A lot of what I’m learning and doing is concerned with biological structures, their internal architecture and how this helps them to deal with the forces that they experience in general use. This week, I was introduced to teeth, which it turns out are more subtle than I gave them credit for. Yes they have enamel and slightly softer (well, less hard) dentine inside that, all surrounding the pulp cavity which houses nerves and blood vessels, but the whole tooth/truth is more complex than that: it’s all to do with keeping your teeth or jaw from falling to pieces when you accidentally bite your fork.
Both dentine and enamel in teeth are made of the protein collagen and the calcium mineral hydroxyapatite (just like bones), although enamel contains a far higher proportion of mineral – up to 96% of the weight of enamel is hydroxyapatite. Dentine, being less highly mineralised, is less hard than enamel.
Despite the extreme hardness (or elasticity, a related property) of enamel, it is not the same throughout the tooth (Fig. 1).
In fig. 1, only the enamel is coloured, with red representing regions of enamel which are more than twice as resistant to denting (deformation) than the darkest blue regions. The trend for decreasing hardness/elasticity from the tooth surface to the enamel-dentine junction (EDJ; the bottom limit of the coloured enamel) corresponds to a higher degree of mineralisation for the hardest parts of the tooth.
Interestingly, the trend is continued in dentine, with the lowest values of elasticity closest to the pulp wall, the thick black line at the bottom of fig. 1.
As we can see, the values for elasticity in dentine (Fig. 2) are about ten times lower than those for enamel (Fig. 1). The elasticity of dentine is similar to that of bone. The difference from enamel is due in large part to the lower mineralisation of dentine – a 1% decrease in the degree of mineralisation can reduce the elasticity of the material by 3 GPa (Cuy et al., 2002). In fact, highest values for elasticity of dentine are found in the centre of the dentine, with a very narrow band of slightly less elastic dentine right next to the EDJ (Angker et al., 2003).
The third mineralised part of a tooth is known as cementum and is found surrounding the dentine of the tooth root. This is slightly less hard than dentine and is connected to the soft and unmineralised periodontal ligament which lies between the tooth and the bone of the jaw (Naveh et al., 2012).
Putting all of the above information about different hardnesses/elasticity together, you get an image that looks like Fig. 3:
It basically shows all the different layers and sub-layers of the tooth. The dark red is enamel and the bright red is dentine, with the yellow in between being the soft region of upper dentine. The pale blue is the cementum, and the beige region is a softer region between dentine and cementum. The darker blue surrounding the whole of the tooth root represents the periodontal ligament.
Being the softest part of the tooth complex, the periodontal ligament deforms first when the tooth comes into contact with food, with the force being transferred from the hardest parts of the enamel down to the root. The precise shape of the root and thickness of the periodontal ligament at different points control how the tooth moves in the socket during a bite, which helps to dissipate force and helps prevent unexpected movements which could damage the tooth or cause it to loosen in its socket.
During harder bites the softer transition zones or interphases between the three mineralised materials act as shock absorbers for the tooth. These reduce stress build-up at distinct boundaries between enamel and dentine or dentine and cementum, which could easily result in the tooth cracking in two when you accidentally slam your mouth shut on your fork.
As humans, we only have enamel on the exposed part of our teeth (the crown). Animals which spend a lot of time chewing vegetation however, have enamel, dentine and cementum exposed at their crowns. The different hardnesses of these three materials result in them eroding at different rates and creating sharp ridges which are great for grinding down tough plant material to make digestion easier (see Fig. 4).
The ridges in the fossil mammoth tooth are easily visible, and the same principle is also seen in modern elephants, cows, sheep etc.
I hope you enjoyed that whistle-stop tour of a tooth, I can only apologise that it’s been so long since my last post – it’s been a little hectic this end but I hope to be back on track this week. We’ll have to wait and see!
Angker, L., Swain, M. V., & Kilpatrick, N. (2003). Micro-mechanical characterisation of the properties of primary tooth dentine. Journal of Dentistry, 31(4), 261–267. Retrieved from http://www.sciencedirect.com/science/article/pii/S0300571203000459
Cuy, J. L., Mann, A. B., Livi, K. J., Teaford, M. F., & Weihs, T. P. (2002). Nanoindentation mapping of the mechanical properties of human molar tooth enamel. Archives of Oral Biology, 47(4), 281–291. Retrieved from http://www.sciencedirect.com/science/article/pii/S0003996902000067
myfossilfind.com. Available from http://myfossilfind.com. Accessed 19:46 17/11/2013.
Naveh, G. R. S., Lev-Tov Chattah, N., Zaslansky, P., Shahar, R., & Weiner, S. (2012). Tooth–PDL–bone complex: Response to compressive loads encountered during mastication – A review. Archives of Oral Biology, 57(12), 1575–1584. Retrieved from http://www.sciencedirect.com/science/article/pii/S0003996912002488.