No statistically significant difference was detected between round and rectangular wires when examining the change in the bacterial count of A. actinomycetemcomitans (Fig. 11). The decrease was ascending, signifying that the greatest decrease was observed after 3 weeks. With regard to round wires, both NiTi and CuNiTi wires exhibited the same bacterial count in the second and third weeks. Round FeCrNi wires led to a lesser decrease in all the three periods compared with the other two types.
With regard to rectangular wires, the least decrease was observed with CuNiTi wires. However, no statistically significant difference was detected between the three types. Moreover, the difference between the change in bacterial count of A. actinomycetemcomitans in both round and rectangular wires was nonsignificant. The least decrease in bacterial counts was detected in this type of bacteria after 3 weeks with CuNiTi wires (Table 3).
The round wire revealed the same results when the change in the bacterial count of Klebsiella spp. was inspected (Fig. 12). In the first week, the least decrease was observed with NiTi wires. In the second week, the least decrease was observed with CuNiTi wires followed by FeCrNi and finally NiTi wires. In the third week, bacterial counts decreased similarly with both NiTi and CuNiTi wires but less than with FeCrNi wires.
With the use of rectangular wires, the bacterial count of Klebsiella spp. decreased. The greatest decrease was observed with CuNiTi wires followed by NiTi and then FeCrNi wires. In the second week, the bacterial count was the same with NiTi and CuNiTi wires but less than that with FeCrNi wires. In the third week, the least decrease was observed with NiTi wires followed by CuNiTi wires and finally FeCrNi wires.
A comparison between the results of round and rectangular wires revealed no statistically significant difference, except the decrease in the bacterial count at the third week with NiTi wires (Table 4).
In-vitro release of nickel from orthodontic appliances has been noted using microscopic analysis of corrosion and chemical analyses of orthodontic components when exposed to an artificial oral environment 22–24. Release of nickel is reported to vary with the composition and manufacturing details of the appliance components 25 and between archwire alloys and mechanical straining 26.
The surface area ratio of two dissimilar alloys is a very important factor as it affects the galvanic corrosion behavior. An unfavorable area ratio, which consists of a large cathode and a small anode, might lead to a greater corrosion rate from the anodic alloy. When this is applied to the orthodontic field, it is difficult to determine the real surface area ratio between brackets and archwire in clinical use. Accordingly, an in-vitro study was carried out.
The effect of corrosion products on bacteria in the oral cavity was not given the importance it deserved. This study was therefore conducted to accomplish this objective.
First, the fluid containing corrosion products was prepared. In this study the electrochemical measurement method was used to produce galvanic corrosion products of various orthodontic wire-bracket couples.
Normal saline solution (0.9% NaCl) was used because it is a corrosive agent. As corrosion testing of dental material should be carried out at a standard temperature of 37°C 27, the solution temperature was controlled at this temperature with a thermostat to simulate the oral condition. This temperature (37°C) represents the normal temperature of the oral cavity 28. Static immersion tests were carried out at different periods (7, 14, and 21 days), and at the end of each period the specimens were removed.
Three different wire alloys, namely, CuNiTi, NiTi, and FeCrNi, and one type of the most commonly used bracket made of iron–chromium–nickel (FeCrNi) were tested in a reference saline solution.
Thirty adult patients were selected to participate in this study. The ages of all patients ranged from 17 to 25 years. Banded and bonded attachments were inserted. After 1 month, samples were taken from the proximal surfaces of all index teeth to standardize the site of sample collection. This site was selected because the incidence of hyperplasia was greater in the posterior areas of the mouth than in the anterior and was greater interproximally than at the center of the crown 29. Moreover, Tzannetou et al.30 had concluded that phosphatase activities in gingival crevicular fluid might be a useful means for monitoring tissue responses to orthodontic treatment.
Paper points of size 30 were used as they are rigid enough and can be easily inserted into the bottom of the gingival sulcus. The paper point was kept in the gingival sulcus for 15 s to allow sufficient time for absorption of the gingival crevicular fluid 31. Thereafter, the samples were transported to the microbiological laboratory within 2 h.
Thioglycolate broth and tryptone soy broth were used as transporting and nourishing media for anaerobic and aerobic bacteria, respectively. Nutrient agar was used for all bacterial cultures. MacConkey’s agar was used for culture of Klebsiella spp. 31.
With regard to the difference between the two strains of staphylococci bacteria, S. epidermidis was coagulase negative, whereas S. aureus was coagulase positive. Klebsiella bacteria tested negatively in both indole and methyl red tests, whereas it tested positively in Voges–Proskauer’s test. Identification of different types of anaerobic bacteria was carried out using anaerobic profile index.
The following Gram-negative and Gram-positive bacteria were investigated: S. epidermidis and S. aureus (Gram-positive aerobic bacteria), A. actinomycetemcomitans (Gram-negative anaerobic bacteria), and finally Klebsiella spp. (Gram-negative aerobic bacteria).
They were selected because they increase in number in cases of periodontitis 1. It was also proven that appliance-free young individuals initially infected with A. actinomycetemcomitans had a higher risk of experiencing more gingival inflammation than did individuals who were not infected with the bacterium during a 3-year observation period 32,33.
A colony counter was used to count bacteria before adding the ion-containing fluid (T0). The number of each type of bacterium was fixed among all the samples to ensure accuracy of the results. Bacteria were counted 1 week, 2 weeks, and 3 weeks after addition of galvanic corrosion product-containing solutions.
Bearing in mind the fact that this is an in-vitro study and salivary flow is absent, the results denote the direct relationship between corrosion products and bacterial counts. It was revealed that the decrease in bacterial counts was proportional to the incubation period. Accordingly, it was proved that the longer the period of immersion, the more the reduction in bacterial count.
Many index sites were selected. The reason was the change in bacterial count before and during treatment in the same mouth. In-vivo studies revealed an increase in both the S. epidermidis and S. aureus counts of the gingival sulcus during the course of the orthodontic treatment. The correlation between the increase in microbial count and duration of follow-up was insignificant at the upper canine and lower second molar. However, this increase was significant at the upper first molar, upper second molar, lower canine, and lower first molar 34. These results agreed with those of Türkkahraman et al.35 and disagreed with those of Petti et al.36 and Speer et al.37.
In other studies, A. actinomycetemcomitans increased significantly during the course of the orthodontic treatment. The correlation between the increase in microbial count and duration of follow-up was insignificant at the upper canine and lower canine. However, it was significant at the upper first molar, upper second molar, lower first molar, and lower second molar 35. These results agreed with those of Ho 20, Paolantonio et al.38, Sallum et al. 39, Türkkahraman et al. 35, Naranjo et al.31, Leung et al. 33, Petti et al.36, and Thornberg et al.18. However, the results disagreed with those of Sinclair et al. 40 and Speer et al. 37.
In-vivo studies showed an absence of Klebsiella spp. before and 1 week after the fixed orthodontic treatment; it increased significantly along the course of the orthodontic treatment. The correlation between the increase in microbial count and duration of follow-up was significant at the upper canine, upper first molar, upper second molar, lower canine, and lower first and second molars. These results were in agreement with those of many other studies 35,38 and disagreed with those of others 37.
From all the previously mentioned results, specific bacteria found in periodontitis cases had to be collected from different index sites.
In this study no surface polishing of orthodontic wire or bracket was carried out by any common polishing methods, the reason being that wire-bracket couples are practically fixed in the oral cavity without surface polishing, which leads to real corrosion.
It was proved that galvanic corrosion potential varied with time of immersion in saline solution. In NiTi wire alloys and FeCrNi (StSt) wire alloys coupled with stainless-steel brackets, the corrosion potential moved in a more desirable direction with a longer immersion period; in contrast, the corrosion potential of the CuNiTi wire alloy coupled with FeCrNi moved in a less desirable direction after starting immersion 41. In addition, no significant potential difference could be found between the FeCrNi wire and NiTi wire-coupled alloys 42.
The previously mentioned findings support the results of the current study. The longer the immersion time, the more the number of corrosion products released, and hence the more the bacterial count decreased. This explains why the bacterial count was intended to be equal when comparing the couple alloys. Moreover, our results are supported by the studies by Darabara et al. 12 and Schiff et al. 43.
From the present study, it was found that the orthodontic NiTi wire coupled with a stainless-steel bracket has the best corrosion resistance when compared with other specimens, whereas the orthodontic CuNiTi wire coupled with a stainless-steel bracket had the least corrosion resistance. These findings are in agreement with those of other studies 44–47. This, in turn, reexplains the greater decrease in bacterial count after 3 weeks of immersion.
With regard to the CuNiTi wire, TiCu2 is a phase present in the alloy. Copper constitutes the precipitated phase and plays an important role in the corrosion rate 48.
The orthodontic FeCrNi wire-coupled alloy showed the second highest decrease in bacterial count. This can be explained by the formation of passive film. This passive film is composed of Cr2O2, which precipitates on the surface of the FeCrNi wire and prevents further oxygen diffusion, resulting in increased corrosion resistance 49–51.
The NiTi wire-coupled alloy also formed a passive film that mainly consisted of several oxides of TiO2, TiO, and Ti2O5, which proved to have good biocompatibility with the NiTi alloy. No significant difference was observed between the results of the present study and those reported in references 52–55.
The decrease in bacterial count was higher with rectangular wires than with round ones. This can be attributed to the larger surface area of the rectangular wire. A significant difference was found between the electrochemical parameters of both rectangular and round wires. The results revealed that both NiTi and FeCrNi orthodontic wires had a similar trend for corrosion resistance, but the CuNiTi wire showed less desirability 41.
- The longer the immersion period of the wire bracket couple, the greater the decrease in bacterial count.
- CuNiTi wires exhibited the highest decrease in bacterial counts, followed by FeCrNi wires and finally NiTi wires.
- Rectangular wires showed a greater decrease in bacterial counts compared with round wires.
Many factors are encountered when considering the oral environment. Among these are the different pH values and various temperatures caused by cold or hot intakes, besides normal ones. Accordingly, when an in-vivo study testing galvanic corrosion behavior is carried out, none of the factors must be overlooked.
The author would like to express his deep gratitude to those who assisted him in this work: Professor Dr Ibrahim Hamed Mahmoud, Professor of material engineering, Department of chemical engineering, Faculty of Engineering, Minia University; and Dr Mohammed Sayed Mohammed, Lecturer of microbiology and immunology, Faculty of medicine, Minia University.
Conflicts of interest
There are no conflicts of interest.
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