Sensitive Teeth, Toothpaste Formulation, and Particle Size
Email: “Here is a Reminder from Your Dentist”
I: Delete… Sorry, covid.
……. until recently when my teeth got too sensitive. I finally went to the dentist only to find five teeth to be filled.
Lying on the dental chair, I got curious: How can teeth be sensitive? Well, it means that temperature and acid molecules get to reach the nerves, located in the pulp.
Therefore, to cause teeth sensitivity, there must be open pathways from the enamel, through the dentin, to the nerves in the pulp. Enamel is the hardest substance in the human body, and it covers the outer surface of your teeth. It is made mostly of calcium phosphate.
How can enamel develop holes?
Here are some of the most common causes:
- Acid Erosion, which could be caused by acidic foods/drinks and/or acid reflux.
- Cavity caused by bad microbiome.
- Stress, which reduces immune system, leading to gum and other inflammations.
- Brushing teeth too often with abrasive toothpastes and toothbrushes.
How do we treat sensitive teeth?
Up to now, the most recommended way is, simply, to brush your teeth.
The magic is KNO3, mostly known as gunpowder. It is also used as a thickening agent in soups and stews in West African cuisine. We discovered that it also has a desensitizing effect in recent decades and can clog tubules in teeth. But it has disadvantages including formulation difficulties and taking a long time before exerting a desensitizing effect. Therefore, we are still looking for better solutions, for example, new desensitizing agents, e.g. amino acid, and capillary sealing method, which focuses on sealing the tubules and remineralizing the teeth.
Now, does it really work?
One of the key factors is particle size distribution. If we want the desensitizing agents to work, the particles containing those agents need to be small enough to pass through enamel and dentin to reach the nerves. Even if we just want to clog the tubules, the particles need to be small enough to get into the tubules.
Well, it sounds like we just need to make very small particles, right? Well, not so simple.
Don’t forget, abrasion is the primary reason for brushing teeth – to get the plaques off. It is partially achieved by having silica particles as the abrasives in toothpaste. When the particles get too small, we may lose the strength of abrasion.
So, what is a good particle size range?
This question is actually quite complicated. Let’s start with some real numbers.
First, pore sizes of enamel and dentin: A healthy enamel has a pore size on the order of 10nm. Sensitive teeth, however, must have holes way larger than 10nm for signals/molecules to easily reach the nerves. Dentin has a pores size prevalently ranging from 0.400 to 100μm.
Here are two examples of toothpaste for sensitive teeth from Colgate and P&G. The reported particle sizes were 2 -10 μm and 0.25 – 5.0 μm, respectively. Both were characterized by Mastersizer. The particles were made of fused silica composition comprising desensitizing agents.
Another method is capillary sealing. For example, one application mixes nano-sized apatite carbonate particles and silica particles, ranging from 1 nm to 500 nm. It was discussed that an average particle size too small (below 1nm) would induce agglomeration and increase viscosity; whereas, particles too large greatly reduce the effect of remineralization.
Now let’s come back to the question: what is a good particle size range?
You see, the answers can be quite different. It all depends on the individual’s habits and goals. For example, I would focus on remineralizing and tubules sealing but care less about plaque removal or antimicrobial efficacy. This is because 1) tend to brush my teeth 2-4 times a day and water-floss; 2) my dentist says my teeth are very clean; and 3) my enamels are worn off. So, my sensitive teeth could be likely due to over-cleaning and/or acid. But it could be a very different story for you. This is why we see such a variety of products on the grocery shelves – to meet every individual’s specific needs. And that’s the beauty of consumer products.
By the way, you may also want to pay attention to enamel surface loss. A study shows that brushing induces enamel surface loss regardless of whether the toothpaste has a desensitizing or anti-erosive claim. As the study suggested, a toothpaste may need a higher concentration of calcium and phosphate, a higher weight percentage of solid particles, and a smaller particle size to slow down enamel loss.
Further reading