition, while other crystals are deposited between the fibers.

The total porosity of the dentin is about 21%. The reason for this is the presence of dentinal tubules in the dentin, which account for 6.5% of porosity. The dentinal tubules are wider and denser in the area of the dentin that is more directed toward the pulp.
This leads us to the conclusion that dentinal tubules occupy a larger volume of the inner layer of dentin compared to the outer. The intertubular dentin, located between the dentinal tubules, is responsible for 15% of the total dentin porosity.
Dental crystals are forms of partially soluble calcium phosphate, better known as hydroxyapatite, which in its pure state has the formula Ca10 (PO4) 6 (OH) 2. The hydroxyapatite crystal has a hexagonal shape in cross section. It contains the central nucleus or C axis of the hydroxyl ion, around which the calcium and phosphate ions are arranged in the shape of a triangle.


Progression of the root caries

Gingival recession is an apical migration of the marginal gingiva that leads to exposure of the dental root.

When the root surface of the tooth is exposed, it finds itself in a whole new environment: the root surface, which used to be an anaerobic microenvironment located below the gingival tissue, now becomes exposed to oral cavity conditions: high oxygen levels and diverse oral microflora.
The cementoenamel junction, normally located below the marginal gingiva, is now exposed to the oral microflora. As this area is very difficult to clean properly, it soon becomes a place for stagnation and accumulation of the biofilm.

In the presence of a cariogenic root biofilm, fermentable carbohydrates from food are converted to organic acids.
Organic acids reduce the pH value of the root biofilm, creating an enviroment for the onset of demineralization: a carious lesion is initiated on the root surface.
Vast majority of lesions begin along the cementoenamel junction or in the area of the cervical third of the root covered by acellular cementum.

The caries process develops in two phases:
1. Demineralization phase
2. Degradation phase of the organic matrix.

As mentioned earlier, the microbiological niche of the root surface represents an environment with a lack of oxygen and limited availability of carbohydrates in relation to the environment that prevails above the marginal gingiva, in the oral cavity.
With the recession of the gingiva and the exposure of the root surfaces to the conditions of the oral cavity, the microenvironment of the root surface changes. Biochemical and structural differences between enamel, cementum, and dentin modify the pattern of caries spread and progression.

The processes of demineralization of cement and dentin are very similar.
For this reason, many authors view them as a whole and call them chemical root mineral behavior.
The critical pH value below which the enamel demineralization process begins is 5.4. Unlike enamel, the critical pH value below which the process of cement and dentin demineralization begins is higher: between 6.8 and 5.8 in some studies. This means that the minerals in the composition of cement and dentin are more soluble than the minerals in enamel. This phenomenon explains why the process of demineralization of the root surface is easier and more progressive compared to enamel.
Certain types of carbohydrates that are safe for enamel, and do not have a cariogenic potential, will be cariogenic for the root surface. For example, sweeteners that contain lactose as a bulking agent are not cariogenic for enamel but are for cement and dentin.
Likewise, it has been proposed that the digestion of starch products would be sufficient to cause root caries but not sufficient to cause the enamel caries.

As the demineralization process progresses, the second phase of the root caries process occurs: degradation of the organic matrix in the composition of the root tissue.
Damaged organic collagen matrix serves as scaffold for bacterial colonization.
From these places the bacteria continue their invasion of the deeper layers of tissue and lead to the progression of the carious lesion.


Lesions in cement

The process of demineralization and degradation of the organic collagen matrix progresses very rapidly through the cementum and invades the undrelying dentin.
The reason for the very rapid progression of cement caries is the damage or complete removal of the cement layer of the exposed root surface due to the aggressive and inadequate technique of teeth brushing, which leads to mechanical damage to the cement layer. Similar lesions occurs with root debridment procedures: root scaling and planning.


Lesions in dentin

When the carious process affects the dentin, it demineralizes and destroys the organic collagen matrix, built mainly of type I collagen.
Demineralization primarily affects the intertubular dentin (dentin that fills the space between the dentinal tubules).
As the demineralization of the above-mentioned dentin progresses, the formed microcavitations in the dentin increase in size. The process of demineralization and destruction continues to progress and in advanced lesions, almost complete intertubular dentin is demineralized.

Bacteria and their products reach deeper parts (layers) of dentin and reach open dentinal tubules. By reaching the bacteria in the dentinal tubules, an unobstructed path of bacterial penetration is achieved. If the carious process doesnt stop, the bacteria reach the pulp through the dentinal tubules and lead to endodontic infection.

Root caries differs from enamel caries also in terms of lesion shape. While carious lesions in the enamel represent narrow and deep defects, lesions of the root surface represent wide, shallower defects.
The reason for these differences is that the root caries process spreads laterally.

The thickness of hard dental tissues in root area is significantly reduced in comparison to the occlusal surfaces, so the path of infection to the pulp is much shorter.
The faster progression of caries, the greater risk of spreading the infection to the dental pulp, which leads the need for endodontic treatment(root canal).

Another specificity of carious lesions located on these surfaces is difficulty in providing a dry working field, necessary for proper restorative treatment (dental filling).
Caries of the gingival third and root caries represents therapeutic challenge. Treatment of these lesions is much more demanding compared to the treatment of occlusal caries.