Esthetic dentistry has been a popular topic in the United States as far back as the late 1800's. In its early years it consisted primarily of recontouring teeth by shading them with India ink for patient approval and then removing the shaded tooth structure. (Haywood, 1992) Also in the late 1800's and early 1900's various mixtures involving hydrogen peroxide, ether, and heat or electricity were used to remove tooth stains. Subsequently, use of these agents evolved to more general esthetic use. Hydrogen peroxide has been used in vital tooth bleaching in a dental setting since the 1930's. In the 1960's an antiseptic containing carbamide peroxide used on oral soft tissues was noted to produce whitened teeth. This discovery provided the foundation that led to the famous 1989 paper published by Haywood and Heymann comprising the first study of nightguard vital bleaching. Whitening your teeth and creating your perfect smile can now be achieved either through your dentist or at the local drugstore.
Our paper will briefly detail some background regarding tooth staining then move on to outline the various types of vital tooth bleaching materials and methods available, discuss some of the potential effects that bleaching can have on both pre-existing and planned restorations, report the effect bleaching can have in removing orthodontic bracket markings, outline the indications for and materials/methods/potential side effects of internal (nonvital) tooth bleaching, and finally, discuss the side effects, or after-effects, that can occur with bleaching protocols. We hope that the information provided is responsive to the various questions and concerns expressed by our classmates following previous lectures on the topic.
Etiology of tooth discoloration
The types of tooth discoloration include: extrinsic stains, age-related color changes and intrinsic stains. Extrinsic stains are considered to be superficial changes and may be caused secondarily by colored food, drinks (tea, coffee, cola) and tobacco products. Thinned enamel and darkened dentin due to deposition of secondary dentin are considered age-related color changes. Intrinsic stains are predominantly caused by medications given systemically during tooth development (most commonly tetracycline), fluorosis, systemic conditions, dental caries and restorative materials (such as old amalgam and endodontic materials left in a pulp chamber). (Helsby, 1991).
Indications for successful tooth bleaching
According to Samarawickrama, et al, external bleaching is appropriate for teeth with yellow discolorations associated with aging and mild and uniform fluorosis. Although bleaching can be successful with blue/gray stains and tetracycline-induced stains, longer treatment times are required generally. Internal bleaching is indicated for discoloration resulting from root canal treated teeth and teeth that have been intrinsically stained due to tetracycline.
Contraindications for successful tooth bleaching
Tetracycline stains covered with porcelain or composite resin veneers cannot be bleached successfully. Similarly, very dark or black stains resist bleaching as do small, deep, discolored or opaque white spots. Fluorosis can result in superficial white, brown and yellow spots. These types of stains are best removed physically (enamel micro-abrasion) or managed restoratively (Samarawickrama, 1996).
The relationship between hydrogen peroxide and carbamide peroxide
At this point it is important to understand the relationship between carbamide peroxide, the most common vital bleaching agent currently used, and hydrogen peroxide, the active molecule in the bleaching process. Generally hydrogen peroxide comprises about 35 percent of carbamide peroxide. Urea comprises the remainder of the carbamide peroxide. So a 10 percent carbamide peroxide solution, equates to approximately a 3.5 percent hydrogen peroxide solution. The hydrogen peroxide breaks down to water and oxygen, and in the process generates free radicals that oxidize larger pigmented molecules in a tooth into smaller less visible molecules. The urea breaks down into ammonia and carbon dioxide.
There is a wide range of bleaching products and techniques that can be used to successfully lighten teeth. In addition to the mechanical stain removal techniques such as microabrasion and laser bleaching, there are many solutions that are applied to the teeth passively and rely on chemical reactions to lighten the teeth. These solutions include home-made mixes, in-office commercial products some of which require a special light, take-home commercial products used in trays, and finally the fast growing over-the-counter market products.
Dentist "home-made" bleaching mixes include such combinations as:
Professional In-Office Bleaching Products (both non-tray and tray products)
Professional In-Office Bleaching Products
| Company | Brand | Type | Percent | Time |
|---|---|---|---|---|
| Ultradent Products | Opalescence Xtra | Hydrogen Peroxide | 35% | As needed |
| Opalescence Xtra Boost | Hydrogen Peroxide | 38% + activator | As needed | |
| Discus Dental | White Speed | Hydrogen & Carbamide Peroxide | 18% H2O2 + 22% H2O2 and carbamide per |
30 minutes |
| ZOOM! | Hydrogen Peroxide | 25% | 3 - 20 min sessions | |
| Denstply | Illumine' | Hydrogen Peroxide | 30% | 15-60 min |
| BriteSmile | Hydrogen Peroxide | 15% | 60+ min |
| Brand | Light Activation | Heat Activation | Misc |
|---|---|---|---|
| Opalescence Xtra | Yes | Yes | |
| Opalescence Xtra Boost | No | No | Syringe to syringe mixing, 7.0 pH |
| White Speed | No | No | Custom Trays, equivalent to 13.2%H2O2 |
| ZOOM! | Whitening Lamp | Yes | |
| Illumine' | No | No | Syringe to syringe mixing, Custom trays |
| BriteSmile | Yes | Yes | 7.0 pH |
Professional Take Home Bleaching Products
| Company | Brand | Type | Percent | Time | Misc |
|---|---|---|---|---|---|
| Ultradent | Opalescence | Carbamide Peroxide | 10 % | 8-10 hrs at night | Reg, mint, or melon |
| Opalescence F | Carbamide Peroxide | 15 or 20% | 8-10 hrs at night | Reg, mint or melon | |
| Opalescence PF | Carbamide Peroxide | 15 or 20% | 8-10 hrs at night | Potassium nitrate & Fl | |
| Discus Dental | Nite White Excel 2 | Carbamide Peroxide | 10, 16, or 22% | 10-overnight 16-4hrs 22-1hr |
Regular or Cherry |
| Nite White Excel 3 | Carbamide Peroxide | Same | Same | Hydrogen Peroxide 1% | |
| Day White Excel 2 | Hydrogen Peroxide | 7.5 or 9.5% | 30 min 2xday | ||
| Premier Dental | Perfecta Bravo | Hydrogen Peroxide | 9% | 30 min 1xday | |
| Densply | Nupro Gold | Carbamide Peroxide | 10% or 15% with Fl | ||
| Procter & Gamble | Crest White Strips | Hydrogen Peroxide | 6.5% | 30 min 2xday | 3 week supply |
Over the Counter Products
| Company | Brand | Type | Percent | Time | Misc. |
|---|---|---|---|---|---|
| Procter & Gamble | Crest White Strips | Hydrogen Peroxide | 5.3% | 30 min 2xday | 2 week supply |
| Colgate-Palmolive | Colgate Simply White | Carbamide Peroxide | 18% (6% H2O2) | 30 sec 2xday | Brush-on gel |
Studies of the effectiveness of new products on the market are not widely available likely due to the time involved in planning, conducting, and publishing study findings. Additionally, there have been very few new studies on current, more established products. Most of the studies available compare the weaker concentrations on the market, which are commonly used in at-home gel nightguard bleaching preparations. An in-office study comparing usage time of a 15 percent hydrogen peroxide solution showed no statistical difference in tooth shade change between 30 and 60 minute application times (Papathanasiou, 2001). Another study revealed that a 5.3 percent hydrogen peroxide preparation (Crest Whitestrips, OTC) yielded equivalent shade change to a 10 percent carbamide peroxide tray system (Gerlach, 2000). Clinical evaluation of 10-15 percent carbamide peroxide preparation showed no difference in final tooth shade attained in a two week period versus a six week period (Matis, 2000). Another study concluded that a 20 percent carbamide peroxide and a 7.5 percent hydrogen peroxide solution gave equivalent results in terms of tooth lightness and sensitivity (Mokhlis, 2000), but in fairness, this study was not carried out using manufacturer's directions. Overall, one can conclude from the various study findings that shorter time periods for in-office procedures (such as 30 minutes) are probably just as good as longer time periods, bleaching preparations with similar amounts of hydrogen peroxide equivalent yield similar results, and finally, that shorter bleaching periods are probably as successful as longer periods in your average extrinsically or age-related stained teeth.
Below are step-by-step instructions for several of the most popular bleaching techniques. Prior to bleaching it is important to note the following for optimal results (compiled from http://ultradent.com ):
At Home Bleaching:
1. Nightguard Vital Take Home Bleaching (Dentist supplied): (10-15% carbamide peroxide, 3.6-10% hydrogen peroxide) (compiled from Opalescence and Nupro Gold tooth whitening system):
Advantage: Decreased cost over in-office bleaching procedures.
Disadvantage: Patient compliance.
Over-the-counter home bleaching:
Advantage: Cost
Disadvantage: Patient Compliance/abuse/overuse potential, no dentist supervision.
Colgate Simply White Clear Whitening gel:
Crest Whitestrips:
In-Office Bleaching:
1. Opalescence Xtra Boost (http://ultradent.com): 38% Hydrogen peroxide
Advantage: performed in the office, takes one hour, requires no
bleaching trays, immediate and rapid results
Disadvantage: Possible rebound of color if home care is not followed,
may take more than one visit to obtain desired results, possible
sensitivity, more expensive than take home protocols
2. BriteSmile (http://britesmile.com)
Advantages: Whitens in one hour, whitens both arches in one treatment,
low sensitivity, whitening lasts for years, uses state of the art
light-activated technology.
Disadvantages: Cost, availability (only selected dentists perform this procedure)
Bleaching has been shown to have various effects on existing restorations in a patient's mouth, as well as potential effects on restorations to be placed following bleaching. Detailed below are discussions under the headings of the various types of restorative material.
Composite Restorations
In considering the effects of bleach on composite restorations there are several aspects to consider. We will consider the effect of bleaching on bond strength, surface roughness and hardness, and bacterial adherence to bleached composites.
The Effect of Bleaching on Bond Strength
There have been many reports regarding the relationship between bleaching agents and the bond strength of composite materials to enamel following bleaching. Many investigators have reported a severe decrease in the average bond strength of composite to bleached versus unbleached enamel (Ben-Amar, 1995; Titley, 1993). Some researchers have reported that only when composite was bonded immediately after bleaching completions was the bond strength significantly reduced (Dishman et al, 1994). Other researchers have given examples of ways to counteract the adverse bleaching effects so that no statistical difference in bond strength was observed. Some examples of counteracting mechanisms are: exposing the enamel specimens to artificial saliva, water, or saline solution (Swift and Perdigao, 1998). Another suggestion was to use water-clearing solvents, for example acetone, ethanol, or acetone-based adhesive systems. Finally, it has been suggested that one remove the superficial enamel layer (Sung et al, 1999).
Various theories have been proposed to explain why the bond strength might be affected on bleached teeth. Changes in the enamel structure are one explanation--for example a loss of mineral content and increased porosity manifested as an over-etched appearance with a loss of the prismatic enamel form (Josey et al, 1996). It has been noted that the resin tags are reduced in number, less defined and shorter in bleached enamel (Titley et al, 1991). Also, it has been proposed that theoretically the enamel pores, dentin, and dentinal fluid could act as a peroxide/oxygen reservoir resulting in oxygen concentrating on the surface of enamel and preventing the complete cure of some resin tags (Titley et al, 1993). In this last case, alcohol-based bonding agents may minimize the oxygen's inhibitory effects through the interaction of alcohol with the residual oxygen.
Consistent with the preceding oxygen inhibition theory, Sung et al. (1999) wondered if newer bonding agents might have overcome the detrimental effects of bleaching on enamel. They evaluated OptiBond with an ethanol carrier, All-Bond 2 with an acetone carrier, and One-Step with an acetone carrier for the shear bond strength of a hybrid composite to enamel that was treated with 10% carbamide peroxide bleaching system. The groups using OptiBond, an ethanol-based bonding agent demonstrated no statistical decrease in bond strength between the bleached and unbleached group. The All-Bond 2 and One-Step bonding agents, both acetone based, demonstrated a significant reduction in bond strength among the bleached specimens when compared to the unbleached controls. This supports previous research, which suggested an alcohol primer may interact with the residual oxygen on the enamel surface and may permit higher bond strength. More research comparing ethanol-based bonding agents is suggested. This data along with previous experiments suggest that the use of alcohol-based bonding agents may result in less compromised composite bond strength when restorative work is to be completed immediately after bleaching. Sung et al (1999) still suggest postponing dental treatment for several days because many studies have shown an increase in the bond strength upon waiting.
Another study (Perdigao et al, 1998) evaluated the effects of carbamide peroxide bleaching agent on interfaces formed by two one-bottled dental adhesives to etched enamel. They proposed two null hypotheses that vital bleaching with commercial 10 percent carbamide peroxide gel would not increase the oxygen in the superficial layer of enamel or induce ultra-morphological changes in resin-enamel surfaces. The study used an Energy Dispersius Spectrometer at three depths and three different areas on the tooth (incisal, middle, and cervical third) to obtain the relative concentrations of oxygen, calcium, and phosphorous. Enamel was also etched and bonded with one of the three adhesives and a composite resin. Small enamel/resin sticks were removed and processed for transmission electron microscope analysis. The results showed no significant changes in the relative oxygen concentration of the bleached and unbleached-but-etched enamel. The calcium and phosphorus content of the enamel, however, did show significantly decreased relative concentrations in the bleached enamel. Bleaching also resulted in morphological alterations in the most superficial enamel crystallites, they were short and randomly disposed and the characteristic alignment of the crystallites was lost. In this study, the material containing acetone solvent (Prime & Bond 2.1), effectively wet and enveloped the enamel crystallites without any visible empty areas. The Syntac with water-based adhesive left empty spaces in the hybridized area of enamel under the SEM. Again, acetone has been suggested as the best solvent for carrying resin into conditioned tooth surfaces. This study (Perdigao et al, 1998) reports that no changes in the relative concentrations of oxygen were found in the enamel bleached with 10 percent carbamide peroxide. This indicates that the surface roughening or removal suggested by others is not needed. Reduction of bond strength caused by bleaching is therefore likely not related to inhibition of resin polymerization by oxygen accumulation in enamel. This does not mean however, that the dentin is not acting as an oxygen reservoir and affecting bond strength.
The Perdigao, et al. study also questioned the role of urea in surface changes. Urea is a denaturing agent for proteins and is released when carbamide peroxide breaks down. Urea is a known deproteinizing substance and may attack the intra-prismatic area of enamel. Urea may remove enamel proteins present in the intercrystallite spaces, and in this process of deproteinization, any mineral elements associated with enamel proteins are also removed. This could be why a decrease in the calcium and phosphorus concentration is observed in bleached enamel. This study refers to an article (Olson et al, 1998), which reported enamel deproteinization could be elicited by an ADA accepted bleaching gel. This is definitely an area that needs more research.
Overall, in considering bond strength, there is a variety of research with varying results. Despite the differing results, all the articles reviewed for this paper suggested using a dental adhesive with an acetone/ethanol carrying agent if restorative work had to be completed immediately, or preferably, waiting 24 hours to two weeks after bleaching to do any bonding procedures.
Bleaching Effects on Surface Color, Roughness and Hardness of Composites
The next area of concern with bleaching and composites is the effect of bleaching on the actual surface of the pre-existing composite. Cooley and Burger (1991) reported that with use of a colorimeter, 10 percent carbamide peroxide gels somewhat lightened the color of composite resins giving statistically significant results, but all the changes were not seen clinically. Carbamide peroxide bleaching gels may also roughen composite surfaces as detected by profilometry and SEM but again no clinically significant result could be observed (Cooley and Burger, 1991).
Garcia-Godoy (2002) evaluated the effects of three bleaching gels: Rembrandt XTRA Comfort Regular 12%, Ultra 30%, and Opalescence 10%, on two resin-based composites Esthet-X and Z-250. The authors used surface profilometry and micro-hardness measurement as a baseline before bleaching, the bleaching gels were then used according to manufacturer instructions. Measurements of surface and hardness were taken again following bleaching. Surface roughness, microhardness and micromorphological effects were measured and observed. The bleaching agents had no significant affect on the surface roughness, micro-hardness or micromorphology of the composite materials. Even with 30 percent carbamide peroxide no apparent effect was noted in the restorative materials tested. The authors concluded that no significant differences were found in the materials they tested , but that the effects of different bleaching agents must be tested on the different restorative materials on the market to determine their safety on each material's surface integrity.
Langsten et al (2002) evaluated changes in surface roughness of hybrid and microfilled composites after exposure with higher concentrations of carbamide peroxide. The study selected a hybrid composite, Prodigy, and microfilled composite, Silux Plus, for evaluation with Opalescence 20% and Opalescence Quick 35%. Surface roughness measurements were taken before and after treatment by mechanical profilometry. Stastical analysis did not detect a difference in the surface roughness in either the hybrid or microfill after exposure to the different concentrations of carbamide peroxide. The study concluded that higher concentration carbamide peroxide bleaching products have minimal to no surface effects on hybrid or microfilled composites.
Bacterial Adherence to Bleached Composites
Another aspect considered relative to composite material and bleaching was bacterial adherence to bleached surfaces of composite resin in vitro. Mor et al (1998) found a 10 percent solution of carbamide peroxide caused a significant increase in the adherence of Streptococcus mutans and Streptococcus sobrinus. A 10 percent solution of hydrogen peroxide caused significant increase in surface adherence of Streptococcus mutans and Streptococcus sobrinus after three days and seven days. Lastly, a decrease in adherence of Actinomyces viscosus was found after treatment with 10 percent hydrogen peroxide for seven days. This study indicates that bleaching agents may affect the adherence of certain cariogenic microorganisms to the outer surfaces of composite resin restorations. Since bacterial adherence to dental hard tissues and restorations is one of the most important events in the pathogenesis of dental caries, this is an important consideration. The surface roughness may not be clinically noticeable, but if it increases plaque accumulation, this could lead to secondary caries. If changes in the surface characteristics occur with bleaching it could have a direct effect on bacterial adherence to these surfaces. If this change in microenvironment shifts to a plaque layer with higher levels of Streptococcus mutans and Streptococcus sobrinus, a more cariogenic environment, a greater tendency toward caries progression may occur. More research into the duration of this effect following bleaching, and whether it is a result of surface changes or chemical composition changes, is required.
Amalgam Restorations
Amalgam is still widely used for restoring premolars and molars, therefore, it is likely many patients desiring teeth bleaching will have amalgam restorations. The concern is that a prolonged bleaching regimen with certain home bleaching products has the potential to increase mercury release from amalgam restorations (Robertobello et al, 1999). Hummert et al (1993) demonstrated that the amount of mercury released into carbamide peroxide solutions was as high as 30 times that seen with saline solution, and depended heavily on the type of bleaching agent applied and the type of amalgam. One study by Rotstein et al (1997) suggested that prolonged treatment with bleaching agents may cause microstructural changes in the amalgam surface and an increase in surface mercury levels in vitro. Other studies have investigated the amount of mercury release from amalgam restorations based on both the brand of amalgam as well as the brand of bleaching agent. One study attempted to measure the effects of three carbamide peroxide products, Opalescence, Nite White, and Platinum on mercury release from amalgam. The study constructed amalgam specimens in acrylic blocks, aged them for one week at 37 degrees Celsius (body temperature), and applied one of the three bleaching agents and a control solution for either 8, 40, or 80 total hours of bleaching. The specimens were then cleaned with a toothbrush, rinsed, and mercury testing was completed using a Gold Film Mercury Vapor Analyzer. The results showed there was no significant difference between the bleaches and control at 8 and 40 hours. However, at 80 hours Opalescence showed a marked increase in mercury release over the other agents tested (Robertobello, 1999.) This information is relevant in that Opalescence is one of the top selling tooth bleaching products. More research is necessary to determine the long-term effects of bleaching on amalgam restorations.
In another study, Rotstein et al (2000) studied the effects of mercury release in vitro on four commercial brands of amalgam. Unfortunately the brands of amalgam studied did not include Dispersalloy or Tytin, which are the brands of amalgam students are accustomed to using at the University of Minnesota. The study found that amalgam restorations exposed 48 hours to 10 percent carbamide peroxide showed higher concentrations of mercury release, which were statistically significant for all four amalgam brands tested: Megaloy, Mega+, Nongama 2, and Valiant PhD. The implications of this study and the research by Robertobello et al (1999) are two fold: First of all, mercury vapor release will vary depending on the type of amalgam as well as the type of bleaching agent applied. Secondly, the dental professional should be knowledgeable of both the recommended bleaching agent as well as the brand of amalgam used (which may not be possible in the case of patients with older, pre-existing amalgams). In fact, Perdigao and Swift (1998) suggested that not all combinations of amalgam and bleaching agents result in higher mercury vapor levels; it is likely that only specific combinations of materials produce substantial levels of mercury vapors. Also, the amount of mercury vapor released from amalgam restorations will vary substantially among individuals, particularly depending on the extent of bleaching (based on the nature of the tooth stain and personal expectations) and amount of amalgam present in the individual's mouth. For example, an individual who uses carbamide peroxide once a year and has only one buccal pit amalgam will most likely have less risk of mercury vapor release compared to individual with 17 amalgams who "power" bleaches his or her teeth every three months. More long-term research is necessary on this topic to ensure the safety of long-term tooth bleaching on amalgam restorations.
Another potential risk of bleaching patients with amalgam restorations is discoloration. In a case study published earlier this year, Haywood (2002) described a greening of the tooth-amalgam interface after bleaching treatment. The researcher found caries in the area of the green discoloration, which was only on a small segment of an amalgam restoration. Ironically certain parts of the amalgam did not demonstrate the green discoloration in the tooth, and in these areas no decay was found. Also, the greening did not occur in other amalgam restorations within the same patient's mouth. The report stated that the cause of the green discoloration is unclear, and more research on the topic is necessary. The implication of this study is that dentists should be prepared to replace amalgam restorations with greening to remove possible decay (Haywood, 2002).
In conclusion, many studies show that carbamide peroxide bleaching may increase the release of mercury vapor from amalgam restorations. Therefore, the bleaching of teeth restored with amalgam should be approached with caution. Dental professionals should also keep in mind that green discoloration of the restoration may occur with bleaching and these green areas may contain decay (Haywood, 2002 and Swift, 1997).
Ceramic Restorations
No effects on the color or physical properties of porcelain or other ceramic materials have been reported (Swift, 1997; Swift and Perdigao, 1998).
Temporary Restorations
Hydrogen peroxide and carbamide peroxide both cause microscopic changes to IRM restorations. Also, hydrogen peroxide may cause macroscopic changes to IRM, resulting in cracking and swelling. This is an important finding as Crest White Strips contain hydrogen peroxide as the active ingredient. On the other hand, IRM appears unaffected by carbamide peroxide on the macroscopic level (Swift, 1997; Perdigao and Swift, 1998). Methacrylate temporary restorations become orangish when exposed to carbamide peroxide (Robinson et al, 1996). However polycarbonate crowns and bis-acryl composite temporary restorations do not discolor upon bleaching (Perdigao and Swift, 1998).
Enamel staining after orthodontic treatment is rare, but has been reported. Clinical signs that have been reported include white spot lesions around the bracket area. This discoloration has been suggested to be a result of hypomineralization around the bonded brackets. Another reason for staining may be due to acid etching of the enamel. Acid etching creates pores in the enamel creating routes for bonding resins to penetrate deep into the enamel surface. Corrosion products from the metal brackets may penetrate into the pores and leave a stain after removal of the appliance (Hodges et al, 2000). Another reason for color changes after staining may be due to the reflection of natural light. A small layer of enamel may be removed during the debonding process that may alter the way light reflects off the tooth (Hintz et al, 2001).
There has been limited research into treating orthodontically stained teeth. A conservative approach that has been suggested involves thorough polishing using pumice mixed with hydrochloric acid, baking soda, and a Vivadent green cup. This procedure is only effective if the stain is superficial and hasn't penetrated too far into the enamel surface (Bishara et al, 1987). Hintz et al (2001) studied the effects of applying 10% carbamide peroxide gel to orthodontically-bonded/debonded teeth. It was concluded from this study that orthodontically treated teeth do respond to 10% carbamide peroxide bleaching. There was a longer waiting period before noticeable color changes were detected, however. One reason for this may be due to resin tags remaining in the etched enamel after debonding procedures. This may delay penetration of the peroxide gel into the enamel surface. Finally the most aggressive approach to treating orthodontically stained teeth is the application of porcelain veneers (Hodges et al, 2000).
Perhaps the best way of treating orthodontically stained teeth is through prevention. Clinicians should detect color changes early during orthodontic treatment. It is also important to know the corrosion capabilities of various metals used in orthodontics. Finally, the clinician should be aware of teeth that may be more susceptible to staining. For example, developmental anomalies of enamel may be a risk factor for enamel staining (Hodges et al, 2000).
Discoloration of the teeth can happen either naturally or iatrogenically. The natural discoloration may be involved with pulp necrosis, intrapulpal hemorrhage, calcific metamorphosis (irregular dentin), aging, developmental defects (endemic fluorosis and systemic drugs). The inflicted/iatrogenic discoloration can be due to various chemicals and material used in dentistry. Endodontically-related discoloration can be associated with incomplete removal of obturating materials from the pulp chamber, remnants of pulpal tissue, intracanal medicaments (phenolic or iodoform-based materials), and metallic coronal restorations.
In order for the internal bleaching to achieve the best outcome, dentists must individually diagnose and determine the prognosis in each case. Internal bleaching is most favorable in cases that involve pulpal necrosis, intrapulpal hemorrhage, remnants of pulpal tissue left in a tooth's pulp chamber, intracanal medicaments left in a tooth's pulp chamber, and tetracycline staining. When the prognosis of a case is questionable other methods of bleaching should be considered.
There are many different internal bleaching materials on the market for dentists to choose from. Three of the most popular materials are hydrogen peroxide, carbamide peroxide, and sodium perborate. Hydrogen peroxide is a powerful oxidizer that must be handled with care because it is unstable, will lose oxygen quickly, and may burn tissue on contact. Carbamide peroxide systems (3-15 percent) are mainly used for external bleaching and have been associated with varying degrees of damage to teeth and surrounding mucosa when used internally. They may adversely affect the bond strength of composite resins and their marginal seal. Therefore, carbamide peroxide should be used with caution as well. The safest and most easily controlled material for internal bleaching is sodium perborate. This agent is available in powder form or in various combinations. When fresh, it contains 95 percent perborate, corresponding to 9.9 percent oxygen. Sodium perborate is stable when it is dry, but in the presence of acid, warm air, or water it decomposes to form sodium metaborate, hydrogen peroxide, and nascent oxygen. Because sodium perborate is easily controlled and safer than concentrated hydrogen peroxide solutions, it is the material of choice for internal bleaching.
Some of the indications for internal bleaching technique are 1) discolorations of pulp chamber origin, 2) dentin discolorations, and 3) discolorations that are not amenable to external bleaching. Some contraindications are 1) superficial enamel discolorations, 2) defective enamel formation, 3) severe dentin loss, 4) presence of caries, and 5) discolored composites. If internal bleaching is indicated and the prognosis is favorable, then either thermocatalytic or walking bleach technique could be applied. These two techniques are described below.
The thermocatalytic technique involves placing the oxidizing agent in the pulp chamber, followed by applying heat. However, potential damages from this technique include the possibility of external cervical root resorption due to irritation to cementum and PDL, increased brittleness of coronal tooth structure due to heat, and chemical burns to the soft tissues. Because of such harmful effects, a safer alternative technique called "walking bleach" is used. The steps for walking bleach are as follows:
In conclusion, sodium perborate and walking bleach technique should be used in all situations requiring internal bleaching because it's safe and effective.
The side effects/adverse effects covered in our research were almost exclusively published in research focused on night guard vital bleaching and internal bleaching of endodontically-treated teeth. At this time published research findings are focused on these two most commonly used and easily controlled for (in formal research studies) methodologies. We hope more research will be forthcoming on some of the more heavily used over-the-counter bleaching regimes, since cost and convenience often drive consumers to this option. It follows that by having consumer's direct their own tooth whitening, there is much potential for improper usage and over-usage that may create their own set of adverse outcomes of which consumers, as well as their dentists, should be aware. After all, we will likely find ourselves called upon as "front line" examiners to identify these problems for further study in the research community, to educate our patients regarding potential adverse outcomes, and finally to diagnose and treat any resultant pathologies or damage.
Our review of the published research found that much debate still surrounds many of the noted tooth bleaching side effects/adverse effects. This field is still by most measures pretty "young" given that most techniques in use today have only been available since 1989 or even more recently, and have been widely used for even less time. As a consequence long term data is lacking and appropriate measures for evaluating bleaching side effects have yet to receive general agreement. Qualifiers having been outlined, we determined most published side effects to fall into the following categories: tooth sensitivity, gingival irritation, gastrointestinal mucosal irritation, changes to enamel and dentin hardness and surface stucture, changes associated with internal bleaching, co-carcinogenic effects with tobacco, and free radical generation and release. In the ensuing discussion we will define each of these areas, discuss controversial conclusions, where appropriate note ways to counter or minimize the effect, and also note what manufacturers of the various bleaching products used in our case presentation say with respect to these potential effects in their product inserts.
Tooth Sensitivity
Tooth sensitivity is probably the most well-known and established side effect of the various bleaching procedures. It is generally described as a sensitivity to hot and cold that occurs early in the bleaching process and will generally disappear within 24 to 48 hours of cessation of bleaching. The scientific literature varies widely in the reported prevalence of this effect (from zero to 100 percent) and often doesn't quantify how many of those noting sensitivity may have experienced tooth sensitivity pre-treatment. Furthermore, sensitivity is not well distinguished as to its degree. Generalities drawn from the data do support that sensitivity is experienced to some degree in a significant percentage of patients undergoing bleaching procedures with 10 percent carbamide peroxide solutions and whitening strips, and that the degree of sensitivity in some people is sufficient to prompt their decision to cease treatment.
It has been hypothesized that the sensitivity is due to the bleaching agent's ability to easily penetrate through enamel and dentin due to the acidic properties of bleaching agents, prolonged contact time, and presence of greater amounts of carbopol in bleaching solutions (Basting et al, 2001). Penetration into the dentinal tubules causes reversible pulpitis (Pohjola et al, 2002). Differences in sensitivity noted between different brands/formulations of bleaching products and between different patients could additionally be attributable to the fit of the bleaching tray, variations in the bleaching solution base or components--with some additives promoting more sensitivity than others, variations in thickening agent added to the carbamide peroxide, pH of the solution, chemical by-products of the solution, tray material, exposure time, solution concentration, patient medical condition, pulp size, patient gender, patient age and patient tooth characteristics (Leonard et al, 2001).
Sensitivity can be eliminated most quickly and reliably through stopping bleaching activity. For patients that still want that "Hollywood Smile" a variety of alternatives can be tried. Some practictioners advocate lessening exposure time each day, alternating exposure days with nonexposure days, or treating the teeth with high dose fluoride during treatment. Additionally, some companies have developed night guard solutions (NiteWhite Excel 2Z and Rembrandt XtraComfort) that have additives designed to limit sensitivity such as potassium nitrate and sodium fluoride. These new formulations have been shown in clinical trials to actually result in lower patient reports of sensitivity (Pohola et al, 2002).
Manufacturer's inserts for the three materials used in our case study do note tooth sensitivity as a potential side effect and advise a patient to discontinue the bleaching procedure and consult with their dentist should sensitivity be experienced.
Gingival Irritation
Gingival irritation is a commonly noted side effect with the various bleaching regimens studied. The irritation is presumed to result from escape of the bleaching material from the tooth area into the gingival margin area where salivary flow is typically low, allowing the material to sit relatively undisturbed. Some researchers also noted that tray fit can contribute to this problem either by permitting escape of bleaching material or by tissue reaction to the tray itself. The hydrogen peroxide can cause an acute inflammatory reaction and some of the other components of the bleaching solution can dehydrate tissues, both potential causes of patient discomfort. Overall, research reviewed for this project did not disclose any long term concerns regarding gingival tissue health and most noted that gingival irritation was quickly resolved in tissues that were healthy at the start of the procedure. One study noted that gingival irritation was much more likely to occur when the overlying epithelium was abnormally thin or permeable, and therefore it may be best to avoid use of known irritants in such patients (Walsh, 2000).
Common sense drives minimization of this side effect. Limit tissue exposure through good appliance fit and careful delivery of bleaching agent. Avoid use of bleaching solutions--known irritants--in patients with soft tissue inflammatory disorders or atrophic gingival epithelium.
Similar to the discussion of tooth sensitivity, the manufacturer's inserts for the three products used in our case study simply mention that gingival irritation can occur and advise an individual experiencing such irritation to consult with their dentist. The manufacturer's inserts were silent on other side effect or safety issues covered below.
GI Mucosal Irritation/Hyperplasia
Gastrointestinal mucosal irritation, including throat irritation, is rarely mentioned in the literature reviewed for this undertaking, however, it warrants mention from the common-sense standpoint that when bleaching teeth overnight or even with application of other whitening products there is some measure of product that will inevitably be swallowed by the patient/consumer. The amount swallowed will vary with length of time the bleaching product is in the mouth, fit of the delivery device and its ability to isolate the solution on the teeth from the rest of the oral environment, and the concentration of the solution. The composition of what is swallowed will vary with the ingredients of the solution and the chemical activity of those components over the time they are in the mouth. One study comparing various night guard products details simple throat irritation that is hypothesized to originate from the dehydrating effects of the anhydrous glycerine or other ingredient used to "carry" the active bleaching ingredient (Pohjola et al, 2002). Still others have attributed epithelial irritation to an acute inflammatory reaction following exposure to hydrogen peroxide (Tam, 1999). In general, throat irritation, when mentioned in the bleaching side effect profile, is reported by trial participants to be minor and transitory in nature and is similar to that irritation experienced in the gingiva.
While no other significant gastrointestinal adverse effects have been disclosed in human bleaching trials, several studies have looked at the issue of ingestion of the bleaching solutions and hydrogen peroxide in rodents, and researchers have reviewed data relating to human ingestion of peroxide solutions as reported to regional poison control centers. The rodent studies have generated much controversy regarding safe exposure levels and effects of exposure since examiners noted gastric tissue hyperplasia with chronic exposure and gastric tissue ulcerations with acute exposure. Acute exposure findings were used to set safety factors for daily exposure limits extrapolated to humans that many other researchers concluded were inappropriate for numerous reasons (Li, 1996). The chronic exposure findings have also been called into question relative to the experimental design and relevance of findings to humans (Walsh, 2000).
In the past several years since concerns were raised in the mid-1990's, it seems that most researchers have concluded that human ingestion of small amounts of hydrogen peroxide solutions equal to or less than 10 percent carries no significant risk of long term adverse effects, although many note that it may cause irritation to gastric mucous membranes that will ultimately be repaired by the body. Consistent with the opening comments for this section, more study should be directed at this area due to the potential for higher ingestion with the less customized and potentially less controlled solution delivery methods (generic trays, paint-on products, etc.) and potential for over-use noted with the growing over-the-counter product market.
Changes to Enamel and Dentin Hardness and Surface Structure
Many studies have had findings suggestive of 10 percent carbamide peroxide solutions causing morphological changes to the surface of enamel. A few studies have countered with findings of no significant alternations. The question appears to remain as to whether the changes are clinically significant and or permanent.
One of the more recent and comprehensive studies looked at changes to enamel and exposed dentin in vivo using fragments from extracted teeth that were fixed to the teeth of subjects participating in a nightguard bleaching trial. This study attempted to include the potential remineralizing effects of saliva, which were excluded in previous in vitro studies. This study found that following a three week bleaching regimen, enamel microhardness was statistically significantly lower as compared to placebo treated enamel. Exposed dentin microhardness did not differ significantly between the two treatment groups. The study author postulated that the lack of effect on dentin was due to its relatively lower inorganic material content, and that the effect on enamel was due to the changes in its relatively higher mineral content due to the bleaching agent's acidity. This paper concluded with a warning that bleaching agents should not be applied on early carious lesions due to their damaging effects (Basting, 2001).
Other studies on the effects of bleaching agents on enamel have looked at the difference in enamel effects of regular bleaching products and those containing potassium fluoride additives. Enamel surface hardness was shown to decrease significantly with traditional bleaching agents, but increase significantly with application of newer, fluoride-containing products (Akal, 2001). Another study attempted to evaluate the longer-term effects of bleaching product application on enamel. This study was fairly small and looked at bleaching of teeth that were later extracted as part of a complete denture treatment plan. Its findings led to the conclusion that bleaching does result in long-term alteration of the enamel surface with exposure of the enamel prisms, which could further lead to bacterial or stain penetration (Bitter, 1998). This study concluded with the need to warn patients of the potential for enamel attrition although it added that long-term consequences have yet to be conclusively determined. Still other research papers have mentioned the appearance of isolated case reports involving misuse/overuse of bleaching products with severe enamel attrition resulting. Overall, this area will benefit from further study and will be very important to our practices as we maintain our surveillance of our patient's enamel integrity.
Non-Vital Bleaching Complications
As discussed in the preceding section of this paper, most non-vital bleaching is done using the "Walking Bleach" technique. Agents advocated for use in this procedure are hydrogen peroxide/carbamide peroxide, sodium perborate, or a combination of the two compounds. Complications that have been documented in connection with this procedure include external cervical root resorption, periradicular inflammation, increased dentin permeability, and alteration of dentin's microstructure. Additionally, many experts believe that internal bleaching causes increased brittleness of coronal tooth structure, although this suspected result has not been demonstrated conclusively through research findings.
Thus far research has demonstrated that teeth treated with hydrogen peroxide (generally a 30 percent concentration is used) fare the worst in tests of tensile strength, shear strength, and microhardness of dentin. The results are generally better and close to the control group results for sodium perborate. Finally results were mixed for a combination of hydrogen peroxide and sodium perborate (Chng, 2002). External cervical root resorption induced by internal bleaching is not well understood as to its mechanism but has been routinely observed. Hydrogen peroxide is hypothesized to be the likely trigger for the resorption due to its irritating effects on dentin and ability to seep through dentin tubules (Walton, 1996). It is for this reason that our Endodontics curriculum advocates the sodium perborate technique. Potential mechanisms whereby bleaching agents affect dentin include hydrogen peroxide's ability to dissolve inorganic material, oxidize proteins, and the effects of acidic pH on dentin integrity‹30 percent hydrogen peroxide pH is 1.7 compared to sodium perborate's pH of 9.6 and the combination's pH of 6.5 (Chng, 2002).
As one can see, there are few conclusive findings in this area aside from simple elucidation of clinically observed effects and their higher correlation with hydrogen peroxide usage. Very little is known with certainty concerning the mechanisms for, and implications of, the noted effects. Similar to the above noted other side effects/adverse effects we will need to watch for new findings in this area and apply them to patient care accordingly.
Co-Carcinogenic Effect With Tobacco Component
In our previous coursework and in instructions to many bleaching products, it is advised that bleaching patients do not smoke while bleaching and in some cases for specified periods following bleaching. This recommendation follows from some older study data suggesting a co-carcinogenic effect between hydrogen peroxide and a specific tobacco compound. A 1986 hamster cheek-pouch study by Weitzman et al concluded that hydrogen peroxide could induce pathologic changes generally associated with neoplastic tissue transformation. The study also noted that hydrogen peroxide could further augment carcinogenic changes previously associated with a chemical compound called DMBA (9,10-dimethyl 1,2 benzanthracene), a tobacco-associated compound. These study findings have been widely criticized due to their design. This study and subsequent similar studies have not shown any carcinogenic or co-carcinogenic activity at hydrogen peroxide concentrations of three percent or less (Walsh, 2000). Therefore, while further study is warranted given the potential overuse/overexposure with over-the-counter product market growth, it does not appear that this is an area of definitive concern at this time.
Free Radical Generation and Release
As noted in the opening to this paper, carbamide peroxide and hydrogen peroxide are the most commonly used active ingredients in tooth whiteners. Chemically, carbamide peroxide contains approximately 35 percent hydrogen peroxide and it decomposes to form hydrogen peroxide and urea in aqueous solution. Hydrogen peroxide, as a compound, may be endogenously produced or enter our body from a wide range of sources such as tooth whiteners, wine, fruit juices and coffee (Li, 1996).
The reason why hydrogen peroxide is considered as a risk factor to our health is because it is a highly oxidative compound and easily decomposes into hydroxyl radicals. As a free radical with an unpaired electron, the hydroxyl radical readily attacks other molecules in its proximity and produces a new free radical and so on. The resulting damage, referred to as oxidative stress, leads to molecular and cellular dysfunction. The destruction of essential macromolecules by oxygen-based reactants is the basis of some diseases and is also believed to be involved in the processes of aging (Raha, 2000).
As hydrogen peroxide is capable of producing free radicals (oxygen species with an unpaired electron) which are highly reactive, it can damage proteins, lipids, and nucleic acids. Thus, hydrogen peroxide is potentially carcinogenic and mutagenic, and can cause many degenerative diseases (Li, 1996). Several studies have shown the involvement of reactive oxygen species including hydroxyl radical and hydrogen peroxide in colon cancer, breast tumors and stroke (Haklar, 2001; Floyd, 1990).
In addition to the carcinogenic potential of the hydroxyl free radical, it also may impact the process of aging. Studies find that oxidative stress and life span in different vertebrate species are inversely related. Animals that have long life spans produce oxygen radicals at a low rate. Across species, the longer the life span, the lower the rate of mitochondrial oxygen radical production. A low rate of free radical production can contribute to a slow aging rate both in animals that conform to the rate of living theory of aging and in animals with exceptional longevities. A high rate of DNA repair together with a low rate of free radical production near DNA are the two main characteristics found in long-living animal species (Perez, 1998).
Because of all the abovementioned possible biological effects associated with hydrogen peroxide, the toxicology, especially the carcinogenicity, of hydrogen peroxide with the use of tooth whitening has been investigated intensively. The carcinogenicity of hydrogen peroxide has been studied by a number of investigators. Their results are controversial. Most investigators found no evidence of carcinogenicity of hydrogen peroxide and a few even showed an anticarcinogenic effect, while several studies reported carcinogenicity or co-carcinogenicity of hydrogen peroxide (Li, 2000).
Although hydrogen peroxide is capable of causing biological damage through free radicals, it is not a surprise to find that the animal studies indicates a lack of carcinogenicity of three percent hydrogen peroxide in tooth whiteners (Li, 2000). To assess the risk of carcinogenicity of hydrogen peroxide, several factors have to be considered. These include the amount and rate of the production of endogenous hydrogen peroxide; the existence of hydrogen peroxide in air, water, and food (Li, 1996); human body's antioxidant defensive mechanisms; and finally the amount of hydrogen peroxide taken up from applied tooth whitener. Considering the fact that carcinogenicity occurs only when the oxidative potential overwhelms the antioxidant defenses, intuitively carcinogenosis requires several simultaneous developments to cause any observable effects: direct contact of hydrogen peroxide with tissues, the failure of normal human antioxidant defenses, the access of free radicals to target DNA, and the failure of damaged DNA to repair itself (Li, 2000). Quantitatively, studies show that the daily production of hydrogen peroxide by the human liver alone is about 6.48 grams per day (Rowacc, 1969). Presumably this amount is effectively managed by a healthy person. The exposure dose of hydrogen peroxide in a two-arch bleaching treatment with gels of 10 percent carbamide peroxide would be 3.52mg per day, which is almost 2000 times less than or just 0.054% of the daily production of hydrogen peroxide in the liver without considering other sources of hydrogen peroxide (Li, 2000). Therefore, the negligible amount of hydrogen peroxide swallowed is far from overwhelming our body's natural antioxidant system when it functions normally. From this one can conclude that the chance of getting cancer simply from bleaching teeth when it is properly used is very slim. However, if body's antioxidant mechanism is greatly challenged by other means, the swallowed amount of hydrogen peroxide from dental bleaching may exacerbate an existing oxidative crisis.
In addition to vitamin E and carotene, several other antioxidants have been studied and are reported to play a protective role in reducing free radicals. Ingestion of viable probiotics is associated with anticarcinogenic effects. Butyrate is one of such protective agents and is associated with lowering colon cancer risk through inhibition of hydrogen peroxide in human colon cells (Wollowski, 2001). Melantonin acts as free radical scavenger and exerts its protective action against a vast array of conditions such ischemia/reperfusion injury, toxin exposure, and other conditions, where free radical damage is a component of the condition (Reiter, 2002). In addition, Japanese-style fermented soy sauce has been studied and is reported to have an effect on inhibiting carcinogenesis by reducing hydrogen peroxide concentration in human polymorphonuclear leukocytes (Kataoka,1997).
As has been the theme throughout our research review, we see that there are many aspects peripheral to this relatively young field of contemporary tooth bleaching techniques that are in need of more research. This likely explains why, after numerous lectures on the topic in our first three years of dental school, so many of us have lingering questions regarding the more long term aspects of this dental procedure. Some of these questions involve how long the color change lasts, the safety of these treatments, and the long-term effects of these products on our patient's dentition. We hope this discussion, if nothing else, has at least served to help you categorize the various potential effects bleaching can have on a patient's restorations, teeth, and overall health status, as well as better understand the current status of knowledge in each of these areas. The good news is that we have learned how to monitor the scientific literature to keep abreast of findings in this field. The bad news is that there remains much to be known and that only time and further study will add to our knowledge base. As noted in the sections on restorations and side effects, we have much concern over the potential impact on our patient's health and dental integrity of the growth of the over-the-counter products and their high abuse/overuse potential and lack of scientific scrutiny. It will be particularly important for us to watch for signs of problems indicative of bleaching agent damage and advise our patients with these signs, or with bleaching product questions, of the potential for damage with misuse.
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