Tooth Whitening with an Experimental Toothpaste Containing Hydroxyapatite Nanoparticles

Published In: BMC Oral Health
Publication Year: 2022

Study Design

This was an in vitro laboratory study designed to assess the whitening effect of hydroxyapatite nanoparticles (nano-HAP) in toothpaste formulations of two different concentrations. The experiment simulated manual toothbrushing on bovine enamel and analyzed both whitening efficacy and durability under mechanical force.

Funding Sources

The study was funded by the Department of Conservative Dentistry and Periodontology, University Hospital, LMU Munich, Germany.
Hydroxyapatite materials were provided free of charge by Fluidinova.
The authors declared no competing interests.

PICO Framework – what was studied and how?

Population:
Bovine incisors stained with coffee, simulating extrinsically discolored teeth.

Intervention:
Toothpaste containing 10 wt% or 1 wt% hydroxyapatite nanoparticles (nano-HAP) applied via manual brushing.

Comparison:
Toothpaste without nano-HAP (negative control)
Water (blank control)

Outcomes:

• Changes in tooth color (ΔE, ΔL, Δa, Δb) using spectrophotometry

• Durability of whitening after mechanical stress (hydrodynamic shear force)

• Surface coverage and adhesion of nano-HAP (via SEM)

In Paragraph Form:
This study evaluated whether toothpastes containing nano-hydroxyapatite (at 1% and 10%) could whiten teeth compared to non-HAP toothpaste and water. Tooth color was assessed after three applications and again after simulated oral mechanical forces. Surface-level observations confirmed particle adhesion and enamel coverage, which contributed to color changes.

Inclusion and Exclusion Criteria

Inclusion Criteria:

• Bovine teeth with intact enamel

• Color stability across three baseline measurements (ΔL, Δa, Δb < 0.05)

Exclusion Criteria:

• Teeth not meeting color stability or shade A3 threshold

• Teeth with surface defects or enamel damage

Demographics and Study Design

A total of 40 tooth samples were randomly assigned to four groups:

• 10 wt% nano-HAP: n = 10

• 1 wt% nano-HAP: n = 10

• No HAP (negative control): n = 10

• Water (blank control): n = 10

Color was measured after three applications (HAP 1–3), and again following hydrodynamic shear force (HSF) to simulate mechanical wear. Additional 16 samples were analyzed with scanning electron microscopy (SEM) to evaluate enamel surface coverage.

Primary Outcome Variables and Results

Tooth Color Change (ΔE)
The 10 wt% nano-HAP group achieved the most significant and perceptible whitening effect.

• The 10 wt% group increased from ΔE = 2.76 → 4.47 over three uses.

• The 1 wt% group increased from ΔE = 1.62 → 2.55.

• Whitening reduced slightly after HSF application but remained visible (ΔE = 4.09 for 10 wt%).

Key Statistical Findings:

• Two-way mixed ANOVA showed a significant main effect of both HAP concentration and application frequency on ΔE, ΔL, Δa, and Δb values (P < 0.05).

• No significant interaction between the two variables.

• SEM imaging confirmed greater particle adhesion and coverage with 10 wt% nano-HAP.

Conclusions

The study demonstrated that hydroxyapatite nanoparticles in toothpaste can deliver a visibly perceptible whitening effect, particularly at a concentration of 10 wt%.
This effect was enhanced with repeated use and remained effective even after simulated brushing forces.
The whitening performance was dependent on concentration, suggesting that a 10 wt% formulation is both effective and safe for daily use.

Discussion: Strengths and Limitations

Strengths:

• Simulated real-world brushing with force control

• Eliminated variables such as fluoride and abrasives

• Controlled for hydration and color measurement positioning

• Provided visual confirmation via SEM imaging

Limitations:

• In vitro design used bovine teeth, not human enamel

• Short duration—long-term whitening not assessed

• Used artificial saliva, which lacks the complexity of human saliva

• Does not account for real-world factors like pH, diet, and temperature changes

Citation

Shang R, Kaisarly D, Kunzelmann KH. Tooth whitening with an experimental toothpaste containing hydroxyapatite nanoparticles. BMC Oral Health. 2022;22:331.
PubMed Link: https://pubmed.ncbi.nlm.nih.gov/

Simple Chart: Tooth Color Change (ΔE Values)