It has been reported that installment of fixed orthodontic appliances might lead to the alteration of the salivary electrolytes (1), i.e. increase in salivary nickel or chromium concentrations. However, most of the authors (2-10) reported no increase in salivary levels of nickel and chromium in patients after installment of orthodontic appliances.
It is well known that oral cavity is an ecosystem which is known to cause biodegradation of metals which usually occurrs through the process of electrochemical breakdown. Besides, it is known that various orthodontic components such as nickel and chromium can cause hypersensitivity reactions in the oral cavity, cytotoxicity, and dermatitis. Furthermore, they might have significant mutagenic and possibly carcinogenic potential (5). Luckily, most of the orthodontic patients do not have visible reactions on the oral mucosa caused by orthodontic materials, probably due to the influence of saliva.
Staffolani et al. (11) reported that a daily release of Ni, Cu and Cr in vitro conditions in acid pH was lower than a daily dietary intake of these metals and that this findings support the use of orthodontic appliances by patients.
Gürsoy et al. (12) reported that group 4 with recycled brackets and recycled archwires released higher amounts of Cr, Fe and Ti compared to other three groups. Ni release was simmilar in groups 1 (new brackets and new archwires) and 2 (new brackets and recycled archwires) and in groups 2,3 (recyled brackets and new archwire) and 4. Furthermore, the amounts of Cu, Cr and Ti ions released from groups 3 and 4 were significantly greater than the amounts released from other two combinations.
Kuhta et al. (13) reported that all investigated appliances (stainless steel, nickel-titanium and thermo NiTi) released measurable quantities of all ions examined and the change in pH had a very strong effect (up to 100 fold) on the release of ions. Also, the release of ions was dependent on wire composition, but it was not proportional to the content of metal in the wire. The largest number of ions was released during the first week of appliance insertion. The same authors (13) concluded that levels of released ions were sufficient to cause delayed allergic reactions.
Therefore, the aim of this study was to compare salivary levels of nickel (Ni), titanium (Ti), chromium (Cr), cobalt (Co), copper (Cu) and zinc (Zn) prior to and six months after the installment metallic or ceramic brackets.
Materials and methods
This study has been approved by The Ethics Committee of the School of Dental Medicine, University of Zagreb, Croatia (No 36/2014). All patients were referred to the private dental practice Fiziodent, Zagreb, Croatia. Informed consent was collected according to guidelines from the WMA Declaration of Helsinki. Inclusion criteria were as follows: patients without systemic diseases who were not taking any medications, permanent dentition, no amalgam or metallic crowns in the oral cavity, no prior orthodontic treatment and no local therapy (fluoride, probiotic, and mouthwash). Study group included 33 males and 51 females. Forty two patients had metallic braces (Mini Sprint Bracket, Forestadent Bernard Forster GmbH, Pforzheim, Germany) and 42 had ceramic ones (Pure, Ortho Technology Inc., Tampa, FL, USA). We used NiTi squared or round archwires whose diameter was changed every 4 to 6 weeks. All ligatures were made of rubber. Orthodontic appliances were set from first to first molar in both jaws and there were not any patients with extracted teeth.
Orthodontic appliances have been inserted into oral cavities of all the patients and six months after that, salivary samples were taken and studied. Subsequently, salivary electrolytes were determined. All participants rinsed their mouth with distilled water and each subject gave 5mL of unstimulated saliva sample between 8AM and 11AM, which was then divided into 6 parts. All participants were instructed to refrain from eating, drinking (except water) and brushing their teeth two hours prior to saliva collection. Salivary samples were kept at -20°C until assayed in plastic containers. Samples were analyzed with high-resolution mass spectrometer with inductively coupled plasma in order to analyze Ni, Cr, Ti, Co, Cu and Zn levels. Before analysis, samples were warmed at room temperature and diluted ten-fold. A calibration curve was made from the standard solution which was made with the same reagents as in the samples. The average of three measurements was used for each sample.
Statistical analysis: Normality of distribution was tested by the Kolmogorov-Smirnov test. Since salivary concentrations of all electrolytes were not normally distributed, non-parametric methods were used in further analysis. Difference in salivary concentrations of electrolytes before and after orthodontic therapy was assessed by the related samples Wilcoxon signed rank test. A difference in salivary concentrations of electrolytes between different brace materials was assessed by the Mann Whitney test. The results were expressed as median (range) and p value lower than 0.05 (p<0.05) was considered statistically significant.
No difference in baseline salivary concentrations of any of the studied electrolyte between patients with metallic and non-metallic braces was found. Salivary concentrations of titanium were significantly higher after the treatment in patients with metallic and non-metallic braces, respectively. Salivary concentrations of chromium and zinc were significantly higher before the treatment in patients with metallic and non-metallic braces, respectively (Table 1.).
*Wilcoxon signed rank test
Lages et al. (14) have measured salivary levels of nickel, chromium, copper and iron in 90 orthodontic patients with metal and esthetic brackets. The same authors (14) reported a significant increase in salivary levels of nickel and chromium in patients with metallic brackets compared to the group with esthetic brackets. However, there were no significant differences in the salivary iron and copper levels between the patients with metallic and those with ceramic brackets. This finding is contrary to ours as we found no differences in salivary electrolytes with metallic and ceramic brackets. Talic et al. (15) reported that installment of fixed orthodontic appliances lead to non-toxic salivary nickel level increase during the therapy, unlike salivary chromium level which remained stable. This finding is not in concordance with our results as we found decreased chromium levels and no change in salivary nickel concentration 6 month after the insertion of orthodontic appliance. Buczko et al. (16) found out that increased salivary nickel levels were noticed in 37 patients after bracket installment. Amini et al. (4) concluded that salivary nickel level might increase in patients treated with conventional and metal-injection molding (MIM) brackets, but that the increase is higher in those having conventional brackets. This finding is also in contrast to ours. Furthermore, Khaneh Masjedi et al. (17) compared conventional and new NiTi arches (epoxy coated and copper NiTi) in 42 orthodontic patients and concluded that NiTi arches might increase salivary nickel levels. Furthermore, epoxy-coated NiTi followed by copper NiTi archwires might release less nickel compared to conventional NiTi ones. Nayak et al. (18) reported a significant increase in salivary chromium and an insignificant decrease in salivary nickel after insertion of orthodontic appliances. This finding of an increase in salivary chromium level is completely opposite to our finding. Nayak et. al (18) stated that although these levels were below recommended daily doses, allergenic potential of these metals might be taken into account. Singh et al. (19) found significant differences in salivary nickel and chromium levels before, one week and three weeks after fixed orthodontic appliance installment. The highest level was noticed after one week; however, salivary levels of chromium and nickel have become stable after three weeks and were higher than initial levels. This finding is in contrast with ours as there was no increase in salivary nickel and chromium in our patients.
Salivary electrolytes serve as component of antimicrobial properties of saliva. Copper is a part of histatine, antimicrobial salivary peptide as well as part of superoxide dismutase (antimicrobial enzyme) as well as lysyl oxidase which takes part in collagen synthesis. Zinc is part of superoxide dismutase which is known antioxidant. It is possible that zinc levels might be significantly decreased due to the increased antimicrobial salivary activity (20, 21). Therefore, decreased salivary copper levels would be expected, which was not found in this study. At this point, we do not have any explanation why salivary chromium levels decreased after the insertion of orthodontic appliances. Significantly increased titanium levels might point to a higher release of titanium from archwires regardless of bracket type that has been inserted.