dentine physiology and hypersensitivity

Question 1

describe the innervation to dentine?

what is the sensory supply

Answer 1

• Plexus of Raschkow in cell free area
• Small & large myelinated / unmyelinated fibres
  
  • Pain detection
• Most sensitive near ADJ
• Quite sensitive near pulp
  
  • increased sensitivity when pulp inflamed – toothache!!

sensory supply = Maxillary and mandibular branch of cranial nerve V

• nerve endings are predominantly nociceptive

Question 2

how are nerves supplying dentine formed?

Answer 2

• Nerve fibres enter through apical foramen during dentinogenesis
• First fibres located close to blood vessels (controlling blood flow)
• Autonomic / sympathetic innervation then follows
• Nerve plexus of Raschkow not established until root formation complete

Question 3

what are the theories for dentine sensitivity?

Answer 3

• odontoblasts as nerves
• intra-tubular nerve endings
• hydrodynamic mechanism

Question 4

describe the ‘odontoblast as nerves’ theory of dentine sensitivity

is this the correct theory?

Answer 4

• odontoblasts are of neural crest origin
  
  • can transduce impulses and therefore ‘act as nerves’
• however no real evidence
  
  • membrane potential of cell is too low
  • highly unlikely

Question 5

describe the ‘intra-tubular nerve endings’ theory of dentine sensitivity

is this the correct theory?

Answer 5

• there are nerve endings within some of the dentinal tubules
  
  • can enter tubules but is not extensive
• not theory
  
  • not many nerves in tubules
  • nerves develop after eruption - newly erupted teeth are sensitive
  • local anaesthetics have little effect on exposed dentine
    
    • would expect a rapid block

Question 6

describe the ‘hydrodynamic mechanism’ theory of dentine sensitivity

is this the correct theory?

Answer 6

• changes to fluid in dentinal tubules -> hypersensitivity
  
  • movement in fluid registered by nerve endings in the plexus of raschkow
  • greater sensitivity at ADJ - more branching of tubules
• believed to be responsible
  
  • explains why local anaesthetics do not block dentine sensitivity

Question 7

describe the structure and morphology of the dentine tubules

how does sensitivity occur?

Answer 7

• The Tubules are typically filled with fluid and the ends of the tubules are occluded by enamel or cementum
• fluid does not move until there are holes in enamel or gaps in cementum
  
  • = exposure of fluid to external stimuli
• Exposure of the tubules is thought to be the cause of dentine sensitivity

Question 8

what are nociceptors?

Answer 8

• receptors that respond to noxious stimuli
  
  • Pain can be a result of stimulation of nociceptors
• Nociceptors are free nerve endings & respond specifically to heat, intense pressure, irritants but not innocuous stimuli

Question 9

what classes of nociceptor are there?

Answer 9

• 3 major classes of nociceptor
  
  • Thermal,
  • mechanical,
  • polymodal
    
    • Polymodal - response to noxious proteins, cytokines, bacterial proteins etc.

Question 10

name the nerve fibres which are responsible for pulpal pain and innervation

Answer 10

• A delta fibres
  
  • myelinated
• C fibres
  
  • un-myelinated

Question 11

what characteristics do A delta fibres have?

what pain are they responsible for?

Answer 11

• Low stimulation threshold
• Fast reaction and conduction
• Communication of short fleeting pain sensation
• Activated by hydrodynamic mechanism
  
  • Fluid movement is mechanical - triggers mechanical nociceptors
• Responsible for sensitivity of dentine

Question 12

what characteristics do C fibres have?

what pain are they responsible for?

Answer 12

• High stimulation threshold
• Slow reaction and conduction
• Located in the pulp
• Communication of persistent, heavy, dull, intense pain, often throbbing in nature and poorly localised
• Responsible for the pain associated with toothache - inflammatory pain

Question 13

what are common initiation factors for dentine hypersensitivity?

Answer 13

Cold>brushing>hot>sweet

Question 14

where is the most common site for dentine hypersensitivity?

why?

Answer 14

• Buccal cervical (premolars)
  
  • thinnest layer of enamel
  • predominant gingival recession -
  • = increased dentine exposure

Order of Predilection

1. Canines and 1st Premolars
2. Incisors and 2nd Premolars
3. Molars

Question 15

how do thermal factors trigger dentine hypersensitivity?

Answer 15

• Cold temperatures cause outflow of fluid → sensitivity.
• BUT high temperatures are generally found to have less affect on sensitivity.
• HIGH TEMPERATURES AFFECT LESS AS: it is thought to be because it causes relatively slow inward movement of dentinal fluid.

Question 16

how do mechanical factors trigger dentine hypersensitivity?

Answer 16

Evaporation from the surface causes outflow of fluid → sensitivity

Brushing causes direct movement of dentinal fluid → sensitivity

Question 17

how do chemical factors trigger dentine hypersensitivity?

Answer 17

• Such as acidic substances
  
  • fruit juices
• Hyperosmotic solutions will induce an osmotic pressure, which in turn causes flow of dentinal fluid (- the hydrodynamic theory)
• acids removes the smear layer which has formed to protect the exposed dentine, making the tubules patent to the pulp again.

Question 18

how can sensitivity increase?

what is the cause?

Answer 18

• due to widening of dentinal tubules
• inflammation

Question 19

how does widening of dentinal tubules increase hypersensitivity?

what is the cause?

Answer 19

• Allows greater outflow of fluid, therefore increasing sensitivity
• cause : consumption of acidic foods
  • acid can demineralise peritubular dentine and widen tubules

Question 20

how does inflammation cause an increased sensitivity?

Answer 20

pulpal inflammation occurs when:

1. Plaque builds up on the exposed dentine
2. Bacteria may start to penetrate the dentine tubules
3. Odontoblasts die causing increased dentine permeability
4. Bacterial toxins can then leak into the pulp, resulting in inflammation

• Pulpal inflammation can cause the stimulation of C fibres in the pulp, which produces a duller, more persistent pain.

Question 21

how can dentifrices be used for dentine hypersensitivity?

Answer 21

• Strontium salts
  
  • occlude dentine tubules
  • Strontium Acetate and Strontium Chloride found in Sensodyne
• Potassium Salts
  
  • Occlude dentine tubules
  • Potassium Nitrate found in Colgate Sensitive
• Salts dissociates & ions diffuse down the dentine tubule.
  
  • Ions surround nerve fibres preventing depolarisation by maintaining charge. Therefore prevent neural response by chemical intervention
    
    • Or blocking of the tubules
    • Prevent fluid flow
• silica is often added to toothpastes
  
  • occludes dentine tubules
  • are resisistant to acidic erosion and mild abrasion
    
    • longer lasting

Question 22

what clinical treatments can be used for dentine hypersensitivity?

Answer 22

• Sodium Fluoride Varnish - Duraphat (5% NaF)
  
  • Secondary effect → reduces sensitivity
  • Applied by dentist as topical application
  • Occludes dental tubules
  • Semi-permanent, eventually removed by brushing & dietary acids
• Glass ionomer
  
  • Hard restoration used to treat gum recession
  • Permanently bonded to tooth
  • Dentine tubules occluded
  • Much more resistant to wear
• More extreme clinical treatments - root canal therapy, extraction.

Question 23

how does exposed dentine become sensitive?

Answer 23

1. A trigger causes dentinal fluid to flow OUT of the dentinal tubules.
2. This results in a pressure change across the dentine.
3. This causes activation of intradental nerves