This in-vitro investigation was designed to form a simulated clinical use using RaCe

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1 Cairo Dental Journal (25) Number (3), 367:374 September, 2009 A Comparative Study of Deformation and Fracture of Two Rotary Nikel Titanium Instruments after Simulated Clinical Use (In vitro) El-Maghraby D; (1) Fawzy M (2) and Nour El-Deen M (2) 1. Postgraduate candidate. 2. Assoc. Prof. of Endodontics, Faculty of Dental Medicine (Girls), Al-Azhar University. Abstract This in-vitro investigation was designed to form a simulated clinical use using RaCe rotary system and HERO Shaper rotary system and check for the deformation and fracture after each use of files using loupe and Dental Operating Microscope (DOM). A total of 100 mesiobuccal root canals from lower 1 st molars were used. They were with average medium curve from 20 0 to 40 0 determined according to Schneider method. Samples were classified into groups I and II. Group I consists of 50 root canal prepared by RaCe rotary system. Group II consists of 50 root canal prepared by HERO Shaper rotary system. No fracture occurred in this study. Deformations were detected in the MAF assigned for examination as follows: In group I (RaCe): 4 MAF were deformed out of 10, 2 of the files were detected by the loupe and all of them were detected by the DOM. In group II (HERO Shaper): 5 MAF were deformed out of 10, 1 of the files was detected by the loupe and all of them were detected by the DOM. There was no statistical difference between the 2 systems regarding deformation. INTRODUCTION Successful root canal treatment depends amongst other factors on the removal of infected dentin and organic tissues through chemomechanical instrumentation of the root canal system. Thus, the cleaning ability of any root canal instrument is of importance for the outcome of root canal treatment (1). Since the introduction of highly flexible alloy to endodontics, Nikel Titanium (Ni-Ti) hand files and rotary instruments have achieved widespread popularity because of superiority in preparing curved root canals compared with stainless steel (SS) instruments (2). Accordingly, rotary Ni-Ti root canal instruments have become an important part of endodontic armamentarium. RaCe system is one of such instruments that possesses twisted areas alternating with straight areas; this design reduces the tendency to screw into the root canal (3). Also the manufacturer of HERO Shaper Ni-Ti rotary system claimed its durability and resistance to fracture due to its big inner core cross-section and flexibility which

2 (368) El-Maghraby D, et al. C.D.J. Vol. 25. No. (3) maintain an even distribution of forces into cutting areas (4,5). Little is known about the physical parameters governing rotary root canal preparation. However, these factors are crucial because Ni-Ti rotary files have an increased risk of fracture compared with K-files. Consequently, a benefit versus risk analysis must be done for rotary Ni-Ti instruments, addressing the reasons and the clinical consequences of instrument fracture (6). Some factors have been implicated in the separation of Ni-Ti endodontic instruments, amongst are: operator proficiency, method of use, rotational speed, design of the instrument, and number of sterilization cycles (5, 7-10). Despite the evident advantages of the new techniques, Ni-Ti instruments may undergo failure by fatigue when used in curved canals due to the tension / compression cycles to which they are subjected when flexed in the region of maximum curvature of the canals (11). Moreover, sterilization of endodontic instruments either for new files or for their reuse involves repeated exposure to heating and cooling cycles which may affect the properties of these Ni-Ti instruments and lead to deformation and instrument separation as a result of cumulative effect of clinical use. (12,13) Several studies have been conducted to examine and evaluate the Ni-Ti Instruments deformation and fracture using SEM and Stereo-microscope (14,15). Sben et al, suggested visual examination of files before using in the canal and that the unwinding of the RaCe instruments might be discernable before breaking (16). Accordingly, we intended to reproduce the complex clinical situation affecting the stresses and strains imposed on the instrument. Hence, we can evaluate the cumulative effect on two rotary Ni-Ti systems possess triangular cross-section and fixed taper using magnifying loupe and dental operating microscope. Materials and Methods One hundred mesiobuccal root canals obtained from freshly extracted lower first molars were used. The teeth were stored in 0.5% Sodium Azide in fresh water until use. The root canals of the chosen teeth were measured to have an average medium curvature ranging from 20 o to 40 o according to the Schneider method (17). The selected teeth were decapitated using a diamond disk and the mesial root was separated from the distal root. The mesial roots were cut coronally to establish 14mm length. The root canal length was determined by inserting # 10 K- files into the canal until the file was seen from the apical foramen and then 0.5mm was reduced from this length to establish the working length. Each root was fixed in a ring filled with pink wax. The 100 specimens were classified into two groups: -Group I (50 roots) -Group II (50 roots) Ten sets from each of the selected 2 rotary Ni-Ti systems were used: RaCe* HERO Shaper** The instruments were used as following: Group I (50 root canals): The mesiobuccal root canals were mechanically prepared by using RaCe Ni-Ti rotary instruments. Group II (50 root canals): The mesiobuccal root canals were mechanically prepared by using HERO Shaper Ni-Ti rotary instruments. Before use; the assigned files for evaluation from the two systems were inspected by the loupe # magnification 6 and Dental Operating Microscope ## (DOM) at magnification 16. Snap shots were taken from the computer connected to the DOM. The shots were taken at magnification 6 and 16. Each set of files for both systems was initially * (FKG Dentaire, La-chaux-de-fonds, Switzerland) ** (MicroMega, Besancon, France) # Neitz Japan ## ZIES Germany

3 A Comparative Study of Deformation and Fracture (369) packed in sterilization bag for autoclaving at 121 C for 20 minutes at 1 atmospheric pressure. All manufacturer recommendations were followed in the preparations. One set of instruments was assigned to prepare 5 root canals in each group in a simulated clinical use. Sodium hypochlorite (2.5%) was used as an irrigating solution 2 ml after each file entering the canal & each instrument was lubricated with Glyde File Prep. The contra-angle reducing handpiece* with a minihead was mounted on an air motor; its reduction is 1:32 to give 562 rpm, which matches the manufacturer recommendation speed rate from 300 to 600 rpm. The coronal 4mm of the mesiobuccal root canals was mechanically prepared with Gats Glidden drills # 2 and 3. Root canal preparation was completed for group I with RaCe up to file #25 taper 0.04 taper to complete apical preparation as master apical file (MAF). Root canal preparation was completed for group II with HERO Shaper up to file #30 & taper 0.04 taper to complete apical preparation (MAF). After each root canal mechanical instrumentation the files were scrubbed under running water & then introduced in the ultrasonic cleaner for 10 minutes cycle. After completion of the cycle, the instruments were dried with towels and wrapped in gauze and inserted in sterilization bags. The bags were exposed to one cycle in the autoclave at 121 o C for 20 minutes at 1 atmospheric pressure. These steps were repeated after the preparation of five root canals by each set of files for each of the two systems. File # taper (MAF) RaCe from each set used for group I and file # taper (MAF) HERO Shaper from each set used for group II were assigned for examination. The Magnifying loupe (magnification 6) and DOM (magnification 16) were used for files examination after each use for detection of deformation (unwinding, roundation & elongation) and fracture. Statistical Analysis Statistical analysis was performed with SPSS 16.0* (Statistical Package for Scientific Studies) for Windows. Qualitative data were presented as frequencies and percentages. Chi-square (x 2 ) test was used for studying the comparisons between different qualitative variables. Reliability analysis using Cronbach s alpha was used to measure the agreement between Loupe and Dental Operating Microscope evaluations. The significance level was set at P Results Group I In the present study; no fractured MAF of group I was detected. Four RaCe files (MAF) out of 10 files were deformed (40%). Two of them were seen by the loupe and all of them were seen by the DOM. All the deformed files were detected at the fifth use (Fig.1&2). Fig. (1) Photomicrograph showing straightening in 2 nd flute of RaCe file after 5 uses as detected by: a.loupe (6X) and b. DOM (16 X) * (ANTHROGYR SAS, SALLANCHES, France) SPSS, Inc., Chicago, IL, USA.

4 (370) El-Maghraby D, et al. C.D.J. Vol. 25. No. (3) Fig. (2) Photomicrograph showing elongation and roundation of the apical 3 rd flute after 5 uses as detected by: a.loupe(6x) and b. DOM (16X). Group II: Root canals of group II prepared with HERO Shaper system. In the present study; no fractured HERO Shaper file was detected. five HERO Shaper files (MAF) were deformed, 1 of them only was seen by the loupe (10 %) and all of them were seen by the DOM (50 %). One file was deformed at 4 th use and 4 files were seen deformed at the 5 th use (Fig.3). Comparison between the two systems I. Loupe evaluation Two deformed RaCe files (20%) were seen while one deformed HERO Shaper file (10%) was seen by the loupe. This difference was not statistically significant (P = 0.531), Fig.4. II. Dental Operating Microscope evaluation Four deformed RaCe files (40%) were seen while five deformed HERO Shaper files (50%) were seen with the DOM. This difference was not statistically significant (P = 0.653). All deformed RaCe files were deformed at fifth use, 1 HERO Shaper file was deformed at fourth use and 4 HERO Shaper files were deformed at fifth use. This difference was not statistically significant (P = 0.343). Fig. (3) Photomicrograph showing unwinded flute in the middle 3rd after 5 uses as detected by: a.loupe (6X) and DOM (16X). The deformation in this file was easily detected by naked eye.

5 A Comparative Study of Deformation and Fracture (371) the cumulative effect of simulated clinical use on two Ni-Ti rotary systems which may cause deformation or fracture. Magnifying tools that have clinical significance were used. Fig. (4) Histogram showing percentage of deformation for RaCe & HERO Shaper rotary systems using the loupe. Agreement between loupe and DOM evaluation RaCe system: two deformed files were seen by loupe and DOM while 2 files were seen by DOM only. Cronbach s alpha measurement was denoting good agreement between the two methods of evaluation. HERO Shaper system: one deformed file was seen by loupe and DOM while 4 files were seen by DOM only. Cronbach s alpha measurement was denoting fair agreement between the two methods of evaluation. Discussion For the evaluation of root canal preparation two models have been used constantly; either extracted human teeth or simulated root canals with strictly defined root canal curvatures in term of angle and radius (18). A major drawback of using rotary instruments in resin blocks is the great heat generated, which may soften the resin material and lead to binding of cutting blades, and separation of the instrument (19). Natural teeth root canals with its dentin hardness differences reflect more the effective preparation proficiency of different rotary files which are subjected to natural environment as in clinical use rather than resin blocks with its different texture. It also helps the operator to be exposed to different hardness degrees to be able to manage different clinical situations and canal obstructions (19). This study addresses Tripi et al showed that dentin deposits which were created during preparation of root canals adhere tenaciously in surface cracks, despite extensive ultrasonic cleaning before SEM examination. Consequently, it may not be possible to remove such deposits from instruments after being used in the root canals of one patient before clinical use on a subsequent one. These lodged dentine chips seem to cause a wedging action on the machining cracks, which may lead to their propagation from localized tensile stress during clinical use and eventually result in instrument fracture (20). However, in the current study all instruments were exposed to ultrasonic cleaning after each use (5 cycles) which may be effective in removing dentin deposits that helped in absence of instrument fracture and possibly delayed the occurrence of deformation to the 4 th or 5 th use. However, SEM examination is recommended to confirm the efficacy of ultrasonic cleaning under the conditions of this study. In the present study no fractures occurred in the MAF assigned for detection either for the RaCe system or for the HERO Shaper system after being used 5 times. This may be related to the fact that RaCe system files are electropolished to seal the machining cracks as to prevent the wedging action of dentin chips (21).This observation is in line with the prevailing concept of an improved resistance of a smooth surface to fatigue-crack initiation (14). However, HERO Shaper files are not electropolished and no fracture occurred while using them, this may be related to its design with a big inner core and longer pitch for enhanced flexibility (22). There was no statistical difference between the two systems regarding the deformed instruments. The RaCe system; 4 files out of 10 were deformed after detection with DOM which were all deformed after the 5 th use. This

6 (372) El-Maghraby D, et al. C.D.J. Vol. 25. No. (3) may be due to the benefit of the alternating cutting edges and the electropolishing of the RaCe system (21). The HERO Shaper system; 5 files out of 10 were deformed, 1 file was deformed after the 4 th use and 4 files were deformed after the 5 th use. This may be related to the fact that HERO Shaper instruments have a large inner core which favors resistance to breakage and longer pitch for enhanced flexibility, which probably enables the HERO Shaper files to negotiate curved canals easily (22). These obtained results corroborate Parashos et al study on discarded rotary Ni-Ti files obtained from 14 endodontists in four countries with known history of usage; concluding that most instruments deformations occurred at the 5 th usage (23). Accordingly deformations occurring after multiple uses (fourth or fifth use) may be due to cumulative effect of the factors at which the instrument were exposed to; including the different root canal parameters, sterilization cycles, number of uses and experience of the operator which is an important affecting factor differing greatly from one operator to the other. Further more; the results of this study support the literature that multiple uses of rotary Ni-Ti instruments are clinically acceptable and they do not support the routine single use of rotary Ni- Ti instruments to prevent fracture, as it is a complex multifactorial clinical problem (6,9,). Several studies have been conducted to examine and evaluate the Ni-Ti instruments deformation and fracture using SEM and Stereo-microscope (16,24). In the present study we used magnifying aids of clinical significance (loupe and DOM). The results reflect the importance of the loupe in detecting major deformations and the DOM in detecting the minor deformations. It also gives a notation about the importance of regular check up of the files after use and using visual aids specially ones with high magnification powers as the DOM. Hence, it will help much in assisting the operator in deformations detection and proper selection of the files to be discarded avoiding its breakage in the patient s mouth. From the results of this study; it was shown that some files deformations (RaCe) were easier for detection using the loupe, probably due to simple uncomplicated design (twisted areas with straight areas), while other deformations (HERO Shaper) needed higher magnification for detection probably due to complicated design (continuous flutes which may mask the deformation of 1or more flutes). That is why it is the choice of the operator in selecting the suitable package of instruments according to the skills and availability of suitable instruments and visual aids in the operating field. To prevent breakage of a Ni-Ti instrument, appropriate rotational speeds and continuous pecking motion in the root canal are recommended (25). Most of the reported deformed parts of the files were always located at the apical one third. In other words, maximum deformation always occurred at the segment located at the maximum curvature of the canal. Thus the in and out pecking movement used in clinical practice makes the segment of the file subjected to maximum fatigue vary continuously, which may increase the life span of the instrument (12).A longer pecking distance gives the instrument a longer time interval before it is once again passing through the highest stress area (25). This type of movement was used in this study which probably was one of the factors that increased the instrument resistance to fracture. Silvaggo & Hicks showed heat sterilization (through use of autoclave or dry heat sterilizers) did not increase the likelihood of instrument fracture (13). However, their study did not include the other variables the file is subjected to during routine clinical usage with its cumulative effects over the life span of the file. In this study the instruments were exposed to 6 sterilization cycles (autoclave) and 5 cycles of ultrasonic cleaner. Thompson & Dummer; in their work on simulated root canals, they recommended the use of lubricants to decrease the generation of sticky flakes and irrigation for evacuation to prevent clogging the instrument (26). In the present study the lubricant (Glyde) and the irrigating solution (Sodium hypochlorite

7 A Comparative Study of Deformation and Fracture (373) 2.5%) were used as manufacturer recommendations which corroborate their work. Busslinger et al showed that a statistically significant amount of titanium was lost from the Ni-Ti instruments after immersion times of 30 and 60 min in 5% Sodium hypochlorite. However, it is not of clinical significance, since clinically the files do not have in situ contact time of 30 min with sodium hypochlorite (27). Despite the short exposure time of the instrument to Sodium hypochlorite and the concentration used, which is not a full strength solution; the effect of Sodium hypochlorite on Ni-Ti rotary instruments in use cannot be dismissed (28). Conclusion 1- There was no fractured instruments in the two systems used in this study. 2- Most of the file deformations occurred after the 5 th use. 3- The Loupe was effective in determination of massive file deformation and the Dental Operating Microscope was effective for minor deformations detection. REFERENCES 1. Bertrand MF, Lupi-Pegurier L, Medioni E. Curved molar root canal preparations using Hero 642 rotary Nikel- Titanium instruments. Int Endodon J 2001; 34: Walia H, Bronthy Wa, Gerstein H. An initial investigation of the bending and torsional properties of NiTinol root canal files. J Endodon 1988; 14: Troian CH, So MVR, Pigueiredo JAP. et al. Deformation of RaCe and K3 endodontic instruments according to the number of uses. Int Endodon J 2006; 39: Turpin YL, Chagneau F, Bartier O. et al. Impact of two theoretical cross section on torsional and bending stresses of Nikle-Titanium root canal instruments. J Endodon; 2000; 26: Turpin YL, Chagneau F, Bartier O. et al. Impact of torsional and bending intertia on root canal instruments. J Endodon 2001;27: Yard GM, Bou Dagher FE, Machtou P. Cyclic fatigue of ProFile rotary instruments after clinical use. Int Endodon J 2000; 33: Yard Gm, Kulkarni GK. Failure of ProFile Ni-T- instruments used by an inexperienced operator under access limitations. Int Endodon J 2002; 35: Blum JY, Cohen A, Machtou p. Analysis of forces developed during mechanical preparation of extracted teeth using ProFile Ni-Ti rotary instruments. Int Endodon J 1999; 32: Martin B, Zelada G, varela P. et al. Factors influencing the fracture of Ni-Ti rotary instruments. Int Endodon J 2003; 36: Berutti E, Chandussi G, Gaviglio I. et al. A comparative analysis of torsional and bending stresses in two mathematical models of nickel-titanium rotary instruments: ProTaper versus ProFile. J Endodon 2003; 29: Prutt JP, Clement DJ, Carnes DL. Cyclic fatigue testing of Ni- Ti endodontic instruments. J Endodon 1997; 23: Craveiro MC, Guimor M, Buono VT. Fatigue resistance of Engine-Driven Rotary Ni-Ti Endodontic instruments.j Endodon 2002; 28: Silvaggo J, Hicks ML. Effect of heat sterilization on the torsional properties of rotary Ni-Ti files. J Endodon 1997; 23: Cheung GSP, Peng B, Bian Z. et al. Defects in ProTaper S1 instruments after clinical use. Fractographic examination. Int Endodon J 2005 ; 38: Shen y, cheung GSP, Bian Z. et al. Comparison of defects in ProFile & ProTaper systems after clinical use. J Endodon 2006; 32: Sben Y, Winestock E, Chung GS. et al : Defects in NiTi instruments after clinical use. Part 4: Electropolished instruments. J Endodon 2009; 35 :

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