Friday, September 9, 2011

Mechanism of tooth bleaching

Bleaching is a decolourisation or whitening process that can occur in solution or on a surface. The colour producing materials in solution or on a surface are typically organic compounds that possess extended conjugated chains of alternating single or double bonds and often include
heteroatoms, carbonyl, and phenyl rings in the conjugated
system and are often referred to as a chromophore.
Bleaching and decolourisation of the chromophore can
occur by destroying one or more of the double bonds in
the conjugated chain, by cleaving the conjugated chain, or by
oxidation of other chemical moieties in the conjugated
chain. Hydrogen peroxide oxidises a wide variety of
organic and inorganic compounds

The mechanisms of
these reactions are varied and dependent on the substrate,
the reaction environment, and catalysis. In general, the
mechanism of bleaching by hydrogen peroxide is not well
understood and it can form a number of different activeoxygen species depending on reaction conditions, including
temperature, pH, light and presence of transition metals.
Under alkaline conditions, hydrogen peroxide bleaching
generally proceeds via the perhydroxyl anion (HO2
). Other
conditions can give rise to free radical formation, for
example, by homolytic cleavage of either an O–H bond or
the O–O bond in hydrogen peroxide to give H + OOH and
2 OH (hydroxyl radical), respectively. Under photochemically
initiated reactions using light or lasers, the formation
of hydroxyl radicals from hydrogen peroxide has been
shown to increase.
The mechanism by which teeth are whitened by oxidising
materials such as hydrogen peroxide and carbamide peroxide
are currently not fully understood. Considering the
available literature, evidence points towards the initial
diffusion of peroxide into and through the enamel to reach
the enamel dentine junction and dentine regions. Indeed, in
vitro experiments by a number of authors have demonstrated
the penetration of low levels of peroxide into the pulp
chambers of extracted teeth after exposure times of 15–
30 min from a range of peroxide products and solutions.
The levels of peroxide measured in these experiments is
considerably much lower than that needed to produce pulpal
enzyme inactivation.
As peroxide diffuses into the tooth, it can react with organic
coloured materials found within the tooth structures leading
to a reduction in colour. This is particularly evident within
dentine as demonstrated by McCaslin et al.who showed,
using hemi-sectioned human teeth mounted on glass slides,
that following external bleaching with carbamide peroxide,
colour changes occurred throughout the dentine. Indeed, the
treatment of dentine specimens with 10% carbamide peroxide,
5.3% and 6% hydrogen peroxide has been shown to give
a significant reduction in yellowness and an increase in
whiteness.35,36 In addition, Sulieman et al. showed using
sectioned extracted teeth stained internally with black tea
chromophores that significant bleaching occurred within the
dentine, particularly on the buccal surface where a 35%
hydrogen peroxide gel had been applied.
For tetracycline stained teeth, the colour is derived from
photo-oxidation of tetracycline molecules bound within the
tooth structures. In some cases, it is possible to bleach these
teeth to give significant and long lasting tooth whitening.
The mechanism by which peroxide affects the tetracycline
stain is considered to be by chemical degradation of the
unsaturated quinone type structures found in tetracycline
leading to less coloured molecules. However, in contrast
there appears to be a paucity of information available in the
literature regarding the nature and chemical composition of
the coloured materials naturally found within the dental hard
tissues and the mechanistic effects of peroxide on these
structures. Thus, this is clearly an area that requires further
research if the chemical mechanistic aspects of tooth
bleaching are to be significantly resolved.


Post a Comment


Related Posts Plugin for WordPress, Blogger...