Influence of Electrochemical Polishing on the Mechanical Properties of K3 Nickel-Titanium Rotary Instruments

Plasma Electrolytic Polishing of Porous Nitinol Structures

In this study, for the first time, the application of plasma electrolytic polishing (PEP) of porous Nitinol structures, mimicking a trabecular bone structure, that were additively manufactured, is reported. The cube-shaped samples were polished in a diagonal position three different times. The effect of PEP was evaluated in terms of the polishing depth, the effect on sample chemical composition and a possible shift of the phase transition temperature using microscopy, the energy dispersive X-ray spectroscopy (EDX), and the differential scanning calorimetry (DSC) techniques, respectively. The obtained results demonstrated that the PEP technique is suitable for polishing porous structures up to a certain depth into the sample inner structure and does not have any influence on the chemical composition and the phase transformation temperatures. However, small changes in the specific enthalpy were observable among the investigated samples. These changes could be attributed to the sample chemical inhomogeneity, measurement error, and/or differences in sample size and shape.

Plasma Electrolytic Polishing of Nitinol: Investigation of Functional Properties

A novel, environmentally friendly, fast, and flexible polishing process for Nitinol parts is presented in this study. Nitinol samples with both superelastic and shape memory properties at room temperature were investigated. The chemical contamination and surface roughness of superelastic Nitinol plates were examined before and after plasma electrolytic polishing. The shift in phase transformation temperature and tensile strength before and after the polishing process were analysed using Nitinol wire with shape memory properties. The obtained experimental results were compared to the data obtained on reference samples examined in the as-received condition. It was found that plasma electrolytic polishing, when the right process parameters are applied, is capable of delivering Nitinol parts with extremely high surface quality. Moreover, it was experimentally proven that plasma electrolytic polishing does not have a negative impact on functionality or mechanical properties of polished parts.

Qualitative Assessment of the Surface Topographic Changes of XP-endo Shaper and TruNatomy files after exposure to Sodium Hypochlorite and Ethylenediaminetetraacetic Acid

OBJECTIVE

TruNatomy and XP-endo Shaper are recently introduced file systems showing increased fatigue resistance rate. The present study aims to evaluate the surface topographic changes and nickel (Ni) and titanium (Ti) elemental loss of XP-endo Shaper (XPS) and TruNatomy (TN) files on exposure to conventionally used root canal irrigants; [5.25% sodium hypochlorite (NaOCl) and 17% ethylenediaminetetraacetic acid (EDTA)] at a 10 minute time frame using atomic force microscopy (AFM) and energy dispersion X-ray spectroscopy (EDX) analytical techniques.

METHODS

Twelve samples for each of XPS (30/.04 taper) and TN (26/.04 taper; prime) instruments were dynamically exposed to 5.25% NaOCl, 17% EDTA separately for 10 minutes and in combination of 5.25% NaOCl (8 minutes)+17% EDTA (2 minutes) for a total of 10 minutes. Post exposure, the files were subjugated to AFM and EDX analysis. Independent t test and one-way ANOVA were used for statistical analysis, and the level of significance was set at 0.05.

RESULTS

XPS and TN showed a significant increase of surface roughness (Ra) and roughness mean square (RMS) on exposure to various irrigants (P<0.05) using AFM analysis. Increased overall roughness was observed with TN in comparison to XPS (P<0.05). Elements Ni and Ti loss was found in both XPS and TN files using EDX analysis. Both files exhibited Ni and Ti loss with the loss of Ni content higher for TN after exposure to 17% EDTA. Loss of Ti was seen for both files on exposure to a combination of 5.25% NaOCl+17% EDTA.

CONCLUSION

After exposure to root canal irrigants, the surface roughness was lesser in XPS compared to TN files. 17% EDTA caused significantly higher surface roughness in both file systems when compared to 5.25% NaOCl. TN exhibited overall higher elemental (Ni and Ti) loss on exposure to 17% EDTA and 5.25% NaOCl+17% EDTA in comparison to XPS files.

Shot peening increases resistance to cyclic fatigue fracture of endodontic files

The objective of this study was to assess the resistance to fatigue fracture of conventional nickel–titanium files after undergoing shot peening. Forty NITIFLEX endodontic files, number 30, were divided into two groups; one was submitted to shot peening treatment and the other was not. All instruments were tested for fatigue fracture in simulated canals with a TRI-AUTO ZX endodontic motor. One file of each group was subjected to a residual stress analysis by XRD. Finally, the fractured surface was observed and elemental analysis performed by means of SEM and EDX. Roughness analysis was made by focal variation microscope. The shot peening group showed greater resistance to fatigue fracture; there was no difference in the length of the fractured fragments. XRD results showed the presence of residual compression stresses in the file submitted to shot peening, a decrease in the interplanar spacing, and an increase in the full-width-at-half-maximum and the microstrains. SEM and EDX showed a ductile fracture with zones of fatigue and an equiatomic ratio between the nickel and titanium. Surface roughness increased after the file was subjected to the shot peening procedure. In conclusion, shot peening increases the resistance to fatigue fracture due to the presence of residual compression stresses in files manufactured from a conventional nickel–titanium alloy.

Evolution of Nickel-titanium Alloys in Endodontics

To improve clinical use of nickel-titanium (NiTi) endodontic rotary instruments by better understanding the alloys that compose them. A large number of engine-driven NiTi shaping instruments already exists on the market and newer generations are being introduced regularly. While emphasis is being put on design and technique, manufacturers are more discreet about alloy characteristics that dictate instrument behavior. Along with design and technique, alloy characteristics of endodontic instruments is one of the main variables affecting clinical performance. Modification in NiTi alloys is numerous and may yield improvements, but also drawbacks. Martensitic instruments seem to display better cyclic fatigue properties at the expense of surface hardness, prompting the need for surface treatments. On the contrary, such surface treatments may improve cutting efficiency but are detrimental to the gain in cyclic fatigue resistance. Although the design of the instrument is vital, it should in no way cloud the importance of the properties of the alloy and how they influence the clinical behavior of NiTi instruments.

CLINICAL SIGNIFICANCE

Dentists are mostly clinicians rather than engineers. With the advances in instrumentation design and alloys, they have an obligation to deal more intimately with engineering consideration to not only take advantage of their possibilities but also acknowledge their limitations.

Fracture resistance of dental nickel-titanium rotary instruments with novel surface treatment: Thin film metallic glass coating

BACKGROUND/PURPOSE

Dental nickel-titanium (NiTi) rotary instruments are widely used in endodontic therapy because they are efficient with a higher success rate. However, an unpredictable fracture of instruments may happen due to the surface characteristics of imperfection (or irregularity). This study assessed whether a novel surface treatment could increase fatigue fracture resistance of dental NiTi rotary instruments.

METHODS

A 200- or 500-nm thick Ti-zirconium-boron (Ti-Zr-B) thin film metallic glass was deposited on ProTaper Universal F2 files using a physical vapor deposition process. The characteristics of coating were analyzed by scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry. In cyclic fatigue tests, the files were performed in a simulated root canal (radius=5 mm, angulation=60°) under a rotating speed of 300rpm. The fatigue fractured cross sections of the files were analyzed with their fractographic performances through scanning electron microscopy images.

RESULTS

The amorphous structure of the Ti-Zr-B coating was confirmed by transmission electron microscopy and X-ray diffractometry. The surface of treated files presented smooth morphologies without grinding irregularity. For the 200- and 500-nm surface treatment groups, the coated files exhibited higher resistance of cyclic fatigue than untreated files. In fractographic analysis, treated files showed significantly larger crack-initiation zone; however, no significant differences in the areas of fatigue propagation and catastrophic fracture were found compared to untreated files.

CONCLUSION

The novel surface treatment of Ti-Zr-B thin film metallic glass on dental NiTi rotary files can effectively improve the fatigue fracture resistance by offering a smooth coated surface with amorphous microstructure.

Influence of Surface Roughness on the Fatigue Life of Nickel-Titanium Rotary Endodontic Instruments

INTRODUCTION

The goal of the present study was to evaluate the influence of surface grooves (peaks and valleys) resulting from machining during the manufacturing process of polished and unpolished nickel-titanium BR4C endodontic files on the fatigue life of the instruments.

METHODS

Ten electropolished and 10 unpolished endodontic files were provided by the manufacturer. Specimens were from the same batch, but the unpolished instruments were removed from the production line before surface treatment. The instruments were evaluated with a profilometer to quantify the surface roughness on the working part of the instruments. Then the files were subjected to rotating bending fatigue tests.

RESULTS

Analysis with the profilometer showed that surface grooves were deeper on the unpolished instruments compared with their electropolished counterparts. In the rotating bending fatigue test, the mean and standard deviation for the number of cycles until fracture (NCF) were greater for instruments with less pronounced grooves. Student t test revealed significant differences in all tests (P < .05).

CONCLUSIONS

The results from the present study showed that the depth of the surface grooves on the working part affected the NCF of the instruments tested; the smaller the groove depth, the greater the NCF.

Influence of combined cyclic fatigue and torsional stress on the fracture point of nickel-titanium rotary instruments

INTRODUCTION

The combined influence of cyclic fatigue and torsional stress on rotary nickel-titanium instruments has been little investigated. The aim of this study was to determine possible differences in the fracture point of rotary nickel-titanium instruments depending on the application of cyclic fatigue only (CO) or in combination with torsional stress (CT).

METHODS

A novel custom-designed testing device was constructed. The device used a 2-pin design to test files under cyclic fatigue stress and allowed the additional application of defined torsional stress to the lateral aspect of the file by cutting into a dentin block. Files were tested dynamically at an amplitude of 2 mm at 0.0625 Hz using a programmable computer-controlled system. Three rotary NiTi systems were tested at 30° under CO or CT (with an added 1-Ncm torsional load): Revo-S (Micro-Mega, Besancon, France), Vortex (Dentsply, York, PA), and Profile (Dentsply) of tip sizes 25 and 35. For each file type, 10 new files 25 mm in length with a 0.04 taper size were tested. A total of 120 files were tested: 60 for CO and 60 for CT. The mean fragment length (MFL [in millimeters from the shaft to the fracture point]) was measured under 10× magnification with an electronic gauge to assess the location of the fracture. One-way analysis of variance, the Tukey Honestly Significant Differenct (HSD) test, and the Student's t test for paired samples were used for statistical analysis.

RESULTS

All fractures, regardless if CO or CT was used, occurred within the area of the curvature. The addition of a torsional load (CT) resulted in a mean 1.09-mm statistically significant difference between CO and CT (P < .0001, CO MFL = 17.78 mm [standard deviation ± 1.08 mm, n = 60]; CT MFL = 16.69 mm [SD ± 0.54 mm, n = 60]), relocating the fracture point toward the area where torsional load was applied. There was a statistically significant difference between the 3 file systems when they were tested either in the CO mode (P < .01) or the CT mode (P < .05). Statistically significant differences also existed for both instrument sizes (ie, 25 [P < .01] and 35 [P < .0001]).

CONCLUSIONS

CT compared with CO resulted in statistically significantly different MFLs. All fractures remained within the area of the curvature, but with the addition of a torsional load, the location of the fracture moved in the direction of the additionally applied torsional stress. This suggests that stress was distributed from the area in which the torsional load was applied toward the area undergoing cyclic fatigue.

Influence of electrochemical polishing on the mechanical behaviour of nickel-titanium rotary files

The purpose of this study was to evaluate the influence of electrochemical polishing on the cyclic fatigue, cutting efficiency and surface topography of Miltex nickel-titanium rotary files. A group of files were submitted to electrochemical polishing. New files and polished files were tested for flexural fatigue and for cutting efficiency. Scanning electron microscopy observations of all files were carried out to compare the surface morphologies of polished and new files. The t-test was used to compare the groups for cutting efficiency. Scanning electron microscopy analysis indicated that the polishing resulted in more regular surfaces in relation to the new files. Electropolished files attained a significantly higher number of cycles to fracture than new unpolished files. No statistical difference exists between the groups in relation to the cutting efficiency (t-test, P < 0.05). The polished files keep their integrity for a significantly higher number of cycles to fracture than the new files, without compromising cutting efficiency.

Qualitative analysis of files of four different rotary systems, before and after being used for the twelfth time

The aim of this study was to make a comparative evaluation of the surface topography of the cervical third and tip of four different rotary systems, before and after being used 12 times, in resin blocks with simulated root canals with standardized 45° curvatures, and analyzed by scanning electronic microscopy (SEM). A total of 1,440 resin blocks with simulated root canals and standardized curvatures were used. The blocks were divided into four groups and prepared by the crown down technique according to the manufacturers' recommendations: Group 1 (G1)--K3®, Group 2 (G2)--Protaper Universal®, Group 3 (G3)--Twisted Files®, and Group 4 (G4)--Biorace®. After each preparation, the instruments were washed and autoclaved. A total of 24 instruments were selected, being three new instruments and three after having been used for the 12th time, from each group. These instruments were analyzed by SEM. To perform the evaluation, 24 micrographs were observed by 12 evaluators who attributed a score to the images according to a template. In the qualitative analysis of the cervical region and tip of new instruments, it was observe that the files of Groups G4 and G2 were the least rough and those of G1, the roughest. In the same manner, the cervical region of the instruments was qualitatively analyzed after the 12th time of using them, and there was no statistically significant difference at a level of 1% (ANOVA P < 0.01) between G2 and G4 as well as between G1 and G3. As regards the tip, it was observed that there was no statistically significant difference, at a level of 1%, between the groups: G1 and G3, G1 and G4, and between G3 and G4, and there was statistically significant difference at a level of 1%, between group G2 and the other groups, with G3 having the greatest roughness (ANOVA P < 0.01).

Alteration in the inherent metallic and surface properties of nickel-titanium root canal instruments to enhance performance, durability and safety: a focused review

The expanded use of nickel-titanium (NiTi) rotary instruments in root canal procedures has led to the development of a wide variety of shapes, designs and applications. Root canal anatomy has not changed, however, and the same challenges exist in both initial treatment and the revision of unacceptable treatment. These challenges include application with high levels of achievement and low to no levels of adverse effects, such as instrument fracture, root canal wall ledging, dentine wall perforation and so forth. To that end, many manufacturers have been seeking ways to alter the presently available and wide range of root canal instrument designs, with a focus on altering the surface of the alloy or altering the alloy microstructure with post-machining or post-twisting heat treatment. This focused review will address the impact that these modifications have had on instrument flexibility, resistance to cyclic fatigue and cutting efficiency.