Dentinal Sensitivity

Theories of dentinal sensitivity:

Direct nerve innervations theory

Direct nerve innervations theory states that the dentin contains nerve endings that respond when it is stimulated.
Some nerves penetrate a short distance into some tubules.
This theory is rejected because.
The dentinal nerves do not extend beyond the inner dentin.
Some nerves occur within some tubules but dentin sensitivity does not depend on the stimulation of such nerve endings.
A newly erupted tooth does not possess such nerve endings and yet it is sensitive.
Application of local anesthetics or silver nitrate to exposed dentin does not eliminate dentin sensitivity.

Transduction theory states that the odontoblasts serve as receptors and are coupled to nerves in the pulp.
Transduction theory was supported because the odontoblast is of neural crest origin, it retains an ability to transducer and propagate an impulse.
But it was rejected because.
1. Absence of synaptic relationship between the odontoblast and pulpal nerves.
2. There are no neurotransmitter vesicles in the odontoblastic process to facilitate the synapse
3. The membrane potential of odontoblasts measured is too low to permit transduction.
4. Local anesthetic and protein precipitants do not abolish sensitivity.

Fluid or hydrodynamic theory states that the tubular nature of dentin permits fluid movement to occur within the tubule when a stimulus is applied.
This fluid movement either inward or outward stimulates the pain mechanism in the tubules by mechanical disturbance of the nerves closely associated with the odontoblast and its process.
When a stimulus is applied to dentin, there is fluid displacement within the tubule and this distorts the tire’s local pulp environment and is sensed by the free nerve endings in the plexus of Raschkow.
These nerve endings may act as mechanoreceptors as they are affected by the mechanical displacement of fluid.
This theory is supported because.
When dentin is first exposed, small bells of fluid can be seen on the cavity floor.
When a cavity is derived with air or cotton wool, a greater loss of fluid is induced, leading to more movement and more pain.
Profuse branching of the tubules at the DEJ is responsible for increased sensitivity at the DEJ.
Pain is produced by thermal change, mechanical probing of hypertonic solutions, and dehydration.

Dentin Sensitivity Mechanisms

1. Color:

2. Dentinal tubules:

There is a reduction in the diameter of dentinal tubules with age due to the continued deposition of intratubular dentin.
Gradually, there is a complete closure of the tubule.
This obliterates the pulp chamber.

3. Sclerotic dentin:

4. Development of dead tracts:

Dentinal tubules are emptied by complete retraction of the odontoblast process from the tubule or through the death of the odontoblast with age.
The dentinal tubules then become sealed off at their pulpal ends.
The tubules are filled with fluid or gas.
In-ground sections, it may entrap air and appear black in transmitted light and white in reflected light.

5. permeability:

As age advances, dentin becomes less permeable.
It occurs due to tubular occlusion, smear layer formation, and lack of tubular communication between primary and secondary dentin.

6. Sensitivity:

Dentinal tubules are found throughout normal dentin.
They are longer than the dentin and are thick because they curve through the dentin.
The course of tubules follows a gentle curve in the crown but is less curved in the root.
Thus, it resembles a gentle S-shape curve.
These curvatures are called “primary curvatures”.
Starting at right angles from the pulpal surface first convexity of this curve is directed towards the root apex.
Tubules end at right angles to DEJ and DCJ.
Tubules also exhibit “secondary curvatures”.
These curvatures are minute, regular, and sinusoidal in shape.
They are present over the entire length of the tubules.

Dentin Curvature Diagram

They appear as fine lines or stations in dentin.
They run at right angles to dentinal tubules.
They represent a normal, rhythmic, linear pattern of dentin deposition in an inward rootward direction.
Its course indicates the growth pattern of dentin.
The distance between lines varies from 4-8 mm in the crown to much less in the root.
The daily increment decreases after a tooth reaches functional occlusion.
They are seen in conventional and ground sections.

These lines result from a coincidence of the secondary curvatures between neighboring dentinal tubules.
They are caused by accentuated deficiencies in mineralization.
They are demonstrated in ground sections.
They represent hypocalcified bands.
They are accentuated because of disturbances in the matrix and mineralization process.

Neonatal Line is a wide contour line.
Neonatal Line separates the dentin formed before birth and that formed after birth.
Neonatal Line reflects the disturbance in mineralization created by the physiologic trauma of birth.
The dentin matrix formed prior to birth is usually of better quality than that formed after birth.

Dentin Pertitubular Structure