Enamel has long been the primary concern when it comes to bleaching-related damage. But could the problem go deeper than that?
A new study in the Journal of Esthetic and Restorative Dentistry (June 2010, Vol. 22:3, pp. 179-184) has found that long stretches of bleaching may harm dentin.
The in vitro study found that the length of bleach application time to dentin using 10% carbamide peroxide was inversely correlated with dentin fracture toughness.
Most studies that have looked at the effect of bleach focused on surface effects such as hardness or surface morphology changes, said lead author Laura Tam, D.D.S., an associate professor of restorative dentistry at the University of Toronto, in a DrBicuspid.com interview.
"Fracture toughness is an intrinsic property that I consider more important than surface effects because fracture toughness indicates tooth fracture resistance," she added.
Previous research
A previous study by the same author (Journal of Dental Research, December 2007, Vol. 86:12, pp. 1193-1197) reported a decrease in dentin fracture toughness after the application of peroxide bleaching products.
That study tested the hypothesis that dental bleaches significantly affect dentin structural integrity. Dr. Tam and her colleague looked at compact test specimens composed of human dentin and bleach. They then applied 16% or 10% carbamide peroxide, 3% hydrogen peroxide, or a control material containing 0.1% sodium fluoride directly or indirectly to dentin through enamel for 6 hours a day over 2 or 8 weeks.
— Laura Tam D.D.S.
"The in vitro reduction in dentin fracture toughness caused by the application of peroxide bleaches was greater for the direct application method, longer application time, and higher bleach concentration," they concluded.
The aim of the new study was to further investigate this decrease by evaluating the effect of different peroxide application times.
The researchers looked at 60 extracted human molars that were subjected to bleach or placebo gel. Molars with visible caries, cracks, and fractures were excluded. The fracture toughness specimens were randomly distributed into five groups.
Within the groups, bleach (10% carbamide peroxide Opalescence, Ultradent) was applied to the specimens for 0 hours in group A, 84 hours in group B, 168 hours in group C, 252 hours in group D, and 336 hours in group E. The placebo was applied for 336 hours in group A, 252 hours in group B, 168 hours in group C, 84 hours in group D, and 0 hours in group E.
The bleaching time periods of 0, 84, 168, 252, and 336 hours were chosen to represent bleaching regimens of 0, 2, 4, 6, and 8 weeks, respectively, assuming overnight bleaching for 6 hours a day.
Fracture toughness testing was done 24 hours after the end of bleaching using tensile loading at 10 mm/min.
Dentin fracture toughness after 252 and 336 hours was significantly reduced compared to the 0- and 84-hour bleach times, the authors noted.
Limit bleaching time
The results suggest that the length of bleaching time is associated with dentin fracture toughness, and a relatively progressive reduction in dentin fracture toughness would be expected over the course of 336 hours of bleaching, the authors wrote.
"Changes in the structural integrity of dentin should be taken into consideration when bleaching is used as a treatment for whitening teeth," Dr. Lam and her colleagues wrote. "The results suggest that it would be prudent to minimize the length of time for clinical bleaching procedures when dentin is directly exposed to bleach."
They did point out that since bleach was applied directly to the dentin surface as opposed to the enamel surface, the rate of dentin fracture toughness decrease over time was far greater than in the clinical situation where bleach is applied primarily to the enamel surface.
However, some circumstances in which dentin could be directly exposed to bleach include gingival recession and occlusal attrition, Dr. Tam noted. Decay can also be included, but patients should not be bleaching their teeth if they have caries, she warned.
Also, although enamel is the intended target for bleach application, dentin can be exposed to bleach indirectly if it penetrates through the enamel to the dentin, Dr. Tam added.
"Enamel and dentin have no regenerative capabilities, and repeated bleaching treatments have the potential for cumulative negative effects," she said. "Bleach teeth only in appropriate situations, and use the shortest treatment time necessary to achieve results."
Bruce Matis, D.D.S., a professor in the department of restorative dentistry at the Indiana University School of Dentistry who has done research on the effect of bleaching on fracture toughness of resin composites (Operative Dentistry, November 2009, Vol. 34:6, pp. 703-708), noted that although Dr. Tam's study is done according to good scientific principles, in vivo findings are sometimes much different than in vitro findings.
"I am not yet convinced that there is a loss of fracture toughness in dentin with the use of tooth-whitening agents, but I accept that may in fact be true," he told DrBicuspid.com. "For that reason and a host of others, I think we need to be more careful in dispensing tooth-whitening agents outside the professionals' control."
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