Dentin is one of the toughest materials in the human body. Using synchrotron radiation, researchers at the Charité-Universitätsmedizin Berlin have revealed the nanostructures and interactions between its organic and inorganic components that give dentin its durability.
A bone-like substance, dentin comprises inorganic carbonated hydroxyapatite (cHAP) nanoparticles embedded in an organic matrix of collagen protein fibers. The mechanical coupling between the collagen protein fibers and the mineral nanoparticles enables dentin to withstand extreme forces.
Compression stress found within dentin can explain why damage or cracks in enamel don’t extend catastrophically into dentin. The researchers used samples of human teeth to measure how the nanoparticles and collagen fibers interact under humidity-driven stress.
“It was the first time we succeeded in precisely determining not only the lattice parameters of the cHAP crystals contained within the nanoparticles, but also the spatially varying size of the nanoparticles themselves. This also allowed us to establish the degree of stress they are generally able to withstand,” said Dr. Paul Zaslansky of Charité’s Julius Wolff Institute for Biomechanics and Musculoskeletal Regeneration.
The researchers used both laboratory experiments and measurements obtained using the Helmholtz-Zentrum Berlin’s synchrotron radiation source BESSY II, a device that produces radiation frequencies ranging from terahertz to hard x-rays.
During the experiments, the researchers increased the compressive stress inside the dentin samples, which also were dried by heating them to 125°C. The collagen fibers then shrunk as result, leading to huge stress exerted on the nanoparticles.
The ability to withstand forces up to 300 MPa is equivalent to the yield strength of construction grade steel and comparable to 15 times the pressure exerted during the mastication of hard food, which usually is well below 20 MPa.
Also, the heat did not destroy the protein fibers, suggesting the mineral nanoparticles also have a protective effect on collagen. Analysis showed a gradual reduction in the size of the cHAP crystal lattices as one moves deeper into the tooth as well.
“Tissue found near the dental pulp, which is formed during the later stages of tooth development, contains mineral particles that are made up of smaller cell units,” said Zaslansky.
The nanoparticle length shows the same trend, with the mineral platelets situated near bone on the outer parts of the root measuring approximately 36 nm in length, while those found near the pulp are smaller, only 25 nm long. This design could be used as a model for new materials development, including dental restoration materials.
The researchers noted that dentin’s morphology is more complex than they expected. While enamel is strong but brittle, the organic fibers in dentin appear to exert exactly the right pressure on the mineral nanoparticles required to increase the material’s repetitive, cyclic load-bearing capacity, as long as the tooth remains intact.
Bacteria that cause decay can soften and dissolve the material, though, and produce enzymes that destroy collagen fibers. Teeth then become more fragile and can break more easily.
“Our findings highlight an important reason for doctors to keep teeth moist during dental procedures, such as when inserting dental fillings or installing crowns,” said Zaslansky. “Avoiding dehydration may very well prevent buildup of internal stresses, the long-term effects of which remain to be studied.”