Potential Local Adverse Effects Associated With Bleaching

Tooth bleaching requires direct contact of the material with the enamel surface. Although the gingival contact with the bleaching material is not intended, it often occurs when the material is applied at home by consumers. For some products which are meant for home use, such as, the strips, the gingival contact is inevitable. Some bleaching regimens also involve a continued contact period of up to 7 to 8 hours (overnight).

Potential adverse effects on Enamel
The effects of bleaching on enamel were primarily examined in-vitro using extracted human or bovine teeth. Bleaching seems to have minimal or no effects on enamel micro hardness and mineral contents, however, the results on enamel surface change are inconsistent. Most scanning electron microscopic studies showed little or no morphologic changes of enamel surfaces associated with bleaching, whereas several studies reported significant alteration of enamel surfaces, including depression, porosity and erosion, after bleaching. In most cases, however, the observed enamel surface alterations varied with the bleaching products used and seemed to be associated with those patients using acidic pre-rinse or gels of low pH. Studies have shown that some soft drinks and fruit juices are capable of causing comparable or greater demineralization and surface alteration of the enamel than those reported for bleaching agents. To date, no clinical evidence of adverse effects of professional at-home bleaching systems on enamel has been reported.

Potential adverse effects on gingiva
H2O2 (hydrogen peroxide) at high concentrations is an irritant and also cytotoxic. In cell culture experiments, H2O2 is cytotoxic at concentrations ranging from 1.7 to 19.7µg/ml. At concentrations of 10% or more, H2O2 is potentially corrosive to the mucous membrane or skin, causing a burning sensation and tissue damage. Studies also reported that commercial peroxide-based gels induced toxicity.
During the in-office bleaching procedure, which routinely involves the use of H2O2 at a concentration of 25% or more, adequate barriers are necessary to protect the gingiva from mucosal damage. If a leakage exists, serious tissue burn can occur. Because of this potential risk, local anesthesia should not be used for in-office bleaching, so that the patient can alert the dentist when the gel seeps through the barriers to cause a burning sensation or pain. However, simply relying on the patient’s response is inadequate; clear instructions to the patient to report any discomfort, careful examination of the seal of the barrier after gel application, and frequent monitoring of the seal throughout the bleaching process are all necessary to minimize the risk of gingival damage and irritation.
Gingival irritation is also common in at-home tooth bleaching. A study found a higher(33.3%) prevalence of gingival irritation in patients using strips of 6.5% H2O2 compared with those using at home tray bleaching with 10% carbamide peroxide (3.5% H2O2). A separate study found that 50% of patients using the 6.5% H2O2 strips reported gingival irritation, which was about 3 times of those using the 5.3% strips (16.7%). These data indicate that the risk of gingival irritation in at-home bleaching is associated with the H2O2 concentration in the bleaching gel, that is, there is a higher prevalence of gingival irritation while using bleaching gels of higher peroxide concentrations. In most cases, the gingival irritation is mild to moderate, tends to be transient, and dissipates when the application discontinues. So far, studies on professional at-home bleaching reported no significant or permanent gingival damage.