Bleaching Mechanism :

The mechanism is oxidation / reduction process called "Redox process".

In this process the oxidizing agent has free radical with unpaired electrons, which it gives up, becoming reduced. The reducing agent(i.e. the substance being bleached) accepts the electrons and becomes oxidized.

Reducing agent                       Oxidising agent

Tooth                            ;             Bleaching material

After the process

Tooth is oxidized                       Bleaching material is reduced

(Organic pigmentation of tooth oxidized)

In addition to the chemical effect other mechanisms include

cleansing of tooth surface
temporary dehydration of enamel during the bleaching process,
change of enamel surface.

Bleaching is least effective when the original tooth color is grayish and may require custom bleaching trays. Bleaching is most effective with yellow discolored teeth. If heavy staining or tetracycline damage is present on a patient's teeth, and whitening is ineffective (tetracycline staining may require prolonged bleaching, as it takes longer for the bleach to reach the dentine layer), there are other methods of masking the stain. Bonding, which also masks tooth stains, is when a thin coating of composite material is applied to the front of a person's teeth and then cured with a blue light. A veneer can also mask tooth discoloration.

In-chair whitening is faster and more effective in comparison to the take-home bleaching options. Some clinicians also make custom bleaching trays for you, which can take up to a week to create, so that after the whitening treatment is completed, you are able to use these trays in the future for maintenance of your bleaching with at home kits or for the use of desensitising products.

Light-accelerated bleaching :

In modern times of today, every individual is making concious efforts to look desirable and presentable. when everyone is in persuit of beauty, no wonder patients comes to dentists desiring perfect smile like billboard models. Having asthetic dentistry evolved largely inpast few years, there are different invasive and non invasive techniques to enhance patient's esthetic dental appearance.

Speaking of Non- invasive in terms of non exposure of inner layers of tooth like dentine, pulp, with relatively not major operative work, many cases can be managed by procedures like Bleaching giving individual whiter and pleasant esthetic appearance retaining his/her natural teeth as they are.

peroxide concentrationNanoparticle catalysts for reduced hydrogen :

A recent addition to the field is new light-accelerated bleaching agents containing lower concentrations of hydrogen peroxide with a titanium oxide nanoparticle-based catalyst. Reduced concentrations of hydrogen peroxide cause lower incidences of tooth hypersensitivity.[39] The nanoparticles act as photocatalysts, and their size prevents them from diffusing deeply into the tooth. When exposed to light, the catalysts produce a rapid, localized breakdown of hydrogen peroxide into highly reactive radicals. Due to the extremely short lifetimes of the free radicals, they are able to produce bleaching effects similar to much higher concentration bleaching agents within the outer layers of the teeth where the nanoparticle catalysts are located. This provides effective tooth whitening while reducing the required concentration of hydrogen peroxide and other reactive byproducts at the tooth pulp.

Internal bleaching is a process which occurs after a tooth has been endodontically treated. This means that the tooth will have had the nerve of the tooth extirpated or removed through a root canal treatment at the dentist or by a specialist endodontist. Internal bleaching is often sought after in teeth which have been endodontically treated as tooth discolouration becomes a problem due to the lack of nerve supply to that tooth. It is common to have this internal bleaching done on an anterior tooth (a front tooth that you can see when smiling and talking). A way around this is by sealing off the bleaching agent inside the tooth itself and replacing it every few weeks until the desired shade has been achieved. The amount of time between appointments varies from patient to patient and with operator preference until the desired shade has been achieved.[40] Even though this is a great option, the disadvantage of this treatment is a risk of internal root resorption of the tooth that is being internally bleached. This may not occur in every patient or every tooth, and its occurrence is difficult to determine prior to completing the treatment.

At home tooth whitening products are available from dentists or 'over the counter (OTC). At home whitening methods include; over the counter strips and gels, whitening rinses, whitening toothpastes, and tray-based tooth whiteners. OTC products can be used for milder cases of tooth staining. Home-based bleaching (following manufacturer's instructions) would result in less tooth sensitivity than in-office bleaching.

Strips and gels
The plastic whitening strips contain a thin layer of peroxide gel and are shaped to fit the buccal/labial surfaces of teeth. Many different types of whitening strips are available on the market, after being introduced in the late 1980s. Specific whitening strip products have their own set of instructions however the strips are typically applied twice daily for 30 minutes for 14 days. In several days, tooth colour can lighten by 1 or 2 shades. The tooth whitening endpoint does depend on the frequency of use and ingredients of the product.
Whitening gels are applied onto the tooth surface with a small brush. The gels contain peroxide and are recommended to be applied twice a day for 14 days. The tooth whitening endpoint like that of the whitening strips.

Rinses
Whitening rinses work by reaction of the oxygen sources such as hydrogen peroxide within the rinse and the chromogens on or within the tooth. It is recommended to use twice a day, rinsing for one minute. To see an improvement in shade colour, it can take up to three months.

Toothpastes
Whitening toothpastes are different to regular toothpastes in that they contain higher amounts of abrasives and detergents to be more effective at removing tougher stains. Some whitening toothpastes contain low concentrations of carbamide peroxide or hydrogen peroxide which help lighten tooth colour however they do not contain bleach (sodium hypochlorite). With continuity of use over time, tooth colour can lighten by one or two shades.

Tray-based
Tray-based tooth whitening is achieved by wearing a fitted tray containing carbamide peroxide bleaching gel overnight or for two to four hours a day. If manufacturer's instructions are followed, tooth whitening can occur within three days and lighten teeth by one or two shades. This type of tooth whitening is available over-the-counter and professionally from an oral health professional.

Baking soda
Baking soda is a safe, low abrasive, and effective stain removal and tooth whitening toothpaste. Tooth whitening toothpaste that have excessive abrasivity are harmful to dental tissue, therefore baking soda is a desirable alternative. To date, clinical studies on baking soda report that there have been no reported adverse effects. It also contains acid-buffering components that makes baking soda biologically antibacterial at high concentrations and capable of preventing growth of Streptococcus mutans. Baking soda might be useful for caries-prone patients as well as those who wish to have whiter teeth.

Now Moving Towards some invasive operative techniques of Esthetic dentistry, treatment modalities which help in changing patients anterior esthetic appearance are:

Composite Restorations

Laminates and veneers

Orthodontic treatments

Dental composite restorations are widely used tooth coloured esthetic restorations and almost completely replacing old contemporary Silver amalgam and dental cement restorations making Dental practice Mercury free.
Dental composite resins (better referred to as "resin-based composites" or simply "filled resins") are types of synthetic resins that are used in dentistry as restorative material or adhesives. Dental composite resins have certain properties that will benefit patients according to the patient's cavity. It has a micro-mechanical retention property that makes composite more effective for filling small cavities where amalgam fillings are not as effective and could therefore fall out (due to the macro-mechanical retention property of amalgam). Synthetic resins evolved as restorative materials since they were insoluble, of good tooth-like appearance, insensitive to dehydration, easy to manipulate and reasonably inexpensive.

Dental composites, or resin-based composites, are synthetic materials that combine polymeric matrix with a dispersion of glass, mineral, or resin filler particles and/or short fibers by coupling agents. Just like dental amalgam, they are used to restore tooth structure lost through trauma, caries, or other diseases. Composites can also be used as cements to cement crowns and veneers, etc. While the amalgam is phasing out in dentistry, composites have become one of the most widely used esthetic restorative materials

Setting mechanisms of resin composite Types of setting mechanisms/Polymerisation:

Chemical cure (self-cure / dark cure)

Light cure

Dual cure (setting both chemically and by light)

1. Chemically cured resin composite is a two-paste system (base and catalyst) which starts to set when the base and the catalyst are mixed together.

2. Light cured resin composites contains a photo-initiator (e.g. camphorquinone) and an accelerator. The activator present in light activated composite is diethyl-amino-ethyl-methacrylate (amine) or diketone.They interact when exposed to light at wavelength of 400-500 nm, i.e, blue region of the visible light spectrum. The composite sets when it is exposed to light energy at a set wavelength of light. Light cured resin composites are also sensitive to ambient light, and therefore, polymerisation can begin before use of the curing light.

3. Dual cured resin composite contains both photo-initiators and chemical accelerators, allowing the material to set even where there is insufficient light exposure for light curing. Chemical polymerisation inhibitors (e.g. monomethyl ether of hydroquinone) are added to the resin composite to prevent polymerisation of the material during storage, increasing its shelf life.

Filler types and particle size:

Resin filler can be made of glasses or ceramics. Glass fillers are usually made of crystalline silica, silicone dioxide, lithium/barium-aluminium glass, and borosilicate glass containing zinc/strontium/lithium. Ceramic fillers are made of zirconia-silica, or zirconium oxide.
Fillers can be further subdivided based on their particle size and shapes such as:

1. Macrofilled filler
Macrofilled fillers have a particle size ranging from 5 - 10 μm. They have good mechanical strength but poor wear resistance. Final restoration is difficult to polish adequately leaving rough surfaces, and therefore this type of resin is plaque retentive.

2. Microfilled filler
Microfilled fillers are made of colloidal silica with a particle size of 0.4 μm. Resin with this type of filler is easier to polish compared to macrofilled. However, its mechanical properties are compromised as filler load is lower than in conventional (only 40-45% by weight). Therefore, it is contraindicated for load-bearing situations, and has poor wear resistance.

3. Hybrid filler
Hybrid filler contains particles of various sizes with filler load of 75-85% by weight. It was designed to get the benefits of both macrofilled and microfilled fillers. Resins with hybrid filler have reduced thermal expansion and higher mechanical strength. However, it has higher polymerisation shrinkage due to a larger volume of diluent monomer which controls viscosity of resin.

4. Nanofilled filler
Nanofilled composite has a filler particle size of 20-70 nm. Nanoparticles form nanocluster units and act as a single unit. They have high mechanical strength similar to hybrid material, high wear resistance, and are easily polished. However, nanofilled resins are difficult to adapt to the cavity margins due to high volume of filler.

5. Bulk filler
Bulk filler is composed of non-agglomerated silica and zirconia particles. It has nanohybrid particles and filler load of 77% by weight. Designed to decrease clinical steps with possibility of light curing through 4-5mm incremental depth, and reduce stress within remaining tooth tissue. Unfortunately, it is not as strong in compression and has decreased wear resistance compared to conventional material.

Classification of resin composites according to handling characteristics This classification divides resin composite into three broad categories based on their handling characteristics:

Universal: advocated for general use, oldest subtype of resin composite

Flowable: fluid consistency, used for very small restorations

Packable: stiffer, more viscous material used solely for posterior parts of the mouth Manufacturers manipulate the handling characteristics by altering the constituents of the material. Generally, the stiffer materials (packable) exhibit a higher filler content whilst fluid materials (flowable) exhibit lower filler loading.

1. Universal:
This is the traditional presentation of resin composites and performs well in many situations. However, their use is limited in specialised practice where more complex aesthetic treatments are undertaken. Indications include: the restoration of class I, II and III and IV where aesthetics is not paramount, and the repair of non-carious tooth surface loss (NCTSL) lesions. Contraindications include: restoration of ultraconservative cavities, in areas where aesthetics is critical, and where insufficient enamel is available for etching.

2. Flowable:
Flowable composites represent a relatively newer subset of resin-based composite material, dating back to the mid-1990s. Compared to universal composite, flowables have a reduced filler content (37–53%) thereby exhibiting ease of handling, lower viscosity, compressive strength, wear resistance and greater polymerisation shrinkage. Due to the poorer mechanical properties, flowable composites should be used with caution in high stress-bearing areas. However, due to its favourable wetting properties, it can adapt intimately to enamel and dentine surfaces. Indications include:
restoration of small class I cavities, preventive resin restorations (PRR), fissure sealants, cavity liners, repair of deficient amalgam margins, and class V (abfraction) lesions caused by NCTSL. Contraindications include: in high stress-bearing areas, restoration of large multi-surface cavities, and if effective moisture control is unattainable.