Dentin- Microscopic Structure, Properties, Types and Functions

Dentin

Dentin is a hard tissue, which is also mildly elastic in nature.

Also called substantia eburnea

Forms the bulk and provides general form and shape to the tooth

Covered by the enamel in the crown and by the cementum in root

Determines the shape of the crown and also the number and size of the roots

Both the dentin and the pulp are embryologically derived from dental papilla

Physical Properties

COLOR- Pale yellow, contributes to the colour of the crown of the tooth

HARDNESS- Dentin is harder than bone and cementum but softer than enamel

VISCOELASTIC- Even though dentin is rigid, it is elastic in nature, which permits slight deformation.

STRENGTH- Higher compressive and less tensile strength than enamel

PERMEABLE- The tubular structure of dentin makes it permeable

RADIOLUCENT- Dentin is more radiolucent than enamel because of its reduced mineral content

SENSITIVITY- Dentin differs from enamel in that it is sensitive and forms throughout life at the expense of the pulp

The modulus of elasticity-1.67 x 106 PSI.

Density 22.1 gm/mm

KHN 68

Chemical Properties

Dentin is made up of approximately 70% inorganic material, 20% organic material and 10% water by weight

50% inorganic material, 30% organic material and 20% water by volume

The inorganic portion is in the form of calcium hydroxyapatite crystals

crystals are similar in shape to those of enamel, they are smaller in size, rich in carbonate and poor in calcium when compared with those in enamel

The collagen fibres, constituting 90% of the organic matrix, are mainly made up of type I collagen and and ground substance of mucopolysaccharides, glycosaminoglycans, proteoglycans, and phosphoproteins with small amounts of citrate, chondroitin sulfate, insoluble protein and lipid.

• Read About

1. Dental Pulp- Morphology, Histology, Structure and Function
2. Tooth Enamel- Composition, Properties, Structure and Functions

Structure of Dentin

A) DENTINAL TUBULES

The dentinal tubules are sigmoid (‘S’) shaped curved Structures which run perpendicularly from the pulp toward the periphery

Near the root tip, incisal edges and cusps, the dentinal tubules are almost straight

The first convexity of the doubly curved dentinal tubules is directed towards the apex of the tooth

They are perpendicular to dentinoenamel junction and cementoenamel junction

Two bends making up the S-shape are called the primary curvatures

Secondary curves- During the deposition of dentin, the odontoblast makes slight undulations that creates wavy dentinal tubules. Waviness of the dentinal tubules is called secondary curves.

The thickness of dentin is between 3 to 10 mm

The diameter of the dentinal tubules is larger at the pulpal side (1.5 to 3 microns) than at the dentino-enamel junction (one-micron)

The number of tubules per unit area at the pulpal end is nearly four to five times more than at the outer surface of the dentin

The ratio between the outer and the inner surfaces of dentin on an average is 5:1

Therefore, the tubules are farther apart in the outer layers and closer together near the pulp

Number-  Pulpal : outer surfaces of the dentin- 4:1

Number per sq. mm – 50,000- 90,000

The root dentin has lesser tubules compared to the crown

The dentinal tubules show more branching at their terminal parts

The lateral branches are called as canaliculi or microtubules

These microtubules originate at right angles to the main tubules every one to two microns along its length

The dentinal tubules contain the odontoblastic processes and a thin organic sheath lining the dentinal tubules. This layer is called lamina limitans

A few dentinal tubules may extend through the DEJ into enamel for several millimeters called enamel spindles

CLINICAL IMPLICATIONS OF DENTINAL TUBULES

Make the dentin permeable, providing a pathway for the invasion of caries

Microscopic examination of infected dentin shows that the dentinal tubules are packed with microorganisms well ahead

Drugs and chemicals present in a variety of dental restorative materials can also diffuse through the dentin and create pulpal injury


B) PERITUBULAR OR INTRATUBULAR DENTIN

Dentin that immediately surrounds the dentinal tubules and is termed peritubular dentin or intratubular dentin

More highly mineralized (9%) & lacks  collagen fibres than inter-tubular dentin

The intratubular dentin is formed after the intertubular dentin formation occurs

Approximately two times as thick in outer dentin than in inner dentin

C) INTERTUBULAR DENTIN

Main Body of Dentin- Present between the dentinal tubules or peritubular dentin

Located between the Peritubular Dentin

About one half of its volume is organic matrix, mainly collagen fibrils.

Highly mineralized- Retained after decalcification

D) PREDENTIN

Innermost Dentin- Non Mineralized

Located Adjacent to Pulp

First formed Dentin

Width 2-6 micron

About one half of its volume is organic matrix, mainly collagen fibrils.

As collagen fibres undergo mineralization at pre-dentin – dentin junction ,predentin becomes dentin and a new layer of predentin forms circumpulpally 

E) ODONTOBLASTIC PROCESSES

Cytoplasmic Extension which extend into the dentinal tubules

The cells are present in the peripheral pulp at the pulp- predentin border

Diameter of 3–4 μm when it enters the dentinal tubule tapers to about 1 μm as it extends further into the tubule.

Cell bodies of the odontoblast are about 7 microns in diameter and about 40 microns in length.

Therefore, the odontoblastic processes become narrow to about half the size of the cell as they enter the dentinal tubules.

F) TYPES OF DENTIN

G) PRIMARY DENTIN

Primary dentin is formed prior to the complete eruption of the teeth and root completion

It forms the major bulk of the dentin

It is composed of mantle dentin and circumpulpal dentin

Primary dentin fulfills the initial formative function of the pulp

Mantle Dentin

First formed dentin close to DEJ

20 micron thick below the DEJ

Type III collagen fiber (Vankorff’s fibres)

Fibers are larger, loosely packed & perpendicular to DEJ

Less Mineralized

Shows fewer defects than circumpulpal dentin

Circumpulpal Dentin

Formed dentin close to Pulp

Bulk of tooth

Collagen fiber Diameter 0.05 micron

Fibers are smaller, closely packed

More Mineralized

H) SECONDARY DENTIN

Formed after root completion and eruption

Regular, Narrow band around the pulp

Few dentinal tubules than primary

Bend of tubule at junction of primary and secondary dentin

Grows slowly and reduce the size of pulp cavity

Grows on floor and wall of pulp cavity

I) Tertiary Dentin/reactive / reparative / irregular Secondary dentin

Formed in response to various stimuli such as attrition, caries or restorative dental procedure.

Protect the pulp from the inward spread of noxious materials along the dentinal tubules (like bacterial toxins etc.)

Reactionary or Regenerated Dentin

1) If odontoblasts are survived after exposure to the operative procedures, abrasion, erosion or caries and produce dentin then this dentin is known as reactionary or regenerated dentin.

2) Irregular appearance with fewer tubules

Reparative Dentin

1) Formed by the newly differentiated odontoblast-like cells that replace the original odontoblasts that have been destroyed by insult/stimulus.

2) Irregular dentin with fewer and more twisted tubules.

Newly differentiated odontoblast-like cells may get entrapped in the matrix they form to produce osteodentin

J) SCLEROTIC DENTIN/TRANSPARENT DENTIN

External stimuli such as slowly progressing caries, attrition or erosion.

Fatty degeneration of Odontoblasts and its process

Laying down of apatite crystals later forms a fine meshwork

Obliterating the lumen of the tubule completely

Tubule is completely filled with mineral

The refractive indices are equalized and becomes transparent

it appears light in transmitted light and dark in reflected light.

K) DEAD TRACTS

Odontoblastic cell processes in the dentinal tubules are degenerated, leaving behind empty, air-filled tubules referred to as “dead tracts”.

Appear black in transmitted light and white in reflected light.

Demonstrate decreased sensitivity.

Probably the initial step in formation of sclerotic dentin.

More often seen in the region of narrow pulp horns in coronal dentin

More commonly in older teeth and demonstrate reduced sensitivity

L) INTERGLOBULAR DENTIN

Calcification of dentin in some areas occurs in a form of globular pattern.

These globules fuses together to form homogenous substance

Sometimes globules in some areas failed to fuse. Area of organic matrix between the globules remain uncalcified or partially mineralized. These areas bonded by the curved outlines of the adjacent globules.

M) INCREMENTAL LINES OF VON EBNER

Striations in dentin

Perpendicular to dentinal tubules

Reflects the daily rhythmic recurrent deposition of dentinal matrix

Indicates- Growth pattern of dentin

N) CONTOUR LINES OF OWEN

Some  incremental lines are accentuated because of disturbance in matrix and mineralization process, such lines are called contour lines of OWEN

O) NEONATAL LINES

Hypocalcified area seen in those teeth, where dentin is partly formed before birth and partly formed after birth.

All deciduous teeth and first permanent molars.

Represent the disturbance in mineralization due to the abrupt change in environment 

P) TOME'S GRANULAR LAYER

Adjacent to Cementum. increases in amount from the cementoenamel junction to the apex of the root

The granular layer represents the looped terminal portion of the dentinal tubules in the root dentin

similar to the branching and beveling of the dentinal tubules at the dentinoenamel junction

Q) DENTINO-ENAMEL JUNCTION

The junction between the enamel and dentin is irregular and is described as scalloped. The convexities face the dentin whereas the concavities face enamel.

R) PREDENTIN PULP JUNCTION

Made up of dense collagenous fibers and is present between the uncalcified dentin (predentin) and pulp.

S) DENTIN PREDENTIN JUNCTION

Dentin predentin junction is the interface between the calcified and uncalcified newly formed dentin called predentin

T) DENTINO-CEMENTUM JUNCTION

There is a smooth line junction between the dentin and cementum in permanent teeth.

The cementodentinal junction in deciduous teeth, however, is sometimes slightly scalloped. The attachment of cementum to dentin is quite firm.

Dentinal Sensitivity

Exposed dentin is very sensitive. Three main theories have been proposed to explain dentin sensitivity

A) Direct Neural Stimulation Theory

B) Transduction Theory

C) Hydrodynamic or Fluid Theory

A) Direct Neural Stimulation Theory

This theory states that the nerve endings inside the dentinal tubules are directly stimulated and cause sensitivity/pain.

Drawbacks

Even though nerve endings are seen inside the dentinal tubule, they are confined to the inner dentin and are absent in the outer dentin

Topical application of the local anaesthetic to the surface of dentin does not eliminate dentin sensitivity.

B) Transduction Theory

This theory states that various stimuli stimulate the odontoblastic process which conducts the impulses to the nerve endings located in the inner dentin

Drawbacks

Neurotransmitter vesicles are absent in the odontoblastic process

There is no synaptic relationship between the odontoblastic process and the nerve endings.

C) Hydrodynamic or Fluid Theory

Dentin sensitivity is due to the movement of fluid inside the dentinal tubules

Stimuli such as heat, osmotic pressure and drying can cause outward movement of fluid

Cold stimulus causes inward movement of fluid

Causes mechanical disturbance of the nerve endings which are present in the inner dentin and are close to the odontoblasts and their process

The free nerve endings in the plexus of Raschkow sense these disturbances, resulting in sensitivity/pain

This widely accepted theory proposes that nerve endings act as mechanoreceptors