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    The interest in bleaching teeth has continued to grow, particularly in North America. Although the process of "whitening" teeth has been around over a decade and a half, little is known about the mechanism. The same can be said about the postoperative sensitivity (POS) that commonly accompanies the procedure.

     The mechanism of bleaching is actually attributed in part to the dissolution of enamel rod prisms. Based upon a Brazilian study it has been shown that the carbamide peroxide attacks and totally dissolves isloted groups of enamel rod prisms; thereby exposing the underlying dentin. The whitening that one observes during the bleaching process is actually due to chromatic changes in the dentin more so than in the enamel.

    Now since an open avenue exists between the facial surface of the tooth and the surface of the dentin, the potential for POS can occur. As both patient and clinician recognize, the teeth undergoing bleaching can experience considerable pain upon the introduction of cold liquids.

    Specifically, the cold water us transfered through the open channels (previously occupied by the enamel rod prisms) contacting the exposedodontoblastic processes. These odontoblastic processes are then subjected to a reduction in pressure which according to Brannstromm (1972) thereby creating pain. This pain can be quite acute. Incidentally, the dissolution of enamel rods has been demonstrated  using scanning electron microscopy (SEM). Addional numerous other studies have demonstrate that bleaching reduces the microhardness of the enamel surface. Over a period of time the enamel fortunately remineralizes and the hardnes goes back up. Unfortunately some studies  have demonstrated that the microhardness and remineralization may not necessarily return to the orginal pre-bleached state.

    Now that the mechanism of bleaching has been established, better guidelines can be generated for the purpose of enhancing the effectiveness of the bleaching action. This knowledge also is quite helpful in prescribing a regimen for reducing  or eliminating POS. For example, the home bleaching procedure can be coupled with the application of desensitizing agents such as GLuma ir Benzylalkonium chloride. Finally, the rate of enamel rod dissolution  and achievement of the bleaching effect is related to concentration of the peroxide as well as the time of application.