Letter to the editor

Abstract

Authors’ responseWe appreciate Dr Gautam’s interest in this technical topic.The purpose of our study was to characterize the fracture pattern of in-vivo fractured wires. The fact that this pattern proved to be brittle should not be misinterpreted as evidence of a hydrogen embrittlement mechanism involved during service. Embrittlemment requires an increase in hardness. In contrast to Dr Gautam’s argument, his first reference provides evidence of an increase of Vickers hardness from 450 to 540 when the wires were immersed in the fluoride solution for 6 hours.Regarding the comments on the methodology used, we want to note the following.1The previous studies this knowledgeable author refers to are in-vitro studies, which involved exposure of the archwires to environments bearing no comparison to actual conditions. For example, immersing specimens in an acidulated phosphate fluoride bath is different from the actual use of fluoride-containing solution in vivo, because the handling and manipulation of materials in the laboratory lacks the formation of biofilms on materials, which might decrease the reactivity of materials with the environment. Also, without saliva, laboratory settings exaggerate the extent of induced effects. Moreover, the immersion times in acidulated phosphate fluoride reported in the first reference of Dr Gautam’s letter are far beyond those used clinically, ranging from 4 to 6 hours. Nonetheless, we noted in the final paragraph of our discussion that these solutions should be avoided with titanium-based orthodontic alloys (brackets, and nickel-titanium (Ni-Ti) and beta-titanium archwires).2Basic texts set the requirement for induction of embrittlement to 200 ppm, but there is a considerable dispute about the threshold for this phenomenon to set in. For example, in titanium-aluminum-zirconium alloys, this might be several times greater than that value (1140 ppm),1Kim T.K. Baek J.H. Choi B.S. Jeon Y.H. Lee D.J. Chang M.H. Characteristics of hydriding and hydrogen embrittlement of the Ti-Al-Zr alloy.Ann Nuclear Energy. 2002; 29: 2041-2053Crossref Scopus (11) Google Scholar whereas the study that Dr Gautman cites indicates a value of 500 ppm for work-hardened Ni-Ti alloys. X-ray diffraction cannot show this effect because of its semi-quantitative character. Even if x-ray diffraction indicated the presence of hydrides, the embrittlement hypothesis could not have been resolved, because actual values would not have been available.3The fact that no difference was noted between the location of fracture and the fracture patterns of the 2 Ni-Ti alloys included in the study precludes the assignment of different mechanisms of fracture between them. It has been hypothesized that the superelastic transformation in Ni-Ti alloys might constitute, in itself, a potential source of resistance to fracture and fatigue.2Melzer A. Stöckel D. Performance improvement of surgical instrumentation through the use of Ni-Ti materials.in: Pelton A.R. Hodgson D. Duerig T.W. Shape memory and superelastic tendencies. MIAS, Monterey, Calif1995: 401Google Scholar The increased fatigue endurance observed for a variety of brittle materials has been attributed to phase transformation, which increases the volume of particles, reducing local stress intensity, thereby effectively inhibiting crack growth. However, in contrast to materials in which transformation involves a significant dilatational component with volume increase, the phase transformation in Ni-Ti alloys entails a minute negative volume change, and therefore its contribution to the overall fatigue resistance of the wires is unlikely.3McKelvey AL, Ritchie RO. Fatigue-crack growth in the superelastic endovascular stent material nitinol. Proceedings of symposium II: advanced materials, coatings, and biological cues for medical implants. Materials Research Society; 1998 Nov 30-Dec 4. Available at: http://www.lbl.gov/Ritchie/Programs/NITI/MRSf98/. Accessed July 24, 2007.Google Scholar Even so, the foregoing discussion is not directly connected to our study because we did not investigate the fatigue properties of wires.4The Clausius-Clapeyron equation notes that the rate at which the natural logarithm of the vapor pressure of a liquid changes with temperature is determined by the molar enthalpy of vaporization of the liquid, the ideal gas constant, and the temperature of the system.4The Clausius-Claperion Equation. Available at: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/clausius.php. Accessed July 24, 2007.Google Scholar Unfortunately, Dr Gautam’s second reference is not accessible and has not appeared in a peer-reviewed source. Therefore, it is difficult to relate this effect with the variation of transitional temperature range in Ni-Ti alloys. Whereas it is true that different Af temperatures can result in different force levels exerted by Ni-Ti alloys, the available literature does not support any influence of the Af temperature on the fatigue life of alloys, with respect to the number of loading cycles.5Drescher D. Bourauel C. Sonneborn W. Schmuth G.P. The long-term fracture resistance of orthodontic nickel-titanium wires.Schweiz Monatsschr Zahnmed. 1994; 104: 578-584PubMed Google ScholarWe believe that queries such as Dr Gautam’s contribute to elucidating complex phenomena and assist in effectively conveying the messages of articles to readers; however, the focus should be on the clinical performance of materials and not on arbitrarily defined manipulation of the laboratory aging conditions, which are strikingly irrelevant in the oral environment. Authors’ responseWe appreciate Dr Gautam’s interest in this technical topic.The purpose of our study was to characterize the fracture pattern of in-vivo fractured wires. The fact that this pattern proved to be brittle should not be misinterpreted as evidence of a hydrogen embrittlement mechanism involved during service. Embrittlemment requires an increase in hardness. In contrast to Dr Gautam’s argument, his first reference provides evidence of an increase of Vickers hardness from 450 to 540 when the wires were immersed in the fluoride solution for 6 hours.Regarding the comments on the methodology used, we want to note the following.1The previous studies this knowledgeable author refers to are in-vitro studies, which involved exposure of the archwires to environments bearing no comparison to actual conditions. For example, immersing specimens in an acidulated phosphate fluoride bath is different from the actual use of fluoride-containing solution in vivo, because the handling and manipulation of materials in the laboratory lacks the formation of biofilms on materials, which might decrease the reactivity of materials with the environment. Also, without saliva, laboratory settings exaggerate the extent of induced effects. Moreover, the immersion times in acidulated phosphate fluoride reported in the first reference of Dr Gautam’s letter are far beyond those used clinically, ranging from 4 to 6 hours. Nonetheless, we noted in the final paragraph of our discussion that these solutions should be avoided with titanium-based orthodontic alloys (brackets, and nickel-titanium (Ni-Ti) and beta-titanium archwires).2Basic texts set the requirement for induction of embrittlement to 200 ppm, but there is a considerable dispute about the threshold for this phenomenon to set in. For example, in titanium-aluminum-zirconium alloys, this might be several times greater than that value (1140 ppm),1Kim T.K. Baek J.H. Choi B.S. Jeon Y.H. Lee D.J. Chang M.H. Characteristics of hydriding and hydrogen embrittlement of the Ti-Al-Zr alloy.Ann Nuclear Energy. 2002; 29: 2041-2053Crossref Scopus (11) Google Scholar whereas the study that Dr Gautman cites indicates a value of 500 ppm for work-hardened Ni-Ti alloys. X-ray diffraction cannot show this effect because of its semi-quantitative character. Even if x-ray diffraction indicated the presence of hydrides, the embrittlement hypothesis could not have been resolved, because actual values would not have been available.3The fact that no difference was noted between the location of fracture and the fracture patterns of the 2 Ni-Ti alloys included in the study precludes the assignment of different mechanisms of fracture between them. It has been hypothesized that the superelastic transformation in Ni-Ti alloys might constitute, in itself, a potential source of resistance to fracture and fatigue.2Melzer A. Stöckel D. Performance improvement of surgical instrumentation through the use of Ni-Ti materials.in: Pelton A.R. Hodgson D. Duerig T.W. Shape memory and superelastic tendencies. MIAS, Monterey, Calif1995: 401Google Scholar The increased fatigue endurance observed for a variety of brittle materials has been attributed to phase transformation, which increases the volume of particles, reducing local stress intensity, thereby effectively inhibiting crack growth. However, in contrast to materials in which transformation involves a significant dilatational component with volume increase, the phase transformation in Ni-Ti alloys entails a minute negative volume change, and therefore its contribution to the overall fatigue resistance of the wires is unlikely.3McKelvey AL, Ritchie RO. Fatigue-crack growth in the superelastic endovascular stent material nitinol. Proceedings of symposium II: advanced materials, coatings, and biological cues for medical implants. Materials Research Society; 1998 Nov 30-Dec 4. Available at: http://www.lbl.gov/Ritchie/Programs/NITI/MRSf98/. Accessed July 24, 2007.Google Scholar Even so, the foregoing discussion is not directly connected to our study because we did not investigate the fatigue properties of wires.4The Clausius-Clapeyron equation notes that the rate at which the natural logarithm of the vapor pressure of a liquid changes with temperature is determined by the molar enthalpy of vaporization of the liquid, the ideal gas constant, and the temperature of the system.4The Clausius-Claperion Equation. Available at: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/clausius.php. Accessed July 24, 2007.Google Scholar Unfortunately, Dr Gautam’s second reference is not accessible and has not appeared in a peer-reviewed source. Therefore, it is difficult to relate this effect with the variation of transitional temperature range in Ni-Ti alloys. Whereas it is true that different Af temperatures can result in different force levels exerted by Ni-Ti alloys, the available literature does not support any influence of the Af temperature on the fatigue life of alloys, with respect to the number of loading cycles.5Drescher D. Bourauel C. Sonneborn W. Schmuth G.P. The long-term fracture resistance of orthodontic nickel-titanium wires.Schweiz Monatsschr Zahnmed. 1994; 104: 578-584PubMed Google ScholarWe believe that queries such as Dr Gautam’s contribute to elucidating complex phenomena and assist in effectively conveying the messages of articles to readers; however, the focus should be on the clinical performance of materials and not on arbitrarily defined manipulation of the laboratory aging conditions, which are strikingly irrelevant in the oral environment. We appreciate Dr Gautam’s interest in this technical topic. The purpose of our study was to characterize the fracture pattern of in-vivo fractured wires. The fact that this pattern proved to be brittle should not be misinterpreted as evidence of a hydrogen embrittlement mechanism involved during service. Embrittlemment requires an increase in hardness. In contrast to Dr Gautam’s argument, his first reference provides evidence of an increase of Vickers hardness from 450 to 540 when the wires were immersed in the fluoride solution for 6 hours. Regarding the comments on the methodology used, we want to note the following.1The previous studies this knowledgeable author refers to are in-vitro studies, which involved exposure of the archwires to environments bearing no comparison to actual conditions. For example, immersing specimens in an acidulated phosphate fluoride bath is different from the actual use of fluoride-containing solution in vivo, because the handling and manipulation of materials in the laboratory lacks the formation of biofilms on materials, which might decrease the reactivity of materials with the environment. Also, without saliva, laboratory settings exaggerate the extent of induced effects. Moreover, the immersion times in acidulated phosphate fluoride reported in the first reference of Dr Gautam’s letter are far beyond those used clinically, ranging from 4 to 6 hours. Nonetheless, we noted in the final paragraph of our discussion that these solutions should be avoided with titanium-based orthodontic alloys (brackets, and nickel-titanium (Ni-Ti) and beta-titanium archwires).2Basic texts set the requirement for induction of embrittlement to 200 ppm, but there is a considerable dispute about the threshold for this phenomenon to set in. For example, in titanium-aluminum-zirconium alloys, this might be several times greater than that value (1140 ppm),1Kim T.K. Baek J.H. Choi B.S. Jeon Y.H. Lee D.J. Chang M.H. Characteristics of hydriding and hydrogen embrittlement of the Ti-Al-Zr alloy.Ann Nuclear Energy. 2002; 29: 2041-2053Crossref Scopus (11) Google Scholar whereas the study that Dr Gautman cites indicates a value of 500 ppm for work-hardened Ni-Ti alloys. X-ray diffraction cannot show this effect because of its semi-quantitative character. Even if x-ray diffraction indicated the presence of hydrides, the embrittlement hypothesis could not have been resolved, because actual values would not have been available.3The fact that no difference was noted between the location of fracture and the fracture patterns of the 2 Ni-Ti alloys included in the study precludes the assignment of different mechanisms of fracture between them. It has been hypothesized that the superelastic transformation in Ni-Ti alloys might constitute, in itself, a potential source of resistance to fracture and fatigue.2Melzer A. Stöckel D. Performance improvement of surgical instrumentation through the use of Ni-Ti materials.in: Pelton A.R. Hodgson D. Duerig T.W. Shape memory and superelastic tendencies. MIAS, Monterey, Calif1995: 401Google Scholar The increased fatigue endurance observed for a variety of brittle materials has been attributed to phase transformation, which increases the volume of particles, reducing local stress intensity, thereby effectively inhibiting crack growth. However, in contrast to materials in which transformation involves a significant dilatational component with volume increase, the phase transformation in Ni-Ti alloys entails a minute negative volume change, and therefore its contribution to the overall fatigue resistance of the wires is unlikely.3McKelvey AL, Ritchie RO. Fatigue-crack growth in the superelastic endovascular stent material nitinol. Proceedings of symposium II: advanced materials, coatings, and biological cues for medical implants. Materials Research Society; 1998 Nov 30-Dec 4. Available at: http://www.lbl.gov/Ritchie/Programs/NITI/MRSf98/. Accessed July 24, 2007.Google Scholar Even so, the foregoing discussion is not directly connected to our study because we did not investigate the fatigue properties of wires.4The Clausius-Clapeyron equation notes that the rate at which the natural logarithm of the vapor pressure of a liquid changes with temperature is determined by the molar enthalpy of vaporization of the liquid, the ideal gas constant, and the temperature of the system.4The Clausius-Claperion Equation. Available at: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/clausius.php. Accessed July 24, 2007.Google Scholar Unfortunately, Dr Gautam’s second reference is not accessible and has not appeared in a peer-reviewed source. Therefore, it is difficult to relate this effect with the variation of transitional temperature range in Ni-Ti alloys. Whereas it is true that different Af temperatures can result in different force levels exerted by Ni-Ti alloys, the available literature does not support any influence of the Af temperature on the fatigue life of alloys, with respect to the number of loading cycles.5Drescher D. Bourauel C. Sonneborn W. Schmuth G.P. The long-term fracture resistance of orthodontic nickel-titanium wires.Schweiz Monatsschr Zahnmed. 1994; 104: 578-584PubMed Google Scholar We believe that queries such as Dr Gautam’s contribute to elucidating complex phenomena and assist in effectively conveying the messages of articles to readers; however, the focus should be on the clinical performance of materials and not on arbitrarily defined manipulation of the laboratory aging conditions, which are strikingly irrelevant in the oral environment. Letter to the editorAmerican Journal of Orthodontics and Dentofacial OrthopedicsVol. 132Issue 4PreviewIt was a pleasure reading the article “Why do nickel-titanium archwires fracture intraorally? Fractographic analysis and failure mechanism of in-vivo fractured wires” by Spiros Zinelis, Theodore Eliades, Nikolaos Pandis, George Eliades, and Christoph Bourauel (Am J Orthod Dentofacial Orthop 2007;132:84-9). Full-Text PDF