Dentin and Pulp: Part 1, Dentin Notes

Dentin and pulp cannot be clinically viewed in healthy teeth; they are internal components.
They only become visible when dental pathologies are present.
Dentin and pulp are interdependent and form the dentin-pulp complex as a single functional unit.

Mature dentin is a crystalline material, less hard than enamel but harder than bone.
Composition by weight:
- 70% inorganic (mineralized) material
- 20% organic material
- 10% water
The inorganic component is primarily calcium hydroxyapatite with the chemical formula Ca{10}(PO4)6(OH)2.
Dentin is covered by enamel in the crown and cementum in the root, enclosing the pulp.
It constitutes the bulk of the tooth and protects the pulp.
Dentin's tensile strength provides an elastic foundation for the more brittle enamel.
Dentin gives the enamel crown its underlying yellow hue due to the translucency of enamel; this hue is deeper in permanent teeth.
On radiographs, dentin appears more radiolucent (darker) than enamel but more radiopaque (lighter) than pulp.

With age, enamel loss due to attrition exposes dentin, which appears yellow-white and rougher than enamel.
Exposed dentin can pick up intrinsic stains from beverages, food, and tobacco over time, becoming yellow to black.
Dentin is more permeable (porous) than enamel, making it susceptible to staining.
Attrition in exposed dentin occurs more rapidly than in enamel due to its lower mineral content.
Gingival recession can expose root dentin when cementum is lost.
Gingival grafts can cover exposed tooth root surfaces with grafted oral tissue.

Dentinogenesis is the process of predentin (dentin matrix) formation during the apposition stage of tooth development.
Odontoblasts produce predentin, which matures into dentin.
Odontoblasts form approximately 4 micrometers (\mum) of predentin daily.
The odontoblast's cell body does not become entrapped in the dentin; instead, a cytoplasmic extension remains behind.

Maturation of dentin involves the mineralization of predentin, occurring in two phases: primary and secondary.
Primary Mineralization Phase:
- Calcium hydroxyapatite crystals form as globules (calcospherules) in collagen fibers of predentin.
- Allows for expansion and fusion.
Secondary Mineralization Phase:
- New mineralization areas occur as globules form in partially mineralized predentin.
- Crystals are layered on initial crystals but fuse incompletely.
Incomplete fusion results in microscopic differences in dentin's crystalline form.
- Globular Dentin: Areas with complete crystalline fusion, appearing as lighter rounded areas.
- Interglobular Dentin: Darker, arc-like areas where only primary mineralization occurred, and globules did not completely fuse.

Dentin is avascular; odontoblasts receive nutrition via tissue fluid in dentinal tubules from blood vessels in the pulp.
Each dentinal tubule contains:
- Dentinal fluid
- Odontoblastic process
- Possibly part of an afferent axon (sensory axon)
Myelinated axons may be associated with the odontoblastic process but do not extend to the DEJ or DCJ.
Tubule direction reflects the odontoblast's pathway during predentin apposition.
- Primary Curvature: Large S-shaped curve of the tubule.
- Secondary Curvature: Small, delicate curves within the primary curvature, reflecting daily changes in odontoblast direction.

Peritubular Dentin:
- Forms the wall of the dentinal tubule.
- Highly mineralized after dentin maturation.
Intertubular Dentin:
- Found between the tubules.
- Highly mineralized, but less so than peritubular dentin.

Mantle Dentin:
- First predentin formed near the DEJ and underneath enamel.
- Shows a difference in the direction of mineralized fibers compared to the rest of the dentin.
Circumpulpal Dentin:
- Layer of dentin around the outer pulpal wall.
- Makes up the bulk of the dentin in a tooth; forms and matures after mantle dentin.

Primary Dentin:
- Formed before completion of the apical foramen.
- Characterized by a regular pattern of tubules.
Secondary Dentin:
- Formed after completion of the apical foramen and continues throughout life.
- A dark line marks the junction between primary and secondary dentin.
Tertiary Dentin:
- Also known as reparative or reactive dentin.
- Formed quickly in local regions in response to injury (caries, cavity preparation, attrition, gingival recession).
- Seals off injured area along the outer pulpal wall.
- Sclerotic Dentin (transparent dentin): A type of tertiary dentin often found with chronic caries or aging, where odontoblastic processes die, leaving tubules vacant.

Dentin is generally stable, but resorption can occur in permanent teeth due to idiopathic causes.
- Internal Resorption: Occurs within the tooth.
- External Resorption: Occurs on the external surface of the tooth.

Cariogenic microorganisms enter dentin through dentinal tubules, extending the carious process from enamel.

Medications placed during cavity preparation can promote secondary dentin formation to protect the pulp.
Dental tubules are sealed during cavity preparation to reduce post-restoration sensitivity.
Cutting dentin during cavity preparation produces a smear layer (adherent dental biofilm debris) about 1 micron thick.
- Composition reflects underlying dentin.
- Lowers dentin permeability and is presumed to be protective.

Exposed dentin due to caries, cavity preparation, gingival recession, or attrition can cause pain (dentinal hypersensitivity).
Enamel and cementum may not meet, leaving a gap of exposed dentin at the CEJ in about a third of cases.
Scaling with hand instruments can remove protective layers of cementum and dentin, initiating sensitivity.
Hydrodynamic Theory: Dentinal hypersensitivity is due to changes in dentinal fluid associated with evaporation, fluid movement, and ionic changes.
Vital tooth whitening (bleaching) can also lead to dentinal hypersensitivity if not properly supervised.

Imbrication Lines of Von Ebner:
- Incremental lines or bands in dentin, similar to growth rings in trees or incremental lines of Retzius in enamel.
- Show the incremental nature of dentin formation during apposition, running at 90° to the dentinal tubules.
Contour Lines of Owen:
- Adjoining parallel imbrication lines in dentin.
- Indicate a disturbance in body metabolism affecting odontoblasts.
Neonatal Line:
- Pronounced contour line of Owen indicating the metabolic changes that happen around the time of birth.
Tomes Granular Layer:
- Found in the peripheral part of dentin beneath the root's cementum, adjacent to the DCJ near the DEJ.

With age, the diameter of dentinal tubules narrows due to peritubular dentin deposition.
This narrowing may reduce the pulp's ability to react to stimuli.
Odontoblasts undergo cytoplasmic changes, including a reduction in organelle content.
Dentin becomes more exposed due to attrition and gingival recession, which may or may not lead to dentinal hypersensitivity.