Re: Bones and enamel, why are they so strong?

Good question. Sometimes it is said that tooth enamel is harder than steel, and to some extent this is true, but obviously in many ways, teeth are significantly more fragile. Teeth and bones are made of a collection of calcium-phosphate molecules called hydroxyapatites, of one form or another. In essence they are ceramics, but with different functions, and structures.

In bones, there are significant advantages to being not completely stiff and rigid, like tooth enamel, one of which is that the porus nature of the bone matrix allows blood stem cells to migrate into circulation, where they mature into immune cells, red blood cells, and other such blood products. The meshy nature of bone allows this to occurr. Additionally, having a certain amount of flexibility prevents bones from breaking when stressed and provides shock-absorption from movement and impact, a useful thing.

In contrast, tooth enamel needs to be as hard as possible, to promote resilance and to protect the dentin from bacteria and environmental insult, but also capable of remineralizing after some attack. Hydroxapatites accomplish this task well at high pH, but are susecptable to degradation at low acid pH, like that produced from ascerbic foods and acids (primarily lactic) produced from bacteria that colonize your mouth. Over time, these acids can degrade and destroy tooth enamel, causing cavities.

Tooth enamel and bone are primarily composed of a calcium phosphate matrix called hydroxyapatite. It's elemental formula is HCa5O13P3, and it exists as Ca10(OH)2(PO4)6 To give you an idea of how strong this substance is, it is a comparable structure to that used in many cements and concretes, which esentially use silicates instead of phosphates, because they are organic. In Cement, the general formula of one of the major components is caclium di (or tri) silicate hydrate, where the same basic power of the hydrated bond is employed to produce concrete, instead of enamel.

2 Ca3SiO5 + 7 H2O ---> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ

2 Ca2SiO4 + 5 H2O---> 3 CaO.2SiO2.4H2O + Ca(OH)2 + 58.6 kJ

Unlike bone, tooth enamel has the distinct advantage of having access to many other minerals with which to mineralize. Your bones often contain a large quantity of lead and strontium apatites, which mineralize well, but are not as strong as cacium. Teeth, however often have fluoride available to them in the form of sodium fluoride from fluoridated drinking water, or toothpaste/mouthwashes. Fluoride combines and mineralizes to form hydroxyfluoroapatite, which is even stronger and more acid resistant than regular tooth enamel. This is why municipal drinking water is fluoridated, to promote formation of this apatite.

I hope this addresses your questions. Thanks

-Matt-