Introduction: The aim of this study was to characterize intraorally fractured nickel-titanium (Ni-Ti) archwires, determine the type of fracture, assess changes in the alloy’s hardness and structure, and propose a mechanism of failure. Methods: Eleven Ni-Ti SE 200 and 19 copper-Ni-Ti (both, Ormco, Glendora, Calif) intraorally fractured archwires were collected. The location of fracture (anterior or posterior), wire type, cross section, and period of service before fracture were recorded. The retrieved wires and brand-, type-, and size-matched specimens of unused wires were subjected to scanning electron microscopy to assess the fracture type and morphological variation of fracture site of retrieved specimens, and to Vickers hardness (HV 200 ) testing to investigate the hardness of as-received and in-vivo fractured specimens. Fracture site distribution was statistically analyzed with the chi-square test (α = 0.05), whereas the results of the hardness testing were analyzed with 2-way ANOVA with state (control vs in-vivo fractured) and composition (Ni-Ti SE vs copper-Ni-Ti) serving as discriminating variables and the Student-Newman-Keuls test at the 95% confidence level. Results: The fracture site distribution showed a preferential location at the midspan between the premolar and the molar, suggesting that masticatory forces and complex loading during engagement of the wire to the bracket slot and potential intraoral aging might account for fracture incidence. All retrieved wires had the distinct features of brittle fracture without plastic deformation or crack propagation, whereas no increase in hardness was observed for the retrieved specimens. Conclusions: Most fractures sites were in the posterior region of the arch, probably because of the high-magnitude masticatory forces. Brittle fracture without plastic deformation was observed in most Ni-Ti wires regardless of archwire composition. There was no increase in the hardness of the intraorally exposed specimens regardless of wire type. This contradicts previous in-vitro studies and rules out hydrogen embrittlement as the cause of fracture.
Although nickel-titanium (Ni-Ti) wires have become an integral part of orthodontic treatment in the past 30 years, evidence is lacking to substantiate the empirically noted frequency of wire fracture, especially on the posterior segment of the arch. Only a few reports indicate that retrieved archwires had much higher fracture incidence in contrast to that expected in laboratory experiments that showed that these wires are practically unbreakable. This effect could be the result of several factors, including intraoral aging of wires, complex loading derived from activated wire and masticatory loads, and local conditions such as unerupted premolars. The latter might give rise to increased loading of the wires, which cannot be bent to adapt to the occlusal conditions of the posterior segment and thus are exposed to masticatory forces. Specifically, it can be postulated that cyclic loading along with potent effects from the intraoral environment including pH fluctuations, oral flora activity, and plaque accumulation are implicated in the aging pattern, which could alter the mechanical properties and typical fracture characteristics of the material. Similarly, complex loading derived from the engagement of the archwire in the bracket slots of angulated or rotated teeth, and the formation of galvanic couples between the less noble bracket and wire alloys with various gold-, nickel-, or silver-containing brazing alloys, might also contribute to the environmental conditions that predispose the alloy to microstructural changes. The effect of this multifactorial aging pattern on the performance of Ni-Ti archwires has been shown in a few investigations focusing on the surface characteristics and mechanical properties of retrieved materials. These studies demonstrated that used Ni-Ti archwires have extended coverage by calcified integuments precipitated on the wire surface, increased porosity, and large pore size. This evidence suggests that research approaches with in-vitro methodologies might not substantiate the in-vivo phenomena.
Alternatively, recent evidence shows that hydrogen absorption, an effect known to increase hardness and induce brittle fracture of titanium alloys, could be a mechanism of Ni-Ti wire failure in vitro. Titanium alloys demonstrate an abrupt decrease in their tensile strength and show brittle fracture in laboratory investigations when the absorbed amount of hydrogen exceeds a critical value.
The hypothesis tested in this study was that exposure of the material to the oral cavity exerts potent structural and mechanical effects that impact on the fracture pattern of the wires. Therefore, the purpose of this study was to characterize intraorally fractured Ni-Ti archwires to determine the type of fracture, assess potential changes in alloy hardness and structure, and clarify the mechanism of failure.
Material and methods
The Ni-Ti wires analyzed in the study (Ormco, Glendora, Calif) were retrieved during the regular treatment visits of orthodontic patients to the practice of an author (N.P.). The wires included 2 types: Ni-Ti SE 200 and copper-Ni-Ti (Cu-Ni-Ti) from the same manufacturer to show the effect of different compositions on fracture incidence and pattern. Upon identification of fracture, each specimen was packaged separately, and the following parameters were recorded: (1) name of patient, (2) date of archwire placement, (3) archwire type (Ni-Ti SE 200 or Cu-Ni-Ti), (4) archwire cross-section, (5) dental arch of placement, (6) means of ligation (stainless steel or elastic), (7) date of archwire fracture, and (8) location of fracture. All brackets used with the wires were stainless steel with 0.022-in slot size.
The retrieval procedure yielded 19 Cu-Ni-Ti and 11 Ni-Ti SE fractured archwires with a mean service period of 3 months (range, 2-4 months) of 0.016-in diameter (0.40 mm). The archwires were rinsed with double-distilled water to detach any loosely bound precipitations, and specimens were prepared from areas corresponding to the proximal and distal sides of the fracture by sectioning the archwires with a sectioning machine (Isomet, Bueller, Lake Bluff, Ill) under water coolant. The same procedure was followed for unused archwires matched by manufacturer and size to each retrieved specimen. The retrieved specimens were then subjected sequentially to the following.
Scanning electron microscopy to assess the fracture type and morphological variation of fracture sites of retrieved specimens. For this purpose, wire segments were cut from retrieved wires, and fracture surfaces were examined with a Quanta 200 (FEI, Hillsboro, Ore) at 15 kV accelerating voltage and 100 μA beam current.
Five archwire segments from each group were embedded in epoxy resin. The specimens were ground with water coolant silicon carbide papers from 220 to 2000 grit, polished up to 0.05 mm with alumina suspension (Bueller) in a grinding-polishing machine (Ecomet III, Bueller), and cleaned in an ultrasonic water bath for 5 minutes. Each specimen’s surfaces were used for the assessment of Vickers hardness (HV 200 ) by using a microhardness tester (HMV-2000, Shimadzu, Tokyo, Japan) applying a load of 200 g and 15 seconds contact time. Five readings were taken from each control and retrieved specimen.
The statistical analysis of fracture site distribution in each archwire group (Cu-Ni-Ti or Ni-Ti SE 200) was performed with the chi-square test at the α = 0.05 level of significance. Two-way analysis of variance (ANOVA) was used to analyze the hardness data with state (control vs in-vivo fractured) and composition (Ni-Ti SE vs Cu-Ni-Ti) as the discriminating variables; further group differences were investigated with the Student-Newman-Keuls test (α = 0.05).
Table I gives the location of the fractures of the retrieved wires, indicating a significantly higher fracture distribution of posterior fractures in the midspan between the mandibular premolars and the first molar for both wires types. Because the number of initially placed archwires was not normalized and varied significantly, no conclusion can be drawn on the relative fracture incidence between Cu-Ni-Ti and Ni-Ti SE; therefore, the analysis was limited to the location of fractures in each archwire group.
|Archwire group||Anterior fracture †||Posterior fracture ‡||Total||χ 2||P|
|Ni-Ti SE retrieved||1||10||11||7.36||<.01|
Figures 1 through 3 show representative fracture surfaces of in-vivo fractured Ni-Ti and Cu-Ni-Ti archwires. Most of both wire types had fracture surfaces with fibrous surface patterns ( Figs 1 and 2 ). Figure 2 shows the fracture origin in higher magnification: a fracture propagates in mainly a quasi cleavage manner ( Fig 2 , A ), although torsion might also have contributed to it, as shown in Figure 2 , B . Few wires had the fracture pattern of Figure 3 , where the fibrous zone is constrained in an area less than half of the whole cross-section area, and the rest had shallow dimples, a characteristic pattern of tension fracture.
The ANOVA table for hardness in Table II shows that hardness was not affected by intraoral exposure, where the interaction was insignificant, implying an effect only from the composition of wire alloys. Cu-Ni-Ti wires had higher hardness than their Ni-Ti SE counterparts, in which no changes in hardness were observed between as-received and intraorally fractured wires ( Table III ).
|Dependent variable: Vickers hardness (HV 200 )|
|Source of variation||DF||SS||MS||F||P ⁎|
|Material × state||1||297||297||3.3||.077|
|Archwire group||HV 200 mean (SD)||Student-Newman-Keuls grouping ⁎|
|Cu-Ni-Ti retrieved||374 (9)||A|
|Cu-Ni-Ti control||367 (12)||A|
|Ni-Ti SE control||313 (9)||B|
|Ni-Ti SE retrieved||309 (6)||B|
The fractographic analysis of the retrieved specimens is consistent with a brittle fracture as evidenced by the absence of extensive plastic deformation associated with a large reduction in cross-section diameter due to necking during the in-vitro testing of archwires in tension. In the few cases where dimples were observed, they were much shallower than their in-vitro tested counterparts reported in the literature. This observation can be explained by the fact that the extent of plastic deformation and consequently the extent of dimples are heavily constrained by the high strain rates developed in the oral cavity.
In the broader materials science research, especially in the application of alloys to metallic tanks used for storage of hydrogen fuel, alloy systems involving nickel or titanium were found to absorb hydrogen. This property results in an undesirable effect termed “anodic slip dissolution” or “hydrogen embrittlement,” which involves an increase in hardness and a decrease in the plastic deformation of the alloy, thereby establishing a brittle fracture pattern. The mechanism underlying this effect at the atomic scale is postulated to derive from a reduction in the tensile stress required to separate atoms.
In the orthodontic literature, hydrogen absorption and subsequent embrittlement of Ni-Ti archwires were reported by Yokoyama et al. These authors proposed that absorption of hydrogen sets in via interatomic diffusion with a direction from the surface to the bulk material, and that hydrogen accumulates at grain boundaries and dislocations near impure atoms. Furthermore, it was hypothesized that the diffusion of hydrogen is enhanced by loading, probably because of the associated expanded interatomic space and increased density of dislocations. This evidence was mostly derived from in-vitro evidence and a case study of a wire failure.
However, the results of our study, which is the first to systematically analyze in-vivo fractured specimens, show that hydrogen absorption as the leading cause of failure is unlikely for the following reasons.
The results of the hardness testing did not show an increase in hardness for the retrieved materials, a condition required for hydrogen-driven embrittlement.
The location of fracture showed a statistically significant preference for the area corresponding to the midspan of the mandibular premolars and the first molar. The higher frequency of posterior failures can be partially explained by the greater interbracket distance in the posterior teeth and the eruption sequence in the mandibular arch, where the most favorable scheme is that the second premolar is the last tooth to emerge in the arch mesial to the first molar. Therefore, it is likely that initial levelling and alignment often begin before premolar eruption, thus exposing the wire segment at the midspan of the premolar and the molar to masticatory forces. The effect from these forces is exacerbated because Ni-Ti archwires cannot be bent to bypass the occlusion with the cusps of the maxillary teeth.
In support of the foregoing hypothesis, in-vitro approaches and, more specifically, archwire tensile tests substantially underestimate the loading pattern of archwires intraorally. The latter is a highly complex entity, which includes forces acting on the wire as a result of engagement to rotated teeth, the effect of ligation and notching of wire surface that might act as nuclei for degradation, and, most importantly, masticatory forces. The masticatory-forces hypothesis gains further validity by the observation that a complete masticatory cycle (sequential opening and closing) of a healthy patient lasts approximately 800 ms, with the closing movement having a duration less than 400 ms because the closing cycle is accelerated. Assuming a 10 to 20 mm jaw movement during chewing, it is expected that the jaw velocity exceeds 2000 mm per minute, a value several hundred times higher than the standard rate used in conventional in-vitro testing. The importance of this relates to the fact that high loading rates induce a stiff body response and a brittle fracture because this loading pattern eliminates the time available for the absorption of the energy through plastic deformation that occurs during slow loading.
Loading of the wire induced by the engagement into the slot and masticatory forces might later change the microstructure of the alloy, involving a reduction in grain size at the compressed locations, which extended beyond the near-surface region. Changes in grain size have been documented in the Ni-Ti metallurgy as stress-induced martensite, where the martensitic transformation occurs below the transition temperature range when external stress is applied. Thus, work-hardened martensitic transformation of the Ni-Ti wires might also account for the brittle fracture pattern observed, because martensite fractures with a typical brittle mode.
Wires exposed in the oral cavity have a distinctive aging pattern that is unique and cannot be simulated with in-vitro testing. Used wires also have a distinctive aging pattern involving more crevices, presumably due to the tensile forces produced locally from the wire engagement, surface defects, and the precipitation of proteinaceous matter that, at later stages, is calcified. The mechanical effects arise from force vectors and can induce changes in the microstructure of the alloy as a reduction in grain size at the compressed locations that extend beyond the near-surface region.
Laboratory evidence suggests that an embrittlement effect sets in when hydride levels are about 200 ppm. Consequently, studies with analytical techniques such as x-ray diffraction to assess the hydride formation have a high degree of false positive information. This is because, although lack of evidence of hydride implies no hydrogen embrittlement, identification of this compound cannot establish the active role of this mechanism as the main cause of failure.
Although hydrogen absorption might not be the main cause of failure clinically, other factors have been shown to have a direct, detrimental impact on the integrity of the alloy surface. An interesting observation pertains to the described potent titanium-induced degradation of its alloys. Specifically, 0.2% acidulated phosphate fluoride, which contains 0.2% sodium fluoride and 0.17% phosphoric acid, has been shown to affect the corrosion resistance of titanium alloys through the formation of hydrofluoric acid, which dissolves the passive oxide film on the surface of alloys. Disruption of the oxide passive film of titanium eliminates its immunity to corrosion and makes it prone to degradation, possibly accelerating failure. On the basis of this evidence, the use of fluoride in an acidulated formulation should not be suggested for patients treated with titanium-containing alloys including Ni-Ti wires, titanium brackets, and beta-titanium wires.