Dental enamel, as the most highly mineralised tissue in the human body, consists of 96 wt% inorganic materials, which are mainly organised in the form of carbonated hydroxyapatite crystals. Other components of enamel include remnants of the organic matrix and loosely bound water molecules [2]. Enamel has an acellular and avascular structure without the ability to regenerate or repair itself. These features reflect the exclusive processes, which occur during the formation of enamel. Enamel formation (amelogenesis) happens during three primary stages, namely, cytodifferentiation, matrix secretion and maturation.

Amelogenesis is initiated by secretion of the organic matrix from the ameloblasts (enamel-forming cells) into the extracellular space adjacent to the dentinoenamel junction (DEJ). This protein-rich matrix controls enamel biomineralisation by self-assembling into supramolecular assemblies that initiate, regulate and organise the precipitation and growth of the hydroxyapatite crystals. Enamel matrix proteins are noncollagenous proteins composed of hydrophobic amelogenins and non-amelogenin proteins including ameloblastin, enamelin and tuftelin [4]. At the time of mineral deposition, enamel proteinases, enamelysin (MMP 20) and Kallikrein 4 actively participate in the selective degradation and removal process of the protein-rich matrix [5]. Throughout the long maturation stage, the initially protein-rich matrix of enamel is broken down by proteases and replaced by mineral deposition onto the pre-existing apatite crystals, allowing them to grow in width and thickness. After the maturation stage, enamel-forming cells (ameloblasts) disappear thereby preventing any further enamel deposition or repair [5]. This makes the preservation and care of dental enamel of paramount importance.

The result of the maturation stage of amelogenesis is a very hard, highly mineralised tissue, which consists of extremely long high-aspect-ratio hydroxyapatite crystals, and small amount of organic components and water. Enamel crystallites are principally composed of calcium and phosphorus as hydroxyapatite (HAp), Ca₁₀(PO₄)₆(OH)₂, with traces of sodium, magnesium, chlorine, carbonate, potassium and fluoride. The organic matrix of mature enamel constitutes 1–2 % of total enamel and functions as the glue for the apatite crystallites [2].

Structurally, the HAp crystals are tightly packed in three levels of hierarchical organisation, which explains some aspects of enamel’s mechanical strength [6, 7]. At the largest structural level, bundles combining dozens of enamel rods form the Hunter–Schreger bands. These bands are approximately 50 μm wide and are visible because of the different directions that adjacent bands of prisms reflect or transmit the light [8]. At the next level, enamel rods (prisms) and interrods (interprismatic substance) are visible which represent the primary and fundamental structural units of tooth enamel. Enamel rods are 5–8 μm in diameter cylindrical-like structures [9] that run continuously through the enamel from the dentinoenamel junction (DEJ) to the outer surface of enamel. Close to the DEJ, they are initially in a tortuous course, which follows the movement of the ameloblasts during initial cell growth and enamel deposition [2]. The diameter of the rods almost doubles from dentinoenamel junction to the surface of enamel [10]. In longitudinal section, each rod is near cylindrical in shape and made up of tightly packed apatite crystals with their long axis approximately parallel to the longitudinal axis of the rod (Fig. 17.1). A thin rod sheath surrounds each rod and separates it from adjacent rods by an interrod substance [11]. The line of demarcation between rod and interrod enamel can be identified by the change in crystallite orientation [2].

In cross section, enamel rods have a fish-scale- or keyhole-like pattern. The keyhole shape is made of a head and tail portions (Fig. 17.1). The head is rounded and defined by a thin proteinaceous rod sheath about 0.5 μm thick [2]. However, the rod tail is less defined and becomes continuous with the interrod enamel [9]. At the last structural level, the nanoscale level, each rod consists of extremely long carbonated HAp crystals which are thin ribbon-like structures with around 60–70 nm wide and ∼20 nm thick [12]. In the central part of the rod, the apatite crystallites are arranged with their long axis (c-axis) parallel to the longitudinal axis of the rod, whereas in the tail portion they diverge from the longitudinal axis of the rod by up to 65° [8]. Each calcium phosphate unit cell in the maturing enamel appears hexagonal. However, in fully mature enamel, the crystal outline looks more irregular due to the interaction with other growing crystals during the last stages of crystal growth.

Upon tooth eruption, although most of the enamel organic matrix is removed during mineralisation and maturation by the activity of enamel proteinases [5], some proteins are retained between the HAp crystallites and in the rod sheath [13]. In fact, some of the matrix proteins are resistant to protein degradation due to their strong binding to the HAp crystals and are retained in the mature enamel. These matrix proteins remain trapped between the crystallites and around the rods in mature enamel. This remnant matrix of mature enamel is a multicomponent protein/peptide mix formed by the association of different monomeric proteins [14]. Those proteins lying between crystallites have the function of ‘gluing’ HAp crystallites together, maintaining the hierarchical structure and enabling enamel to sustain the mechanical loading during function. The presence of remnant matrix proteins in mature enamel has been shown to influence its optical and mechanical properties. The minor components of enamel, protein remnants and water, have a profound plasticising effect [9]. Enamel is now known to be much more flexible and softer than its major component, crystalline HAp [15]. Structural changes in these protein components can reduce the ability of enamel to dissipate stresses [16]. In the later parts of this chapter, the effect of protein matrix modification of enamel on its mechanical response will be discussed. The presence of this organic matrix along with the mentioned hierarchical organisation of enamel has been shown to regulate the mechanical properties of enamel by forming a structure whereby flexural strength and elastic modulus decrease with increasing hierarchical dimension along with a change from linear–elastic to elastic–inelastic mechanical behaviour. This complex structural arrangement also makes enamel the hardest tissue in the human body [16]. Mechanical properties vary at different regions of enamel. The surface layer of enamel is stiffer, denser and less permeable than the rest of the enamel [8]. In spite of its hardness, enamel is extremely brittle especially when the underlying resilient dentin is lost.

Enamel is translucent and varies in shade from light yellow to grey white. At the incisal edge of a recently erupted tooth, enamel appears bluish white [11]. In cervical areas, enamel reflects the yellow colour of the underlying dentin, which determines the tooth colour along with the enamel thickness and translucency. The enamel translucency increases with age and therefore transmits the yellow colour of the underlying dentin and appears darker. Despite the fact that enamel is composed of tightly packed hydroxyapatite crystals, enamel is selectively permeable by way of the protein ‘glue’ structure to certain substances such as calcium and fluoride for remineralisation of demineralised enamel. Fluoride ions can penetrate the enamel from saliva and form fluorapatite crystal-rich layers at the surface, which are larger and more chemically resistant to bacterial acid dissolution [11]. Ions can penetrate enamel either from saliva, food and beverages, and then become incorporated into the interprismatic region or internally from the pulp through the DEJ and change its chemical composition [10]. In addition to small inorganic molecules, enamel is also permeable to larger molecules associated with stains and pigments. These large molecules diffuse through pores found at the prism boundaries and interprismatic enamel [8].

Dentin and pulp tissue are generally regarded as a single functional and histological unit, termed the dentin–pulp complex [2]. However, we will only consider dentin in this chapter, as the subject of this book is about calcium phosphates. Dentin is a highly organised biological structure composed of complex protein assemblies and organised mineral components, which collectively form a rigid and durable mineral-rich biocomposite [17]. Dentin is formed through the process of dentinogenesis, by odontoblasts that differentiate from ectomesenchymal cells of the dental papilla. The dental papilla primarily induces the formation of dentin until it is finally surrounded by secreted dentin, thus forming the dental pulp [2]. A complex group of synchronised biological events regulates the formation and maturation of dentin. The main stages of dentinogenesis include the cytodifferentiation of the odontoblasts, the formation of mantle dentin, the control of mineralisation of the primary dentin organic matrix and, finally, the secretion of secondary and tertiary dentin. Primary dentin is the outermost layer of the dentin, which includes the mantle and the circumpulpal components. Secondary dentin is a layer that is secreted after the root formation. Tertiary dentin is the type of dentin that is secreted after full formation of the tooth, as a response to a stimulus, such as carious attack or wear.

Compositionally, dentin is a hydrated tissue comprised of approximately 50 vol% of carbonated hydroxyapatite minerals, 30 vol% of collagen and noncollagenous molecules and the remainder fluids. 90 wt% of the organic phase in dentin is almost exclusively composed of collagen type I, although other types of collagen have also been identified [18]. The remainder of the dentin organic matrix includes noncollagenous structures, of which proteoglycans (PG) are of a greater structural and mechanical importance [17]; other dentin matrix proteins, such as phosphoproteins and γ-carboxyglutamate-containing proteins, are believed to be more involved in mineral–matrix binding events [19]. Decorin and biglycan, two members of the small leucine-rich repeat (SLRP) family, are the PGs predominantly expressed in dentin. The most frequently found GAGs, in turn, are chondroitin 4-sulphate and a relatively lower content of chondroitin 6-sulphate.

Structurally, dentin is composed of packed dentinal tubules, measuring about 1–2 μm diameter surrounded by a hypermineralised layer called peritubular dentin, and a softer intertubular matrix, where the organic material is concentrated [20]. Dentinal tubules contain cytoplasmic extensions of odontoblasts, which lie in the pulp and originally were involved in the secretion of dentin matrix. In addition to this odontoblastic process, dentinal tubules contain dentinal fluid, which contains proteins and proteoglycans. The peritubular dentin is a highly mineralised composite material constituted of phosphorylated proteins [21, 22], proteoglycans and glycosaminoglycans [22], which lacks collagen fibrils [22, 23]. The intertubular dentin, on the other hand, is composed of supramolecular aggregates of collagen molecules, which are assembled into type I fibrils interconnected by noncollagenous components and water [24]. This array of organic molecules forms a hydrated organic network serving as a scaffold for the nucleation and growth of carbonated apatite mineral crystals [25]. The result of this organic–inorganic interplay is an intricate biocomposite that exhibits outstanding longevity and serves as a tough and resilient foundation for the brittle enamel, as well as a protective layer for the living pulpal soft tissue. This foundation prevents the propagation of catastrophic cracks from the brittle enamel further into the dentin [3, 26].

Dentin and enamel are bound strongly at their common interface, which is called the dentinoenamel junction (DEJ). Dentinoenamel junction is a hypermineralised area that appears like a well-defined scalloped area under the microscope. The presence and the form of this area have an important role in the structural and functional integrity of enamel and dentin. Due to the presence of dentinal tubules, dentin is far more permeable than enamel (Fig. 17.2).

Cementum is a mineralised avascular connective tissue, which is primarily composed of HAp minerals (65 wt%), with the addition of organic matrix (23 wt%) and water (12 wt%). The composition of cementum is very similar to bone. HAp minerals in cementum have a uniform small plate shape. The organic matrix is composed of collagen and noncollagenous proteins. Type I collagen fibrils comprise about 90 % of the organic cementum matrix. During cementogenesis, cementum is secreted by cementoblasts and after the completion of tooth development; it covers the roots of the tooth tightly in an interlocking manner. There are two types of cementum, which includes cellular and acellular cementum. Cellular cementum, which covers the apical third of the root, has an organised lacunar structure similar to bone and includes cementocytes. Acellular cementum covers the cervical portion of the root and has no organised structure and cells. It is assumed that cellular cementum anchors the periodontal ligament (PDL) fibre bundles and the acellular cementum has a more adaptive role [2]. Cementum, along with alveolar bone and periodontal ligament, forms the dental periodontium, which is the attachment apparatus of the tooth. Collagen fibrils within the cementum matrix play a crucial role in the attachment of periodontal ligament fibres to the tooth. The mastication forces exerted on the teeth are directed and distributed into the alveolar bone through these fibres.

In this section, we first discuss the basic mechanical properties of sound enamel, and later we will present the data on the contributing role of each structural element of enamel, including HAp, water and protein remnants in determining the mechanical behaviour of dental enamel.