On the ingredient list, the four materials are indistinguishable: hydroxyapatite, offered for enamel-repair toothpaste, each with a datasheet and a price per kilogram. In 2025, a group at the University of Bologna put four such commercial grades on actual enamel sections and measured what fraction of the surface each one coated.
How different can “hydroxyapatite” be?
The spread answers the question by itself. After a single three-minute application in artificial saliva, “coverage differed between groups ([A] 28.83 ± 7.35% vs. [B] 31.11 ± 3.12% vs. [C] 57.20 ± 33.12% vs. [D] 99.90 ± 0.12%).” The best material covered essentially the whole surface, uniformly, every time. The worst left seven-tenths of the enamel bare. Same INCI name. A 3.5-fold performance range — and note sample C’s standard deviation of ±33 points: it didn’t just underperform, it performed unpredictably.
The authors’ framing is worth keeping: substituted hydroxyapatites “are widely used in oral-care formulations for enamel repair; however, head-to-head comparisons among commercial grades remain limited.” Buyers have been choosing between these materials largely without this data.
What separated the winner from the rest?
Not purity in the naive sense. The two best-covering samples “exhibit lower crystallinity and correspondingly higher carbonate levels: 6.16% and 6.52%, respectively.” The material that behaved most like a perfect laboratory crystal was not the one that bonded to teeth. The paper’s synthesis: materials with “moderate-to-low crystallinity, a Ca/P ratio close to that of dental apatite, measurable carbonate substitution, a spongy particle morphology, and sufficiently negative ζ-potential tend to form more continuous, homogeneous coatings on enamel.”
That list should look familiar — it is a description of biological apatite. Tooth mineral is itself carbonated, imperfect, and substituted, and the powder that resembles it wins. For anyone specifying an eggshell-derived grade, this cuts in an interesting direction: biogenic routes naturally carry carbonate. The “impurity” a purist would calcine away is on the winning side of this particular ledger.
It also reframes what “quality” means on a certificate. Crystallinity is usually reported as a virtue, the higher the better, because it reads like purity’s cousin. Here the two highest-crystallinity powders were the two that barely coated the tooth. The certificate was not wrong; it was answering a different question from the one the formulator was asking. Numbers only protect a buyer when someone has established which direction of each number the application actually rewards — and that mapping, not the datasheet, is where grade selection really happens.
What the test did not show
The limits are real and belong in the record. One application, three minutes, in vitro; the top scorer is a branded commercial material; and the authors call for durability testing under brushing before anyone treats a coating as a repair. This is a starting-line comparison, not a finish-line one.
But the starting line is where purchasing decisions happen. Four drums with the same name spanned 28.8% to 99.9% on the one function the label implies — which means the name tells you almost nothing, and the characterisation data tells you almost everything. A supplier who reports crystallinity, carbonate content, morphology and ζ-potential per lot has effectively pre-run this experiment for you. A supplier who reports “hydroxyapatite, 97% purity” has left you to run it yourself, one consumer complaint at a time.
Sources
Lelli M, Foltran I, Pucci R, Tarterini F. “Comparative Chemical and Physical Characterization of Biomimetic Versus Commercial Hydroxyapatites for Tooth Enamel Repair.” Biomimetics, 2025;10:672 (open access) — pmc.ncbi.nlm.nih.gov/articles/PMC12564015.