Aquafresh Science Academy

Dental caries is a progressive condition characterized by demineralization and destruction of enamel and dentin.

A major factor in the causation of dental caries is plaque, a film that develops on the surface of teeth, made up largely of bacteria. Some of these bacteria, in particular Streptococcus mutans, efficiently metabolize carbohydrates from sugars in the diet to produce lactic and other acids, which demineralize the tooth surface. [Duckworth, 1993]

Caries sites

Caries is a process that attacks enamel, cementum and dentine, gradually destroying the tooth. The enamel is gradually broken down underneath the still-intact surface until the surface collapses and an open cavity is formed.

This results in dental cavities which can occur at a number of sites:
[Collins, 1992; Clarkson, 1991]

1. Pit-and-fissure caries develop initially in the fissures of the teeth, but can spread into the dentine, causing complete breakdown of the tooth.
2. Smooth-surface caries are most common on the interdental surfaces, but can occur on any smooth surface of the tooth.
3. Root caries attacks the dentine directly, which becomes exposed as gingiva recede. Root decay is a widespread problem in adults and appears to be increasing. This may be because more and more adults are retaining their own teeth into old age. Also, investigation and treatment of periodontal disease exposes the root surface, making the root more susceptible to bacterial attack.

The role of pH in tooth decay

pH is a measure of the acidity or alkalinity of a solution where 1 is highly acidic and 14 is highly alkaline. If the plaque pH falls below about 5.5 (e.g. when plaque bacteria produce acid), the tooth enamel begins to dissolve (demineralize). Persistent acid conditions from plaque metabolism will lead to the formation of a carious lesion. Under normal oral conditions closer to pH 7 (neutral), such as periods between meals, enamel tends to re-acquire mineral ions (remineralize) and minor carious lesions may be repaired.

The process of tooth decay

Tooth decay occurs in five stages: [Collins, 1992; Clarkson, 1991]

1. Acid from plaque dissolves some of the hydroxyapatite mineral out of the surface of the enamel. Demineralization can then extend down towards the enamel-dentine junction whilst the surface remains intact. At this time, the appearance of the lesion resembles a white spot. The ‘white spot’ lesion can undergo several changes. It may become a 'brown spot' lesion, an arrested lesion, or a partially remineralized lesion. It may occasionally even disappear as a 'caries reversal'.
2. However, it may also progress to a cavity. As the decay spreads through the enamel, it gradually leaves the surface less well supported. Eventually, the enamel caves in, leaving a break in the surface.
3. If the lesion is left untreated, the decay
   may enter the dentin.
4. If it remains untreated, the decay may
   continue to spread inwards, down the
   dentinal tubules, until it reaches the pulp.
5. If the decay spreads down into the pulp,
   an apical abscess may form, which can
   be extremely painful.

View an animation depicting caries development

 

Streptococcus mutans

A newly cleaned tooth surface is rapidly covered with a glycoprotein deposit referred to as 'pellicle'. The pellicle is derived from salivary constituents, which are selectively adsorbed onto the tooth surface. Major components of the dental pellicle include phospho-proteins (such as statherin), proline-rich proteins and mucins. The formation of pellicle is the first step in dental plaque formation. Dental plaque can be defined as a dense and complex microbial community, in an extracellular matrix, growing on the surface of teeth. Streptococcus mutans is an important plaque organism. It is carried by virtually everyone and is highly acid-tolerant. It is generally accepted as a leading cause of dental caries worldwide and is considered to be the most cariogenic of all of the oral streptococci. S. mutans sticks to the surface of teeth and subsists on a diverse group of carbohydrates. While metabolising sugar and other energy sources, the microbe produces acid that can cause caries in teeth. S. mutans was first described by JK Clark in 1924 after he isolated it from a carious lesion, but it was not until the 1960s that real interest in this bacterium was generated, when researchers began studying dental caries in earnest.