“Teeth rely on enamel, which is arguably one of the most damage-tolerant materials in all of nature,” says Arola. “We are using reverse engineering to try and interpret what nature has done.”
Cracks become confused
Considering it is one of the strongest materials in the body of most mammals, with the exception of specialized tissues such as horns, enamel is especially unique. Composed primarily of the mineral hydroxyapatite, enamel has a natural tendency to be brittle. Cracking is therefore not only common, but expected.
A sectioned lion tooth sample is loaded on the stage of a scanning electron microscope.
“Why would Mother Nature put this material in this region when it’s subject to so many cycles of contact?” asks Arola, who points out that since enamel is acellular, it also can’t repair itself. “The most fascinating thing about enamel is that you can see cracks in the tissue, but they don’t often enable fracture of the tooth.”
To investigate what prevents cracks from propagating, the researchers began by monitoring cracks microscopically. They found that cracks grow inward and are eventually arrested before reaching dentin, the layer directly underneath the enamel. They attribute this to decussation, the pattern inherent in the microstructure of the enamel.
“The crack is confused and blocked by the woven pattern of the enamel rods,” Arola explains. “The woven patterns are important — they are the secret behind this. The crack is trying to follow the path of least resistance, but that keeps changing so it starts undergoing bifurcation or forking.”
Exploring enamel patterns
Since the microstructure of enamel varies among species, the researchers were eager to learn which animals possess the most fracture-resistant enamel. To do so, they correlated the decussation patterns to the functionality of the tooth — in this case, the strength of an animal’s bite, called the bite force quotient. This also takes into account the animal’s body mass.
“We think there are predation requirements — what the animals with high bite forces are eating, along with how they acquire food, requires a higher fracture toughness,” explains Renteria, who received his Ph.D. in 2023 from MSE. “We believe that’s what causes different patterns in the decussation.”
3D segmentations from X-ray microcomputed tomography allow the researchers to view the unique orientation and angles of enamel rods across species. Pictured are the structure of enamel rods in a wolf (left) and lion (right). The purple rods (diazone) travel diagonally while the gold rods (parazone) run lengthwise.
Since molars are subjected to higher bite forces, the researchers obtained molar specimens for various animals including lion, African wild dog, gray wolf, black bear and snow leopard. Using high-resolution 3D imaging technology called X-ray microcomputed tomography, they could clearly see — for the very first time across these species — unique patterns in the enamel microstructure, particularly the angles and orientation of enamel rods. Distinct designs in enamel from animals with the highest bite force quotient, namely lions, African wild dogs and wolves, also became more evident.
“The enamel rods cross each other at a steeper angle, which does a better job of constraining cracks — it takes the crack more energy to keep moving,” explains Grimm. “An African wild dog might be biting into bone, while other animals are eating softer foods, so its enamel is designed more for crack growth resistance.”
“Imagine seeing a crack on the outer surface of an airplane and being told it will stop, the materials will not allow it to penetrate any further,” Arola says. “We don’t have engineering materials like that yet — that would be the holy grail.”