Dentin Structure and Properties Quiz

Dentin Structure

• Dentin forms the major bulk of the tooth.
• In the crown, it is covered by enamel.
• In the root, it is covered by cementum.
• Dentin is yellowish in color.
• Hunter Schreger bands explain why enamel, despite being translucent, doesn't fully reflect the yellow dentin color.
• Some light rays pass through the wavy enamel rods, others are reflected, giving the tooth its natural appearance.
• If enamel rods were straight, the dentin color would be more apparent, which would be undesirable.
• Dentin is an elastic vital tissue.
• The presence of collagen fibers and smaller hydroxyapatite crystals gives it an elastic nature, preventing overlying enamel fracture.
• Dentin is considered a vital tissue due to dentinal tubules containing odontoblastic processes and dentinal lymph.

Physical Properties

• Dentin is softer than enamel but harder than cementum and bone.
• On X-ray, dentin appears greyish.
• Dentin is more radiolucent than enamel and more radio-opaque than cementum.
• Dentin's thickness ranges from 3 to 10 mm.

Chemical Properties

• Dentin is 70-75% inorganic material (hydroxyapatite crystals).
• Dentin is 30-25% organic material (collagen type I, phosphorylated proteins, glycoproteins, and lipids, inactive matrix-metalloproteinase MMPs).

Role of Matrix-Metalloproteinase (MMPs)

• MMPs are found in dentin.
• Odontoblasts produce MMPs during dentin matrix secretion.
• MMPs are suggested to be involved in dentin formation.
• After mineralization, inactive MMPs remain trapped within the calcified matrix, mostly concentrated in the predentin and at the DEJ.

Activation of MMPs

• MMP activation occurs if pH decreases (e.g., during caries or acid etching).
• The consequences of MMPs activation include spreading of caries, especially at the DEJ.
• Another consequence is weakening of the hybrid layer, impacting restoration durability.

Types of Dentin

• Primary dentin: Forms the typical tooth shape.
• Secondary dentin: Continuing dentin deposition by odontoblasts after root formation completion.
• Mantle dentin, circum-pulpal dentin, and secondary (Ir D, RD), are additional types described.
• Predentin is a part of the tooth structure.

Histological Structure

• Odontoblasts are positioned on the pulpal surface (not within the calcified substance).
• A long process (Tomes' fiber) extends from odontoblasts across the dentin thickness, to the DEJ and before the DCJ.
• Odontoblasts have processes that extend into tubules, forming the dentinal tubules.
• Dentinal tubules are built from peritubular dentin, separated from each other by intertubular dentin.
• Number of tubules per unit area is higher on the pulpal surface and decreases outward.
• Tubule diameter is greatest near the pulp and tapers outward.
• Odontoblasts contain organelles, especially near the predentin area.
• Microtubules and intermediate filaments are longitudinal in the process.

Peritubular Dentin

• It is highly calcified dentin with a mineral content greater than 90%.
• Inter-tubular dentin has a lower mineral content (about 70%).
• It forms the internal walls of the tubules.
• It maintains the patency of the tubules, keeping odontoblastic processes vital and unmineralized.

Intertubular Dentin

• Dentin located between tubules.
• It represents the primary secretory product of odontoblasts.
• It has a larger surface area than the tubules, particularly near dentin's periphery.

Odontoblastic Processes

• Odontoblastic processes exist within the dentinal tubules.
• Odontoblastic processes follow the dentinal tubules' course, number, diameter, and even branch.
• Lateral branches originate at right angles to the main process, some ending blindly, others connecting to neighboring processes.
• They have all the cell organelles, most abundant in the predentin area.
• Microtubules and intermediate filaments run longitudinally through the processes.

Incremental Lines

• Reflect variations in structure and mineralization during dentin formation.
• They are hypocalcified rings approximately 5 days apart. Indicated by an increment of 20 microns.
• Neonatal line signifies the separation between prenatal and postnatal dentin formation (deciduous teeth and first permanent molars).

Interglobular Dentin

• Calcification of dentin in certain regions is in a distinct globular pattern.
• Individual globules can fuse to create homogeneous areas.
• Some globules may fail to fuse, creating organic matrix spaces; those spaces are uncalcified or partially mineralized.
• The boundaries between regions are curved.

Tome’s Granular Layer

• Tome's granular layer results from the looping of terminal dentinal tubule portions.
• Odontoblastic processes do not extend to the cemento-dentinal junction, unlike at the dentino-enamel junction (enamel spindle).

Significance of Tome's Granular Layer

• The layer might shield pulpal tissues from PDL-derived insults, as the overlying cementum is thin and permeable.

Dentinal Tubules: Clinical Implications

• Dentinal tubules are permeable. They provide pathways for caries invasion.
• Infected dentin exhibits microorganisms within the tubules.
• Dental restorative materials may diffuse through dentinal tubules, causing pulpal injury.
• Cavity preparation needs sealing of tubules to prevent this.

Dentin Bonding System

• Dentin bonding is challenged by the presence of smear layers.
• Smear layers are formed after dentin exposure, with a superficial loosely attached layer and a deeper one that adheres intensely to the dentin wall, hindering restoration attachment.

Dentin-Bonding: Hybrid Layer

• Resin tags are major bonds between resin-based restorations and dentin.
• Deeper tags weaken bonds.
• Extensive intertubular dentin surface promotes the bond, forming the hybrid layer.
• The hybrid layer is a hybrid of deliquesced (demineralized) inter-tubular dentin and resin. This means it is neither resin nor dentin.
• The hybrid layer's high collagen content can weaken if active MMPs are activated from the acid etch. This would lead to weaker bond durability.
• Acid etches work by demineralizing the dentin, exposing the collagen fibers of the intertubular dentin (hybrid layer). Thereby facilitating an adequate bond between the resin and the dentin.
• The acidity activates MMPs, which degrade the collagen fibers, reducing hybrid layer stability and bond strength.

Dentin’s Enzymatic Inhibitors

• To overcome the collagen/MMPs issue, the dentin bond should have enzymatic inhibitors to hinder MMP activation.

Incremental Lines of Dentin

• The incremental lines of dentin reflect variation in structure and mineralization.
• They are marked by hypocalcified bands, roughly 5 days apart, shown in 20-micron increments.

Interglobular Dentin

• Calcification in certain areas of dentin forms a globular pattern.
• Globules fuse into a homogeneous substance.
• Some globules fail to fuse
• Non-fused globules form organic areas between, and they are either uncalcified or partially mineralized, held together by the curvature.

Dentin Sensitivity

• Dentin sensitivity is highest at the DEJ (dentino-enamel junction).
• Less sensitivity is in intermediate areas.
• High sensitivity close to the pulpal surface.

Theories of Pain Transmission Through Dentin

• Direct neural stimulation: Dentin has nerve endings that respond directly to stimulation.
• Odontoblastic transduction: Odontoblasts can propagate impulses, but there's no synapse with nerves.
• Fluid/Hydrodynamic: Fluid movement in tubules affects nerve endings in the plexus of Raschkow.
• Theory A (direct innervation) and C (fluid movement) are widely accepted.

Fluid/Hydrodynamic Theory Explanation

• Explaining pulp response during exposed dentin/reversal pulpitis.
• Stimulus causes fluid movement in the tubules.
• The fluid movement distorts tissues and activates nerve endings.
• Variations in stimulation affect nerve responses (hot vs. cold).  Hot causes fluid outward movement, which is a smaller stimulus. Cold contracts fluid movement into the tubules, which produces a stronger stimulus.

Test your knowledge on the structure and physical properties of dentin. Learn about its composition, relationship with other tooth structures, and its vital role in dental health. This quiz covers key concepts essential for understanding dentin's function.