Present tooth-bleaching techniques are based upon hydrogen peroxide as the active agent. It is applied directly, or produced in a chemical reaction from sodium perborate or carbamide peroxide. More than 90% immediate success has been reported for intracoronal bleaching of non-vital teeth, and in the period of 1-8 years' observation time, from 10 to 40% of the initially successfully treated teeth needed re-treatment. Cervical root resorption is a possible consequence of internal bleaching and is more frequently observed in teeth treated with the thermo-catalytic procedure. When the external tooth-bleaching technique is used, the first subjective change in tooth color may be observed after 2-4 nights of tooth bleaching, and more than 90% satisfactory results have been reported. Tooth sensitivity is a common side-effect of external tooth bleaching observed in 15%-78% of the patients, but clinical studies addressing the risk of other adverse effects are lacking. Direct contact with hydrogen peroxide induced genotoxic effects in bacteria and cultured cells, whereas the effect was reduced or abolished in the presence of metabolizing enzymes. Several tumor-promoting studies, including the hamster cheek pouch model, indicated that hydrogen peroxide might act as a promoter. Multiple exposures of hydrogen peroxide have resulted in localized effects on the gastric mucosa, decreased food consumption, reduced weight gain, and blood chemistry changes in mice and rats. Our risk assessment revealed that a sufficient safety level was not reached in certain clinical situations of external tooth bleaching, such as bleaching one tooth arch with 35% carbamide peroxide, using several applications per day of 22% carbamide peroxide, and bleaching both arches simultaneously with 22% carbamide peroxide. The recommendation is to avoid using concentrations higher than 10% carbamide peroxide when one performs external bleaching. We advocate a selective use of external tooth bleaching based on high ethical standards and professional judgment.

Tooth Bleaching—a Critical Review of the Biological Aspects:

In vitro degradation of resin-dentin bonds analyzed by microtensile bond test, scanning and transmission electron microscopy.

Nonvital Tooth Bleaching: A Review of the Literature and Clinical Procedures

Effect of bleaching on restorative materials and restorations—a systematic review

Dentin bonding—Variables related to the clinical situation and the substrate treatment

Oxygen inhibits polymerization of resin-based materials. We hypothesized that compromised bonding to bleached enamel can be reversed with sodium ascorbate, an anti-oxidant. Sandblasted human enamel specimens were treated with distilled water (control) and 10% carbamide peroxide gel with or without further treatment with 10% sodium ascorbate. They were bonded with Single Bond (3M-ESPE) or Prime&Bond NT (Dentsply DeTrey) and restored with a composite. Specimens were prepared for microtensile bond testing and transmission electron microscopy after immersion in ammoniacal silver nitrate for nanoleakage evaluation. Bond strengths of both adhesives were reduced after bleaching but were reversed following sodium ascorbate treatment (P < 0.001). Resin-enamel interfaces in bleached enamel exhibited more extensive nanoleakage in the form of isolated silver grains and bubble-like silver deposits. Reduction of resin-enamel bond strength in bleached etched enamel is likely to be caused by a delayed release of oxygen that affects the polymerization of resin components.

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Reversal of Compromised Bonding in Bleached Enamel

Compromised bonding of total-etch adhesives to dentin treated with oxalate desensitizers results from the interference of a surface layer of acid-resistant crystals of calcium oxalate. We hypothesize that effective tubular occlusion and dentin bonding may be simultaneously achieved by depleting dentin surfaces of calcium with acids before desensitizer application. Dentin specimens treated with 4 oxalate desensitizers before or after being acid-etched were bonded with a two-step adhesive. Microtensile bond strengths ( micro TBS) were significantly lower, compared with the control, when oxalates were used before the specimens were acid-etched; in contrast, when oxalates were used after acid-etching. micro TBS were similar to nonoxalate-treated controls. Dentin surfaces and tubular orifices were covered with a surface layer of crystals when desensitizers were applied to fractured dentin and smear-layer-covered dentin before specimens were acid-etched. However, when the dentin was acid-etched prior to the application of oxalate desensitizers, the crystals were largely limited to the subsurface of dentinal tubules, where they did not interfere with subsequent resin bonding.

Integrating Oxalate Desensitizers with Total-etch Two-step Adhesive

One-step self-etch adhesives behave as permeable membranes after polymerization, permitting water to move through the cured adhesives. We hypothesize that osmotic blistering occurs in bonded enamel when these adhesives are used without composite coupling. Tooth surfaces from extracted human premolars were bonded with 5 one-step self-etch adhesives. They were immersed in distilled water or 4.8 M CaCl2, and examined by stereomicroscopy, field-emission/environmental SEM, and TEM. Water blisters were observed in bonded enamel but not in bonded dentin when specimens were immersed in water. They collapsed when water was subsequently replaced with CaCl2. Blisters were absent from enamel in specimens that were immersed in CaCl2 only. Water trees were identified from adhesive-enamel interfaces. Osmotic blistering in enamel is probably caused by the low water permeability of enamel. This creates an osmotic gradient between the bonded enamel and the external environment, causing water sorption into the interface.

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Y.F. Mak

Papers

Reversal of Compromised Bonding in Bleached Enamel

Integrating Oxalate Desensitizers with Total-etch Two-step Adhesive

Osmotic Blistering in Enamel Bonded with One-step Self-etch Adhesives