Sem analysis of working surface in new manual endodontic instruments

Introduction The aim of this study was to analyze working surfaces of new hand endodontic instruments and to check possible existence of dirt or defects on working surface that resulted from manufacturing process using SEM. Material and methods Three sets of new hand instruments: K-File (KF), (18 instruments) (Dentsply Maillefer, Switzer-land) and Hedstorm Files (HF), (18 instruments) (SybronEndo Co, USA) were used. Instruments were analyzed by SEM method at 170× magnification while semi-quantitative EDS analysis was used to determine chemical composition of dirt particles. Fisher test (p < 0.05) was applied in statistical analysis. Results Results showed that none of the instruments was defect-free. The most common defects were metal strips and fretting noticed at the surface of all tested instruments. Debris was present on all KF (100% in apical and middle third) and HF (56% in apical and 56% in middle third) instruments. Pitting was noticed in KF (33% in apical and 39% in middle third) and HF (11% in apical and 6% in middle third) instruments. Corrosion of working surface, metal flash and disruption of cutting edge were marked only in KF group. Conclusion Manufacturing defects were noticed in all instruments and the most common type of irregularity were metal strips and fretting.


INTRODUCTION
Chemomechanical root canal treatment is usually performed with hand endodontic instruments (made of stainless steel or Ni-Ti alloy) or engine-driven Ni-Ti rotary endodontic instruments with adequate and abundant irrigation of canal system [1]. Despite the fact that Ni-Ti rotary instruments have become widely used in endodontic practice due to its efficacy when compared to stainless steel hand instruments (speed, simplicity and instrumentation uniformity), hand instruments are still used in standard endodontic procedure [2,3]. Most manufacturers recommend a combination of hand stainless steel instruments and Ni-Ti rotary instruments when establishing initial patency for curved and / or narrow canals [4]. Stainless steel hand files are much better choice than Ni-Ti rotary instruments for preparation of initial patency mainly due to better tactile sensation of complicated canal morphology, low fracture risk and economic efficiency. Flaws of hand instruments are greater fatigue of practitioner, longer procedure and more frequent instrumentation mistakes (irregularity in intracanal dentin, apical transportation, over-extension, apical perforation, ledging, zipping and canal obtrusion and apical blockage by dentine debris) [2]. Canal preparation with Ni-Ti endodontic instruments secures more appropriate canal shape with less frequent faults caused by instrumentation when compared to hand instruments. Nonetheless, complications such as unexpected deformation and fracture are more frequent [5]. Great number of studies analyzed percentage of fracture incidence of rotary Ni-Ti instruments and their results vary from 0.3% to 23% (Sattapan et al. 2000 [6], Ankrum et al. 2004 [7], Spili et al. 2005 [8], Iqbal et al. 2006 [9], Wu et al. 2011 [10]) while fracture incidence in stainless steel instruments ranges from 0.25% to 6% [8,11,12,13]. Fractured instrument is a serious threat to treatment, irrigation and filling of root canals and it may significantly affect the outcome of endodontic therapy [14].
The most common reason for avoiding engine-driven endodontic treatment in dental practice is higher frequency and unpredictability of rotary Ni-Ti instrument fractures. On the top of that, root canal anatomy itself might make engine-driven treatment even more difficult. This relates to mandibular incisors (due to mesiodistal flattened root canals), very wide canals and apical deltas. In all these situations, hand endodontic technique prevails over engine-driven [15].
The majority of new endodontic instruments are not sterile. Various metal debris and dirt of organic and non-organic origin can be found on their surface. Stainless steel endodontic instruments manufacture process might cause metal strips which, to some extent, stay on the surface of endodontic instruments working parts [16].
It is confirmed that endodontic instruments, due to their design and different manufacture process, may significantly impact deformation and fracture during root canal instrumentation [7][8][9][10].
Stainless steel endodontic instruments are usually made by twisting of various steel profiles around longitudinal axis thus forming blades from vertical wire edges [17]. Irregularities at the instrument surface might increase its vulnerability to fracture. Surface defects seem to be points of tension and can initiate and spread cracks thus potentially highly contributing to possible fractures during instrument activation [18].
The aim of this study was to analyze working surfaces of new hand endodontic instruments and check possible existence of manufacture dirt or defects on working surface using SEM.

MATERIAL AND METHOD
This research was performed on three basic sets (each set consisting of 6 instruments) of new hand stainless steel instruments: K-File, KF (Dentsply Maillefer, Switzerland) and Hedstorm Files, HF (SybronEndo Co, USA). SEM analysis was performed in SEM-EDS laboratory of the Faculty of Mining and Geology, University of Belgrade (JEOL JSM-6610LV, Japan), without any prior preparation.
Microphotographs were taken at 170× magnification but in case of noticeable changes on the instruments and for the purpose of more detailed analysis, they were magnified up to 800×. Apical and middle third of the files were analyzed from two different directions and each side of instrument was analyzed by three images.
Analysis of different irregularities and faults during manufacturing process implied the criteria proposed by Eggert et al. [19]: Score 1 -No visible defect, Score 2 -Pitting, Score 3 -Fretting, Score 4 -Micro fractures, Score 5 -Complete fracture, Score 6 -Metal flash, Score 7 -Metal strips, Score 8 -Blunt cutting edge, Score 9 -Disruption of cutting edge, Score 10 -Corrosion, Score 11 -Debris. Qualitative analysis was performed though obtained results were not quantified. Semi-quantitative EDS analysis determined chemical composition of found dirt. Fisher test (p < 0.05) was used for statistical analysis.

RESULTS
Obtained results were presented in Tables 1-5, Graphs 1 and 2 and Pictures 1-8.
Analysis of SEM microphotographs determined contamination of working surface of tested instruments and subsequent EDS analysis defined its chemical composition. Thus, we divided instruments into two types -instruments contaminated with metal strips and contaminated with debris.
EDS analysis of KF instrument (ISO 20) (Figure 1, Table 1) for Spectrum 1 was performed on a clean part of  Table 2) for Spectrum 1 was performed on a clean part of instrument surface, while Spectrum 2 and 3 were performed on a contaminated surface. The most abundant element in the analysis of all three Spectrums was iron with maximum presence of 71.12 mas% and minimum of 69.53 mas%. Aluminum, silicon, titanium, chrome, manganese and nickel were detected in different mass concentrations. EDS analysis of Spectrums 2 and 3 showed contamination with metal strips.
All tested instruments had some kind of defect on their working surface. New hand instruments did not show any signs of micro fractures, fractures or blunt cutting edges (Tables 3, 4, 5). The most frequent defect types were metal strips and fretting which were detected on the surface of all tested instruments (in 100% of cases) (Tables 3, 4, 5, Figure 3). Fisher test did not show any statistically significant differences between tested instruments at their ends or apical and middle thirds.
Debris was noticed on all KF instruments (100% apical and middle third) and half of the HF instruments (56% apical and middle third) ( Table  3, Graph 1, Figure 4 and 5). After the comparison of debris on different hand instruments (KF and HF), statistically significant difference was noted (p = 0.0029 in apical and p = 0,0029 in middle third).
The presence of pitting was noted in apical and middle third of KF instruments (33% apical and 39% middle third) and HF instruments (11% apical and 6% middle third) (Tables 3, 4, 5, Graph 2, Picture 6). After the comparison of pitting on different hand instruments (KF and HF), both apical and middle thirds showed statistically significant difference (p = 0.0051 end) (p = 0,0045 middle).      Metal flash, corrosion of working surface and disruption of cutting edge were detected only in KF instruments (corrosion 11% apical and 17 % in middle third; metal flash surface 11% apical, 6% in middle third; disruption of cutting edge 2% apical) ( Table 3, 4, 5, Figures 7 and 8).
In HF instrument group, there were no corrosion, metal flash or disruption of cutting edge.

DISCUSSION
Above all, success of endodontic therapy depends on proper instrumentation i.e. biomechanical treatment and tridimensional hermetic root canal obturation.
Design of endodontic instruments, their metallurgical characteristics and surface may complicate endodontic treatment in case instrument deforms or fractures during  use. It is proven that manufacturing defects might cause fracture of new instruments even during their first clinical use [20]. During manufacturing process, working surface of instruments, especially its threads, might have residuals of metal strips and organic and non-organic debris which might have infective and non-specific irritating potential [21][22][23][24].
Results of this study showed that all analyzed instruments had minimum two and maximum five different defects prior to any use. Such results comply with literature data reporting frequent defects of endodontic instruments during their manufacturing process [5,16,19,25,26]. The most common defects on working surfaces of new endodontic stainless steel instruments (KF and HF) in our study were fretting and metal strips.
Fretting on working surface of an instrument during the manufacturing process was noticed in all tested hand stainless steel instruments. Conventional manufacture of instruments by twisting the wire (of quadrangular profile for K-type file and milling of circular profile for Hedstorm file) causes surface irregularities such as traces of milling and metal flash (especially on blades) which might compromise efficiency of instrument blade and potentially cause problems related to corrosion and fracture [20,25,26]. Clinical importance of fretting on instrument surface reflects in its easy screwing (due to friction that is present because of uneven surface) which as a consequence leads to greater incidence of fracture [27]. Greater incidence of HF file fracture is explained with different design of this file which implies different activation in root canal. HF instruments have increased incline of blades compared to the instrument axis (60° and 65°) while KF has significantly smaller angle (25° and 40°), therefore, manipulation must be very careful [28].
Presence of metal strips, shown in all tested groups in 100 percent, just confirms the complexity of endodontic instrument manufacture. This finding complies with the result of Chianella et al. study that confirmed the presence of such contamination in 96.3% of all new tested instruments [28]. This type of defect is very significant since it decreases the blade efficacy. Apart from that, metal strips on active surface of instrument might stick to dentin root canal walls or slip into periapical tissue during instrumentation. Van Eldik reported possible contamination of periapical tissue with metal strips that were transferred by instruments which significantly reduced tissue reparation [29].
Pitting on working surface of instrument was noticed in small percentage of instruments (KF, HF), and it could be explained by specific technological process of manufacturing just like the presence of metal flash and blade damage in KF grupi. Bonetti Filho et al. also draw attention to potential pitting on new instruments [30].
Debris was present in KF tested groups in 100% and HF in 56% (apical and middle) which confirmed that manufacturing clean endodontic instruments was a very complex procedure. As opposed to the study of Lopes et al. which combined acetone and ultrasonic cleaning to obtain clean and dry instruments, this study analyzed the instruments immediately after the removal of their packaging and without any prior preparation [25]. Thus, SEM analysis tested the quality of their final processing and packaging conditions. Remains of grease (used in manufacture process), epithelial cells, hair and parts of fabrics might be found on the surface of new instruments after the manufacturing process and inadequate packaging. This potentially may compromise the success of endodontic treatment. Study of Roth et al. determined biological contamination in 13% of new hand stainless steel endodontic instruments made by different manufacturers thus proving the possibility of new instrument contamination by live microorganisms. (S. epidermidis, Paenibacillus species and three fungal species) [31].
Problems in manufacturing process might arise due to the quality of wire used, since oxide and carbides particles might be incorporated in alloy during manufacturing, thus creating more brittle zones that represent key points for micro defects development [32]. Corrosion factors (irrigators, disinfects and sterilization solutions) and torsion and cyclic pressure during instrumentation might cause corrosion and further propagation of these defects [32].
Review of EDS analysis showed mass percentage of elements present in stainless steel alloy and exact composi-  Great abundance of chrome on spectrum of clean surface (18.03 mas% and 18.76 mas%) and nickel (7.27 mas% and 8.08 mas%) confirms the significance of these elements in improvement of instrument features. This type of alloy provides good mechanical features and is resistant to corrosion. In order to avoid unwanted effects during instrumentation, manufacturers developed new stainless steel alloys which are characterized by greater flexibility. As a result, state of the art ferritic steel has 12-18% of mass share of chrome [30]. Due to great affinity of chrome to bond carbon and create brittle chromium carbide, increase in mass share of carbon leads to decrease in corrosion resistance. In order to prevent unwanted chrome carbide, new alloys are enriched with titanium which has a greater affinity toward carbon that results in stabilization of ferritic steel [30].

CONCLUSION
The results of this study showed that all tested instruments had manufacturing defects (two or more), and that the most common types of defects were metal strips and fretting. Debris on working surface indicated the necessity to sterilize instruments before their first use. These facts could be warning sign to all practitioners to carefully manipulate files even during first use and perform good observation of working surface in order to prevent possible complications during endodontic treatment.
Ve ći na no vih en do dont skih in stru me na ta nij e ste ril na i na nji ho voj po vr ši ni se mo gu na ći razli či ti me tal ni osta ci, ne čisto će, organskog i neorganskog porekla. Pro ce s pro iz vod nje endodontskih instrumenata od nerđajućeg čelika može do vesti do prisustva sit nih opil ja ka -me ta la ko ji se u ma njoj ili ve ćoj me ri za drža va ju na po vr ši nama rad nih de lo va endodontskih instrumenata [16].