Ask a Dentist: Tooth Whitening

Tooth Whitening. (Gerd Altmann)

Back in the 1970s, orthodontists were seeing that some of their patients who were wearing braces had very poor oral hygiene and thus had periodontal (gum) issues—the gums were swollen and inflamed. To help with this condition, the orthodontists would apply low doses of hydrogen peroxide (H2O2) in its known capacity as a wound cleansing and healing agent.

When used for some time, as an unexpected consequence, the teeth would actually appear somewhat whiter. Oral surgeons, who were also using peroxide after surgery, also noticed a similar effect.

Companies took note and developed products specifically for tooth whitening.

Hydrogen peroxide (HP) naturally occurs in the human body as it is an essential component in the immune system. This may be the reason that studies undertaken have never shown any adverse effects in HP use for whitening teeth in the human body. However, higher concentrations can readily burn the gingiva (gums), and these concentrations should be avoided. Oxygen is released within seconds of contact with enamel.

Our teeth collect stains throughout life. For the majority, they are “extrinsic stains,” such as stains on the outsides of the teeth (enamel). We already know the staining properties of many of the things we eat as we have to deal with the results of spilling or getting them onto clothing or tablewear. These strongly colored foods are considered to be highly chromogenic.

A chromophore is that part of a molecule responsible for its color. Chromophores are responsible for the absorption of light waves of various wavelengths, and as these molecules grow, the more chromophores there are, the more light is absorbed, and these groups appear darker, which results in the adverse discoloration of both dentin and enamel.

Protein Inside Plaque Feeds the Issue

Chromophores have a high affinity to protein inside plaque in the acquired biofilm or calculus that grows on the tooth surface. Organic chromophores are small organic molecules like tannins, such as from coffee, tea, red wine, or fruits.

Inorganic chromophores are colored metal ions like copper, iron, and magnesium. Organic and inorganic chromophores may also be present in combination: in hemoglobin where the organic component is combined with a colored iron (inorganic) ion.

Oral care products such as stannous fluoride and chlorhexidine can cause stain. Chlorhexidine is often used after oral surgery in the mouth, much as for the purpose of the original peroxides. But instead of whitening, they tend to stain the teeth over time.

The enamel of the teeth is somewhat porous, so once chromophores enter into the microscopic pores (grooves and imperfections in the structure of the enamel), they become very difficult to remove with normal brushing. But conversely, this enamel porosity also makes it possible to whiten (bleach) the teeth.

Intrinsic stains form below the surface of the enamel and into the dentin, resulting from tetracycline absorption or fluorosis, as well as trauma-induced bleeding in the teeth.

The dentin under the outside enamel of the tooth is darker and dentin will darken with age. If you lose enamel, you will show more dentin. Tooth discoloration is caused by a combination of intrinsic and extrinsic chromophores, sometimes in addition to aging effects. As with eye, hair and skin color, there is a wide variety of inherited tooth color.

Office Tooth Whitening

This is normally done with moderate to high concentrations of hydrogen peroxide (H2O2). In Europe, however, the maximum concentration of H2O2 can only be 6%, yet the systems and methods of application allow for a successful whitening to be achieved in those countries.

There is a direct relationship between the concentration of H2O2 (HP) used and the length of time required for the whitening, but there is a similar correlation between the sensitivity generated with the higher concentrations.

As mentioned, hydrogen peroxide (HP) in high concentrations will burn the gums. So, with in-office bleaching, a “liquid dam” is applied to all the gingival (gums) surfaces surrounding the teeth. This is a soft gel covering that is set using a curing light, and it protects the gingiva from the peroxide, which can then be painted onto the outer surface of the enamel. Vitamin E oil is sometimes applied to the lips.

In-office whitening will not be provided for pregnant women or children under 18 years.

An activating light is then pointed at the area. There have been many versions of these over the years and the current generation is usually LED based. Blue light irradiation enhances the whitening process.

The light speeds up the breakdown of the peroxide to the components of molecular oxygen, oxygen ions, hydrogen ions, and free radicals. Oxygenation, ions, and free radicals cause physical and chemical breakdown of molecules that cause coloring, and it is needed to physically remove these molecules from the structure of the tooth by diffusion.

These reactive molecules attack the long-chained, dark-colored chromophore molecules in the dentin, breaking down the conjugated double carbon bonds: breaking them into smaller, less colored, and more diffusible molecules. To summarize, the chromophores are broken into smaller single bonds which reflect light, and the tooth appears lighter.

Sensitivity can occur with tooth whitening, and this is due to dehydration of the teeth. In fact, when the teeth are necessarily kept dry during office procedures, they will also appear somewhat whiter, and this is why the shade (color) of the teeth is generally recorded at the beginning of procedures to assess the true shade of the teeth.

For sensitivity, a dentist may recommend amorphous calcium phosphate products that block the dental tubules, preventing fluid flow. Over-the-counter painkillers can also help.

HP has a relatively short duration of action with this process, and so for Home Whitening products, carbamide peroxide (CP) is used. With carbamide, the maximum effect is at two hours, but it will continue longer overnight.

Carbamide peroxide breaks down into oxygen and urea. Urea has a high PH value and helps to kill bacteria.

Carbopol and glycerin may be used with the CP to stabilize or thicken the solution, making it more suitable for storage. Any whitening agent should be taken out of a fridge for at least an hour before use.

Impressions are taken of the upper and lower teeth, and special trays are made from the models. In the past, reservoirs were included to allow greater amounts of the CP to be held, but it became evident that this was unnecessary and could lead to greater sensitivity and gingival irritation. Small amounts of CP are inserted from a syringe into each tooth indentation in the trays, which can be kept in the mouth for hours at a time, or overnight. One can choose to just apply the CP to the six front teeth or to the entire arch.

Often in-office whitening is used as a “jump start,” and one then continues the process at home. For tetracycline staining, four to six months may be required. Sensitivity of the teeth is also common with home applications of CP, especially if the concentration used is high. In this case, one simply skips a day or so before continuing with the regimen. The frequency is thus self-determined. Naturally, if there is gum recession or actual cavities in the teeth, this will also result in sensitivity.

A purchased 10% carbamide peroxide product used for tooth whitening generally would yield a maximum of 3.6% hydrogen peroxide, while the commonly used products containing 16% carbamide peroxide would normally be releasing less than 6% hydrogen peroxide.

A single discolored tooth can result from the death and necrosis of the pulp. Hemosiderin often forms after bleeding in the root canal. This is handled by opening the back of the tooth and inserting peroxide directly into the chamber and sealing it for a week or so. However, a coronal barrier seal is first required to prevent progress of bleaching agents and their derivatives travelling from the pulp chamber down the root-filled tooth. When the desired color is achieved, the cavity behind the tooth is resealed.

There is a gradual relapse of all these whitening procedures over time, but that time could be numbered in many years. The majority of professional whitening procedures are very successful. One has the option of continuing with the home regime as a “top-up” at any time.

However, after whitening there is a reduction in bond strengths of composite resin of between 25% and 50% for a period of two weeks, as the released oxygen will affect the ability to bond to teeth after whitening. So if a filling is required, whitening should be suspended for at least that period of time.

Whitening will not change the color of existing composite fillings in the teeth or change the color of a crown, etc. These fillings will need to be changed later to match the new color—or temporary restorations placed until the desired shade is reached.

Returning to the original use of HP for those wearing fixed orthodontic braces on the teeth, now rubber trays can be used to fit over the braces to do the same thing. For those using the Invisalign clear tray system for orthodontics, these trays can also be used to house the whitening agent overnight. Special formulas of whitening agents have been developed for this purpose with 10% CP. This will also make the gingiva healthier for someone who is not strict with oral hygiene.

Whitening strips are available impregnated with HP that one can apply directly to the teeth, such as high adhesion 9.5% HP whitening strips, for two hours a day for 10 days.

Whitening pens for direct application to the teeth are also available. It is no doubt wiser to choose the well-known companies from which to purchase products as hopefully they are more stringent on the ingredients that they contain.

So, if whitening the teeth is the goal, there are now many avenues to achieve that goal, but avoid harsh abrasive toothpastes.