8. Prevention
_____As according to Proposition 65, now law in California, the clinician is rendered responsible for the materials he uses; it is in his interest to select the least hazardous polymers: while some of these have already been reported to be hazardous, these are still used in direct contact with human tissues.
_____To protect himself, the average clinician could use the material’s extract in water according to an existing ISO standard, but the tests require sophisticated means and specific knowledge.
_____Determination of impurities has been the object of two ISO standards, one for a monomer5, and the other for water quality.6 The first requires an expensive spectrophotometer endowed with selectors for continuous variations wavelength, while the second a rather sophisticated chemical analysis. Fortunately, the color-change exhibited by the strong oxidizing agent used in both cases leads to a simplified method. In the process, in the diluted, purple solution of potassium permanganate, the MnVII ions are reduced to the colorless MnII ones while the released ingredients are oxidized.
_____To prepare a disclosing solution, to 1 liter of distilled water are added 1.6 mg potassium permanganate ACS reagent grade. After stirring, the solution is left in closed container for 24 hours and then placed in adequate containers along with the various polymeric products to be tested, such as adhesives, plastic appliances and elastomeric ligatures. The containers are either 25 ml capped test tubes or the dents in a porcelain egg-holder. The evolution of the solution’s discoloration in time is sketched in Fig. 8.1.
_____The reaction proceeds in two ways, according to the pH of the solution. In a neutral environment, the MnVII ions are reduced to manganese oxides (MnII, III) that precipitate giving the solution a brownish tent. By adding few drops of an acid such as phosphoric, sulfuric or hydrochloric (muriatic) to pH 1-2, the solution’s color further changes to clear as the manganese is further reduced to colorless salts (MnII), Fig. 8.2, as shown for the testing of polyurethane elastomeric ligatures.
_____To test adhesives and restoratives, these are spread thin on a Teflon surface and cured according to their manufacturer recommendations. After cure, the foils are reduced to particles by using a coffee grinder. The particles are then immersed in the permanganate solution in the presence of phosphoric acid, as shown in Fig. 8.3a & 8.3b. Among adhesives, restoratives and sealants, the unfilled samples (sealants) showed, as expected, the fastest changes in color. Interestingly, the cold cure Jet®, from which unreacted methyl methacrylate could presumably easily leach, released less ingredients than most samples. Fuji’s compomer was found to leach more reducing ingredients than the resin-based adhesives.
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_____In alternate tests, the monomers were added as viscous liquids as shown in Fig. 8.4.
Similarly, elastomeric ligatures, chains or separate modules, were cut to weigh 1 g each and immersed as shown above in Petri dishes with an acidified gel of potassium permanganate. The discoloration has been photographed before and after removing the ligatures, as shown in Fig.8.5.
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_____Interestingly, the same test has shown that the Invisalign retainers, made of polycarbonate, also show leaching, as seen in Fig. 8.6. If it is considered that these have to be used up to 40 times in a treatment, it is obvious that for some, this may constitute a problem.
_____To test retainers, these can be either first minced by cooling them in liquid nitrogen followed immediately by their crushing them with a hammer, or directly immersed in an acidified potassium permanganate solution (pH 2) in a ceramic tray as shown in Fig.8.7. In this experiment, retainers and a mouth-guard (designed for a short wear and having a high content of plasticizers) were subjected to the same gel. While the mouth-guard leached significantly, see aura around it, the orthodontic retainers tested didn’t show any alarming leach. The least leaching was the C+ retainer from Raintree Essix, claimed to contain (or be based upon) polypropylene, a polymer known for very little water affinity.
_____While almost all the polymeric devices examined leach in some extent, the ones that exceed are the resins cured in situ, as shown in Fig. 8.8 where several adhesives and restoratives (cured according to their manufacturer’s recommendations and molded in identical hemispheres) were immersed in a permanganate gel. As it can be seen, even after 48 hours and repeated change of the gel, all of these still leached considerably. While differing in speed, some oxidation-reduction reactions took place in all cases, as shown by the discoloration of all the samples except the control. As in a neutral environment the reaction is slow and the discoloration more difficult to assess due to the suspended particles of manganese oxides, an acid environment is needed as the shade changes are easy to disclose.