1. What Is Dental Plaque, the Sticky Film That Rinsing Cannot Remove?
The sticky, slimy substance on the surface of teeth is called dental plaque. It is described as a mass formed by gathered bacteria, called a biofilm [1]. It resembles the slime in a kitchen drain, but in the mouth, it sticks firmly to the tooth surface. Because it has strong adhesion, it cannot be removed by rinsing with water [3]. The basic method is to remove it mechanically by rubbing with a toothbrush, interdental brush, or floss [2]. In other words, a toothbrush is a tool for "scraping off a film that water cannot wash away." Just as oil stains on a dish do not come off with water alone but require rubbing with a sponge, toothbrush bristles act as that sponge. However, teeth are not flat surfaces like dishes; they have complex shapes with bumps and gaps. This raises the question of where exactly the bristles touch the dirt.
2. How the Tips of Toothbrush Bristles Clean Teeth
To answer the main question first: it is thought that having many thin tips makes it easier for them to touch the uneven tooth surfaces and gaps. Let us look at the reasons in order. According to an explanation in the journal of the Japanese Society of Periodontology, the part of the toothbrush that removes plaque most effectively is the tip of the bristles [4]. It is not the middle or the root of the bristle, but the tip that is the main actor. Toothbrushes with "tapered" bristles, where the tips are processed to be thinner, are described as reaching easily between teeth and to the edge of the gums [4]. Lion, a manufacturer, also introduces "ultra-fine bristles" that are thin and flexible, claiming they reach inside the periodontal pocket to scrape out biofilm [5]. However, this is an explanation by the manufacturer itself. Combining the fact that tips are the main actors with the fact that thin tips reach narrow places gives us the initial answer. Since no materials directly compared the thickness or number of individual bristles in this context, we keep this as a "possible view." However, because the tips are so important, the way you apply force changes the result.
3. Why Pressing a Toothbrush Hard Makes the Bristle Tips Bounce
You might think that applying more force removes more dirt, but the explanation in the Japanese Society of Periodontology journal says the opposite. It states that if brushing pressure is too strong, the bristle tips splay, and biofilm cannot be removed efficiently [4]. Because the most effective tips lift off the tooth surface, pressing harder may actually be a loss. A guide from Isogo Ward in Yokohama City suggests using force that bends the bristles slightly without splaying them as a benchmark. It recommends placing the tips at the boundary between teeth and gums and moving them in small increments for 1 to 2 teeth (about 5 to 10 mm, or 0.2 to 0.4 inches) [3]. Toothbrushes with splayed tips have lower cleaning efficiency and tend to accumulate bacteria, so replacing them once a month is the recommended guideline [3]. So, would stiffening the bristles help remove dirt without needing force?
4. Do Stiffer Toothbrush Bristles Clean Teeth Better?
In an experiment published in the journal of the Japanese Society for Disability and Oral Health, evaluating a flat model showed that harder bristles had higher cleaning efficiency, while soft bristles needed larger movements [7]. However, this result was from a flat model, not the actual mouth, and it is not a recommendation for hard bristles. A guide from Nerima Ward on choosing actual toothbrushes recommends "medium" hardness among "hard," "normal," and "soft." It advises choosing "soft" if there is bleeding from the gums [6]. The Yokohama Isogo Ward guide recommends nylon bristles with rounded tips [3]. While hard bristles show high cleaning efficiency in the flat model, they also raise concerns about the burden on the gums. "Hardness" seems to be something chosen based on the balance between cleaning ability and gentleness to the gums. By the way, when did nylon start being used for toothbrushes?
5. How Toothbrush Bristles Changed from Pig Hair to Other Materials
Toothbrush bristles were once made from the stiff hair of pigs (or boars). A commentary from the dental museum at a US university states that DuPont launched a toothbrush with nylon bristles in 1938, which was a turning point [8]. Nylon is introduced as having characteristics such as low water absorption and the ability to be manufactured with precise hardness levels [8]. Animal hair varies in thickness and stiffness from strand to strand. In contrast, nylon allows for mass production of bristles with the same hardness, making it easier to process tips to be thin or to align them to the same length. However, this is a view inferred from the descriptions in the sources, as there is no material directly linking tip processing and material. Regarding the "first synthetic bristle in the world," since synthetic bristles were also announced in another country in the same year, we only note here that "DuPont launched nylon bristles." Nevertheless, toothbrushes have limits. Next, we discuss places the bristles cannot reach.
6. Why a Toothbrush Alone Leaves Plaque Between Teeth
A guide from Nerima Ward states that the spaces between teeth are easy to miss, and plaque cannot be completely removed with a toothbrush alone. Therefore, it is recommended to use dental floss or interdental brushes [6]. No matter how thin the tips become, there is a limit to reaching narrow gaps. It is also explained that completely removing plaque every day is realistically difficult [1]. The goal is not "zero" but to rub it away daily so it does not accumulate too much. To summarize, the reason toothbrush bristles are thin and numerous is thought to be a design to scrape off the bacterial film that water cannot remove, reaching into narrow spaces with the tips. To make the most of these tips, it is important to move them in small motions with light force, rather than pressing hard.
7. How to Observe Toothbrush Bristles Bending at Home
Things you can do at home: If you have an unused toothbrush, gently place it on your palm to observe how the bristles bend, then apply a little force and compare. You can see with your eyes how the tips change when you press harder. Do not test this on your face or gums; use only your palm. Compare old and new: Place a new toothbrush next to one used for about a month and compare how splayed the tips are. If they are splayed, it is a sign to replace it. To research more: Your local government website may have guides on brushing and choosing toothbrushes.