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The goal of this study was to determine the chemical composition of the passivation layer of three clinically available orthodontic miniscrews at different depths.
The miniscrews used were Aarhus Mini-Implant (AAR), IMTEC Ortho (IMT), and VectorTAS (VEC). The chemical compositions of the as-received miniscrews were determined by X-ray photoelectron spectroscopy (XPS). Data was acquired before etching the miniscrews with argon, as well as after etching at depths of 10 nm, 20 nm, 30 nm, and 80 nm.
The elements found in all miniscrews were mainly C, O, and Ti. Also found were other metals in small amounts, and other trace elements. All three miniscrews showed very different characteristics in surface composition. IMT had the greatest increase in Ti, as well as the most titanium metal at 80 nm. VEC remained stable at all tested depths and contained no titanium metal at 80 nm. AAR was an intermediate between the two.
The passivation layer of the orthodontic miniscrews has different compositions depending on the brand, as well as the depth analyzed. VEC appeared to have the largest passivation layer, and IMT appeared to have the thinnest passivation layer.
Beginning in the late 20th century, many types of skeletal anchorage devices were introduced, including prosthodontic implants, zygoma ligatures, palatal onplants and implants, retromolar implants, miniplates, and surgical screws (1). Surgical screws, which became known as temporary anchorage devices (TADs), have become increasingly popular because their small size allows for more placement sites in the oral cavity and reduces bone-healing time (2). In addition, TADs are easy to insert and remove, can be loaded immediately after insertion, and can provide absolute anchorage for many types of orthodontic treatment with no need for special patient compliance (3–6).
In current clinical orthodontic practice, TADs are widely used for mesial or distal movement of teeth, anterior retraction or protraction of teeth, intrusion or extrusion of teeth, correction of midlines, transversal problems, open bite or deep bite, skeletal anchorage for mandibular or maxillary advancement, and several other specialized cases. In special cases, TADs, also known as orthodontic miniscrews, can be used as an alternative to orthognathic surgery (7). These devices can be applied to several areas, such as the maxillary or mandibular base, maxillary or mandibular alveolar region, palate, zygoma, and retromolar region. Over the last decade, numerous studies about TADs have led to considerably improved clinical outcomes and to the changing of treatment protocols (8).
Although the dramatic developments and improvements of orthodontic miniscrews have been substantial, their biocompatibility is an important question for orthodontists today. Biocompatibility means that the tissues of the patient who comes into contact with the materials do not suffer from any toxic, irritating, inflammatory, allergic, mutagenic, or carcinogenetic reaction (9, 10). Very limited data are available in the orthodontic literature regarding the biocompatibility of commercially available orthodontic miniscrews. Materials used in the oral cavity must be nontoxic and biocompatible, have good mechanical properties, and be able to resist corrosion (11). The most common metals and alloys used in dentistry can be exposed to a process of corrosion in vivo that makes them cytotoxic; therefore, the biocompatibility of these materials must be studied before their use (12, 13).
In dentistry, orthodontics is the branch in which the problem of biocompatibility is most recognized since the patients are young and consequently are more susceptible to developing inflammatory reactions. In addition, alloys may have toxic effects (14). It is important to evaluate the biocompatibility of orthodontic miniscrews and TADs because their materials are inserted directly into the periodontal tissues and alveolar bones. Metallic ions released from these miniscrews may cause a reaction such as inflammation or necrosis in adjacent tissues, including the oral mucosa, gingiva, or alveolar bone (8). In addition, using an animal model (15), metal ion release has been associated with clinical implant failure, osteolysis, cutaneous allergic reactions, and kidney lesions due to haematogenous transport, hypersensitivity, and cytotoxicity (16).
As miniscrews are inserted into the biological environment, a phenomenon may occur in which the proteins uncoil and are adsorbed by the metallic surface. The protein may become denatured depending on the charge of the metallic surface. Furthermore, the charge depends on the characterization of the miniscrew (17).
The goal of the current study is to determine the nanoscale chemical surface composition of three clinically available orthodontic miniscrews. The information obtained can be used to help the biosafety evaluation, as well as for further testing on the cytotoxicity of orthodontic miniscrews. The null hypothesis to be tested is that there will be no difference in the nanoscale chemical surface composition of the tested orthodontic miniscrews.
The orthodontic miniscrews used in this study were Aarhus Mini-Implant (AAR; Medicon, Tuttlingen, Germany), IMTEC Ortho (IMT; 3M Unitek, IMTEC, Ardmore, Oklahoma, USA), and VectorTAS (VEC; Ormco, Glendora, California, USA). All three types of miniscrews are Ti-6Al-4V alloys. All miniscrews were packaged individually by the manufacturers. Packages were only opened immediately prior to X-ray photoelectron spectroscopy (XPS) analysis and were handled carefully using nitrile gloves, touching only the head of the miniscrew, to reduce the potential for further contamination. The same procedure was used for scanning electron microscope (SEM) analyses.
The chemical compositions of the as-received miniscrews were determined using XPS analysis. This method of analysis provides the energy distribution of electrons emitted as a result of the interaction between the biomaterial and the incident X-rays. Their analysis provides qualitative (which elements are present) and quantitative (the relative concentration of each spectral peak component) information, as well as binding state analysis (bonds in which the elements are involved) (18).
The XPS analyses were performed using a Kratos Axis Ultra X-ray Photoelectron Spectrometer (Wharfside, Manchester, UK). The spectrometer was calibrated to the Au 4f and Cu 2p lines and the detection limit was 0.1 at%. Noteworthy parameters include a base pressure of 2×10−9 torr, an X-ray gun emission set to 15 mA, and an X-ray gun anode set to 15kV, which equates to a power setting of 225W. The type of anode used was aluminium monochromatic X-rays. The elements detected were observed using both survey and high resolution spectra. Additional factors included a detector normal to the surface for the take off angle geometry (15 degrees), a hybrid lens (magnetic and electrostatic), the use of a charged neutralizer during data acquisition set to 2 A and 4V, and aperture set to 700×300 μm. The greater aperture dimension was oriented along the length of the screw. A pass energy of 160eV was used during the survey scan, while a pass energy of 20eV was used during the high resolution scan. The estimated sampling depth of XPS analysis was 10nm. Data were acquired before etching the miniscrews with argon, as well as after etching at depths of 10, 20, 30, and 80nm. Standardizing of argon depth profiling was achieved by ablation of a standard. The area analysed on all miniscrews was identical, all of which occurred in the threads of the miniscrews. Three samples (n = 3) from each manufacturer were tested. Results of XPS were analysed using Casa Software Ltd and average values were reported. Prior to analysing the data, the XPS binding energy values were charge-corrected and calibrated to that of uncharged carbon (CH–CH) at 285.0eV.
A JEOL JSM-7600TFE SEM was used to image the surface structure and topography. All miniscrews imaged were new, and only the threads of the miniscrews were imaged. Using the secondary electron imaging detector, the accelerating voltage for all miniscrews was 10kV, and magnifications were 10 μm scale and 1 μm scale. The beam current used was 100 nA.
The XPS results were analysed by calculating the mean and standard deviation for each brand, using multivariate analysis of variance followed by Bonferroni post hoc test. All statistical analyses were carried out using the OriginLab software system version 9.2.
The miniscrews were initially characterized before etching with argon ions, followed by testing at 10, 20, 30, and 80nm depths. Figure 1 illustrates the per cent composition of the five main components [carbon (C), oxygen (O), titanium (Ti), other metals, and trace elements] for the three clinically available miniscrews that were tested at five different depths. The other metals include silicon, aluminium, vanadium, and chromium. The trace elements (less than 1 per cent) include nitrogen, phosphorus, calcium, and argon (from the etching procedure). All three miniscrews showed a similar pattern in the per cent composition of both the other metals and trace elements. Because the miniscrews were characterized at a lower depth, there appeared to be a gradual increase in the percentage of other metals, with IMTEC having the highest percentage of nearly 20 per cent at a depth of 80nm. The percentage of trace elements, in all three miniscrews, was consistent throughout all depths.
All three miniscrews showed different characteristics in surface composition. Carbon showed no significant statistical difference in per cent composition without etching. The carbon is present due to contamination of the miniscrew, which occurred during the manufacturing process prior to packaging, as well as during the first seconds when the miniscrew came into contact with the room’s atmosphere as the sealed package was opened (18). Also important is that throughout testing, the Aarhus miniscrew had the highest standard deviation, whereas the VectorTAS miniscrew had the lowest standard deviation.
The Aarhus miniscrew showed a very large drop in the amount of carbon after etching, followed by a nearly stabilized percentage of around 20–25 per cent from 10nm on. However, as stated above, the very large standard deviation is an important factor. Oxygen levels showed a dramatic increase after etching, followed by stabilization, but at a depth of 80nm, the percentage of oxygen decreased slightly. The amount of titanium in terms of its pattern was similar to oxygen. After etching a large increase in titanium was seen, followed by stabilization, but at 80nm rather than a slight decrease there was a slight increase in percentage of titanium.
The IMTEC miniscrew showed a large drop in the amount of carbon after etching, followed by stabilization, but ended with a very slight decrease in percentage at 80nm. The amount of oxygen in this particular miniscrew is unique in that the two other miniscrews showed an increase after etching but the IMTEC miniscrew showed completely different results. Not only was there no significant change from no etch to a depth of 10nm, but there appeared to be a gradual decrease in the percentage of oxygen at a lower depth, as opposed to an increasing percentage. The IMTEC miniscrew also had a unique characteristic for titanium. In all miniscrews, there was a common large increase in percentage after etching, but rather than stabilization in the IMTEC miniscrew, there was a continuous, gradual increase in titanium. Also important is that the amount of titanium was significantly higher at each depth compared with the Aarhus or VectorTAS miniscrews.
There are three unique aspects to the last miniscrew, the VectorTAS. After etching, carbon, oxygen, and titanium all stabilized and there was a very small standard deviation at each tested depth. The amount of carbon dropped significantly after etching, to about 10 per cent, and remained at that percentage throughout each tested depth. The amount of oxygen increased slightly after etching, to about 55 per cent, and remained at that percentage throughout each tested depth. Lastly, the amount of titanium increased after etching and remained at approximately 25 per cent throughout each tested depth.
Figure 2 shows high resolution data of the peaks for titanium without etching, at depths of 30 and 80nm. The colour of the peaks is arbitrary and is only relevant to show distinction between the different peaks. The y-axes of these images are labelled as arbitrary units because the importance of these peaks comes from the peak position and intensity. The x-axes of these images show the binding energy, with units being electron volts (eV). The binding energy provides the peak position, which allowed the researchers to determine the composition of each specific peak.
Titanium will often produce two different peaks for each high resolution image. The peak at a lower binding energy has a greater intensity and is referred to as Ti 2p 3/2, whereas the peak at a higher binding energy has a lower intensity and is referred to as Ti 2p 1/2. For the purposes of this research, the Ti 2p 3/2 peak was the important peak and was the one examined. Also, it is important to point out that XPS high resolution spectra can detect the multiple states of titanium compounds, such as: titanium oxide (TiO) at 454.8eV, dititanium trioxide (Ti2O3) at 456.1eV, titanium hydroxide (TiOH) at 457.5eV, titanium dioxide (TiO2) at 458.8eV, and Ti metal at 454.0eV. The per cent composition of titanium by depth is presented in Table 1, which shows the specific compound of titanium, the expected peak position with the average binding energy (found using database literature) (19), and in what per cent it existed, at various depths.
Before etching, the Aarhus miniscrew showed two different titanium environments, with the most intense peak at 459eV, indicating TiO2. At 30nm, the most intense peak position remained at 459eV, but there was a clear change in the titanium characteristics as the peak became much wider and a third environment appeared. At 80nm, three environments remained but a shift downward occurred as the most intense peak position became 454eV, indicating titanium metal.
The IMTEC miniscrew showed three different titanium environments before etching, with the most intense peak at 459eV, indicating TiO2. At 30nm, a large shift occurred, in which the most intense peak decreased to 454eV, indicating titanium metal. In addition, there was also an increase in the number of environments from three to four. The 80nm high resolution results were very similar, indicating that titanium metal remained as the most prominent titanium compound.
The VectorTAS miniscrew, before etching, showed two different titanium environments, with the most intense peak at 456eV, indicating Ti2O3. At 30nm, a shift occurred, in which the most intense peak increased to 459eV, indicating TiO2. There was also an increase in the number of environments from two to three. Unlike the Aarhus and IMTEC miniscrews, the 80nm high resolution results demonstrated a prominent peak at 459eV, indicating TiO2.
All three miniscrews examined in the present study contained aluminium (Al), vanadium (V), and chromium (Cr). The Aarhus miniscrew was found to contain silicon in addition to the other metals. Although the focus was on metal ions, it should also be noted that trace elements of nitrogen, phosphorus, and calcium were found in the miniscrews. All three miniscrews contained nitrogen, while only the Aarhus and VectorTAS miniscrews contained phosphorus, and only the IMTEC and VectorTAS miniscrews contained calcium.
The SEM images shown in Figure 3 are used to assess the differences in topography among the miniscrews because in order to improve bone anchoring, dental implants may have their surfaces treated with some kind of procedures. These procedures can include titanium plasma-spraying, grit-blasting, acid-etching, anodization, or calcium phosphate coatings (20). Using SEM at a magnification of 10 μm, three different microtopography profiles were observed. The IMTEC miniscrew has a rough surface, with grooves wider than 1μm; the surface topography looks like has been treated using grit-blasting with coarse particles (Figure 3A). On the other hand, the VectorTAS miniscrew has a very smooth microtopography, indicating no grit-basting procedure (Figure 3B). The Aarhus miniscrew images displayed the presence of a great amount of microparticles on the surface (Figure 3C and and3D).3D). This fact may explain the high variation of carbon composition because there is more area for adventitious carbon attachment. The composition of these particles should be a silica compound because according to XPS analysis Aarhus is the only miniscrew with approximately 5 per cent Si in the no-etch condition. It is important to note that these silicon-based microparticles were sprayed on the miniscrew surface to create desirable roughness, but many of them became attached to the titanium oxide layer. This Si contamination only occurred on the surface, which is supported by the XPS showing no presence of this element at the 30nm depth.
Biocompatibility of orthodontic miniscrews is a principal concern because they are inserted directly into the periodontal tissues and alveolar bones. Patients may have negative reactions, such as inflammation or necrosis of the oral mucosa, gingiva, or alveolar bone, to these miniscrews (21). These reactions are highly correlated with the stability of the TiO2 film layer. Additionally, metallic ions released from orthodontic miniscrews can affect these reactions, and therefore, the contamination of orthodontic miniscrews from undesirable metals should be studied (8). According to the results obtained from this study, several titanium compounds are present at the outermost layer (Figures 1 and and2)2) of the tested miniscrews.
The titanium oxidation states are: +IV for TiO2, +III for Ti2O3, +II for TiO, and 0 for Ti (metal). The most stable Ti oxide, however, is TiO2, which has Ti in oxidation state +IV. The TiO2 film layer is the principal foundation for biocompatibility of orthodontic miniscrews. This oxide film is a strong and stable layer that can form spontaneously, for example when in contact with air during the manufacturing and packing process, and prevents the diffusion of the oxygen from the environment, consequently providing corrosion resistance (22). The adherent the oxide film, the higher the miniscrew corrosion resistance (23).
The alloys used in orthodontic appliances, such as the Ti-6Al-4V alloy, rely on the formation of this passive oxide film to resist corrosion; however, the titanium dioxide layer can be disrupted by chemical and mechanical attacks and thus is not infallible (24). Saliva can cause corrosion of miniscrews and other orthodontic appliances because it is an electrolytic solution and contains acids from bacteria, yeast, fungi, and viruses.
TiO2 is chemically stable and inert and is, therefore, ideal for insertion into tissue or bone (25). Additionally, TiO2 has photocatalytic properties (in its anatase form) that prevent the harbouring of pathogenic microorganisms (26). This TiO2 layer, also referred to as the passivation layer, exists in all three orthodontic miniscrews tested in this experiment.
Analysing Figure 1, it is possible to calculate the depth of the TiO2 layer in the three miniscrews studied. The atomic ratio O:Ti for this oxidation state is 2:1, after the etch has removed adventitious carbon on surface. After etching, the ratio between the amount of O and Ti for Aarhus is approximately 2:1 at 10, 20, and 30nm. However, at 80nm the ratio is nearly 1:1. Based on the same assessments, the VectorTAS miniscrew had the largest passivation layer, which still existed at a depth of 80nm. The percentages obtained by XPS high resolution and described in Table 1 confirm these results. The miniscrew with the smallest TiO2 passivation layer was the IMTEC miniscrew, which lost the TiO2 layer at a depth of only 10nm. One of the hypotheses to explain this difference could be the surface treatment made by the manufacturer, prior to packaging, to create enough roughness to obtain bone anchoring, as shown in Figure 3A. In fact, the abrasion using a grip-sanding with coarse particle can remove a great amount of substrate (TiO2). The opposite can explain why VectorTAS has the largest TiO2 layer, due to the fact that these miniscrews were only machined, as shown in Figure 3B.
Additionally, the XPS high resolution spectra in Figure 2 can be used to analyse the existence or non-existence of the protective passivation layer. Without etching, a large peak at 459eV can be seen in all three miniscrews, indicating the protective TiO2 layer. At a depth of 30nm, major changes can be seen. A large peak at 459eV was visible in both the Aarhus and VectorTAS miniscrews, while the IMTEC miniscrew showed a large peak at 454eV, which is indicative of titanium metal. There was no peak at 459eV, signifying that the protective oxide film had been lost. At a depth of 80nm, the deepest depth tested in this experiment, the Aarhus miniscrew had also lost the 459eV peak and gained the 454eV peak. Similar to the IMTEC miniscrew, at a depth of 80nm, the Aarhus miniscrew also contained titanium metal. However, a large peak at 459eV persisted in the VectorTAS miniscrew XPS spectra, indicating the biocompatible TiO2 layer continued to exist.
The protective TiO2 film layer can be improved through the use of anodic oxidation, in which a constant voltage is applied to the miniscrew. By changing the anodizing voltage, the colour and thickness of the anodizing layer changes as well. As the anodizing voltage increases, the anodizing layer thickness increases. An anodizing voltage of 20V causes a blue colour, which is the colour of the Aarhus miniscrew used in this study. An anodizing voltage of 60V causes a yellow colour, which is the colour of the VectorTAS miniscrew used in this study (27).
The titanium dioxide layer over the miniscrew surface is both directly and indirectly related to a second means of biocompatibility: the release of toxic metal ions. Any metal or alloy that is implanted directly into the oral cavity, much like TADs, is a potential source for toxicity (28).
The orthodontic miniscrews studied are manufactured as a Ti-6Al-4V alloy. The small percentages of both aluminium and vanadium atoms in the alloy are potentially toxic because of the corrosion fatigue that miniscrews experience in bodily fluids (29, 30). The results from this study demonstrated that while all three miniscrews exhibit a similar escalation in percentage of other metals (Al, V, and Cr) as the depth increases, the actual percentage of other metals is higher as the TiO2 layer becomes thinner (Figures 4–6).
Some studies have determined that the most detrimental components in alloys are cobalt (Co) from the Co–Cr alloy, nickel from stainless steel, and vanadium from the Ti-6Al-4V alloy (31). The miniscrews tested in this study did not contain cobalt or nickel, and therefore the focus remains on vanadium, aluminium, and chromium.
The acute and chronic effects of vanadium when absorbed in large amounts have been well documented. Due to its interference with mitosis and chromosome distribution, vanadium has the potential to be cytotoxic, particularly for fibroblasts and macrophages (32, 33). Vanadium can instigate local and systemic reactions and can inhibit cellular proliferation (16). In addition, vanadium can be bound by certain iron proteins, such as ferritin and transferrin, which subsequently affects the distribution and accumulation of vanadium in the body (28, 34).
IMTEC is the only miniscrew of the three in this study that showed vanadium on the surface prior to etching (Figure 4). Various studies have indicated that the pattern of vanadium release may be associated with the electrochemical and mechanical behaviour of the surface oxide film (15). When Ti-6Al-4V alloys are inserted into the oral cavity, changes in the alloy’s protective surface oxide ensue and these changes may have an influence on the release of alloy corrosion products. These changes occur because the concentration of chloride ions in serum and interstitial fluid causes a corrosive environment for metallic materials. In addition, body fluid consists of an assortment of amino acids and proteins that influence metallic corrosion (31).
This has led to the investigation of dietary intake of several elements, including vanadium. The dietary intake of a certain element may vary based on the specific individual’s eating and drinking habits, as well as their geographical location, but studies have shown that the amount of vanadium released from a Ti-6Al-4V alloy miniscrew was far below the daily intake of this element through food and drink (35). Another study, conducted by Gioka et al. (36), measured in vitro traces of vanadium released from Ti-6Al-4V orthodontic brackets, and it was determined that the vanadium release was insignificant. These authors noted that the long-term release of vanadium may be higher than the first weeks, but in contrast to the long-term biomedical applications of titanium alloys in orthopaedics, the orthodontic use of titanium alloy miniscrews has a very short and limited service life. Consequently, the minimal levels of vanadium release should not be a cause for concern, as they likely will not reach toxic levels.
It has been recognized that the near surface region, within 10 μm depth, is the most important in biodegradation and release phenomena (37). However, we noticed that in the outermost layer tested in this study (0.08 μm), several differences among the metals could be found. While vanadium is the most hazardous metal element found in the three miniscrews tested in this study, a significant amount of aluminium was also detected in all miniscrews, as illustrated in Figure 5. Aarhus and VectorTAS are consistently below 4 per cent at all depths studied, whereas with IMTEC, the percentage increased as the depth increased, reaching 14.38 per cent at a depth of 80nm. It is also important to be aware that aluminium may be associated with osteomalacia, pulmonary granulomatosis, and neurotoxicity (38–40). Additionally, aluminium has been linked to Alzheimer’s disease, and aluminium intoxication is a known cause of dialysis encephalopathy syndrome (41). Similar to vanadium, however, the amount of aluminium released is minute and will likely not reach toxic levels due to the short service life of orthodontic miniscrews.
Chromium could be considered a trace element (less than 1 per cent) that is expected in the IMTEC miniscrew at depths of 30 and 80nm, as shown in Figure 6. Trace elements are particularly important because they are fingerprints of the surface treatments that are performed, for example, anodic oxidation. Chromium is normally introduced to improve both the mechanical strength and the surface passivation property of the alloy (Co–Cr alloy), or it could be considered a contamination due to the titanium rod cutting procedure during manufacturing (Ti-6Al-4V). In vitro studies indicated that high concentrations of chromium in the cell could lead to DNA damage (42). However, this does not seem to be a hazard for miniscrews used in orthodontics because the major risk is related to chromium dust inhalation, which can occur with workers during machining (43). It should be noted that the presence of elements with potentially hazardous effects, especially vanadium, has increased the level of awareness in adopting alternatives, such as the development of new titanium alloys containing niobium (Nb) as a beta stabilizer, Ti-6Al-7Nb (36). This new alternative is superior with regards to metal ion release because the implant materials are more corrosion resistant and biocompatible with human body organs and fluid and can, therefore, remain in the body for much longer periods of time without the concern of toxicity due to metal ion release (29).
The results of this study indicated, for the first time in the literature, both a quantitative and qualitative difference in the outer layer of each tested miniscrew, which was determined using XPS. The null hypothesis was rejected. The VectorTAS miniscrew appeared to have the largest passivation layer, and the IMTEC miniscrew appeared to have the thinnest passivation layer. Further research should be conducted to investigate whether an inflammatory process might be related to the variation in the miniscrews’ outer layer composition.
We would like to thank Dr K. McEleney of the University of Manitoba Chemistry Department for his help with XPS. We would also like to thank Medicon, 3M Unitek, and Ormco for supplying the miniscrews.