Caries Management - Science and Clinical Practice. Группа авторов

Oral Microorganisms

       Colonization of the Mouth in the Newborn

       Plaque: Development and Metabolic End Products

       Plaque Stagnation Areas

       Plaque Composition and Structure in Stagnation Areas

      In simplified terms, dental caries develops because certain bacteria in the oral cavity ferment carbohydrates (sugars) into organic acids,¹ which in the case of lactic acid may result in dissolution of dental hard tissue.² However, in reality the etiology and pathogenesis of caries are much more complex and will be comprehensively discussed in the following chapters. All oral tissues, especially the dental hard tissues, microorganisms, and the saliva interact not only in the physiology of the oral cavity, but also in the caries process. Therefore it is important to know their composition, structure, and functions to understand the caries process.

      This chapter will deal with basic knowledge about the oral cavity focusing on the teeth, saliva, and oral microbiology, primarily from the perspective of caries disease. The subsequent chapters will build further on this knowledge. Age-related changes in dental hard tissue as well as in the salivary glands will also be touched on, as will related diseases and conditions other than caries.

      In particular this chapter will cover:

      • the structure of teeth,

      • the functions of saliva,

      • changes in the dental hard tissues and saliva with aging,

      • dental plaque and its role in caries, and

      • the interaction between tooth structure, saliva, and plaque in the oral cavity.

      Teeth

      The structure of the coronal part of the teeth is as follows.³ The enamel is the outermost layer covering the dentin, which in turn covers the pulp (Fig. 1.1). In the roots the outer layer consists of cementum, covering the dentin, which covers the pulp.

      Fig. 1.1 Hemi-sectioned molar showing the major components of the tooth. The dentin forms the bulk of the tooth and encloses the pulp chamber and root canals. The enamel covers the dentin in the coronal part of the tooth and the cementum covers the dentin in the roots. EDJ: enamel–dentin junction.

      Tooth Development and Tooth Emergence

      Human beings have two sets of teeth: those belonging to the primary dentition, and those belonging to the permanent dentition. The conditions influencing the start of mineralization of the individual teeth, when the crowns are formed, the time for eruption, and when the roots are fully formed were mapped during the first half of the last century.⁴ Teeth start to develop late in embryonic development. The first tooth type to erupt is most commonly a primary incisor in the lower jaw, which normally happens when the child is 6–8 months old (Fig. 1.2a). All teeth in the primary dentition are fully erupted when the child is about 2½ years old,⁶ and approximal contact between first and second primary molars is seen about 1 year later.⁷

      The first permanent teeth to erupt are either the central incisors or the first molar teeth; this happens in about 90% of children between 5 and 6 years of age.⁸ The last permanent tooth to erupt is the third molar, which happens at the age of around 18 years. Thus, during a period of 18 years, different teeth erupt into the oral cavity, and between the ages of 5–6 and 12 years the child has a mixed dentition consisting of primary as well as permanent teeth (Fig. 1.2b).

      Macromorphological Terms

      Professionals know where caries develops: in the primary dentition it develops mainly on the approximal and occlusal surfaces and occasionally on smooth surfaces along the gingival margin; in the permanent dentition it develops primarily on the occlusal surfaces, foramen cecum, and later, on approximal surfaces. In the elderly, caries also develops on root surfaces. The following paragraph will describe macromorphological terms related to these caries-prone sites of the teeth.

      Occlusal Surfaces

      In a simple model, Carlsen (1987) suggested dividing the crowns of teeth into lobes—from one (e.g., incisors) to five (e.g., some molars) in number.³ Often molar teeth have five lobes, each with an essential cusp. Three of them (Fig. 1.3a) are the facial lobes, namely the mesiofacial, centrofacial, and distofacial lobes, which are separated on the occlusal surface by the mesiofacial and distofacial interlobal grooves. These interlobal grooves run down to the facial surface. In particular, the mesiofacial interlobal groove can end cervically in a (sometimes deep) tract called the foramen cecum.

      The remaining two lobes are placed lingually: the mesiolingual and distolingual lobes separated on the occlusal surface by the lingual interlobal groove. The facial lobes are separated from the lingual lobes by the mesial and distal interlobal grooves. Where the interlobal grooves meet, a tract called the fossa arises. Thus molar teeth often have at least three fossae: the mesial, central, and distal fossae (Fig. 1.3a). On each lobe there are also several intersegmental grooves. On the marginal ridge, particularly in molars, grooves termed margino-segmental grooves run downward along the approximal surface. Premolars have normally two lobes, one buccal and one lingual, separated by the mesiodistal interlobal groove.

      Fig. 1.2a,b Development and growth patterns of the teeth in both dentitions.⁵

       Fig. 1.3a,b

      a Occlusal aspect of a permanent first molar. MF, CF, and DF are the mesiofacial, centrofacial, and distofacial lobes. The ML and DL are the mesiolingual and distolingual lobes. Black arrows point to fossae areas and red arrows point to margino-segmental grooves.

      b Lingual aspect of a first permanent molar. The cervical enamel line/cemento-enamel junction separates the crown from the root. FC: location of foramen cecum.

      The total number of grooves, intersegmental grooves, and fossae on the occlusal surface are termed the “groove–fossa system,” replacing the classical term “pits and fissure system.” To build a bridge between the two classification systems, it has been suggested that the groove–fossae systems can be fissurelike or groovelike, where “fissurelike” is defined