Oral Biology I: Dentin Structure

Oral Biology I: Dentin Structure

• Dentin is the primary component of a tooth, forming the bulk of the tooth structure.
• In the crown, dentin is covered by enamel.
• In the root, dentin is covered by cementum.
• Dentin is yellow-ish in color.
• Hunter Schreger bands cause the translucent nature of dentin. Some light rays pass through the wavey structure, while others are reflected, and this is why teeth appear naturally.
• Dentin is an elastic vital tissue. Collagen fibers and hydroxyapatite crystals give it its elasticity.
• Dentin is vital due to dentinal tubules, which contain odontoblastic processes and dentinal lymph.
• Dentin thickness ranges from 3-10 mm.
• Dentin is harder than bone/cementum but softer than enamel.
• In X-rays, it appears as a greyish shade. It's more radiolucent than enamel but more radio-opaque than cementum.
• Dentin is 70-75% inorganic material (hydroxyapatite crystals) and 30-25% organic material (collagen type I, phosphorylated proteins, glycoproteins, and lipids.
• MMPs in dentin are produced by odontoblasts and are suggested to be involved in dentin formation.
• After mineralization of the collagen matrix, inactive forms of MMPs remain trapped in the calcified matrix, mostly concentrated in the predentin and DEJ (dentino-enamel junction).
• MMPs activation can occur if the pH decreases (such as during caries or acid etching), causing further caries spread particularly at DEJ and weakening the hybrid layer, which affects restoration durability.
• Types of dentin include primary (basic tooth structure), secondary (produced after root development), and mantle (first layer of dentin produced).
• Dentin's histological structure includes dentinal tubules and odontoblasts.
• Odontoblasts are aligned on the pulpal surface, not inside the calcified substance of the dentin.
• Odontoblastic processes (Tomes' fibers) extend from odontoblasts and traverse the dentin until near the DEJ/DCJ (dentino-cemental junction).
• Dentinal tubules are the unit structure of dentin, built up by peritubular dentin.
• Dentinal tubules are separated by intertubular dentin.
• Number of dentinal tubules per unit area varies in thickness.
• Dentinal tubules taper from the pulpal surface outward.
• They are more closely packed near the pulp than in the peripheral layer.
• Branches (lateral/terminal) of odontoblastic processes may extend into the enamel, producing enamel spindles, making the DEJ sensitive to pain.
• Dentin tubules course/shape varies from the cusp/incisal edge, the cervical area and the mid portion/apical areas.
• The S shape of the curvature is due to odontoblast crowding during pulp recession. This can slow caries propagation.
• The microscopic picture of a TS (transverse section) of dentinal tubules shows odontoblastic processes (Tomes' fibers) and preodontoblasitc space (containing dentinal lymph).
• Peritubular dentin (halo) is highly calcified (greater than 90% mineral content), which helps maintain the patency of the dentinal tubules and vital odontoblastic processes and keeps them from being mineralized.
• Intertubular dentin represents the greater surface area, especially at the dentin's periphery. It's the primary secretory product of odontoblasts.
• The odontoblastic process follows the dentinal tubule in course, number, diameter, and branches. It contains cell organelles (especially predentin area).
• Microtubules and intermediate filaments run longitudinally through the odontoblastic process/Tomes' Fiber .
• The hybrid layer is a mixture of demineralized intertubular dentin and resin, neither a resin nor a tooth, but a combination.
• High collagen content in the intertubular dentin can be degraded by activated MMPs, decreasing the hybrid layer strength and restoration durability.

Dentin bonding

• Acid etching demineralizes dentin, exposing collagen fibers in the intertubular dentin, allowing proper bonding between resin-based restorations and dentin.
• Acidity also activates MMPs, which degrade collagen, thus decreasing the bond strength.
• Adding enzymatic inhibitors to a dentin bond would prevent MMPs activation and thus enhance the durability of the bond.
• Dentin bonding is affected by the presence of a smear layer, a superficial layer on the canal wall that is loosely attached, and a deeper layer that occludes dentinal tubules which adheres strongly to the canal wall hindering proper attachment of the restoration.

Incremental Lines of Dentin

• Incremental lines reflect the variations in dentin structure and mineralization during dentin formation. Bands represent 5-day growth increments (20 microns).
• Neonatal lines separate prenatal from postnatal dentin, and they are found in deciduous teeth and first permanent molars.

Interglobular Dentin

• Calcification of dentin in certain areas occurs in a globular pattern.
• Globules fuse to form homogeneous substance.
• Sometimes globules do not fuse forming an area of organic matrix between the globules.
• Some of these areas remain uncalcified or partially mineralized. Curved outlines bond adjacent globules.

Tome's Granular Layer

• Results from looping of the terminal portion of dentinal tubules.
• This layer occurs instead of the long odontoblastic processes extending to the cemento-dentinal junction (unlike the enamel spindle).
• Protects pulpal tissue from the PDL (periodontal ligament) by creating a thin but highly permeable cemental covering.

Clinical Implication of Dentinal Tubules

• Reason for dentin permeability and pathway for caries.
• Infected dentin displays dentinal tubules packed with microorganisms.
• Drugs and chemicals in dental restoration treatments can diffuse through dentin and cause pulp injury.

Course of Dentinal Tubules

• At cusp/incisal edge: straight
• At cervical area: S-shaped
• Mid portion/apically: Straight

Dentin Sensitivity and Innervation

• Dentin sensitivity is high at the DEJ (dentino-enamel junction), and it is less sensitive as you move away from this area.
• High near the pulpal surface

Theories of Pain Transmission Through Dentin

• Direct neural stimulation: Nerve endings within the dentin respond to stimuli, as confirmed by recent research.
• Odontoblastic transduction theory: Odontoblasts, of neural crest origin, can transduce and propagate signals. This theory is questionable due to no synapse between the cells and nerve fibers.
• Fluid (hydrodynamic) theory: Fluid movement through the dentinal tubules affects nerve endings in the Raschkow plexus, causing pain. This, and direct neural stimulation, are the most accepted theories.

Pulp Response Due to Exposed Dentin

• Fluid movement through dentinal tubules can stimulate nerve activation.
• In hot stimuli, dentinal fluid expands, causing inward movement. This alleviates nerve pressure.
• In cold stimuli, the fluid contracts which causes outward movement to occur, causing nerve pressure. This leads to a stronger reaction compared to the hot stimulus reaction.

This quiz focuses on the fundamental aspects of dentin structure, a vital component of dental anatomy. Explore the properties of dentin, its relationship with enamel and cementum, and the unique characteristics that contribute to its function in teeth. Test your knowledge about the composition, function, and radiographic appearance of dentin.