Updated: Oct 12
The hardest substance in the human body is tooth enamel. You may be thinking that is impossible because you've chipped a lot of teeth but never broken a bone before. How can your enamel be harder than bone?
The answer lies in what they are made out of and how much of it they contain. Despite having the hardest substance in our body, it does not make it immune to fractures, chips, and cracks. This article was written to answer all of those questions.
Table of Contents:
What our enamel and bones are made of
Both our enamel and bones are composed of the same type of mineral called hydroxyapatite (Hap), which is the naturally occurring mineral form of calcium apatite. You may not know what hydroxyapatite is but the word calcium should ring a bell.
As you were growing up, you probably remember hearing about how you should drink milk to get calcium to help build strong bones and teeth. That was absolutely true and it still stands to this day because the calcium does help build strong teeth and bones.
In fact, if you don't get enough calcium in your diet, the hydroxyapatite becomes weaker because it becomes calcium deficient. For you to understand better, the formula for hydroxyapatite is: Ca10(PO4)6(OH)2. The entire structure contains a lot of calcium and if it is missing some of it, the structure will become compromised with its structural integrity weakened.
It is due to the presence of the mineral hydroxyapatite, which gives our enamel and bones their strength. So technically the "hardest substance" in our body is actually the hydroxyapatite and not so much the teeth or bone.
Why is enamel harder than bone?
Despite our teeth and bones containing the same exact mineral, enamel has more hydroxyapatite than bone which is the primary reason why it is harder. Enamel has about 96% hydroxyapatite while bone has roughly 70%. The composition in bone can vary depending on which bone in your body you're talking about.
The difference in strength and hardness is due to the fact that enamel simply has a greater percentage of the hardest substance, hydroxyapatite. Hopefully that answers your question of which is the hardest substance in our body. Your teeth is stronger and harder than all of the bones in your body.
With that being said, teeth are not bone despite sharing a lot of similarities.
Why can enamel still chip despite how strong it is?
Enamel may be the hardest biological substance within our body but that has no effect on its brittleness. It is not due to the hardness that your teeth can chip but rather due to how brittle it is. As a general rule of thumb, harder substances tend to be more brittle.
In order to have a complete understanding, we need to explain what hardness and brittleness of an object means.
The hardness of an object refers to its resistance to plastic deformation, which is a change in size or shape when subjected to a force. As an example, concrete is very hard which makes it unlikely for its shape to change if you press on it or scratch it. On the other hand, pizza dough is not hard at all and you can alter its shape just by touching it and moving it around.
To be more precise, hardness can be measured by three metrics:
Scratch hardness. If you scratch it, will it leave a mark?
Indentation hardness. If you press on it, will it leave an indentation?
Rebound hardness. If you dropped an object from a certain height, will it bounce?
If you apply these three hardness traits to enamel, it would be considered very hard because your enamel does not deform while you're chewing nor can you scratch it with a fork. You also can't change the shape of enamel just by pressing on it. If it did, your tooth would look different after every meal.
Mohs Scale of hardness
In fact, enamel has a Mohs scale of hardness of 5, which makes it harder than steel and a lot of other metals as well. Here is a list of some of the metals' Mohs scale number:
Gold = 2.5-3
Platinum = 3.5
Iron & Nickel = 4
Steel = 4-4.5
Diamond = 10
To summarize, enamel is very hard since it is stronger than most metals with the exception of diamond. Since it is harder than steel, you can't leave a scratch on your enamel if you try scratching it with a stainless steel fork!
The brittleness of an object refers to its propensity to fracture instead of elastic deformation when it is subjected to stress. This means that if you apply force to enamel, it won't leave an indentation nor a scratch, but it will fracture or chip instead if enough force is applied. On the other hand if you have a material that is not brittle like pizza dough, it won't fracture but it would deform instead if you applied stress or force to it.
In general a material that is hard will often be more brittle but a material that is not as hard will be less brittle. They are correlated in opposite directions.
Brittleness is the reason why your enamel may be the strongest, but it does not make it immune to fractures, cracks, and chips. Any type of heavy impact with enough force will break your teeth. The same can be said for bones because older individuals tend to sustain hip fractures when they fall.
Well, now you should know that it is your enamel which is the hardest part of the human body. The reason for its hardness and strength is due to its high concentration of hydroxyapatite, which is a mineral that is composed of a lot of calcium. This mineral is tougher than even steel and that gives hydroxyapatite the crown for being the hardest biological substance within the human body.
Nevertheless, just because enamel is hard it does not make it immune to fractures because brittleness has nothing to do with hardness! Hopefully you've learned a thing or two today but don't forget to go for your 6 month dental check ups so that you can learn more interesting facts from your preventative dentist.
Author: Written by Dr David Chen, a dentist in long island city.