Cutting efficiency loss of root canal instruments due to bulk plastic deformation, surface damage, and wear

Comparative Analysis of Endodontic 0.15 Stainless-Steel K-Files: Exploring Design, Composition, and Mechanical Performance

To establish a glide path, smaller files (up to size 0.15) with tapers of 2% are commonly used as pathfinding files. They pre-shape the root canal space before transitioning to larger taper endodontic instruments, aiming to prevent procedural errors. This study aimed to compare the design, metal wire composition, and mechanical characteristics of seven different ISO size 15 stainless-steel hand files (K-File and C-File+). Ninety-one new stainless-steel ISO 15 K-files were mechanically tested. All files were inspected for deformations before the assessment. Dental operating microscope, scanning electron microscope (SEM), and optical microscope analyses were conducted on four randomly selected instruments from each group, and two instruments per group underwent an energy-dispersive X-ray spectroscopy (EDS) analysis. Buckling mechanical tests were performed using an Instron universal testing machine, and microhardness was assessed using a Vickers hardness tester. The statistical analysis employed the nonparametric Mood's median test, with a significance level set at 0.05. The instrument design analysis unveiled variations in the active blade area length and the number of spirals, while maintaining consistent cross-sections and symmetrical blades. Distinct tip geometries and surface irregularities were observed. While the energy-dispersive X-ray spectroscopy confirmed similar compositions, the buckling strength and microhardness values exhibited variability across for all tested files. Notably, the Dentsply ReadySteel C-File+ recorded the highest buckling value (2.10 N), and the Dentsply ReadySteel K-File exhibited the lowest (1.00 N) (p < 0.05). Moreover, the Dentsply ReadySteel K-File recorded the highest microhardness value (703 HVN), while the SybronEndo Triple-Flex had the lowest (549 HVN) (p < 0.05). While similarities in cross-section design and metal wire composition were noted among the files, variations in the number of spirals and mechanical performance were also observed. Thus, all of these factors should be considered when selecting suitable files for an efficient root canal treatment.

A critical analysis of research methods and experimental models to study the physical properties of NiTi instruments and their fracture characteristics

The aim of this review is to provide a critical overview of the physical properties (surface hardness, cutting efficiency, bending properties, flexibility and cyclic fatigue resistance) of NiTi instruments. Frequently used experimental models regarding these aspects will be presented and discussed with regard to their strengths and weaknesses. For all these aspects, a plethora of experimental models have been described. Based on a critical appraisal and especially taking the appropriate translation of experimental findings to clinical endodontics into account, suggestions for future research based on clearly defined and valid experimental methodologies will be provided. Up to now, very few attempts have been made to assess which particular physical properties of NiTi instruments exert an impact on the clinical outcome of root canal treatment. Departure from merely focusing on physical properties and fracture characteristics towards more biological aspects in terms of treatment outcome is essential.

Cutting efficiency of heat-treated nickel-titanium single-file systems at different incidence angles

Cutting efficiency of Reciproc R25 (REC) and Reciproc blue R25 (REB) at different inclinations was evaluated. Sixty new files were tested at 90°, 70° and 45° of inclination in relation to the sample (n = 10), using a customised machine. All files were activated in reciprocation against standardised gypsum blocks for 120 s. Cutting efficiency was determined by measuring the block weight loss with an analytical balance and measuring the length of the block surface cut using a digital calliper. Data were statistically analysed (two-way ANOVA, Bonferroni t-test) with the significance level set at P < 0.05. There was no difference for REC among the tested angles. REB had no statistical difference between 90° and 70°; however, its cutting efficiency significantly increased at 45°. There was a significant difference between REC and REB at 45° only. Under these conditions, increased file inclination to 45° and blue heat treatment improved cutting efficiency of reciprocating files.

Mechanical behavior of deep cryogenically treated martensitic shape memory nickel-titanium rotary endodontic instruments

OBJECTIVES

The aim of this study was to investigate the role of deep cryogenic treatment (DCT) on the cyclic fatigue resistance and cutting efficiency of martensitic shape memory (SM) nickel-titanium (NiTi) rotary endodontic instruments.

MATERIALS AND METHODS

Seventy-five HyFlex(®) CM instruments were randomly divided into three groups of 25 each and subjected to different DCT (-185° C) conditions based on soaking time: DCT 24 group: 24 h, DCT 6 group: 6 h, and control group. Each group was randomly subdivided for evaluation of cyclic fatigue resistance in custom-made artificial canals (n = 15) and cutting efficiency in plexiglass simulators (n = 10). The cyclic fatigue resistance was measured by calculating the number of cycles to failure (NCF) and cutting efficiency was measured using the loss of weight method.

RESULTS

Increase in NCF of instruments in DCT 24 group was highly significant (P < 0.01; Tukey's honest significant difference). There was no difference in weight loss of plexiglass simulators in all the groups (P > 0.05; one-way analysis of variance). In conclusion, deep dry cryogenic treatment with 24 h soaking time significantly increases the cyclic fatigue resistance without affecting the cutting efficiency of SM NiTi endodontic instruments.

MATERIALS AND METHODS

Seventy-five HyFlex(®) CM instruments were randomly divided into three groups of 25 each and subjected to different DCT (-185° C) conditions based on soaking time: DCT 24 group: 24 h, DCT 6 group: 6 h, and control group. Each group was randomly subdivided for evaluation of cyclic fatigue resistance in custom-made artificial canals (n = 15) and cutting efficiency in plexiglass simulators (n = 10). The cyclic fatigue resistance was measured by calculating the number of cycles to failure (NCF) and cutting efficiency was measured using the loss of weight method.

RESULTS

Increase in NCF of instruments in DCT 24 group was highly significant (P < 0.01; Tukey's honest significant difference). There was no difference in weight loss of plexiglass simulators in all the groups (P > 0.05; one-way analysis of variance). In conclusion, deep dry cryogenic treatment with 24 h soaking time significantly increases the cyclic fatigue resistance without affecting the cutting efficiency of SM NiTi endodontic instruments.

A comparison of the machining efficiency of two brands of stainless steel endodontic hand files

The purpose of this study was to compare the machining efficiency of a flexible stainless steel K-type hand file recently introduced by Brasseler USA ("F-Style files"), with that of a well-studied Endodontic instrument ("Flex-o-files," Maillefer Dentsply), that has been on the market for more than 20 years. The comparison of machining efficiencies of these two brands of files was conducted on both dentin and plexiglas substrates, using an apparatus that allowed a constant force to be applied from each file being tested. The results indicated that the Brasseler instruments had poorer machining efficiency than those of the Maillefer files.

Effect of cryogenic treatment on nickel-titanium endodontic instruments

AIM

To investigate the effects of cryogenic treatment on nickel-titanium endodontic instruments. The null hypothesis was that cryogenic treatment would result in no changes in composition, microhardness or cutting efficiency of nickel-titanium instruments.

METHODOLOGY

Microhardness was measured on 30 nickel-titanium K-files (ISO size 25) using a Vicker's indenter. Elemental composition was measured on two instruments using X-ray spectroscopy. A nickel-titanium bulk specimen was analysed for crystalline phase composition using X-ray diffraction. Half of the specimens to be used for each analysis were subjected to a cryogenic treatment in liquid nitrogen (-196 degrees C) for either 3 s (microhardness specimens) or 10 min (other specimens). Cutting efficiency was assessed by recording operator choice using 80 nickel-titanium rotary instruments (ProFile 20, .06) half of which had been cryogenically treated and had been distributed amongst 14 clinicians. After conditioning by preparing four corresponding canals, each pair of instruments were evaluated for cutting efficiency by a clinician during preparation of one canal system in vitro. A Student's t-test was used to analyse the microhardness data, and a binomial test was used to analyse the observer choice data. Composition data were analysed qualitatively.

RESULTS

Cryogenically treated specimens had a significantly higher microhardness than the controls (P < 0.001; beta > 0.999). Observers showed a preference for cryogenically treated instruments (61%), but this was not significant (P = 0.21). Both treated and control specimens were composed of 56% Ni, 44% Ti, 0% N (by weight) with a majority in the austenite phase.

CONCLUSIONS

Cryogenic treatment resulted in increased microhardness, but this increase was not detected clinically. There was no measurable change in elemental or crystalline phase composition.

Wear resistance of cryogenically treated stainless steel files

Cryogenic treatment, which involves ultra-sub-zero treatment of metal alloys, has been shown to improve the wear resistance of several types of stainless steel. The purpose of this study was to examine the effect of cryogenic treatment on wear resistance of Flex-R and Hedstrom (Union Broach) files. These instruments were attached to an Instron testing machine and underwent 300 push-pull strokes (6-mm movement range, 600-mm/min speed, 1-N loading) against 1.5-mm thick dentin wafers. Their wear was determined by comparing the depth of grooves (without changing file position) cut in acrylic specimens (1.5-mm thick) before and after machining dentin. The mean and standard deviation (n = 20) of the relative change in cutting efficiency was determined for the files. An analysis of variance showed that the cutting efficiency of all files decreased significantly after wearing on dentin (p < 0.0001). However, when comparing the post-/pre-cutting efficiency ratios of the files, it was seen that cryogenic treatment did not affect the wear resistance of the files.

Location of contact areas on rotary Profile instruments in relationship to the forces developed during mechanical preparation on extracted teeth

AIM

The aim of this study was to locate the areas of direct instrument contact with dentine in the root canal system during rotary preparation and to analyse the relationship between these areas and the vertical forces and torque developed during the preparation.

METHODOLOGY

Canal preparations were performed by endodontists either with the step-back (SB) or the crown-down (CD) technique. In order to locate the areas of contact, the instruments were coated with two layers of gold by electro-deposition. They were photographed before and after use, and a coding system, based on mm from the instrument tip, was devised to designate areas of gold removal or instrument wear due to friction. To standarise the conditions of instrument manipulation, the teeth were fixed in the Endographe holder, and this device was used to measure vertical forces and torque.

RESULTS

The results showed that the first series of instruments used for the CD technique (taper 0.06) left 2 +/- 1 mm of the tip with the gold intact, indicating that these instruments and this step of the CD technique are the safest part of the preparation. For all other instruments (taper 0.04 series), the areas of gold removal involved the 3 mm around the tip and this finding was independent of the order of instrument use and preparation technique (SB or CD). The differences between the two techniques were significant in terms of the mean area of decolouration and the mean force and torque values. For the SB and CD techniques, the contact areas were, respectively, 10 +/- 3 and 7 +/- 2 mm. The forces and torque values were correspondingly higher for SB vs. CD; the mean values were, respectively, 19 and 21 N for vertical forces and 16 10(5) and 13 10(5) Nm for torque.

CONCLUSIONS

The recorded torque values and the location on the instruments of the areas of contact with dentine during this development of torque i.e. at or near the tip, indicate that great caution should be used with the rotary technique, particularly with the taper 0.04 instruments, regardless of preparation technique.

Cutting efficiency of nickel-titanium endodontic instruments and the effect of sodium hypochlorite treatment

The purpose of this study was to assess the cutting efficiency of nickel-titanium (NiTi) files in the presence and absence of sodium hypochlorite (NaOCl) treatment, and compare them to a conventional stainless-steel K-type file. NiTi files from four manufacturers were randomly selected and exposed to NaOCl for 12 or 48 h, or not at all. Their cutting efficiencies were then assessed, using a new methodology: "mass of Plexiglass cut per energy used over 50 linear cutting motions." In the absence of NaOCl, Brasseler (318 micrograms/J) and Maillefer (280 micrograms/J) NiTi files were most efficient, followed by JS Dental (71.4 micrograms/J) and McSpadden (40 micrograms/J). These differences were significant (p < 0.0001), except for those between the latter two brands. Moreover, NaOCl treatment did not alter the cutting efficiency of any brand of instruments significantly. When compared with conventional stainless steel files, all NiTi files tested were less efficient.

Effects of cleaning, disinfection, and sterilization procedures on the cutting efficiency of endodontic files

The effects of various cleaning, chemical disinfection, and sterilization procedures on the cutting efficiency of endodontic instruments Unifile (De Trey, Bois Colombes, France), (Flexofile Maillefer, Ballaigues, Switzerland), and H-File (Maillefer)) were investigated. The cross-infection control treatment procedures investigated were as follows: chemical disinfection--NaOCl (2.5%) for 12 and 48 h, and NH4 (5%) for 1 and 4 h; ultrasonic cleaning for 4 and 16 cycles of 15 min; and sterilization methods with chemiclave for 5 and 10 cycles of 20 min, Poupinel for 5 and 10 cycles of 120 min at 180 degrees C and glass beads for 10 and 40 cycles of 40 at 250 degrees C. Cutting efficiency was evaluated as the mass of Plexiglas cut per unit of energy expended by the instrument in microgram/Joule. The cutting efficiency decreased from 1 to 77%, depending on the file design and type of treatment procedures. Heat sterilization (Poupinel) did not modify the cutting efficiency of Unifile and Flexofile. The decrease in cutting efficiency was independent of frequency and duration of treatment procedures.

Machining efficiency of nickel-titanium K-type files in a linear motion

The purpose of this study was to compare, in vitro, the machining efficiency of four brands of nickel-titanium files and two brands of stainless steel K-type files. Instruments sizes 15 to 40 were tested in a linear motion simulating the clinical motion used to remove a file from the canal. The tips of the loaded files were in contact with a resin block. The load applied increased with file size. An indentation varnish caliper was used to measure the depth of the groove after 100 back-and-forward motions. The stainless steel instruments with a triangular cross-section were more efficient than the stainless steel instruments with a square cross-section. There was a significant discrepancy between the machining ability of the nickel-titanium K files. The Maillefer instruments were the most efficient.