526HFigures 10.3 (a) and (b) show an example of a highly successful result. If the colour of
a tooth has not significantly improved after three changes of bleach then it is unlikely
to do so, and further bleaching should be abandoned. The maximum number of bleach
applications is usually accepted as 10. Failure of a tooth to bleach could be due to
either inadequate removal of filling materials from the pulp chamber or to 'time
expired' bleaching agent. Both these factors should be checked before abandoning a
procedure.
Slight overbleaching is desirable, but the patient should be instructed to attend the
surgery before the next appointment if marked overbleaching has occurred.
Non-vital bleaching has a reputation of causing brittleness of the tooth. This is
probably the result of previous injudicious removal of dentine (which only needs to be
'freshened' with a round bur), rather than a direct effect of the bleaching procedure
itself.
This method of bleaching has been associated with the later occurrence of external
cervical resorption. The exact mechanism of this association is unclear, but it is
thought that the hydrogen peroxide diffuses through the dentinal tubules to set up an
inflammatory reaction in the periodontal ligament around the cervical region of the
tooth. In a small number of teeth there is a gap between the end of the enamel and the
beginning of the cementum, and in these cases the above explanation is tenable. The
purpose of the 1-mm layer of cement is to cover the openings of the dentinal tubules
at the level where there may be a communication to the periodontal ligament. In the
same way, non-setting calcium hydroxide is placed in the pulp chamber for 2 weeks
prior to final restoration in order to eradicate any inflammation in the periodontal
ligament that may have been initiated.
Clinical studies have demonstrated that regression can be expected with this
technique. The longest study after 8 years gave a 21% failure rate. However, if white
gutta percha has been placed within the pulp chamber then it is readily removed and
the tooth easily rebleached.
The advantages of the technique are many: easy for operator and patient; conservation
of tooth tissue and maintenance of the original crown morphology; no irritation to
gingival tissues; no problems with changing gingival level in young patients
compared to veneers or crowns; no technical assistance required.
527H