JOURNAL OF ENDODONTICS Copyright © 2003 by The American Association of Endodontists
Printed in U.S.A. VOL. 29, NO. 10, OCTOBER 2003
Scanning Electron Microscope Observations of New and Used Nickel-Titanium Rotary Files Satish B. Alapati, William A. Brantley, Timothy A. Svec, John M. Powers, and John C. Mitchell
MATERIALS AND METHODS
The appearances of the tip sections of ProFile 0.04 taper and Lightspeed 25-mm long, ISO size 25, nickel-titanium rotary instruments were compared with a scanning electron microscope in the asreceived condition and after one, three, and six simulated clinical uses to prepare mesial canals of extracted mandibular molars. For the used ProFile instruments, there was some flattening of the characteristic material rollover and minor apparent wear at the edges of the flutes, but there was little change in the tip regions of the used Lightspeed instruments. Deposits on the surfaces of the instruments were attributed to the manufacturing processes and the in vitro preparation of root canals in the extracted teeth. The simulated clinical use did not cause substantial changes in the regions of these two brands of rotary instruments that are involved in the clinical preparation of root canals.
ProFile 0.04 taper (Dentsply Tulsa Dental, Tulsa, OK) and Lightspeed (Lightspeed Technology, San Antonio, TX) rotary nickel-titanium endodontic instruments (25-mm long, ISO size 25) were selected. Instruments from the same package of each product were subjected to one, three, or six uses of simulated clinical instrumentation in extracted teeth. This was performed in the same manner as previously reported by Svec and Powers (11). Sections of approximately 4-mm thickness were cut from the used instruments, as well as as-received instruments serving as controls, with a slow-speed, water-cooled, diamond saw. After cleaning ultrasonically in ethanol, these specimens were examined with an SEM (JSM-820, JEOL Ltd, Tokyo, Japan) at a wide range of magnifications to observe the effects of clinical use on the instruments. Both secondary electron images and backscattered electron images (with high atomic number contrast) were acquired. Qualitative X-ray energy-dispersive spectroscopic (EDS) spot analyses were performed on microstructural features and adherent deposits (see “Results”), using an INCA microanalysis system with an ultra-thin detector window (Oxford Instruments Group, High Wycombe, England).
RESULTS Nickel-titanium alloys have become very popular for endodontic files because of their much lower elastic modulus, compared with stainless steel, which facilities the use of these instruments in curved canals (1). The metallurgy of these alloys (2) and the manufacturing of nickel-titanium endodontic instruments (3) have been discussed. Several recent studies have evaluated the wear of rotary nickel-titanium files under a variety of clinical and laboratory conditions. Most studies have examined ProFile, Lightspeed, or GT instruments (4 –10). Although these instruments do not have the same configuration, they have the same nominal cross-section geometry and should perform and wear in a similar fashion. If wear of these instruments is to be evaluated accurately, it is necessary to control as many variables as possible, namely: speed (rpm), load, rate of penetration, and depth of penetration. This particularly applies when a study is performed with extracted teeth. The purpose of this investigation was to examine new and used ProFile and Lightspeed rotary nickel-titanium files with a scanning electron microscope (SEM). The files were used in the small curved canals of extracted human teeth while controlling the aforementioned parameters.
Figure 1 is a secondary electron image of a region on the cutting tip of a Lightspeed instrument after one simulated clinical use, showing a series of parallel, elongated rectangular features. EDS spot analysis showed that, besides nickel and titanium, these features contained oxygen and traces of carbon and potassium (which are assumed to be surface contaminants). They are interpreted as nickel-titanium oxide precipitates (12) that were elongated into “streamers” during alloy processing. Some flattening of the characteristic “rollover” (1), which occurs at the cutting edge, also can be seen. Figure 2 also shows a region of the cutting tip of a Lightspeed instrument after one simulated clinical use, where a pit in the nickel-titanium alloy surface is evident. A large number of surface grooves, attributed to the manufacturing process, are prominent. Figure 3 presents a ProFile instrument after six simulated clinical uses, showing that there was some flattening of the rollover and minor wear at the edges of the flutes. There was little change in the cutting tip regions of the used Lightspeed instruments. The adherent deposit on the flattened flute in Fig. 3 is associated with 667
Alapati et al.
Journal of Endodontics
FIG 1. Secondary electron image of the cutting tip for a Lightspeed instrument after one simulated clinical use, showing elongated nickel-titanium oxide precipitates (“streamers”) and some flattening of the rollover. Scale bar length is 5 m.
FIG 3. Secondary electron image of the region near a cutting flute for a ProFile instrument after six simulated clinical uses, showing substantial flattening of the rollover and an adherent tooth structure deposit. The black hole near the upper right edge of the flute is a defect in the specimen mounting tape. Scale bar length is 25 m.
FIG 2. Secondary electron image of the cutting tip for a Lightspeed instrument after one simulated clinical use, showing a surface pit and numerous flaws arising from the manufacturing process. Scale bar length is 5 m.
the in vitro preparation of a root canal in an extracted tooth. The secondary electron image of Fig. 4a shows numerous such deposits on the cutting tip of a Lightspeed instrument that was subjected to six clinical uses. Qualitative EDS spot analyses showed that these deposits contained principally carbon and oxygen, indicative of their tooth structure origin. The dark appearance of these deposits in the backscattered electron image of Fig. 4b is caused by their much lower mean atomic number, compared with the nickeltitanium alloy. DISCUSSION The presence of secondary phase particles and surface flaws, such as those seen for the Lightspeed instrument in Fig. 1, can
FIG 4. Secondary electron image (a) of the cutting tip for a Lightspeed instrument after six simulated clinical uses, showing numerous adherent tooth structure deposits, whose dark appearance in the backscattered electron image (b) corresponds to a lower mean atomic number compared with the nickel-titanium alloy. Scale bar length is 25 m.
Vol. 29, No. 10, October 2003
potentially lead to instrument fracture, and surface flaws arising from the manufacturing process also were observed with the ProFile instruments. Present manufacturing technology for these instruments seems to be incapable of avoiding the creation of such surface flaws (8 –10) and rollover, which arises during machining of the highly flexible nickel-titanium alloy (1). As expected, this rollover becomes flattened during clinical use, perhaps with some decrease in cutting efficiency, although further research is necessary to verify this hypothesis. Our results show that tooth structure deposits adhere tenaciously to these rotary instruments after simulated clinical use, perhaps by mechanical retention in surface flaws, despite extensive ultrasonic cleaning. Consequently, it may not be possible to remove such deposits from instruments after in vivo use in root canals. The potential role of these deposits for clinical fracture of nickeltitanium instruments is currently under study.
The authors thank Dr. Wenhua Guo for technical assistance in preparing the digital image prints used for the figures. Dr. Alapati is a graduate student, Dental Materials Science Program, directed by Dr. Brantley, who is professor, Section of Restorative Dentistry, Prosthodontics and Endodontics, College of Dentistry, The Ohio State University, Columbus, OH. Dr. Svec is associate professor, Division of Endodontics, Department of Stomatology; and Dr. Powers is professor and vice chair, Department of Restorative Dentistry and Biomaterials, and director, Houston Biomaterials Research Center and the Graduate Program in Dental Materials Science, University of Texas-Houston Health Science Center, Dental Branch, Houston, TX. Dr. Mitchell is assistant professor, Department of Biomaterials and Biomechanics, School of Dentistry, Oregon Health & Science University, Portland, OR, and former research scientist and senior electron microscopist, Department of Geological Sciences, The Ohio State University.
SEM of Ni-Ti Files
Address requests for reprints to Dr. William A. Brantley, Section of Restorative Dentistry, Prosthodontics and Endodontics, College of Dentistry, The Ohio State University, Mailbox #191, P.O. Box 182357, 305 West 12th Avenue, Columbus, OH 43218-2357.
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