Teeth are the hardest substance in the human body, achieving remarkable strength and durability through a specialized chemical makeup. This structure relies on a significant concentration of mineral compounds. While teeth and bone share components, teeth are chemically distinct, notably lacking the ability to regenerate after damage. The capacity of teeth to withstand immense chewing forces and resist degradation is directly tied to the specific chemical architecture that varies across the tooth layers.
The Defining Chemical Structure: Hydroxyapatite
The foundation of a tooth’s hardness is the highly stable mineral, hydroxyapatite. This compound is a crystalline form of calcium phosphate and is the primary inorganic material in all calcified tissues, including bone. It is composed of calcium ions, phosphate groups, and hydroxyl groups.
Hydroxyapatite forms dense, microscopic, hexagonal crystals. The density and crystalline nature of these structures provide the rigidity and resistance to compression that teeth require. Trace elements, such as magnesium, sodium, and chlorine, can be incorporated into the crystal lattice, affecting its solubility. For instance, incorporating fluoride ions creates fluorapatite, a compound more resistant to acid dissolution than pure hydroxyapatite.
Chemical Differences Across the Tooth Layers
The physical layers of the tooth—enamel, dentin, and cementum—all use hydroxyapatite as their main mineral component. However, they differ significantly in the proportions of mineral versus organic material, which dictates the unique mechanical properties of each layer.
Enamel
Enamel, the outermost layer covering the crown, is the most highly mineralized tissue in the body, accounting for its exceptional hardness. It is composed of approximately 96% inorganic material, primarily hydroxyapatite crystals. This high mineral content makes enamel extremely hard but also brittle, and it lacks the ability to self-repair once fully formed.
Dentin
Located beneath the enamel and cementum, dentin forms the bulk of the tooth structure and is substantially less mineralized. Dentin is composed of about 70% inorganic material, 20% organic matrix, and 10% water. This lower mineral percentage gives dentin greater flexibility compared to enamel. This flexibility prevents the overlying, brittle enamel from fracturing under the stress of biting and chewing.
Cementum
Cementum is a thin, bone-like layer that covers the tooth root and helps anchor the tooth to the jawbone. Its chemical composition is similar to bone, consisting of around 65% inorganic hydroxyapatite, 23% organic matrix, and 12% water. This ratio makes cementum softer than both enamel and dentin, allowing it to function as a mineralized connective tissue.
The Importance of Organic Components (Collagen and Water)
The organic matrix and water are important for providing resilience and preventing failure, balancing the rigidity provided by the mineral component. The organic matrix is concentrated in the dentin and cementum layers, as it is largely absent in mature enamel.
The main organic protein in dentin and cementum is Type I collagen, making up about 90% of the organic matrix. This collagen forms a dense, three-dimensional scaffolding that acts as a flexible framework for the hydroxyapatite crystals. The collagen matrix provides tensile strength and elasticity, allowing the tooth to absorb and distribute mechanical forces.
Non-collagenous proteins, such as phosphoproteins, are also present and help control the mineralization process and crystal growth. Water makes up a significant part of the dentin structure, occupying space within microscopic dentinal tubules. This water content contributes to the overall hydration and shock-absorbing characteristics of the tooth.