A study shows that human teeth lose some enamel hardness after the application of teeth whitening products used in the home. The study suggests that future generations of such teeth whitening products might be reformulated in an effort to reduce these side effects.

The researchers noted that teeth typically could restore their previous hardness after losing small amounts of enamel calcification. This is the first study to show at a nanometer scale, measuring in billionths of a meter, how home teeth whitening treatments affect human teeth.

"There is some significant reduction in nano-hardness of enamel, but we are talking on a very minute scale. So even though it may not be visible to the human eye, it's important for research because that's how we improve products," said Shereen Azer, lead author of the study.

The researchers applied the recommended treatments of five name-brand home teeth whitening products to samples of human teeth and compared the effects to tooth samples that received no teeth whitening treatment. In all cases, the products reduced the hardness of the enamel as well as what is called the elastic modulus or stiffness, a measure of the ability of the tooth surface to bounce back in response to applied force.

Many studies have sought to determine how teeth whitening affects tooth enamel hardness, but results have been inconclusive. Azer said that previous studies measured the loss of enamel hardness in microns, or millionths of a meter, rather than on the nanometer scale used in his study.

Teeth whitening products contain solutions of various strengths of either hydrogen peroxide or carbamide peroxide, which provide the whitening effect. They bleach teeth by producing unstable free radicals that attack pigment molecules in the organic parts of enamel. The reduction in pigment means the molecules no longer reflect light, so the teeth appear whiter.

Enamel, which is almost entirely inorganic and translucent, appears yellow in most teeth because it reflects the color of the dentin underneath, which is naturally yellow.

The study did not test for two other common side effects of teeth whitening, gum irritation and tooth sensitivity.

The researchers used extracted molars to assemble 65 human tooth samples measuring 4 millimeters square and 2 millimeters deep for the study. Ten samples were used in a pilot study that determined they could achieve accurate results for the research under dry conditions rather than wet conditions simulating the presence of saliva.

Of the remaining samples, five were left untreated, and 50 were divided into five groups of 10 each to undergo treatment.

They used whitening strips on two groups of samples and trays filled with whitening gel on three groups. The treatment times included up to 60 minutes once per day or 60 minutes twice per day according to manufacturer recommendations. All treatments lasted for three weeks except for one tray method, which lasted 10 days.

The teeth whitening treatments included both over-the-counter and professionally provided products to be used in the home.

The scientists used a specialized tool to apply force to test the enamel hardness and stiffness (the surface ability to bounce back), and an atomic force microscope to observe the tiny nanometer-scale effects on the teeth.

The average loss of enamel ranged from 1.2 to 2 nanometers on the treated teeth. The control teeth, on average, actually gained 0.4 nanometers of hardness in comparison over the treatment time frame. The surface ability to bounce back from applied force was reduced by an average of between 6 percent and 18.8 percent among the treated teeth, depending on the type of teeth whitening treatment.

Among the different products, most of the reductions in hardness and elastic modulus were similar. However, there was a significant difference between one strip treatment method and one tray method, with the tray method reducing enamel hardness more dramatically than the strip treatment.

Enamel is the hardest structure in the human body. It protects teeth and maintains the integrity of the bite. But enamel is subject to abrasion by certain products and even too-vigorous brushing, which is why it is important to figure out ways to reduce damage to this part of the tooth, Azer said.

The products used in the study were Crest Whitestrips Premium Plus, Crest Whitestrips Supreme, Nite White ACP, Oral B Rembrandt and Treswhite Opalescence. The final three are tray treatments.
References:
1. Shereen Azer, et al. Ohio State University.
2. Image by  AimanStudio..