Human teeth withstand tremendous forces daily, yet their apparent strength can be misleading. The resilience of a tooth comes almost entirely from its outermost layer, the enamel, which is widely recognized as the hardest biological substance in the human body. This mineralized shield protects the softer internal tissues from the constant mechanical and chemical stresses of chewing and diet. Understanding this strength requires an examination of how scientists define and quantify hardness, and how this property interacts with the overall durability of the tooth structure.
Defining Dental Hardness
Hardness in materials science is defined as a material’s resistance to permanent indentation, scratching, and plastic deformation. For general mineral comparison, the Mohs scale of hardness is often used, which ranks materials from 1 to 10 based on which substance can visibly scratch another. While useful for broad context, the Mohs scale is qualitative and lacks the precision needed for dental research. To achieve technical accuracy, dental and material scientists rely on microhardness tests, such as the Knoop or Vickers scales. These methods involve pressing a precisely shaped diamond indenter into the surface of the enamel under a controlled load, and the resulting indentation’s size is measured to calculate a quantitative hardness number. This indentation method provides a specific measurement of a localized area’s resistance to deformation.
Enamel: The Hardest Substance in the Body
The exceptional hardness of human tooth enamel is due to its composition, which is approximately 96% mineral by weight. This mineral content is primarily a crystalline form of calcium phosphate known as hydroxyapatite, which forms a dense lattice structure. This high mineral density sets enamel apart from other hard tissues like bone, which contains significantly more organic material. On the Mohs scale, tooth enamel typically registers a value of around 5, placing it in the same range as materials like steel or titanium. When measured using the Knoop Hardness Number (KHN) scale, enamel values generally fall between 300 and 500. In contrast, the underlying dentin, which is about 70% mineral, is considerably softer, with a KHN value closer to 60 to 70. This makes enamel roughly five times harder than the layer immediately beneath it.
Hardness Versus Durability
Despite its high ranking on the hardness scales, tooth enamel is not indestructible, which highlights the distinction between hardness and durability, or fracture toughness. Hardness measures resistance to surface damage like scratching, while durability measures resistance to breaking or cracking under impact. The crystalline structure that makes enamel so hard also makes it relatively brittle, meaning it has a low fracture toughness. This brittleness explains why a tooth can still chip or fracture despite being harder than many metals. The tooth’s structure compensates for this limitation through the softer, underlying dentin. Dentin is less mineralized and more flexible, functioning as a shock absorber that prevents cracks initiated in the brittle enamel from propagating to the pulp. This dual-layer system ensures the entire tooth can withstand the massive compressive forces generated during chewing without catastrophic failure.
Factors That Reduce Tooth Hardness
The primary threat to the high surface hardness of enamel is chemical wear, specifically demineralization caused by acid erosion. Hydroxyapatite crystals begin to dissolve when exposed to acids that lower the mouth’s pH below a critical level, typically around 5.5. This process strips away the mineral content, weakening the surface layer. Dietary acids from common foods and drinks, such as carbonated sodas, fruit juices, and vinegar-based products, are frequent sources of this erosion. Stomach acid brought into the mouth during episodes of acid reflux or vomiting can cause rapid and severe demineralization, often affecting the inside surfaces of the teeth. Once this mineral loss occurs, the enamel’s intrinsic hardness is reduced, making it more susceptible to subsequent physical wear from forces like grinding or abrasive brushing.