The substance that withstands the most physical stress and abrasion in the human body is significantly harder than bone, despite bone’s role as the support structure. This superior material is not part of the skeletal system but is found on the exterior of our teeth, serving a unique and demanding purpose. The hardest substance the body produces is tooth enamel.
Identifying the Hardest Substance
Tooth enamel is the highly mineralized tissue that forms the outer layer of the anatomical crown of a tooth. Its primary location is strategic, as it is the only bodily substance in direct contact with the external environment during the mechanical action of chewing. Enamel can reach a thickness of up to 2.5 millimeters at the cusps, which are the points on the chewing surface of the molars and premolars.
The function of this tough outer shell is to protect the softer, more sensitive tissues beneath it, namely the dentin and the dental pulp. Without this durable coating, the constant forces of biting and grinding, as well as exposure to temperature changes and acidic foods, would rapidly wear down the inner tooth structure. Enamel’s immense durability allows it to endure decades of use, making it exceptionally well-suited for its mechanical task in digestion.
The Chemical Foundation of Superior Hardness
The hardness of enamel stems directly from its chemical makeup, which is unlike any other tissue in the body. Enamel is the most mineralized tissue, consisting of approximately 96% inorganic material by weight. The remaining small percentage is composed of water and trace amounts of organic compounds.
The dominant mineral component is crystalline calcium phosphate, hydroxyapatite. This hydroxyapatite is arranged in a highly organized, tightly packed crystalline structure that resists compression and abrasion.
This dense, inorganic arrangement is what provides enamel with a Mohs hardness rating of about 5, which is comparable to some types of steel. The minimal presence of softer organic material contributes to its strength but also to its brittleness compared to bone.
How Enamel Differs From Bone
While both enamel and bone contain the mineral hydroxyapatite, their structural organization and composition are fundamentally different. Bone tissue is a dynamic, living structure that is composed of only about 60 to 70% mineral content. The large remaining portion is a flexible organic matrix, primarily collagen, which provides bone with its characteristic resilience and ability to absorb impact without fracturing.
Enamel is an acellular and avascular tissue, meaning it contains no living cells, nerves, or blood vessels once it is fully formed. This lack of a living infrastructure is the main difference, as it makes enamel a rigid, non-flexible shield. Bone’s living nature and high collagen content allow for continuous remodeling and repair, a mechanism that enamel lacks.
Why Enamel Cannot Regenerate
The inability of tooth enamel to self-repair is a direct consequence of its acellular nature. The specialized cells responsible for producing enamel, known as ameloblasts, are present only while the tooth is forming before it erupts into the mouth.
Once the tooth is completely formed and emerges into the oral cavity, the ameloblasts are lost. Therefore, mature enamel lacks the cellular machinery necessary to lay down new tissue, repair cracks, or replace lost material. Unlike bone, which uses osteoblasts and osteoclasts for continuous maintenance, damage to the enamel requires intervention from a dentist. Minor loss of mineral content, however, can sometimes be partially offset by remineralization from saliva and dental products.