Caries progression and dentine changes

Caries Progression and Dentine changes



Caries Progression
Caries progression is caused by an imbalance favouring demineralization. The magnitude of this imbalance will in turn affect not only the progress, but it will also determine the rate at which the caries progress. Naturally, a larger imbalance would imply a faster progress of caries while a smaller imbalance will lead to a much slower caries progression
Imbalance
Caries progression is a continuum resulting from many cycles of mineralization and demineralization. It begins at the atomic level on both the enamel and dentine. Enamel and dentine contain carbonate and hydroxyapatite that are acid soluble and result in loss of mineral structure when exposed to acids

Pathological factors that favour the progression of caries are:
· Acid producing bacteria
· Frequent eating/drinking of fermentable carbohydrates
· Subnormal saliva flow and function

Protective factors that reverse or slow the advancement of caries are:
· Saliva flow and components
· Presence of fluoride with calcium and phosphate
· Antibacterials (chlorhexidine and xylitol)

Earliest clinical sign that is seen by the human eye is a white spot lesion and it itself takes months to occur. Eventually, this results in the formation of cavitation (Featherstone JD, 2008, p. 286-291)

Here, is a short video showing the 5 stages of caries progression
http://www.youtube.com/watch?v=fAiu0_-uTYU



Dentine
Before we can truly appreciate the changes that dentine undergo due to caries, it is necessary for us to first understand the dentine structure of a healthy tooth. Then only can we truly understand and comprehend both the histological and physiological changes that a dentine undergoes from caries.

Dentine
  • Dentine is one of the calcified connective tissue within the body. The inorganic part of it is made out of calcium hydroxyapatite. The inorganic part is deposited on a type 1 collagen framework radiating from the pulp to the enamel. The collagen forms fibrils that have many cross-links to provide a stable framework that is resistant to degradation

  • Dentine comes from the cells of the dental papilla

  • Dentine is made up of very small (smaller than those found in enamel) 10 nm calcium hydroxyapatite crystals with a carbonate ion in 1 out of 5 the phosphates ions. The carbonate ions makes the dentine more susceptible to be dissolved by acids.

  • Half of the dentine by volume is made up of hydroxyapatite and it is arranged to form tubules that radiate outwards. Within the dentinal tubules lie an odontoblast process along with dentinal fluid (consisting of albumins, transferrins, tenascins and proteoglycans)

  • Dentine, unlike enamel, has the ability to react to the progression of caries due to the presence of odontoblasts. Odontoblasts can respond to irritation by depositing minerals in the dentinal tubules.

Table 1: Dentine by composition

Content Percentage present (%)
Inorganic molecules (calcium hydroxyapatite)
- Calcium
- Phosphorus
- Magnesium
- Sodium
- Chloride
- Ash
- Carbon dioxide
(75)

27
13
1
0.5
0.1
30
3
Organic molecules
- Collagen
- Citric acid
- Insoluble protein
- Mucopolysaccharide
- Lipid
(19-21)
18
0.2
0.2
0.2
Water 4-6

Figure 1 : Structure of Dentine
dentine structure


Table 2: Physical properties of dentine and enamel

Physical Property

Dentine Enamel
Specific Gravity 2.14 2.9 - 3
Hardness (knoop number) 63.9 296.1
Young’s modulus (GN/m2) 12 131
Compressive Strength (MN/m2) 262 76
Tensile Strength (MN/m2) 29 - 65 30 - 35
Rigidity Modulus (GN/m2) 0.62 -


Table 3 : Various Types of dentine
Type of Dentine

Brief Description
Primary dentine It is the most prominent dentine in the tooth and lies between the pulp and enamel. It is more mineralized than mantle dentine and has a more compact array of type 1 collagen fibres
Secondary (regular) dentine Secondary dentine is formed after root completion and the tooth is functional. It is responsible for the slow but incremental growth of the dentine (0.8 micrometer a day) in the direction of the pulp
Tertiary dentine (also known as secondary irregular dentine) Tertiary dentine refers to irregular secondary dentine and reparative dentine that is formed as a result of caries. It is produced at a rate of 2.9 micrometers per day (Wennberg et al, 1982)
Circumpulpal dentine The inner portion of dentine that is adjacent to the pulp and consists of thinner fibrils
Mantle dentine Mantle dentine is formed from the pre-existing ground substances that made up the dental papilla. It is only about 5-30 micrometres thick and is a result of the odontoblast process secreting calcium hydroxyapatite in the early stages of tooth development. It is considered part of primary dentine

Intertubular dentine This refers to the dentine that is found between adjacent tubules
Peritubular dentine This refers to the dentine that is found within the tubules
Interglobular dentine This refers to the dentine that is found between 2 globules (globules are the nuclei centres of crystallization)
Predentine This is the latest layer of dentine that has been formed (not to be confused with mantle dentine)





Dentine Caries
Dentine caries is differentiated into early and advanced carious lesions

Table 4: Information from Atlas of Endodontics (Rudolf Beer, et al, 2006)
Early Dentine Carious Lesions Advanced Dentine Carious Lesions
- Histological changes occur without enamel cavitation


- Plaque toxins diffuse through the enamel and cause secondary
(reparative) dentine formation


- Early accumulation of inflammatory cells


- If the cause of the caries is eliminated, partial regression of caries can occur at this stage
- Enamel cavitation has already occurred

- The bacterial destruction expands along the mantle dentine


- The lesion becomes more extensive, undermining the enamel

- Initially there will be a defence mechanism to impede the progress of caries. Later on the bacteria may progress unhindered

- 6 zones can be differentiated
1. Softening and liquefication with excavatable dentine
2. Demineralization with multiple areas of destruction
3. Advancing bacteria penetrate the dentine tubules
4. Hypermineralization
5. The transparent zone is clinically hard
6. Reparative secondary dentine forms on the pulpal walls


Figure 2: Carious dentine
carious dentine






Dentine Changes from caries
  • Caries progression is governed by the intensity of the bacterial acid challenge, the structure of the dentine and the response of dentine. Dentine caries is mainly the result of slow chronic caries

    Figure 3 : Location of dentine sclerosis
    dentine sclerosis

  • The acids produced by carious bacteria dissolve away the inorganic calcium hydroxyapatite crystals. As a result the crystals become smaller and areas that are porous form

    Figure 4: Caries breaking down dentine and making it porous
    porous dentine


  • The increased porous nature of the calcium hydroxyapatite lattice and the dentine tubules allow the acids/bacteria to move further into the tooth and do extensive damage. The first group of bacteria is called pioneer bacteria

    Figure 5: Pioneer bacteria travelling into dentine tubules
    dentinal tubules with travelling pioneer bacteria

  • Underlying dentine is necessary to support the enamel. If the underlying dentine is degraded, it will cause the enamel to be easily broken.

  • Advanced dentinal chances include the decalcification of the walls and the joining of the tubules. The tubules get filled with necrotic debris and adjacent tubules get distorted

  • The exposed collagen fibres that form as a result of the acid attack converts reversibly to their precusors form. Therefore, remneralisation can occur the collagen fibres can reorganize and form a structural framework (Miyauchi et al, 1978)

  • However extensive damage to the collagen fibres results in permanent loss of the collagen framework and irreversible loss of the dentine structure. This is due to the fact that as the caries continues, the exposed collagen becomes increasing less resistant to the bacterial acids



Dentine Changes from reparative mechanism

Table 5: Different dentine reactions
Formation of secondary dentine
  • Dentine is made up of microscopic channels called dentinal tubules which radiate outwards to the cementum and enamel

  • The carious process continues through dentinal tubules to reach the depths of the tooth (The tubules actually allow caries to progress at a much faster rate)

  • As a result, immunoglobulins are carried inside the fluid (within the tubules) to fight the bacterial infection

  • Simultaneously, there is an increased mineralization in surrounding tubules to restrict the passage of caries by constricting the affected tubules

  • More calcium and phosphate are released to allow for more crystals to form a barrier deeper within the dentinal tubules and slow the advancement of caries (Summit, et al , 2001, p. 13)
Formation of tertiary dentine
  • Tertiary dentine refers to the additional dentine that is produced towards the direction of the pulp as a result of caries. It is produced in response to protect the pulp from the advancement of the caries

  • If the odontoblasts still remain it is referred to as reactionary dentine and if the odontoblasts are destroyed it is referred as reparative dentine

  • In reparative dentine, growth factors initiate production of reparative dentine by the cells of the pulp

  • Reparative dentine results in the production of irregular tubules, which diminishes the progress of the caries within the dentinal tubules (Summit, et al, 2001, p. 14)
Odontoblasts
  • Odontoblasts lie between the pulp and dentine and continually produce dentine at the dentinal-pulpal junction.

  • Presence of caries can trigger a biological defence mechanism that leads to the formation of sclerotic and tertiary dentine.

  • If the odontoblasts still remain it is referred to as reactionary dentine and if the odontoblasts are destroyed it is referred as reparative dentine
Odontoblast-like cells
  • When the odontoblasts die, pulpal cells are tasked with maintenace/repairing dentine.

  • Reparative dentine is produced from the newly differentiated odontoblast-like cells (that differentiated from pulpal cells)

Reversible vs Irreversible repair
  • Reversible damage
    The exposed collagen fibres that form as a result of the acid attack converts reversibly to their precusors form. Therefore, remineralisation can occur the collagen fibres can reorganize and form a structural framework (Miyauchi et al, 1978.)

  • Irreversible damage
    However extensive damage to the collagen fibres results in permanent loss of the collagen framework and irreversible loss of the dentine structure. This is due to the fact that as the caries continues, the exposed collagen becomes increasing less resistant to the bacterial acid




Areas for further research / queries / considerations/ controversies
(Featherstone JD, 2008)
  • Do the different type of dentine react differently to the carious progress? or do they as a whole react only in one main manner?
  • With regards to the earlier question, do people with dentinogensis imperfecta II/III display markedly different dentine changes ?
  • At what point does the collagen matrix stop having the abilty to repair/reform itself in order for dentine to regrow?
  • Does the dentine still continue to react even when the pulp has been exposed ? or does the exposure of the pulp signal the end of dentines resistance to the caries?
  • Is it possible to introduce fibroblast (that make type 1 collagen) or specific enzymes into teeth dentine and hope that they reasemble some of the collagen matrix and enable "irreversibly" damaged areas to regow?
  • Most importantly, how significant is the "S" shape of the dentine tubule in determining how dentists perform restorative treatments? Do they account for the "S" shape and drill according ? Or do they merely adopt a fixed approach without consideration for the tubules ?
  • More importantly, is it possible to intoduce chemicals into the pulp via the dentine? so as to intitiate the odontoblast to make more dentine, esp if the caries are rampant ?


References
Featherstone JD (2008). Dental caries: a dynamic disease process. Aust Dent J;53(3):286-91


Frencken JE, Holmgren C, and Mikx F. Atraumatic Restorative Treatment – How to organize and run an ART training course.WHO Collaborating Centre for Oral Health Care Planning and Future Scenarios, College of Dental Science University of Nijmegen.


Miyauchi H, Iwaku M, Fusayama T. (1978). Physiological recalcification of carious dentine. Bull Tokyo Med Dent Univ;25:169-179

Rudolf Beer, Michael A. Baumann, Andrej M. Kielbassa. (2006).Pocket Atlas Of Endodontics.Thieme Medical Publishers.


Summit, James B., J. William Robbins, and Richard S. Schwartz.(2001). Fundamentals of Operative Dentistry: A Contemporary Approach. 2nd edition. Carol Stream, Illinois, Quintessence Publishing Co, Inc.

Wennberg A, Mjör IA, Heide S. (1982). Rate of formation of regular and irregular secondary dentin in monkey teeth.Oral Surg Oral Med Oral Pathol;54(2):232-7





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