Article
At the dental school, the structure of enamel is taught in great detail, but has anybody ever explained why enamel has its wonderfully unique structure? Before I go there, let's remind ourselves about the functions of enamel. It is there to cut, tear and grind the food to aid digestion. It does this very effectively, and if you look after your teeth, they will last you a lifetime. No such claim can be made for any of the dental materials we use to replace enamel. They are always second best. And yet enamel is not a strong material, it has been reported to have a flexural strength in the region of 30–50MPa. This contrasts with zirconia where strengths can reach into four figures. So what is it that makes enamel such a durable material? In order to answer that question, we have to look at enamel, not from its biological or chemical perspective, but from an engineering perspective.
Enamel has a prismatic structure where the rods are always aligned perpendicular to the surface, and you might imagine this is for a reason. Well, the first rule of engineering is that the load will always follow the stiffest part of the structure. Thus, any load applied to a tooth surface is channelled down the enamel prisms into the bulk of the tooth and not around the outside. The load is dissipated into the underlying dentine rather than being spread around the brittle outer shell (Figures 1–3).
This is not what happens with our ceramic crowns, for example, where the load is channelled around the shell of the crown (Figure 3). The enamel prisms have a fish-like cross-section (Figure 2). This ensures that the prisms interlock and will not split apart when under load. In the head of the fish, the hydroxyapatite crystals are densely packed and aligned in the long direction of the prism, thus carrying the main load. However, in the tail, the hydroxyapatite crystals are turned through ninety degrees. This allows some of the load to be spread sideways, and avoids a severe stress concentration under the loading point. It also controls the stiffness of the enamel in the direction perpendicular to the long prism direction. This is vitally important at the enamel/dentine interface. If the stiffness of the materials differs parallel to the interface, then this creates shear stresses that will eventually destroy the interface. In the case of enamel, sufficient numbers of hydroxyapatite crystals are aligned in the direction perpendicular to the prisms to ensure that the stiffness of the enamel is identical to that of the dentine. Therefore, no differential shear stresses are generated at the enamel/dentine interface.
So there you have it, enamel is designed in such a way as to create the most benign stressing conditions that are compatible with its function as a hard-wearing outer layer of the tooth. Does that make it the smartest dental material? Well, I think so.