Tooth whitening is a popular and conservative method of removing dental stains and whitening teeth. It is believed that teeth bleaching is a result of the oxidation of chromogenic materials by free radicals of the peroxide.
There are two principal ways of teeth bleaching:
1. Power bleaching
This form of teeth bleaching involves high concentrations of hydrogen peroxide or carbamide peroxide. The most common teeth bleaching agent for power bleaching is hydrogen peroxide at concentrations of about 30 to 35%. Due to their caustic nature, these tooth whitening materials have the potential to cause soft tissue injuries, which usually heal uneventfully within 1 to 2 weeks. Therefore this way of teeth bleaching is performed only in the dental office with appropriate protection of the oral soft tissues.
Recent studies have shown that intense light sources as used for in-office teeth bleaching elevate the temperatures of the dental pulps and may affect tooth sensitivity and pulpal health. The dental pulp contains nerves, blood vessels, and connective tissue.
Light increases the risk of tooth sensitivity during in-office bleaching, while it may not improve the bleaching effect when high concentrations of HP (25-35%) are employed. Therefore, the authors advised that dentists should use the light-activated system with great caution or avoid its use altogether.
He LB et al. The effects of light on bleaching and tooth sensitivity during in-office vital bleaching: a systematic review and meta-analysis. J Dent 2012; 40(8):644-53.
2. Professional teeth bleaching at home
This is a more common way of bleaching teeth. It involves custom-made bleaching trays and syringes containing a safe bleaching gel. Teeth bleaching gels for home use are usually of thick viscosity, pleasant-tasting, and contain carbamide peroxide at different concentrations. Tooth whitening involves two short office visits. The first one is for taking impressions for the custom-fabricated trays and the second one to pick up the trays and the bleaching agent. One needs to wear the gel-containing trays only two or more hours for a few days. The teeth bleaching results on yellow/brown teeth are most dramatic.
Some patients may experience some sensitivity which usually diminishes during the course of treatment. Some teeth bleaching gels contain potassium nitrate which seems to reduce tooth sensitivity. However, it should be noted that successive applications of even just 10% carbamide peroxide, a supposedly mild formulation, has shown to promote severe cytotoxic effects in the dental pulp.
Lima AF. Toxic effects of daily applications of 10% carbamide peroxide on odontoblast-like MDPC-23 cells. Acta Odontol Scand. 2013 Sep; 71(5): 1319-1325.
Soares DG. Effect of fluoride-treated enamel on indirect cytotoxicity of a 16% carbamide peroxide bleaching gel to pulp cells. Braz Dent J. 2013;24(2):121-7.
Soares DG. Transenamel and transdentinal cytotoxicity of carbamide peroxide bleaching gels on odontoblast-like MDPC-23 cells. Int Endod J. 2011 Feb;44(2):116-25.
Another study demonstrated direct cytotoxic effects on odontoblast-like cells.
Dias Ribeiro AP. Cytotoxic effect of a 35% hydrogen peroxide bleaching gel on odontoblast-like MDPC-23 cells. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009; 108(3): 458-64.
Some patients may have used consumer-available bleaching products, such as paint-on bleaching solutions. These products were shown to adversely affect microhardness compared to controls and regimens with 10% of carbamide peroxide. The study suggested that consumers should be made aware of this effect on enamel.
Leonard RH. Effect on enamel microhardness of two consumer-available bleaching solutions when compared with a dentist-prescribed, home-applied bleaching solution and a control. J Esthet Restor Dent 2005; 17(6): 343-50.
Several studies described a negative effect of bleaching agents on hardness and the surface morphology of enamel.
Lopes GC et al. Effect of bleaching agents on the hardness and morphology of enamel. J Esthet Restor Dent 2002; 14(1): 24-30.
Grobler SR et al. Effect of various tooth-whitening products on enamel microhardness. SADJ 2009; 64(10): 474-479.
Majeed A et al. Effect of four different opalescence tooth-whitening products on enamel microhardness. SADJ 2008; 63(5): 282-284, 286.
Mishima FD. The effect of bleaching on the enamel surface and the tensile force to debond orthodontic brackets. J Orthodont 2009; 36(4): 236-242.
Bodanezi A. Surface Modifications on Aesthetically Restored Teeth following Home Bleaching with 16% Peroxide Carbamide. Eur J Dent. 2011; 5(2): 157–162.
Some authors found increased surface roughness and a higher retention of Streptococcus Mutans (the bacterium that is the main contributor to tooth decay).
Hosoya N. Changes in enamel surface roughness and adhesion of Streptococcus mutans to enamel after vital bleaching. Journal of Dentistry 2003; 31(8): 543–548.
Peroxide bleaching agents do not damage surfaces of existing composite fillings. While they are not bleached as teeth, some composite materials show some color alteration after bleaching procedures. In addition, the bond strength of restorative material to dentin and the flexural strength of dentin have been shown to be reduced after the use of higher concentrations of bleaching agents.
Mourouzis P et al. Effect of in-office bleaching agents on physical properties of dental composite resins. Qunintessence Int 2013; 44(4): 295-302. Vieira C et al. Effect of high-concentrated bleaching agents on the bond strength at dentin/resin interface and flexural strength of dentin. Braz Dent J 2012; 23(1): 28-35. Shinohara MS et al. Shear bond strength evaluation of composite resin on enamel and dentin after nonvital bleaching. J Esthet Restor Dent 2005; 17(1): 22-9; discussion 29.
In severe cases of dental staining, such as tetracycline stains, even a combination of power bleaching and professional teeth bleaching at home may not yield desirable results. In such cases, the only remedy to create a natural beautiful look is the placement of porcelain veneers.
Since it is usually the dentin, underlying the relatively translucent and colorless dental enamel, that gives a tooth it's chroma, whitening agents need to penetrate the deeper surfaces of the tooth structure. Dental enamel is permeable due to the inter-prismatic spaces of its crystalline configuration. While it is not exactly known how peroxide leads to color change, it is widely assumed that a chemical reaction of hydrogen peroxide and organic chromophores (Marcovic I. et al, 2010). This so called Dominant Chromophore Effect Theory assumes that hydrogen peroxide is attracted to areas with high electron density, which is a property of chromatic molecules. Structural change of these molecules makes them "whiter," which in turn lightens the tooth.
Marcovich I et al. Effects of bleaching agents on human enamel light reflectance. Operative Dentistry 2000; 35(4): 405-411.
As stated above, the amount of penetrations of the bleaching agent into the tooth correlates with the amount of tooth whitening. The extent of penetration is depends on
- exposure time (Hanks CT et al, 1993)
- surface area
- concentration of the hydrogen hydroxide (Bowles WH et al, 1987; Joiner A, 2006)
- size of dentinal tubule openings (Camps J et al, 2007)
- location at tooth
- acid etching of tooth surface (Benetti AR et al, 2004; Patri G et al, 2013)
- light activation (Camargo SE et al, 2009)
Hanks CT et al. Cytotoxicity and dentin permeability of carbamide peroxide and hydrogen peroxide vital bleaching materials, in vitro. J Dental Res 1993; 72(5): 931-938.
Bowles WH et al. Pulp chamber penetration by hydrogen peroxide following vital bleaching procedures. J Endod 1987; 13(8): 375-377.
Joiner A. The bleaching of teeth: a review of the literature. J Dentistry 2006; 43(7): 412-419.
Camps J et al. Time-course diffusion of hydrogen peroxide through human dentin: clinical significance for young tooth internal bleaching. J Endod 2007; 33(4): 455-459.
Benetti AR et al. In vitro penetration of bleaching agents into the pulp chamber. Int Endod J 2004; 37(2): 120-124.
Patri G et el. An in vitro spectrophotometric analysis of the penetration of bleaching agent into the pulp chamber of intact and restored teeth. J Clin Diagn Res; 7(12): 3057-3059.
Camargo SE et al. Penetration of 35% hydrogen peroxide into the pulp chamber in bovine teeth after LED or Nd:YAG laser activation. Eur J Esthetic Dentistry; 4(1): 82-88.