Bone extracellular matrix (ECM) composition is best described as which mix?

• A. 23% organics; 77% hydroxyapatite
• B. 77% hydroxyapatite; 23% organics
• C. 50% hydroxyapatite; 50% organics
• D. 65% hydroxyapatite; 35% organics

Answer: (B)

Bone extracellular matrix is a mineralized composite where the inorganic mineral phase provides stiffness and compressive strength, while the organic matrix, mainly collagen, supplies toughness and resilience. In the dry weight of the ECM, the mineral part is the larger portion, typically around three-quarters, with the organic portion making up about one-quarter. So describing the mix as 77% hydroxyapatite and 23% organics fits the idea that mineral content dominates the ECM. This balance explains why bone is strong yet able to absorb some energy, and it matters for how dental materials interact with bone, including bonding and remodeling at the interface. Descriptions that reverse the proportions, making organic the majority, don’t align with the true composition of bone ECM.