1
The Oral Environment and the Main Causes of Tooth Structure Loss

J. Kaidonis, G.C. Townsend, J. McIntyre, L.C. Richards & W.R. Hume

The human oral environment evolved within a Paleolithic (Stone Age) hunter-gatherer setting, reflecting well over 2 million years of evolution from primate origins of even greater antiquity. The interplay between genes and differing environments over many tens of thousands of generations resulted in various oral adaptations that together provided good oral health and function.

Common themes were evident among Paleolithic groups. First, dentitions showed extensive wear when compared to many modern societies, yet the teeth remained functional. Second, although the diet was generally acidic, erosion and the non-carious cervical lesions that are endemic in current populations were not apparent. Third, oral bacterial biofilms were present, yet the prevalence of dental caries and periodontal disease was so low that it could be considered insignificant.

With the advent of farming in human societies about 10,000 years ago and increasingly over the last 400 years, as food manufacture and distribution became more common, changes in our diet have led to profound changes in the oral ecosystem. These changes were sudden from an evolutionary perspective and they have been primarily responsible for the modern oral and many of the systemic diseases now afflicting human populations.

Among those diseases are several that lead to the loss of tooth structure.

The Human Oral Environment in Health

The sialo-microbial-dental complex in a state of balance

The human mouth is centrally involved in the first stages of digestion, as is the case in all species with alimentary canals. The need to chew and swallow a variety of foods in order to survive in different environments caused the evolution of oral and dental tissues, salivary secretions and a unique oral microbial ecology. These elements together can be referred to as the sialo-microbial-dental complex. ‘Sialo’ means to do with saliva. All elements were, until recent centuries, in relative harmony or balance in the great majority of individuals.

Tooth structure, diet, the oral microbial mix, saliva and biofilms1 (dental plaque) are closely associated and interrelated physically, functionally and chemically. Although each is described separately in the sections below, the components have evolved together and function as an integrated system. All contribute to health, and under some circumstances all contribute to disease.

Tooth Structure

Enamel

Dental enamel, the strongest substance in the human body, is a highly mineralized tissue with a well-defined structure. It is formed by the precipitation of crystals of apatite, a compound comprised of calcium, phosphate and other elements, into an extracellular protein matrix secreted by specialized cells called ameloblasts.

The precipitation of apatite is called ‘calcification’ or ‘mineralization’. It commences before the emergence of the tooth in the mouth. The protein matrix dissolves as the crystals grow, leaving a tissue comprised almost entirely of apatite crystals in a unique physical arrangement.

Of central importance to the understanding of the chemical dynamics relating to the inorganic components of tooth structure is the fact that ions can move in and out of apatite crystal surfaces, that is, the crystals can shrink, grow or change in chemical composition, depending on local ionic conditions.

Tooth shape is determined by ameloblast proliferation

Ameloblasts are derived from epithelial cells of the embryonic mouth. In a process that is closely controlled genetically, they proliferate into underlying tissue to create the enamel organ, which has a shape or form unique to each tooth crown, defining the shape of the future dentino-enamel junction.

The underlying tissue, which becomes the dental papilla, is derived from ecto-mesenchymal neural crest cells that have previously migrated into the region of the developing oral cavity. These mesenchymal tissues give rise to all of the other dental tissues: dentine, pulp, cementum and to the adjacent periodontium.

Differentiation of ameloblasts and odontoblasts, which are mesenchymally derived, results from a series of reciprocal interactions between the adjacent cells of the enamel organ and those of the dental papilla, mediated by various signalling molecules and growth factors among and between the two groups of cells.

Calcification and the formation of enamel prisms

Stimulated by the deposition of pre-dentine by adjacent odontoblasts, ameloblasts secrete an extracellular matrix of protein gel made up of amelogenins and enamelins. The local ionic environment of the extracellular fluid is supersaturated with calcium and phosphate, and the matrix proteins form an appropriate lattice for the precipitation and growth of apatite crystals.

Also sometimes called hydroxyapatite (HA), apatite is a molecule with the basic structure Ca10 (PO4)6 (OH)2 highly substituted with other ions, most notably carbonate in place of some phosphate ions, and fluoride in place of some hydroxyl ions. Strontium, sodium, zinc and magnesium may also be present in place of some calcium ions, usually in trace amounts.

Highly carbonated apatite is much more soluble than apatite with low carbonate, a significant factor in post-eruption enamel maturation as carbonate is replaced with phosphate from saliva by ion exchange. The degree of solubility of apatite also varies with the level of substitution between hydroxyl and fluoride ions. Extensive research has demonstrated that optimal levels of fluoride within apatite, most probably achieved also through ion exchange once in the oral environment rather than incorporation during initial formation, lead to the creation of enamel surfaces of very low solubility.

The apatite crystals coalesce within the gel matrix, their orientation being aligned to a strong degree, very probably because of orientation of the matrix protein molecules. The proteins dissolve as the crystals grow, creating a tissue comprised largely of apatite crystals.

As crystal growth continues, the ameloblasts continue to secrete matrix proteins and move away from the developing hard tissue, leaving long, generally parallel apatite crystals in arrays that form enamel rods, or prisms, with a relatively enamelin-rich boundary layer between rods corresponding to the interface between individual ameloblasts in the secreting layer. There is a change in the crystal orientation near the rod boundaries, with individual rods being connected by varying amounts of inter-rod crystallites (Figures 1.1 and 1.2).

Figure 1.1 The surface of a specimen of fractured enamel showing the enamel rods, which consist of bundles of enamel crystals. The rods lie relatively parallel with each other so there is a distinct ‘grain’ along which fracture of the enamel is likely to occur. Note also the spaces between the rods that will be filled with ultrafiltrate in life. Magnification x 4,800. Courtesy of Professor Hien Ngo.

Figure 1.2 (a) Scanning electron micrograph of enamel prisms showing the interprismatic space (yellow arrows) with a periphery of densely packed crystals in parallel arrays. Courtesy of Professor Hien Ngo. (b) Higher magnification scanning electron micrograph showing the space between the apatite crystals, which in life is occupied by an aqueous organic gel matrix, and their orientation. Courtesy of Professor Hien Ngo.

By the time permanent teeth erupt, the enamel is normally 96–98% carbonated HA by weight, and about 85% by volume. The remainder of the enamel, consisting of about 3% protein and lipid and about 12% water by volume, exists in laminar pores between the enamel crystals (see Figures 1.2 (a) and 1.2 (b)).

Each enamel crystal exists within a local hydration shell, and is in ionic equilibrium with it. It is within this local aqueous phase, which itself is in ionic balance with its adjacent environment, that the dynamics of post-eruption maturation will take place, and it is within this phase of enamel, influenced by the adjacent biofilm, that post-eruption demineralization and remineralization may occur under appropriate conditions of local pH.

Filtered tissue fluid moves very slowly outward through enamel in vital, erupted teeth because the pressure inside the tooth is higher than that outside. This tissue fluid is called ultrafiltrate and it contains no protein, only water and inorganic ions. Ultrafiltrate does not appear to be a significant factor in local ion dynamics near the enamel surface, very probably because of its very slow rate of outward flow.

Macroscopic structural elements affecting appearance and strength

Enamel is formed in an incremental manner and fine cross-striations may be seen within the prisms, representing daily increments of matrix production. Larger striations, the striae of Retzius, probably reflect a 7–10-day rhythm in the rate of matrix production and mineralization. Where the striae of Retzius reach the surface, mainly in the cervical region, they can produce distinct grooves or depressions referred to as enamel perikymata. These run circumferentially around the crown giving it a slightly rough surface texture and disperse reflected light in a way that gives the tooth its characteristic appearance.

The orientation of the enamel rods give the human enamel a...