The human body contains diverse tissues, including bones and teeth, both known for their firmness and strength. This often raises questions about their relative toughness and ability to withstand daily forces. Understanding the science behind these structures reveals their unique properties and functions.
Direct Comparison of Hardness
Tooth enamel, the outer layer covering the tooth crown, is the hardest substance in the human body. It allows teeth to withstand the forces of biting, tearing, and grinding food. While bones are strong and rigid, they are not as hard as tooth enamel. Dentin, the layer beneath the enamel, is softer than enamel but harder than bone. This arrangement provides protection and support within the tooth.
Structural and Compositional Differences
The hardness of teeth and bones comes from their unique compositions. Tooth enamel is predominantly inorganic, about 96% minerals, mainly crystalline hydroxyapatite. This high mineral content and the organized arrangement of hydroxyapatite crystals into tightly packed rods contribute to enamel’s hardness and durability. Enamel contains only about 1-2% organic material and 2-4% water.
Beneath the enamel, dentin forms the bulk of the tooth. It is less mineralized than enamel, composed of about 70% hydroxyapatite, 20% organic material (including collagen), and 10% water. This composition makes dentin harder than bone but more flexible and less brittle than enamel, supporting the overlying enamel.
In contrast, bone tissue is a composite of organic and inorganic components. Approximately 60-70% of bone’s dry mass is inorganic mineral, mainly hydroxyapatite, providing firmness and compression resistance. The organic component, largely collagen fibers, provides bone with flexibility and tensile strength, preventing brittleness. This blend allows bone to be strong yet resilient.
Functional Significance of Hardness
The differing hardness of enamel and bone relates directly to their biological roles. Enamel’s hardness protects the tooth’s inner parts from chewing stresses and acid damage. Once formed, enamel contains no living cells, blood vessels, or nerves, meaning it cannot repair itself if extensively damaged. While minor demineralization can be addressed through remineralization with saliva minerals and fluoride, larger chips or cracks require dental intervention.
Bones, with their lesser hardness and organic matrix, are designed for flexibility, structural support, and organ protection. Their composition allows them to absorb shock and resist fracturing. Unlike enamel, bone is a living tissue with specialized cells, including osteoblasts that form new bone and osteoclasts that break down old bone. This continuous process, known as bone remodeling, allows bones to repair themselves after fractures and adapt to mechanical loading. This enables bone to maintain its integrity and perform its functions.