The chemistry of the chompers

I had my first filling in January 2018, and there’s a lot of chemistry in dentistry: X-ray sources , local and general anaesthetics, UV-cured polymer fillings or the older mercury-amalgam. But I’m not interested in any of these: my tooth woes didn’t stop when the benzocaine wore off. After the filling settled in, I developed temperature sensitivity that put the brakes on my three favourite things in the world: eating mini twisters, drinking tea, and breathing cold air on the right side of my face.

A quick checkup (and more x-rays) later, and the filling integrity verified, I left my dentist with a smear of orange-flavoured fluoride gloop and a recommendation to massage my tooth with regular poultices of “sensitive toothpaste”. Toothpaste duly purchased, the active ingredients caught my eye: arginine and calcium carbonate.

The calcium I understand, in a vague way, but arginine? As developed, it seems that people simply tried it and it worked (though with some dubious study ethics). But what’s the mechanism of action? Why that amino acid? These, these are the interesting questions. And to answer these, I wondered: Why do teeth get thermally-sensitive in the first place?

I had initially assumed that the thermal conductivity of my filling was to blame. A tooth is not a warm-blooded mammal, but a cold-blooded lizard – so when the ambient temperature changes, teeth follow (I shied away from a metal filling partly for this reason). But polymer dental filling materials are even less thermally conductive than teeth. If all of them chill out, and at the same rate, then why does this filling-bearing one complain?

At this point, I had no time for research and this post languished as a draft for a year. The tooth slowly got more and more heat-sensitive, culminating in a sleepless weekend followed by a root canal treatment. No infection, just a nerve that had “had enough” from being roommates with a chunk of plastic. Nerve removed, tooth filled in, the arginine and calcium carbonate toothpaste had no further use. But, I developed a taste for it, so it stayed. 

The source of thermal pain in teeth is surprisingly complex, not well understood, and a problem more biomechanical in nature. Less interested in the cause of the sensitivity, attention turned back to the chemistry to address it.

I found an excellent summary of the research from the BSDHT – sister society to the BDA?. Teeth are shot through with microporous tubules that connect nerve to environment, running through the dentine layer of the tooth. The tubule walls are made of dentine, and the positively-charged guanidinium of the arginine will bind the phosphates of the calcium phosphate in the hydroxyapatite. Forming some kind of complex with calcium, phosphate, and salivary glycoproteins, this blocks the tubules. The exact structure of the complex doesn’t yield to a cursory literature search, so if you have any ideas tweet at me!

Not only structural, the arginine raises the pH when digested by bacteria. Oral bacteria will rapidly metabolise free arginine to ammonia, counteracting the acidifying effects of said bacteria. Warfare rages on a microscopic level: battening the hatches against the ravages of a post-paleolithic (arguably) diet, the arginine plugs hold fast, for a time. Succumbing at last to digestion, they thwart the acidic dissolution of the natural tooth by turning the invading bacteria against themselves.

So it seems not only should we brush, it’s quite important what we brush with!


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