Histopathology of Caries

The histopathology of caries varies in both enamel and dentinal lesions. They are made up of several zones that differ in properties and appearance. Here we will discuss the zones present in these lesions.

Enamel Caries (Pre-cavitation)

• Normal enamel is composed of carious structures – enamel prisms (basic unit), prism borders, striations, incremental growth bands (Striae of Retzius) and the surface zone. The changes in these determine the lesion.
• Common sites – smooth surfaces (particular below contact points), in pits and fissures and recurrent around the edges of existing restorations.
• Lesions on smooth surfaces differ slightly from those in pits/fissures pre-cavitation.

Smooth surface Caries

• Site – usually below contact points
• Appearance – surface typically hard and shiny. Usually a white opaque lesion, sometimes brown spot lesion
• Macroscopically, as the lesion progress through the enamel, we see the following:
  • A conical lesion with the apex towards the amelo-dentinal junction (ADJ)
  • Lateral spread at the ADJ
  • A larger dentinal lesion
• These changes are thought to be due to a combination of loss of interprismatic material, roughening of enamel rods and enhancement of Striae of Retzius.

Pit/Fissure Caries

• Site – pits and fissures
• Appearance – commonly sticky brown/yellow lesions, may be chalky white lesions
• Macroscopically, as the lesion progress through the enamel, we see the following:
  • Lesion arising from fissure walls, not base
  • A conical lesion with the base towards the ADJ – the opposite of smooth surface, with the caries following the enamel rods
  • Lateral spread at the ADJ – more tubules involved as base is wider
  • A larger dentinal lesion
• These changes are thought to be due to a combination of loss of interprismatic material, roughening of enamel rods and enhancement of Striae of Retzius.

Microscopic Features of Enamel Caries

Within the enamel lesions described above, there are 4 zones identified pre-cavitation. These are:

1. Translucent zone – the advancing edge of the lesion
2. Dark zone
3. Body of the lesion
4. Surface zone

1) Translucent Zone

This is the advancing edge of the enamel lesion. It is only present in 50% of lesions.

• Pore volume of 1% (compared to 0.1% of normal enamel) – usually at prism boundaries and junctional sites
• Increased concentration of fluoride ions
• Preferential loss of magnesium and carbonate ions
• No protein loss

2) Dark Zone

Lies adjacent and superficial to the translucent zone. Present in up to 95% of lesions.

• Pore volume of 2-4% – smaller pores sit over the larger pores of the translucent zone. Briefly, medium can’t penetrate the smaller pores and so they remain air-filled. This leads to light being scattered when trying to pass through the zone on ground section examination, giving a dark appearance.
• Re-precipitaton of minerals lost in the translucent zone
• Variable thickness – a wider dark zone indicates a slower advancing lesion

3) Body of the lesion

The largest part of the lesion and lies between the surface zone and dark zone.

• Area of greatest demineralisation
• Pore volume of 5% (peripheries) to 25% (at centre)
• Prominent Striae of Retzius
• Appears transparent on assessment

4) Surface Zone

This zone appears almost unaffected in the superficial layers

• Minimal demineralisation (1-4%) until dentine is involved
• Pore volume of less than 5%
• Calcium and phosphate ions re-precipitate and remineralise from deeper zones
• Higher concentration of fluoride also favours remineralisation

Dentinal Caries

Dentine can be affected both pre-cavitation and post-cavitation of the lesion. Unlike enamel, the presence of alive odontoblasts allows for reparative and protective changes to occur. Due to lateral spread at the ADJ, a larger cavity can form very quickly (particularly larger than the apparent enamel lesion). The general shape is a triangle with the apex towards the pulp.

Pre-cavitation

• Acids diffuse through the porous enamel and reach the dentine
• This triggers the pulp to respond to ‘attack’
• 2 main zones form
  • Sclerotic zone  – advancing edge with peri-tubular dentine walling off the lesion
  • Body of the lesion – present between the sclerotic zone and the ADJ

Essentially, there is minimal evidence of bacteria within the dentine at this point – only acid. Any bacteria that have managed to gain access are the pioneering bacteria. These changes are more prominent in slowly advancing lesions.

Post-cavitation

This allows bacteria to readily access the dentine via the tubules. As a result, the lesion forms new zones:

• Sclerotic zone – still the advancing edge with peri-tubular dentine being deposited
• Body, divided into 3 further zones, from innermost to outermost:
  • Zone of demineralisation/decalcification – no/minimal bacteria are present, with acid attacks
  • Zone of penetration – bacteria lie within the dentinal tubules and multiply. This causes widening and softening of the tubules. Dentine typically becomes decalcified in this zone.
  • Zone of destruction – dentine decomposition – grossly discoloured with severe breakdown of tooth structure. There is evidence of liquefaction foci (collapse of tubules due to bacteria multiplying) and transverse clefts (expansion and joining of foci to form large necrotic areas of dentine)

Throughout this process, secondary dentine is also laid down. The quicker the lesion progresses, the more irregularly this secondary dentine is. As the lesion progresses closer to the pulp, the pulp begins to get inflamed and starts to become responsive with changes such as dead tract formation and tertiary dentine deposition. Proteolytic bacteria are more common deep in the lesion, whilst acidogenic are present more superficially due to the presence of carbohydrates.

Summary

• The histopathology of caries varies in pre-cavitated and post-cavitated lesions
• Enamel caries tends to occur on smooth surfaces or pits/fissures
• Pre-cavitated enamel lesions are composed of 4 zones – translucent zone, dark zone, body of lesion and surface zone
• Post-cavitated lesions have several zones in dentine – sclerotic zone and the body (divided in to zones of demineralisation, penetration and destruction)

References and Recommended Reading

• Single Best Answer Questions for Dentistry
• Essentials of Dental Caries
• Shafer’s Textbook of Oral Pathology