Tooth discolouration following either the use of endodontic medicaments containing tetracyclines or systemic tetracycline antibiotics during tooth formation is common and well-documented. It is known that light is responsible for promoting the discolouration of tetracycline in endodontic medicaments. Currently, there have been no studies into the wavelength of light (action spectrum) responsible for this discolouration. Broader studies suggest that numerous factors, including the presence of certain ions, the pH, or oxygen tension, could influence the rate of discolouration. Further, this discolouration is difficult to treat with conventional bleaching methods. In the following thesis, the wavelength of light responsible for promoting the discolouration of the tetracycline molecule is determined, as well as the influence of various ions, oxygen tension and pH. Subsequently, treatment of tetracycline discolouration was undertaken with coherent and non-coherent light activated bleaching systems, to determine the most effective method of treatment.
To determine the wavelength of light responsible for discolouration, a variety of tetracycline derivatives were exposed to UV, visible, and infrared light in solution with distilled water, and the results monitored using analysis of digital photographs and by absorption spectrophotometry from the short wavelength ultraviolet region of the spectrum through to the near infrared region. Exploration of factors that affected the rate of discolouration was undertaken using a similar method. To determine the optimal treatment, a number of bleaching systems – ZOOM!®, Smartbleach 3LT®, and Smartbleach® with a KTP laser were trialled in a variety of combinations on root slices uniformly stained with Ledermix® paste, a tetracycline-containing endodontic medicament. ZOOM!® is a photochemical system based on the photo-Fenton reaction activated with an array of blue LEDs (470 nm), while Smartbleach 3LT® is activated with an intense green LED array (535 nm). Conventional Smartbleach® gel is activated with a KTP laser (532 nm).
The results indicated of the various types of light within sunlight, blue visible light is the main wavelength of light responsible for causing tetracycline discolouration through photo- oxidation reactions, with infrared and UV light playing only a small role. A higher oxygen tension resulted in more discolouration, as did the presence of calcium, strontium, and bismuth ions, all of which bind through chelation to tetracyclines. Conversely, zinc and magnesium ions had a photoprotective effect, limiting discolouration. An acidic pH reduced discolouration, while a more neutral to mid-alkaline pH increased it, which reflects in part the solubility of these hydrochloride salts of tetracyclines.
With regard to the optimal bleaching system, Smartbleach® combined with the KTP laser was found to yield the best bleaching results, dramatically reducing both the yellow colour and increasing the luminosity. While a greater level of change was seen in the samples which were more darkly stained at baseline, these samples also underwent more relapse over the following month.
Overall, blue visible light is responsible for promoting tetracycline-discolouration, and the best system to treat this discolouration is the Smartbleach® system activated by a KTP laser emitting visible green light. The results from this study have implications for clinical practice, including how light exposure of tetracycline materials should be minimised, how agents can be included in products to reduce the extent of photo-oxidation reactions, and how practitioners can optimize the results obtained by choosing different combinations of intense lights with peroxide gels in in-office bleaching treatments.