Cutting ability of K type endodontic files

What meaningful information are the instruments mechanical testing giving us? A comprehensive review

Instruments mechanical strength and flexibility are traditionally tested by running cyclic fatigue, torsional, bending, buckling and microhardness tests. Several cyclic fatigue test models have been used in endodontics, all capable of providing a curved trajectory for the instrument to rotate. The cyclic fatigue testing allowed to identify conditions that may affect the fatigue strength outcomes, such as canal radius and degree of curvature, handpiece static vs dynamic motions, test temperature, kinematics, instrument previously wear and sterilization cycles, or instrument's size and metal alloy features. Due to the international test specifications for both torsional and bending tests, the variations of their models are not as many as for cyclic fatigue. These tests have also identified conditions capable of affecting the outcomes, such as kinematics, instrument's preloading, cross-sectional diameters, or alloy heat treatments. Buckling and microhardness are less common, with the metal alloy being considered to have a major influence on the results. Instruments mechanical testing, having all these individual conditions as independent variables, allowed to understand them and moulded the way the technical procedures are performed clinically. Even though the artificiality and simplicity of these tests will hardly mimic real working situations, and independently of being capable of producing cornerstone knowledge, these tests are also associated with inconsistency, lack of reproducibility and low external validity. Several attempts have been made to increase the generalizability of the outcomes by adding test settings that intend to mimic the clinical condition. Although pertinent, these settings may also add variabilities inherent to their concepts and practical applications in the laboratory environment. Although the actual studies should be seen as laboratory mechanical tests that measure very specific parameters under very particular conditions and that by far do not mimic the clinical condition, the lower validity drawback seems to be possible to be minimized when achieving a comprehensive understanding of the instrument behaviour. A Finite Elements Method and/or a multimethod research approach may lead to superior data collection, analysis, and results' interpretation, which when associated with a reliable confounding factors control and proper study designs may be helpful tools and strategies in order to increase the reliability of the outcomes.

Effects of cleaning, chemical disinfection, and sterilization procedures on the mechanical properties of endodontic instruments

The aim of this study was to test and compare the values of torsional moment, torsional angular deflection, bending moment, and permanent angular deflection of three designs of root canal files (Unifile, Flexofile, and H-File) before and after cross-infection treatment procedures, according to ANSI/ADA specification no. 28. An increase in value for all mechanical properties tested was observed after the treatment procedures, with the exception of Flexofile wherein a decrease in permanent angular deflection was evident. Unifile showed a decrease in torsional moment and bending moment. The changes in mechanical properties after treatment procedures ranged from 0.1 to 63% from the control groups. Generally, the changes in values observed were insignificant and still well within ANSI/ADA specification no. 28. Thus, they do not have any clinical significance.

Cutting efficiency of K-files manufactured with different metallic alloys

The aim of this study was to compare, in vitro, the machining efficiency of different triangular cross-section K-files made of nickel titanium (Nitiflex, Naviflex), titanium (Microtitane), and stainless steel (Flexofile, Flex-R). Ten instruments of each K-file from size 25 to 40 were tested. The cutting efficiency was assessed in a linear motion using an indentation caliper to measure the depth of grooves. The load applied (in grams) was equal to the ISO file size. Each file was allowed to do 100 back-and-forward movements. Files made of stainless steel were the most effective, in particular Flexofile. There were statistically significant differences between Flexofile and Flex-R in all sizes. In the group of nickel titanium instruments, Nitiflex was significantly more efficient than Naviflex in all sizes. The machining ability of titanium files was higher than that of Naviflex but lower than that of Nitiflex and stainless steel files.

Cutting efficiency of Hedstrom, S and U files made of various alloys in filing motion

The cutting efficiency for sizes 25 and 35 stainless steel Hedstrom S and U files from 10 manufacturers, and titanium-alloy Hedstrom S and U files from five manufacturers, tested for linear (filing) motion under standardized conditions were determined. Special plastic samples having well-defined abrasive properties were used as the substrate and constant pressure was applied until the instruments were blunt. The depth of the groove achieved by filing was used to measure cutting efficiency. For both sizes there were significant differences in the cutting efficiency of files made by the various manufacturers (P < 0.001). Hedstrom files made of stainless steel, made by VDW, gave the best cutting efficiency for sizes 25 and 35. Overall, under the conditions of this study. Hedstrom files had better cutting efficiency than S or U files. Likewise, stainless steel files provided better cutting efficiency than instruments made of titanium alloys.

Intra-canal cutting ability of MM1500 files

The aims of this study were to develop a model system capable of monitoring lateral forces during root canal preparation and to measure the cutting ability of files activated by the MM1500 sonic handpiece. Forces were monitored by a calibrated model system which utilized a combination of spring steel beams fitted with strain gauges, these were interfaced through two strain gauge amplifiers to an x-y recorder. Single rooted canine teeth (n = 36; 32 experimental, four control) were mounted in a two-part acrylic mould (which was an integral part of the model system) prior to sectioning horizontally 11 mm from the tooth apex. A 2(4) full factorial experiment with two replications was performed. Four variables were selected for evaluation, load (50 and 100 g), power (air inlet ring half or fully open), file type (Heliosonic or Shaper) and stroke rate (1 or 2 cycles per second). A new file (size 25) was used for 1 min in each canal with water irrigation. The control group was not instrumented. The cross-sectional root canal area was measured before and after instrumentation using image analysis and increase in area was used as an indication of cutting ability. The results showed that the increase in load, power and the Shaper file all produced a significant increase in cutting ability (ANOVA, P < 0.001). However, stroke rate was not found to have a significant effect (P > 0.05). None of the interactions between the variables were significant and there was no significant difference in the control group (P > 0.05). In conclusion, this work has developed a model system to monitor lateral forces and has shown that instrument design and operator usage affect dentine removal from a root canal wall.

Factors affecting the wear of sonic files

The aim of this study was to investigate factors affecting the wear and cutting ability of sonic files. A model system was used and the following variables evaluated, file type; Heliosonic, Rispisonic or Shaper, load; 25, 50 or 100 grams and length of time in use; new, 30 or 60 seconds. A 3(3) full factorial analysis with two replications into the effect of the above variables on the cutting ability of the Heliosonic, Rispisonic and Shaper files powered by the MM1500 sonic instrument was performed. A new file size 25 (Heliosonic and Shaper) or No 3 (Rispisonic) was used for each cut together with water irrigation and the substrate used was 1 mm thick sections of bovine bone. All variables had a significant effect on cutting (ANOVA. p < 0.001). However examination of the F values showed that the most significant variable was load, followed by file type, and time. The most significant interaction was between file type and load followed by time and file type. The interaction between time and load was not significant (p > 0.05). The Rispisonic file was most susceptible to wear during use especially at higher loads and the Heliosonic file cut least. It is suggested that the Shaper file is the better design of the three with respect to cutting ability and wear with use.

Factors affecting the cutting ability of sonic files

The aim of this study was to investigate the cutting ability of sonic files. A model system was developed and the following variables evaluated: file type. Heliosonic or Shaper; file length, 21 or 29 mm; power, air inlet ring opening of half or fully open; stroke length, 2 or 4 mm; stroke rate, one or two cycles per second; and load 50 or 100 g. A 2(6) full-factorial analysis with two replications into the effect of the above variables on the cutting ability of the MM1500 sonic instrument was performed. A new size 25 file was used for each cut, together with water irrigation, and the substrate used was 1-mm thick sections of bovine bone. The differences between the variables were significant (ANOVA, P < 0.001). However, examination of the F-values showed that the most significant variable to affect cutting was load, followed by power, file type, stroke length and stroke rate, with the least significant variable being file length. The most significant interaction was between rate and length of stroke. An increase in stroke rate from one to two cycles per second at a stroke length of 2 mm produced an increase in cutting for both the Heliosonic and Shaper files. However, at the longer stroke length of 4 mm, the same increase in rate resulted in a decrease in cutting for the Shaper files. Therefore, it is suggested that operators should press the file against the canal wall and move it slowly to maximise cutting.

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.

The endodontic file is a disposable instrument

Most cutting and machining tools wear when properly operated on intended substrates. Little is known about endodontic files when machining on human dentin. It was shown earlier that the efficiency of files was very variable when evaluated on Plexiglas substrates. The present study of seven different brands and types of files was undertaken to understand their wear better when machining human dentin. It was shown that all files evaluated rapidly deteriorated when machining dentin. This decline in efficiency was significant but different within, as well as, among brands. It was suggested that endodontic files be disposable.

Properties of endodontic hand instruments used in rotary motion. Part 1. Cutting efficiency

Cutting efficiency of 24 different types of endodontic hand instruments, which are primarily designed for a rotary (reaming) working action, was investigated under standardized conditions. With a computer-driven testing device, resin specimens with simulated cylindrical canals were instrumented using a defined working motion simulating the clinical use of the instruments. Maximum penetration depth was the criterion for cutting efficiency. Sample size was 12 instruments for each type and size (#25 and #35). Nitinol K-files showed the least cutting efficiency. Stainless steel reamers and especially K-files showed better cutting efficiency than Nitinol K-files. Flexible stainless steel instruments displayed the best results. With regard to cutting efficiency, flexible stainless steel instruments were clearly superior to stainless steel reamers and K-files, and especially to Nitinol K-files.

Variations in stroke rate and loading using hand sonic or ultrasonic instrumentation

An in vitro model system was developed to investigate the stroke rate and loading during hand, sonic, or ultrasonic instrumentation. The model system consisted of a mandibular molar tooth attached to a strain gauge and interface to a computer. Ten clinicians instrumented the distal canal of lower molar teeth. Stroke rates used and loads applied were measured using different sized files with the three different preparation techniques. There was no significant difference in stroke rates between different file sizes (p > 0.05) within the same generator, however, stroke rate for hand was significantly slower than for sonic or ultrasonic (p < 0.01). Loading of the canal wall rose with increase in file size which was significant for hand (p < 0.01) and sonic (p < 0.05) but not ultrasonic (p > 0.05). There was a tendency for loading to be higher with hand instrumentation, this however was not significant (p > 0.05).

Effects of sterilization procedures on the cutting efficiency of stainless steel and carbon steel Gates Glidden drills

One-hundred fifty carbon steel Gates Glidden drills were compared with 150 stainless steel Gates Glidden drills before and after sterilization in an autoclave, salt beads, dry heat, or 2% alkaline glutaraldehyde. Cutting efficiency and separation rates of the drills were evaluated. Stainless steel #1 and 2 Gates Glidden drills resisted separation and maintained their cutting efficiency statistically better than the carbon steel #1 and 2 Gates Glidden drills after one sterilization cycle in an autoclave, salt bead sterilizer, or dry heat oven. Glutaraldehyde did not effect either type of drill.

The effects of steam sterilization and usage on cutting efficiency of endodontic instruments

The effects of steam sterilization and usage on sharpness were evaluated on #25 endodontic files. Files were used to instrument 1, 5, and 10 molars. Control groups determined the effect of steam sterilization alone on cutting efficiency of unused files. A cutting efficiency test was performed on an apparatus that compares sharpness of files when used in linear motion. Scanning electron microscopic analysis was performed in each group. Significant differences were found between experimental files used to instrument 1 molar and those used for 5 or 10 molars. The difference in cutting efficiency between the second and third experimental groups was not significant, indicating that most of the decrease in sharpness occurred with use between one and five molars. No significant difference was found between the control groups, indicating no decrease in cutting efficiency by sterilization alone. The scanning electron microscopic analysis supported the statistical data.