"Soft" Means Nothing: Why the Same Word Describes Different Brushes

Two manufacturers. One filament diameter — 0.10 mm. The first prints "soft" on the box. The second prints "ultra-soft." Both are technically correct — because no binding standard connects the word on the package to a specific number.

This is not a marketing trick. It is a structural problem in classification that has existed since synthetic toothbrush bristles were introduced. A patient told by their dentist to "use a soft toothbrush" picks up a "soft" brush — and gets something the dentist never intended.

What ISO 22254 Actually Measures

There is an international standard: ISO 22254:2005 — "Dentistry — Manual toothbrushes — Resistance of tufted portion to deflection." It sounds like it resolves the question. It does not.

The standard measures the stiffness of an entire bristle tuft under load — resistance to deflection in N/cm². Not the diameter of a single filament. Not trim length. Not filament count. The stiffness of the assembled tuft as a physical object.

And the standard itself explicitly states: the measured tuft stiffness may not correspond to the consumer's subjective perception of "soft" or "hard" during brushing — because perception depends on angle, applied pressure, technique, saliva, and water temperature.

Typical filament diameter ranges in the industry: around 0.10 mm (finest), 0.127 mm, 0.152 mm, and thicker. How these numbers map to "ultra-soft," "soft," and "medium" depends entirely on the brand. Where a mass-market brand draws the line for "medium," a premium brand may still be calling it "ultra-soft."

Five Variables That Actually Determine Stiffness

Toothbrush stiffness is not a single number. It is the combined result of five factors acting simultaneously.

Filament diameter. The stiffness of a single bristle scales approximately with the fourth power of its diameter. A 0.15 mm filament is roughly five times stiffer than a 0.10 mm filament of the same material and length. This is fundamental beam mechanics.

Free trim length. Stiffness decreases with the cube of the free filament length. Longer tufts flex more at the tip — the same material and same diameter will feel softer if the free length is greater.

Pack density. The number of filaments per tuft and tufts per head. More filaments means lower load per filament at the same brushing pressure. A denser brush feels softer at the same diameter.

Material. Nylon 6.12, nylon 6.6, and PBT have different elastic moduli — different inherent bending resistance. A PBT filament at 0.15 mm is stiffer than a nylon 6.12 filament at the same diameter.

Tip geometry. A cylindrical filament presses against tissue with its full cross-sectional area. A tapered filament narrows toward the tip — lower contact pressure at the end. This matters both for comfort and for actual loading on the gingival epithelium.

The Curaprox Example

The Curaprox CS 5460 is one of the clearest illustrations of the gap between conventional labeling and actual parameters. 5,460 CUREN filaments at 0.10 mm diameter — ten or more times the filament count of a typical mass-market brush with 500–700 thicker bristles.

Curaprox labels this "ultra-soft." That is technically precise. But if a Chinese OEM brand uses filaments of the same 0.10 mm diameter with lower pack density and standard trim length, the result at the tooth surface will be different — even if the package says "soft" or "extra soft."

Same diameter: 0.10 mm. Different stiffness at the brush head. Because diameter is one of five variables, not the whole formula.

The total contact area between bristles and tooth is the product of filament count multiplied by tip area. At higher density, the load per contact point is lower at the same hand pressure. A 5,000-filament brush is gentler per square millimeter of gum than a 500-filament brush — even if the individual filaments are equally thin.

The Clinical Side

Dentists generally recommend soft bristles — and there is evidence behind that recommendation. A systematic review by Ranzan et al. 2019 (PMC9379007) analyzed 13 studies: hard-bristle brushes produced more soft tissue damage than soft and medium brushes. Soft and extra-soft brushes showed the fewest adverse effects.

But "less damage with soft" compares within a group of studies where "soft" means the same thing across brands. In reality, it does not.

A 2025 narrative review (PMC12111729) confirmed that brushing force, technique, and bristle stiffness interact. Stiffness alone matters — but it is amplified by incorrect technique. A softer brush leaves more margin for technique errors.

How to Actually Read Stiffness

The word on the package is a starting point, not the full answer. What to look for beyond it:

• Filament diameter, if listed. Below 0.12 mm — genuinely soft. At 0.15 mm and above — medium or firm regardless of what the label says.
• Filament count. More filaments means more even load distribution. 5,000 filaments versus 500 is a fundamentally different story.
• Material and tip shape. Tapered or rounded filament tips are gentler on gingival tissue than blunt cylindrical ends.
• Whom the choice actually matters for. Sensitive gums, recession, periodontal disease, orthodontic treatment — look at diameter and density, not at the label.

When specifying the QDRO brush line, we focused on parameters rather than categories. Ultra-soft means Pedex filaments at 0.10 mm diameter with mechanically rounded tips — not just a label. Soft and medium are defined Pedex and DuPont diameters with specified pack density. Because parameters are predictable. Labels are not.

What Follows From This

Manufacturers are not required to disclose filament diameter. So "soft" on a package is always a promise within one brand's coordinate system. Comparing two "soft" brushes from different brands as equivalent is like comparing a "small" cup from two different coffee shops.

What to do:

1. Ask your dentist about the condition of your gums — recession, bleeding, and periodontal status change the requirements.
2. If diameter is listed, read it rather than the word.
3. With sensitive gums, choose a brush with demonstrably fine and dense filaments.
4. Remember: one "ultra-soft" may be firmer than another brand's "soft" — if the first has sparse thick tufts and the second has dense fine ones.

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Sources: ISO 22254:2005 (Dentistry — Manual toothbrushes — Resistance of tufted portion to deflection) · Ranzan N et al., Int Dental J 2019 (PMC9379007, PubMed 30152076) · PMC12111729 (Narrative Review, 2025) · Curaprox CS 5460 product specifications (curaprox.com) · PMC7344766 (Changes in Bristle Stiffness of PBT Toothbrushes, RCT, 2020)