How a tooth gets sick – tooth decay from the inside out

According to the "Oral Health Monitoring" survey carried out in 2010–2012, as many as 99% of Poles aged 35 to 44 suffer from it. By comparison, in Scandinavia only about 15% of society in this age group struggles with the problem. These figures concern tooth decay – the most widespread disease of the teeth in the world and the main reason for visiting a dental practice. Why is decay such a common phenomenon?

Tooth structure and decay

Understanding the way decay attacks and destroys a tooth first requires knowing how teeth are built. In dentistry, three basic parts of a tooth are distinguished:

• the crown (corona dentis) – the enamel-covered part of the tooth visible in the mouth,
• the neck (cervix dentis, collum) – the section of the tooth surrounded by the gum,
• the root (radix dentis) – the part covered with so-called cementum that anchors the tooth in the alveolar ridge.

The mineralised enamel covering the crown is almost entirely built of non-renewable inorganic components, which is why it is the hardest tissue in the human body. Yet for cariogenic bacteria even matter rated 5 on the Mohs scale is no barrier. Why?

The structure of enamel, visible only under a microscope, reveals its similarity to glass. Enamel is built of hydroxyapatite crystals, and the pores separating them are filled with organic components and water, which makes up 2–4% of the enamel.

That is why, despite the remarkable hardness of enamel, the moment cariogenic bacteria reach its surface they begin producing acids. It is the acids – and not the bacteria themselves – that are able to dissolve the hydroxyapatite crystals. This leads to the irreversible enlargement of the pores, that is, the start of a chain reaction: ever less tight enamel becomes an open gateway for bacteria into the interior of the tooth.

How does decay attack?

Once the enamel barrier is overcome, the bacteria attack the much less durable dentine that forms the core of the tooth. The hardness of dentine is already only 2 degrees on the Mohs scale, and organic components this time make up 20% of its composition. Degeneration of the tooth becomes an irreversible and ever-faster process, and the bacteria penetrate deeper and deeper – into the so-called cementum, the tissue that builds the root. This is the part of the tooth most susceptible to destruction because of its weak calcification. Although the cementum lies deep and fixes the tooth in the socket, it can become exposed, e.g. as a result of periodontal diseases or gum recession.

Between the dentine and the cementum lies the pulp – a heavily innervated type of connective tissue that fills the entire interior of the tooth cavity. It is divided into coronal pulp (filling the crown) and root pulp (filling the canal). Thanks to its rich blood supply and immune cells, the pulp plays an alarm-and-defence role, and it is the pulp that sends a signal in the form of pain. Most people go to the dentist only at this stage.

What is tooth decay and what are its four main causes?

Tooth decay is an infectious, transmissible disease of the hard tissues of the tooth, caused by acids produced by cariogenic bacteria that demineralise the inorganic substances in the tooth (enamel, dentine and cementum). At the same time the organic substances are damaged and pathogenic bacteria penetrate the body through the living pulp.

Many types of decay are distinguished: from the bottle caries that affects small children, through the insidious pit-and-fissure caries, acute (so-called wet) caries, chronic (so-called dry) caries, rampant caries, arrested caries, up to secondary caries. Each progresses differently, often at different stages of life. As part of dental prevention we have tools that make it possible to diagnose even so-called hidden caries. For decay to develop, however, four factors must occur simultaneously:

1. Bacteria. Pathogenic bacteria must enter the mouth (among others Streptococcus mutans, streptococci, L. acidophilus). This can happen, for example, during kisses or by using the same unwashed cutlery as an infected person. The bacteria produce acids while metabolising sugars, and an acidified environment promotes enamel demineralisation; they also contribute to the formation of dental plaque.
2. Sugars. Sugars must be present in the mouth – especially refined ones, as well as sucrose, glucose and fructose. They are the main food of plaque bacteria and increase acid production, further acidifying the plaque.
3. Tooth susceptibility. The teeth must be characterised by an increased susceptibility to this disease, determined genetically and environmentally.
4. Time. Time is needed for the bacteria to multiply.

Decay most often develops when the teeth are not cleaned frequently and thoroughly and when preventive treatment and check-up visits are neglected. A contributing factor is also an incorrect diet and, in some cases, disturbances in the secretion of saliva, which has a remineralising effect on small defects and limits the development of bacteria. If you do not know what is favouring decay in your case, the dentist will help identify that factor during the visit.

Symptoms and consequences of decay

The first symptoms of decay most often go unrecognised. Early enamel discolourations are hard to notice, and symptoms such as sensitivity to hot and cold food or an unpleasant smell from the mouth do not raise concern. Toothache is already a sign that the disease has attacked the deep parts of the tooth and reached the pulp.

The effects of neglected and untreated decay are not only pain and loss of teeth. Frequent complications are infections, abscesses, headaches and sinus pain, periodontal diseases and, in rarer cases, complications affecting the joints, kidneys or heart – because of bacteria entering the bloodstream. That is why dental prevention should be a permanent part of everyday health care, and early conservative treatment allows the disease to be stopped before root canal treatment or loss of the tooth becomes necessary.

Studies

1. Deep Caries Management: EFCD-ESE-ORCA S3-Level Clinical Practice Guideline

   Schwendicke F., Kosan E., Banerjee A., Baysan A., Bjørndal L., Ceballos L., Duncan H. F., Herbst S. i wsp. (2026) · International Endodontic Journal · S3-level clinical practice guideline

   In deep caries, strategies that preserve the pulp and selective tissue removal are recommended in appropriately chosen teeth, rather than aggressive excavation at any cost. Early lesions can often be arrested or remineralised, while cavities require clinical assessment.

2. Fluoride toothpastes of different concentrations for preventing dental caries

   Walsh T., Worthington H. V., Glenny A. M., Marinho V. C. C., Jeroncic A. (2019) · Cochrane Database of Systematic Reviews · Cochrane systematic review and meta-analysis

   Fluoride toothpastes prevent caries more effectively than non-fluoride ones, and higher concentrations usually give greater protection. In children, the amount and concentration should be matched to age and caries risk to limit swallowing.

3. Effect of Fluoride Varnish in Preventing Dental Caries of First Permanent Molars: A 24-Month Cluster Randomized Controlled Trial

   Wang Z., Rong W., Xu T. (2022) · International Journal of Environmental Research and Public Health · cluster randomised controlled trial

   A programme of regular fluoride varnish in children reduced the increase in caries compared with standard care. The effect was practical but did not replace daily brushing, diet control or check-ups.