Human Tooth ~ Too Common yet so Unique

Real Life Samples Taken from KJM hospital, Mumbai

Do you know that Human tooth is considered as the strongest component in a human body?

Perhaps this information you read in standard 6 while attending the general knowledge classes. Fortunately I got the opportunity to test the same during my under graduation studies. The following blog is little bit different from general entertainment flicks. In this post I have tried to explain human tooth and its properties by looking at them with the perspective of a Materials Science engineer.

Human tooth is indeed the strongest component in a human body but what fascinated me the most was its unique biomechanical properties exhibited at the dentin enamel junction commonly known as DEJ. Have you ever experienced chipping of the tooth. Yes, chipping of the tooth, not breaking of the human tooth because it is an arduous task to break a human tooth into two halves. In most of the cases tooth gets chipped from the top because cracks cannot propagate further inside the tooth due to the DEJ. I realized the fact and the reason when I was just browsing through a highly acclaimed scientific journal, Nature Publication. I came across a paper by V. Imbeni, J. J. Kruzic, G. W. Marshall, S. J. Marshall, and R. O. Ritchie (The names are according to the order appeared on the journal paper). They published the result of the experiments done on the adult human tooth to explain the unique biomechanical properties of human tooth. They found the toughness and hardness variation of the human tooth on moving away from the DEJ. If you move outwards from DEJ then obviously you are in Enamel section so you will have harder material, whereas if you are moving from DEJ towards tooth root then you are in Dentine which has lower hardness as compared to hard enamel.

fig 1: Hardness Variation from DEJ, positive axis represent
dentin whereas negative side represent enamel.

Figure 1 showing the variation of hardness with variation in distance from DEJ. As you can see hardness drastically drops on reaching the dentin region but toughness properties are superior which makes it soft as well as tough material. Human tooth is indeed a beautiful specimen at microscopic range. It has lots of beautiful pores as shown in figure 2 filled with a material which is present in our bones, you guessed it right, that material is none other than a compound of calcium and phosphorus which is commonly known as Hexa Apetite.

fig 2: SEM images of various sections of human tooth
(a) Dentin, (b) Enamel, (c) DEJ.

I really feel lucky sometimes when I think about my past. Got through JEE and studied in one of the internationally acclaimed university known as Indian Institute of Technology, Bombay commonly known as IIT and widely appreciated by IIT brand name. It gave me such a great opportunity to work with some of the high end instruments; Atomic Force Microscope is one of them. AFM images of human tooth reveal the surface morphology of the enamel and the dentin. The uneven surface due to acid etching clearly comes out when we get the result from AFM. Etching of an inorganic material is done with the help of an acid so we used Hydro-Phospheric acid to etch the mineral part of the human tooth. Figure 3 show the result of the AFM where the uneven pits formed due to acid etching which can be observed in the form of hills and valleys. It is indeed easy to represent data on paper and making comment on them but when it comes to do the experiment then you realize the real hardship behind it. It took me several weeks to get the two pictures which you can see at the bottom. It was not only hard to do the setup but it was difficult to get an appointment from the lab instructor. Yes, it took me around three months to test my samples and by that time you feel exhausted because half of your work is stuck in between due to awaited results.

fig 3: AFM images of various sections of human tooth (a) Dentin, (b) Enamel.

Indeed if I do not show the beautiful and magnificent images of cracks formed because of the Vickers Hardness Test then it would be unfair from my side. Moreover toughness test revealed why we get chipped tooth and not a broken tooth when we fall on our mouth. Figure 4 are the optical images of the enamel near DEJ on human tooth. The colorful picture is due to optical diffraction happening inside the optical microscope. If you observe the images closely then you would notice that cracks do not impinge the DEJ rather they move sidewards as if avoiding the movement inside dentin. The reason as I said earlier is the supreme and yet unique biomechanical quality of DEJ which prohibits the crack propagation inside the dentin.

fig 4: Optical Microscope Images of various indentation on human tooth
showing the direction of prorogation of crack near DEJ regions.

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