The durability of human teeth often leads to comparisons against common engineering materials like steel. It is reasonable to ask if the hardest substance in the human body can stand up against something so robust. The answer requires understanding how hardness is measured and the complex, layered structure of the tooth.
The Direct Comparison
Comparing the hardness of a tooth to steel requires a relevant measurement scale. Hardness is typically measured on the Mohs scale, which assesses a material’s resistance to scratching, or the Vickers scale, which measures resistance to indentation. Tooth enamel, the outer layer, registers approximately 5 to 6 on the Mohs scale. This ranking places enamel above many common metals and alloys, including mild steel used in structural applications, which typically scores around 4.5.
The comparison changes when looking at specialized metals used in manufacturing. Certain hardened tool steels, engineered for extreme toughness, can achieve a Mohs hardness of 7.5 or higher. While enamel is harder than the steel found in a simple utensil or common nail, it is not harder than every type of steel in existence. Enamel is surpassed by some of the most advanced, artificially hardened metal alloys.
The Architecture of Tooth Hardness
The hardness of the tooth’s outer layer comes from its unique composition and structure. Enamel is composed of 96% to 98% inorganic material, making it the most mineralized tissue in the body. This mineral content is primarily hydroxyapatite, a crystalline form of calcium phosphate. The high concentration of these densely packed crystals accounts for enamel’s resistance to physical deformation.
The hydroxyapatite crystals are organized into millions of microscopic structures known as enamel rods. This organized, columnar arrangement provides the strength needed to withstand the immense forces generated during chewing. Beneath this hard, brittle enamel is dentin, a slightly softer tissue that forms the bulk of the tooth. Dentin contains a higher proportion of organic material, which gives it flexibility and allows it to act as a shock absorber.
This combination of a hard outer shell (enamel) supported by a yielding foundation (dentin) grants the tooth its overall durability. Without the underlying dentin, the highly mineralized enamel would be susceptible to catastrophic fractures under biting pressure. Dentin prevents the enamel from shattering by dissipating mechanical stress across a larger area.
Why Teeth Still Fail
Even with its superior hardness, the tooth’s integrity is not permanent. The primary threats to a tooth are often chemical, specifically acid exposure, rather than mechanical force. While enamel resists physical wear, it is susceptible to a process called demineralization.
Acids, which can originate from dietary sources or from gastric reflux, dissolve the hydroxyapatite crystals. This chemical erosion begins when the pH level in the mouth drops below 5.5, leaching the minerals out of the enamel structure. The enamel becomes porous and weakened, making it easier for bacteria to penetrate or for mechanical forces to cause damage.
Mechanical wear, which includes attrition and abrasion, is a separate factor contributing to failure. Attrition is the wear caused by tooth-on-tooth contact, such as grinding, while abrasion is the wear caused by external objects like aggressive brushing or chewing on hard items. However, the most constant source of tooth failure is the chemical attack that compromises the hard structure.