Teeth are primarily mineral in content. The enamel is rich in calcium and is more than 95% hydroxyapatite, while dentine contains less hydroxyapatite than enamel. There is in constant demineralization and remineralization in the oral environment and saliva. As such, optimum pH and minerals should be maintained for the processes to be balanced.

Types of dental caries

There four types of caries. These are pits and fissures caries found generally in posterior teeth and maxillary incisors, smooth surface caries found on all intact enamel surfaces of the tooth except those of pits and fissures, root surface caries occurring anywhere on the root surface and secondary or recurrent caries found on tooth surface adjacent to a restoration.

Causes of dental caries

Dental caries is an infectious and transmissible disease. Various factors play a role in the transmission of the cariogenic bacteria and the propagation of the colonies in the susceptible host. As with any infection, when the agent is transmitted, first be able to colonize the host before the disease can be established.

Caries and tooth decay more commonly occur in the rear teeth, which are pitted and fissured – the molars and premolars. The uneven surfaces of these teeth aid in chewing and mastication however, food particles get stuck in them. Since they are in the back of the mouth, they are harder to reach when brushing and so the bacteria in the mouth work on these trapped food particles, turning them into acids, thus dropping the pH of the mouth. When brushing is not done frequently and properly, plaque build up occurs and the acids in it attack the teeth enamel.

Mature tooth enamel is harder than deciduous tooth enamel, hence the increased incidence of dental caries in young children. Aside from this, diet and oral hygiene are other factors that contribute to this distribution.

The acids first attack the minerals of the enamel, the outer stronger barrier. Since the pH of the mouth has dropped and become acidic, tooth demineralization is hastened and remineralization is slowed down. Once the acids have eroded the enamel, cavities are formed. These cavities are the weaker points in the tooth surface and continued demineralization will eventually expose the inner, softer dentin to the acids. As dentin is less resistant to acids, the tooth decay process is hastened, especially when it is left untreated.

The acids will continue to eat away through the layers of the tooth, until finally reaching the inner material of the tooth –the pulp. Because of the opening, bacteria are also able to work their way into the pulp. The presence of acid and bacteria inside the tooth causes inflammation, which in turn causes the tooth pain. When this happens, most patients seek medical advice.

Enamel and dentin structures

To further understand dental caries, the compositions of the enamel and dentin must be understood.

The enamel is the hardest and most mineralized tissue in the body. It contains 96% inorganic material, most of which is the hydroxyapatite and calcium, and only 4% of it is organic material, which include water. The enamel has a protein –rich matrix, which allows the diffusion of fluids, ions, and small-sized molecules. In dentin, only inorganic material comprises only 70%, which is mainly composed of calcium phosphate. Unlike enamel, dentin contains collagen and is not as hard as enamel.

Amelogenesis, or enamel formation occurs before the tooth erupts, after the establishment of the dentin. There are 2 stages of enamel formation: the secretory stage, when cells, known as the ameloblasts, secrete enamel proteins, and the maturation stage, when the ameloblasts now transport substances used in the formation of the enamel. Before the tooth erupts, these ameloblasts are broken down hence there is no way to regenerate enamel once it is eroded.

Dental plaque formation

The formation of dental plaque occurs naturally and is of benefit to the host as it helps prevent teeth colonization by exogenous bacteria. The plaque has a constant composition of bacteria which is maintained by their constant synergistic and antagonistic activities. However, changes in the environment also affect the homeostasis of these bacteria, permitting the activity certain species over the others. For example, when there is prolonged exposure to an acidic environment, aciduric bacteria (e.g. Lactobacillus spp, Streptococcus mutans) flourish and produce more acids, thereby initiating enamel erosion.

The dental plaque is also supersaturated with calcium phosphate at normal oral pH. When the pH becomes acidic or drops below the critical level, it loses its calcium phosphate content, allowing further destruction of the enamel by the acids.

The cariogenic bacteria

Normal oral flora consists of various organisms, of which the Lactobacillus species and Streptococcus mutans are the ones of interest in caries development. When diet is rich in carbohydrates and sugars, these microorganisms ferment the available sources, producing lactic acid. This drops the pH enviroment in the mouth, promoting faster demineralization and slows down remineralization. This process leads to the formation of dental caries and tooth decay by weakening the enamel.

Mutans Streptococci (MS)

Studies have shown that MS play a major role in the transmission of dental caries. Out of all the bacteria, MS strain GS-5 shows the greatest sucrose-dependent adherence to smooth surface. Thus it is more able to colonize the tooth and this plays an important role in the development of caries. However, a study done in 2006 claimed that sucrose level in the diet is not correlated with the bacterial count.

Determination of levels of MS may be done using various tests. Levels detected by these test relate to the levels of bacterial colonization in teeth, and correlated with dental health status and morbidity.

Lactobacillus species (LB)

Lactobacillus species’ role in caries formation has been refuted. Instead, it is now known that LB levels in saliva corresponds to the prevalence of dental caries. LB is both acidophilic and aciduric, meaning, it thrives in acidic environment and it also produces acids, and so it aids in the demineralization of the enamel once a lesion has been formed. Increased bacterial count is seen in patients with clinical caries, although it is part of the “normal” flora. As such, tests for LB detection are not good for caries detection. Instead, LB tests are good indicators of cavitation, sugar intake, and oral health status since strong association has been noted between caries and high or increasing LB counts.