Description of the condition
The importance of teeth and smile in the context of facial attractiveness has been well established. The lower third of the face has a significant influence on the perception of beauty (Mack 1996), and teeth are considered the most important facial feature, followed by eyes (Jørnung 2007). The presence of noticeable discolouration of the teeth can be a physical handicap that can impact upon a person's self image, self confidence, physical attractiveness and employability. The demand for aesthetic dentistry has increased in recent years, particularly for tooth whitening. Surveys undertaken in the United Kingdom (Alkhatib 2004), the United States (Odioso 2000) and China (Xiao 2007) report that between 34% and 52% of adults are dissatisfied with the colour of their teeth. Discolouration may be due to either extrinsic staining on the surface of the tooth or caused by a change to the structural form or composition of the dental hard tissue producing intrinsic discolouration (Haywood 2006).
Extrinsic discolouration is present on the surface of the enamel or acquired pellicle and may arise from chromatogenic agents, the most common found in dietary products e.g. coffee, tea, red wine and tobacco (both traditional and smokeless). Poor oral hygiene can exacerbate and perpetuate extrinsic stains (Boksman 2006). Intrinsic discolouration may be due to a metabolic disease or systemic factors e.g. porphyria, antibiotic tetracycline staining, fluorosis, aging, caries, amalgam restorations, haemorrhage or pulp necrosis (Fasanaro 1992; Haywood 2006).
Discolouration may be due to one or a combination of factors. It is important to identify the cause prior to treatment, therefore an exhaustive clinical examination and history should be undertaken in order to determine, as accurately as possible, the causal agent and to provide the patient with an accurate prognosis before any treatment is administered. Intrinsic stains are more difficult or impossible to eliminate using bleaching agents compared to discolouration caused by external chromatogenic substances (Boksman 2006). Bleaching may be achieved by agents applied professionally or for use at home. This review will focus on professional bleaching (applied in professional clinics only) and assisted bleaching (professional in-practice whitening as a supplement to home bleaching).
Description of the intervention
Most bleaching techniques involve peroxide or peroxide related products. Hydrogen peroxide was used for whitening teeth over 100 years ago, in 1884 by Harwan (Feinman 1987), however, its first use in dentistry was to treat periodontal disease (Wennström 1979). Professional bleaching was the first bleaching technique to be developed (Matis 2009). A higher strength of bleaching agent is used (e.g. 30% to 50% hydrogen peroxide) compared to home-based treatments (e.g. 10% to 22% carbamide oxide or 3% hydrogen peroxide). A current feature of professional bleaching is the use of an activator or accelerant agent to provide power bleaching. Application of heat, light, laser, or a combination, is used to increase the temperature of the bleaching agent in contact with the tooth surface (Weinberg 1997). A systematic review of activators concluded that superiority of accelerant over non-activated bleaching therapies is still debatable (Buchalla 2007). The aim of professional bleaching is to obtain the greatest improvement in as few sessions as possible (Goldstein 1997). Whilst this method of bleaching is faster than home-based techniques, it is more expensive but suitable for those who do not tolerate the use of trays (Boksman 2006). In the United States of America, around 50% of dentists provide professional bleaching, however, only 40% say they are "very satisfied" with the results of this technique (Weisman 2002).
Home bleaching is performed by the individual, and many systems require a custom-made tray worn for a few hours at a time, or overnight, to keep the bleaching agent against the teeth. Assisted bleaching can either be a series of treatments within a practice alone, or a supplement to boost home bleaching. The concentrations of bleach are higher than home bleaching (e.g. 30% to 44% carbamide peroxide) and it is popular because it does not require the gums to be protected. Power professional bleaching uses the highest concentration of bleaching agent and the gingiva require protection with either a rubber dam or resin shield.
In a review of the safety of bleaching procedures, no evidence was found for events such as pulpal necrosis or brittleness fracture (Fasanaro 1992). However, other types of adverse effects or harms, including those due to poor technique, have been reported, and these include soft tissue burns and tooth sensitivity (Haywood 2006; Jorgensen 2002). A direct association between tooth sensitivity and duration and dose of bleaching agent has been reported (Boksman 2006).
How the intervention might work
The bleaching action of hydrogen peroxide is not fully understood (Kihn 2007). However, the underlying chemical theory suggests two possible explanations.
Hydrogen peroxide breaks down into two components, forming a free-radical molecule (HO2-) with high oxidative power, which would break-up macromolecular stains (Fasanaro 1992).
Peroxide opens the carbon-ring of pigments, transforming them into chains, which would give an appearance lighter in colour (Haywood 2001).
When a photochemical accelerator like light or laser is used, the rate of formation of hydroxyl radicals increases (Kashima-Tanaka 2003). Carbamide peroxide has a different chemical mechanism with other intermediary molecules, however, the final free-radical molecule is the same (Haywood 2001). Either hydrogen peroxide or carbamide peroxide final products diffuse into the tooth through the organic matrix of enamel and dentin, due to their low molecular weight, reaching the internal portion of the tooth within minutes (Bowles 1987; Cooper 1992). As soon as chromatogenic agents are transformed by the action of treatment into colourless molecules, the bleaching process reaches a plateau and no extra benefit can be obtained through further administration (Haywood 2006).
Why it is important to do this review
Professionally-applied bleaching treatments have been used for a long time, with a variety of products and different concentrations of active substances, resulting in a large body of research literature. In addition, methods for evaluating the effectiveness of one regimen of in-office bleaching compared to another are not standardised. Despite all of the information available, it is difficult for clinicians to determine which is the most effective treatment for tooth discolouration and the level of potential harms of these treatments. A systematic approach to summarise, organise and critically assess the evidence about the beneficial and adverse effects of in-office, professionally-applied chemically-induced whitening of teeth in adult patients is required to complement another Cochrane systematic review of the evidence for the use of home-based treatment methods (Hasson 2006).