Dental Anatomy: Dentin Structure and Properties

Dentin Structure and Function

• Dentin is a mesodermal derivative, forming the main bulk of the tooth.
• Dentin morphology is determined by the tooth structure.
• The crown of the tooth is covered by enamel.
• The root of the tooth is covered by cementum.
• Dentin contains vital tissue.

Physical Characteristics of Dentin

• Dentin is yellowish in color.
• It is elastic.
• It is hard, but less hard than enamel, and harder than cementum and bone.
• In X-rays, dentin is more radiolucent than enamel, but more radiopaque than cementum.
• Dentin thickness ranges from 3-10mm.
• It is permeable due to its tubular pattern.

Chemical Composition of Dentin

• Dentin is composed of 70-75% inorganic material and 25-30% organic material.
• The inorganic material is primarily hydroxyapatite crystals.
• The organic material includes collagen type I and insoluble proteins, glycoproteins and lipids.

Ground and Decalcified Sections

• Ground sections show the inorganic portion of the dental structure.
• Decalcified sections show the organic portion of the dental structure.

Dental Tubules and Odontoblastic Processes

• Dentin tubules are microscopic channels running through dentin.
• They originate from odontoblasts, which are cells that form odontoblastic processes, which extend into these tubules. Tubules are thick near the pulp, and thin towards the outer end.
• The ratio of tubules per unit area is approximately 4:1 at buccal and lingual walls compared with mesial and distal walls.
• Tubules have lateral branches, and terminal branches that are more prominent in root dentin.
• The tubules are S-shaped, and start at a right angle from the pulpal surface; in the root, incisal edge, and cusp, they are almost straight.

Location of Dentinal Tubules

• The tubules are thicker in the coronal regions than in the root regions; and their number, per unit area, is higher near the pulp.

Odontoblasts and their Processes

• Odontoblasts are involved in secretion and maintenance during dentin production, and extend into the tubules.
• Tomes' processes are the extensions of odontoblasts in the dentinal tubules.
• Odontoblastic processes have microfilaments, vesicles, ribosomes, and mitochondria. They have hydrolytic enzymes and are thicker near the cell body.

Dentinal Tubules Branching

• They form branches, called canaliculi, perpendicular to the tubules.
• Branching is more extensive in root dentin than coronal dentin.
• The number of tubules per square millimeter ranges from 50,000 to 90,000 near the pulp.

Incremental Lines in Dentin

• Von Ebner lines are rhythmic mineral deposits in dentin, running perpendicular to the tubules.
• Owen's lines are accentuated hypomineralized lines that result from secondary curvatures.
• Counter lines are positioned between primary and secondary dentin.
• Neonatal lines are hypocalcified lines, resulting from prenatal factors.

Dentino-Enamel Junction

• The dentino-enamel junction is scalloped, which prevents enamel shearing during tooth function.
• The basal lamina separating enamel and dentin is termed the membrana preformativa during early tooth development.

Types of Dentin

• Mantle dentin is the first formed dentin layer, near the enamel.
• Circumpulpal dentin surrounds the pulp, more mineralized and has parallel and oblique fibers to DEJ, and crowded cells.
• Primary dentin is formed before root completion, with fibers perpendicular to DEJ in crown and parallel to basement membrane.
• Secondary dentin is formed after root completion, with more wavy tubules, decreasing in number and thicker over pulp roof and floor.
• Interglobular dentin is the less mineralized main body of the dentin.
• Peritubular dentin forms the wall of the tubules, is hypermineralized, and has a smaller lumen.
• Predentin is the first-formed, unmineralized dentin and is adjacent to the pulp tissue. Its thickness ranges from 10-40µm.
• Reparative/Tertiary dentin is formed in response to severe stimulus.

Dentinogenesis

• Pattern of formation begins at cusp tips, spreading downward to reach the cervical loop.
• Rate of coronal dentin deposition is about 4µm per day.
• Root dentin formation occurs after the epithelial root sheath of Hertwig forms.
• Root dentin has a different orientation of collagen fibers and less phosphoryl content compared with coronal dentin.

Odontoblastic Process Formation

• Several processes form initially; as dentin forms, cells recede, leaving a single process, known as a Tomes' fiber.

Dentin Matrix Formation

• Mantle dentin forms the initial dentin layer near the enamel, with fibers perpendicular to DEJ.
• Circumpulpal dentin surrounds the pulp, with parallel or oblique fibers to DEJ, and dense cells.

Dentin Mineralization

• Budding matrix vesicles rupture, releasing contents that mineralize the mantle dentin.
• Calcium and phosphate ions crystallize, creating a linear, globular dentin arrangement. This takes place around regions below and near the mantle dentin, and in all remaining circumpulpal dentin of the crown and root regions.

Age-Related Changes in Dentin

• Dentin deposition rate decreases with age.
• Dentin thickness increases with age.

Secondary Dentin

• Regular secondary dentin forms throughout the pulpal surface.
• The tubules have a more wavy course and the number of tubules decreases.
• A demarcation line is present and darkened.

Irregular Secondary Dentin

• Formed in response to severe stimulus at the pulpal end of exposed dentin.
• Tubules are fewer and irregular.
• The subodontoblastic layer will differentiate, replacing degenerated cells to form reparative dentin.

Types of Reparative Dentin

• Types of reparative dentin include osteodentin, atubular dentin, and vasodentin.

Clinically Relevant Considerations

• Exposed dentin should be sealed as damage to 1mm² of dentin can affect ~30,000 live cells.
• Tubular dentin nature allows easy spread of bacteria/caries.
• Trauma can dislodge odontoblasts and cause pain/infection.
• Pulp capping with calcium hydroxide can help form reparative dentin.
• The permeability of dentin tubules is higher near the pulp horns.
• Smear layer should be removed due to toxic products and potential for bacterial infection.
• Hypersensitive dentin may be treated with potassium nitrate to reduce conduction of heat and reduce pain.

Transparent and Sclerotic Dentin

• The affected area of occluded dentin has a uniform refractive index.
• Affected dentin appears translucent under transmitted light.

Dead Tracts

• Severe stimulation/trauma leads to odontoblast destruction, causing empty dentinal tubules.
• These regions appear black, while dead tracts are surrounded by sclerotic dentin.

Dentin Innervations

• Sensory innervation is high near the DEJ and the pulpal surface, becoming less sensitive toward the DEJ, and high near the pulpal surface, with less sensitive regions near the DEJ.

Theories of Pain Transmission in Dentin

• Direct neural stimulation, Odontoblastic transduction, and fluid-dynamic theories describe potential dentinal pain pathways.

Miscellaneous

• A good head and heart are a formidable combination.

This quiz focuses on the anatomy of dentin, highlighting its unique characteristics compared to enamel and cementum, as well as its structural composition. You will explore topics such as the diameter of dentinal tubules, the type of collagen present in dentin, and the mineralization patterns seen in secondary dentin. Test your knowledge of these essential elements in dental anatomy!