The enamel and cementum layer together surround the dentin core, which makes up the principal mass of the tooth. Dentin (dentinum, substantia eburnea) forms a layer up to 3 mm thick. This layer encloses a hollow cavity in the tooth, the pulp cavity (cavum dentis; cavitas dentis; ca-vum pulpae), and the root canal (cavum radicis dentis). The dental pulp (pulpa dentis) lies inside this cavity.
The dentin-forming cells (odontoblasts) produce dentin, a hard bonelike substance that is different than enamel. Between 70% and 80% of the mineralized part of dentin is made up of inorganic hydroxyapatite crystals of calcium phosphate without prismatic alignment, and 20% to 30% is organic substances. The mineral content is relatively uniform, but the fluoride content gradually increases with age. Dentin has an inherent yellow color, is not as hard as enamel, and is highly elastic and hence deformable. Dentin can also be strongly stained by residues of medicines and by the infiltration of blood pigments.
The odontoblast processes are the cell processes of odontoblasts, which run from the boundary layer with the pulp cavity through the dentinal tubules to the dentinoenamel junction (Fig 3-9).The odontoblasts probably originate from cells of the neural crest and have the ability to pass on stimuli via their processes. The transmission of stimuli involves the movement of fluid within the processes. The odontoblasts bordering on the pulp are in synaptic contact with nerve fibers. This is why dentin is particularly sensitive to touch, temperature differences, and chemical influences and accounts for painful dentinal sensitivity.
The odontoblast processes can form new dentin throughout the lifetime of a tooth. Dentin is able to keep growing as long as the pulp is healthy. As a result, the originally large pulp cavity becomes increasingly narrow with advancing age due to dentin growth. In places where the enamel is damaged (eg, by abrasion), new dentin will also form; this is called secondary dentin.
Despite this ability to produce new dentin, however, it is rare for an area affected by caries to become fully sealed with secondary dentin. Generally, dentin is not nearly as resistant to caries as enamel. A caries lesion usually results in a small depression in the enamel; this caries then spreads through the enamel to the underlying dentin, where it hollows out the tooth down to the pulp cavity. Once the pulp is affected, this produces the familiar swollen cheek appearance. By this stage, the tooth is very difficult to salvage.
In summary, the odontoblast processes are responsible for:
- Warning about pain
- The formation of secondary dentin
- Making the dentin permeable to water
The dentinal tubules with the odontoblast processes run in a wavelike pattern through the dentin and crowd together more closely in the area near the pulp (Fig 3-10). This means the odontoblast processes are densely packed in the dentin close to the pulp; preparing a cavity in this area produces a large wound.
With teeth prepared for a crown, the wound area can be as large as 12 mm2; by comparison, pulp removal only produces a wound measuring 3 mm2. On average, up to 40,000 odontoblast processes per square millimeter can be damaged when preparing the base of a normal filling, whereas around 15,000 tubules per square millimeter are opened up in the outer layers of dentin during crown preparation. It is therefore reasonable to call this a "dentin wound."
The continual formation of new dentin is a physiologic aging process by which dentinal tubules can even be fully blocked. However, it does offer a possible means for remedying a caries lesion. This ability of odontoblasts to react to physiologic and pathologic stimuli by forming secondary dentin is a remarkable testimony to the vitality of dentin. This is because odontoblasts still supply dentin even after the mineralization phase, and they can strengthen it by secondary buildup.