Root canal instruments for manual use: a review

Present status and future directions -Management of curved and calcified root canals

Root canal curvature and calcification introduce factors that increase the risk of procedural accidents during root canal treatment. The inability to achieve patency to the apical third, asymmetrical dentine removal leading to transportation, perforation and instrument fracture inside the curved trajectories are some of the procedural problems that might jeopardize the management of intraradicular infection and result in poor treatment outcomes. In fact, curved and constricted canals introduce such complexity that total instrumentation concepts and specially designed instruments have been developed to deal with the challenge. This narrative review seeks to provide and consolidate the principles necessary for understanding the dynamics of curved and constricted canal management and to improve the understanding for future developments in this field.

Nitinol Type Alloys General Characteristics and Applications in Endodontics

A very extensive literature review presents the possibilities and needs of using, in endodontics, the alloys commonly known as nitinol. Nitinol, as the most modern group of engineering materials used to develop root canals, is equilibrium nickel and titanium alloys in terms of the elements’ atomic concentration, or very similar. The main audience of this paper is engineers, tool designers and manufacturers, PhD students, and students of materials and manufacturing engineering but this article can also certainly be used by dentists. The paper aims to present a full material science characterization of the structure and properties of nitinol alloys and to discuss all structural phenomena that determine the performance properties of these alloys, including those applied to manufacture the endodontic tools. The paper presents the selection of these alloys’ chemical composition and processing conditions and their importance in the endodontic treatment of teeth. The results of laboratory studies on the analysis of changes during the sterilization of endodontic instruments made of nitinol alloys are also included. The summary of all the literature analyses is an SWOT analysis of strengths, weaknesses, opportunities, and threats, and is a forecast of the development strategy of this material in a specific application such as endodontics.

What Are the Chances of Resilon to Dominate the Market Filling Materials for Endodontics?

This paper is a literature review with additional virtual analyses of the authors’ own experimental research results. Knowledge from various areas was synergistically combined, appropriately for concurrent engineering, presenting several possible methodological approaches used in research, optimizing the selection of engineering materials and the conditions of their application with particular application in endodontics. Particular attention was paid to the theoretical aspects of filling material strengths, weaknesses, opportunities, and threats SWOT analysis. Attention was paid to the original concepts of Sustainable Dentistry Development in conjunction with Dentistry 4.0, which includes endodontics as an important element. The dentists’ actions, among others, in conservative dentistry, along with endodontics, requires close cooperation with engineers and the enginering sciences. Methods of root canal preparation were described, together with selected tools, including those made of nitinol. Principles concerning the process of cleaning and shaping the pulp complex are presented. The importance of obturation methods, including the Thermo-Hydraulic-Condensation THC technique, and the selection of filling materials with the necessary sealants for the success of endodontic treatment are discussed. The experimental studies were carried out in vitro on human teeth removed for medical reasons, except for caries, for which two groups of 16 teeth were separated. After the root canal was prepared, it was filled with studs and pellets of a filling material based on polyester materials, which has gained the common trade name of resilon or, less frequently, RealSeal (SybronEndo) with an epiphany sealant. The teeth for the first group were obturated by cold lateral condensation. In the second case the obturation was performed using the Thermo-Hydraulic-Condensation technique using System B and Obtura III. The experimental leakage testing was done using a scanning electron microscope SEM and a light stereoscopic microscope LSM, as typical research tools used in materialography. The research results, in a confrontation with the data taken from the literature studies, do not indicate the domination of resilon in endodontics.

Is Gutta-Percha Still the “Gold Standard” among Filling Materials in Endodontic Treatment?

The paper is an extensive monographic review of the literature, and also uses the results of the authors’ own experimental research illustrating the noticed developmental tendencies of the filling material based on gutta-percha. The whole body of literature proves the correctness of the research thesis that this material is the best currently that can be used in endodontics. Caries is one of the most common global infectious diseases. Since the dawn of humankind, the consequence of the disease has been the loss of dentition over time through dental extractions. Both tooth caries and tooth loss cause numerous complications and systemic diseases, which have a serious impact on insurance systems and on the well-being, quality, and length of human life. Endodontic treatment, which has been developing since 1836, is an alternative to tooth extraction. Based on an extensive literature review, the methodology of qualifying patients for endodontic treatment was analyzed. The importance of selecting filling material and techniques for the development and obturation of the root canal during endodontic treatment was described. Particular attention was paid to the materials science aspects and the sequence of phase transformations and precipitation processes, as well as the need to ensure the stoichiometric chemical composition of Ni–Ti alloys, and the vacuum metallurgical processes and material processing technologies for the effects of shape memory and superelasticity, which determine the suitability of tools made of this alloy for endodontic purposes. The phenomena accompanying the sterilization of such tools, limiting the relatively small number of times of their use, play an important role. The methods of root canal preparation and obturation methods through cold side condensation and thermoplastic methods, including the most modern of them, the thermo-hydraulic condensation (THC) technique, were analyzed. An important element of the research hypothesis was to prove the assumption that to optimize the technology of development and obturation of root canals, tests of filling effectiveness are identified by the density and size of the gaps between the root canal wall, and the filling methods used and devices appropriate for material research, using mainly microscopy such as light stereoscopic (LSM) and scanning electron (SEM). The most beneficial preparations were obtained by making a longitudinal breakthrough of 48 natural human teeth, extracted for medical reasons, different from caries, with compliance with all ethical principles in this field. The teeth were prepared using various methods and filled with multiple obturation techniques, using a virtual selection of experimental variants. The breakthroughs were made in liquid nitrogen after a one-sided incision with a narrow gap created by a diamond disc using a materialographic cutter. The best effectiveness of the root canal filling was ensured by the technology of preparing the root canals with K3 rotary nitinol tools and filling the teeth with the THC thermoplastic method using the System B and Obtura III devices with studs and pellets of filling material based on gutta-percha after covering the root canal walls with a thin layer of AH Plus sealant. In this way, the research thesis was confirmed.

Evaluation of the usage-induced degradation of Genius and Reciproc nickel-titanium reciprocating instruments

The aim of this study was to characterize the main features and the usage-induced degradation of the Genius file after four severely curved root canal instrumentations and to compare their properties to the Reciproc files. Brand new and ex vivo used files were analysed by scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDS), differential scanning calorimetry (DSC), X-ray diffraction (XRD), optical metallography, and nano-indentation to disclose their morphological, chemical, mechanical, thermal, and phase composition features. Nano-indentation data were statistically analysed using the Student's t test for normal distribution or the Kolmogorov-Smirnov test for not-normal distributions. SEM analysis showed the presence of micro-cracks near the tip on both files after ex vivo usage test. EDS analysis confirmed that both files are manufactured from an almost equiatomic NiTi alloy. DSC analysis revealed that the transition temperature of the Genius is below 20 °C, while that of the Reciproc is above 20 °C. XRD analysis of Genius files identified cubic B2 austenite with minor peaks of residual monoclinic B19 martensite, while the contemporaneous presence of martensite, austenite and hexagonal R-phase was observed in the Reciproc files. Significant differences in nanohardness and modulus of elasticity (P < .05) were observed in both Genius and Reciproc files before and after use. The collected results showed that both instruments can be safely used as single-use files.

The effects of shape memory alloys’ tension–compression asymmetryon NiTi endodontic files’ fatigue life

Nowadays, NiTi rotary endodontic files are of great importance due to their flexibility which enables the device to cover all the portions of curved canal of tooth. Although this class of files are flexible, intracanal separation might happen during canal preparation due to bending or torsional loadings of the file. Since fabrication and characterization of such devices is challenging, time-consuming, and expensive, it is preferable to predict this failure before fabrication using numerical models. It is demonstrated that NiTi shape memory alloy shows asymmetric material response in tension and compression which can significantly affect the lifetime of the files fabricated from. In this article, the effects of this material asymmetry on the bending response of rotary files are assessed using finite element analysis. To do so, a constitutive model which takes material asymmetry into account is used in combination with the finite element model of a RaCe file. The results show that the material asymmetry can significantly affect the maximum von Mises equivalent stress as well as the force–displacement response of the tip of this file.

Mechanical Preparation Showed Superior Shaping Ability than Manual Technique in Primary Molars - A Micro-Computed Tomography Study

Abstract The aim of the study was to evaluate canal preparation in primary molars with hand files, ProTaper Next and Self-Adjusting File (SAF) by 2D and 3D micro-computed tomography (micro-CT) analysis. Canals of 24 primary molars were prepared with hand files (HF), ProTaper Next (PTN) and SAF (n=8/group). The teeth were scanned before and after root canal preparation and the pre- and postoperative micro-CT images were reconstructed. Changes in 2D (area, perimeter, roundness, minor and major diameter) and 3D [volume, surface area, structure model index (SMI)] morphological parameters, as well as canal transportation and lateral perforations were evaluated (Kruskal-Wallis and ANOVA; a=0.05). SAF presented smaller changes in minor diameter, volume and surface area compared with HF and PTN (p<0.05). PTN presented more circular canals after preparation. 3D analysis revealed greater transportation in HF. PTN and SAF presented more centered canal preparation, especially in curved areas. SAF and HF presented, respectively, the lowest (0.05±0.02 and 0.07±0.04) and highest (0.14±0.11 and 0.29±0.17) apical transportation. There were fewer lateral perforations in SAF (4.2%) and PTN (7.7%) than in HF (47.8%) (p<0.05). In primary molars, mechanical preparation showed better shaping ability than hand files, promoting more centered preparations and lower occurrence of lateral perforations and canal transportation. Clinical Relevance: Manual instrumentation is still reported as the main choice in the primary teeth preparation; however, studies have shown limitations in its use. The morphological characteristics of primary teeth and the limited knowledge of shaping procedures in these teeth using mechanical preparation become a challenge for clinical practice and might impair the predictability of endodontic treatment.

The effect of severe grain refinement on the damage tolerance of a superelastic NiTi shape memory alloy

Nickel-titanium (NiTi) shape memory alloys are widely used for medical components, as they can accommodate large strains in their superelastic state. In order to further improve the mechanical properties of NiTi, grain refinement by severe plastic deformation is applied to generate an ultrafine-grained microstructure with increased strength. In this work comprehensive fracture and fatigue crack growth experiments were performed on ultrafine-grained NiTi to assess its damage tolerance, which is essential for the safe use of this material in medical applications. It was found, that equal channel angular pressing of NiTi for 8 passes route B_C increases the transformation stress by a factor of 1.5 and the yield stress of the martensite by a factor of 2.6, without significantly deteriorating its fracture and fatigue crack growth behavior. The fatigue crack growth behavior at high mean stresses is even improved, with lower fatigue crack growth rates and higher threshold stress intensity factor ranges, however, beneficial contributions from crack closure are slightly reduced.

A comparative evaluation of the increase in root canal surface area and canal transportation in curved root canals by three rotary systems: A cone-beam computed tomographic study

Aim:

The aim of this study was to measure the increase in root canal surface area and canal transportation after biomechanical preparation at 1, 3, and 5 mm short of the apex with three different rotary systems in both continuous rotary and reciprocating rotary motions.

Materials and Methods:

Sixty freshly extracted human mandibular molars with mesial root canal curvatures between 20° and 30° were included in the study. Teeth were randomly distributed into three groups (n = 20). Biomechanical preparations were done in all the mesial canals. In Group 1, instrumentation was done with ProTaper universal rotary files, Group 2, with K3XF rotary files, and Group 3, with LSX rotary files. Each group was further subdivided into subgroups A and B (n = 10) where instrumentation was done by continuous rotary and reciprocating rotary techniques, respectively. Increase in root canal surface area and canal transportation was measured using the preoperative and postoperative cone-beam computed tomography scans.

Statistical Analysis:

The data were analyzed by one-way ANOVA followed by Tukey pairwise multiple comparison tests.

Results:

Increase in root canal surface area was significantly more (P < 0.05) in ProTaper and K3XF groups when compared to LSX group. Canal transportation was significantly more (P < 0.05) in ProTaper group when compared to K3XF and LSX groups. There was no significant difference (P > 0.05) in increase of root canal surface area and canal transportation between continuous rotary and reciprocating rotary techniques for ProTaper Universal, K3XF and LSX groups.

Conclusion:

LSX rotary system showed minimal increase of root canal surface area and minimal canal transportation when compared to ProTaper and K3XF rotary systems.

Endodontic instruments after torsional failure: nanoindentation test

This study aimed to evaluate effects of torsional loading on the mechanical properties of endodontic instruments using the nanoindentation technique. ProFile (PF; size 30, taper 04; Dentsply Maillefer, Switzerland) and stainless steel (SS; size 30, taper 02; Mani, Japan) instruments were subjected to torsional test. Nanoindentation was then performed adjacent to the edge of fracture (edge) and at the cutting part beside the shank (shank). Hardness and elastic modulus were measured under 100-mN force on 100 locations at each region, and compared to those obtained from the same regions on new instruments. It showed that PF and SS instruments failed at 559 ± 67 and 596 ± 73 rotation degrees and mean maximum torque of 0.90 ± 0.07 and 0.99 ± 0.05 N-cm, respectively. Hardness and elastic modulus ranged 4.8-6.7 and 118-339 GPa in SS, and 2.7-3.2 and 52-81 GPa in PF. Significant differences between torsion-fractured and new instruments in hardness and elastic modulus were detected in the SS system used. While in PF system, the edge region after torsional fracture had significantly lower hardness and elastic modulus compared to new instruments. The local hardness and modulus of elasticity of endodontic instruments adjacent to the fracture edge are significantly reduced by torsional loading.

Effect of heat sterilization on surface characteristics and microstructure of Mani NRT rotary nickel?titanium instruments

AIM

To evaluate the effect of repeated dry heat sterilization on surface characteristics and microstructure of Mani nickel-titanium rotary instruments.

METHODOLOGY

Thirty-three new Mani NRT instruments, size 30, taper 0.04 and 25 mm in length were examined. Twenty-seven instruments were divided into three groups for surface characterization by scanning electron microscopy (SEM). In the first group (n = 3), instruments were examined in the 'as-received' condition and after they had been subjected to 11 sterilization cycles. In the second and third subgroups (n = 12), 12 instruments were prepared for cross-section and a further 12 for longitudinal sectional analysis and evaluated in subgroups of three, after 0, 1, 6 and 11 sterilization cycles. The remaining six instruments were analysed with differential scanning calorimetry (DSC), three in the 'as-received' condition and three after being subjected to 11 cycles of sterilization.

RESULTS

Scanning electron microscopy observations indicated the presence of debris, pitting and deep milling marks in both new and sterilized files. After 11 sterilization cycles, debris remained and surface roughness was increased significantly (P = 0.05). DSC analyses showed that the specimens in the 'as-received' condition and after 11 sterilization cycles were in the austenite phase or a mixture of austenite and R-phase at 37 degrees C.

CONCLUSIONS

The machining defects and structural imperfections of new Mani instruments are indicative of the difficulty in manufacturing nickel-titanium endodontic instruments. DSC measurements suggest that Mani instruments are capable of superelastic behaviour under clinical conditions.

SEM Observations and Differential Scanning Calorimetric Studies of New and Sterilized Nickel-Titanium Rotary Endodontic Instruments

Scanning electron microscopy (SEM) and differential scanning calorimetric (DSC) studies were utilized to investigate surface and microstructure of two brands of rotary nickel-titanium (NiTi) endodontic instruments, in the as-received condition and after subjection to 1, 6, and 11 sterilization cycles. A total of 66 ProFile (n = 33) and Flexmaster (n = 33) files were examined. SEM observations indicated the presence of surface imperfections and adherent material in all new and sterilized instruments and an increase in surface roughness of the instruments that underwent multiple sterilizations. DSC measurements showed that the specimens of both brands, in the as-received condition and after 11 sterilizations, were completely austenite in the oral environment temperature, suggesting that they are capable of superelastic behavior in appropriate clinical conditions.

An objective evaluation comparing the physical properties of two brands of stainless steel endodontic hand files

The purpose of this study was to evaluate the physical properties of a flexible stainless steel K-type hand file recently introduced by Brasseler USA (F-Style files; Brasseler USA, Savannah, GA), in comparison to those of a well-studied instrument (Flex-o-files; Maillefer-Dentsply, Ballaigues, Switzerland), that has been on the market for more than 20 yr. The physical properties measured included torque at failure, angular deflection at failure, flexibility, and consistency of diameter at 3 mm from the cutting tip; and the evaluations were carried out on size #10 through #50 files of each of the two brands being tested. The results indicated that the Brasseler instruments were inherently more flexible, but had smaller diameters, lower torque, and angular deflection values at failure, than those of the Maillefer files.

Analysis of forces developed during mechanical preparation of extracted teeth using RaCe rotary instruments and ProFiles

AIM

To compare the torque and load during instrumentation with ProFile and RaCe nickel-titanium rotary instruments.

METHODOLOGY

Thirty human incisor roots 12 mm in length were embedded in epoxy resin, and divided into three groups of 10 specimens each. Instruments employed in each group were as follows: RaCe .02 Step Back Files in group 1; RaCe .04 Step Back Files in group 2; and .04 ProFiles in group 3. Two load cells, whose outputs were connected to a digital oscilloscope, recorded torque and vertical load. Data were analysed by one-way anova.

RESULTS

Torque values were statistically different between groups 1 and 3, and between groups 2 and 3 (torque was higher in group 3 than in groups 1 and 2). Vertical loads were statistically higher in group 3 than in groups 1 and 2.

CONCLUSIONS

Torsional and vertical forces can be evaluated during instrumentation of straight root canals using the device and methods described in this study. When the step-back technique was employed, torque and vertical load obtained with RaCe instruments were lower than that obtained with ProFiles.

Comparative analysis of torsional and bending stresses in two mathematical models of nickel-titanium rotary instruments: ProTaper versus ProFile

During root canal instrumentation, nickel-titanium rotary instruments are subjected to continual stresses inside the canal due to its anatomy and the hardness of the dentin they must cut. They must therefore be both stress-resistant and elastic. This study aimed to compare the mechanical behavior of two nickel-titanium rotary instruments (ProTaper and ProFile) by applying the finite element analysis method to produce a numerical evaluation. The nonlinear mechanical behavior of the alloy was taken into account during the study. The distribution of stresses due to torsional and bending moments was compared in the two experimental models. The ProFile model was found to be more elastic than the ProTaper model. Under equal loads, the ProTaper model showed lower and better distributed stresses than the ProFile model.

Effect of physical vapor deposition on cutting efficiency of nickel-titanium files

The purpose of this study was to investigate possible changes in cutting efficiency of nickel-titanium K-files that had undergone physical vapor deposition coating. Titanium nitride coatings were deposited using different process parameters. A total of 84 nickel-titanium K-files (size 35) were randomly divided into 7 groups of 12 instruments each. Groups A to F (experimental): instruments were coated with titanium nitride using different process parameters regarding substrate temperature, applied voltage, coating thickness, and ion bombardment. Group K (control): samples were not coated with titanium nitride. The cutting efficiency of all instruments was determined in a rotary working motion by means of a computer-driven testing device. Special plastic samples with a cylindrical canal were used, and the maximum penetration depth of the instruments into the lumen was the criterion for cutting efficiency. Instruments of groups A, F, and C achieved significantly greater penetration depths than the uncoated instruments of the control group (p < 0.05). Cutting efficiency of physical vapor deposition-coated nickel-titanium files was increased by up to 26.2% in comparison with uncoated instruments.

Effect of sterilization on the cutting efficiency of PVD-coated nickel-titanium endodontic instruments

AIM

To investigate alterations in cutting efficiency when conventional and titanium nitride (TiN) coated nickel-titanium (NiTi) K-files were exposed to repeated sterilization using an autoclave.

METHODOLOGY

A total of 96 NiTi K-files (size 35) were randomly divided into two groups (A and B) of 48 instruments each. Whilst the instruments of group B were exposed to physical vapour deposition (PVD) creating a coating of a TiN layer, the files of group A were not coated. The instruments of groups A and B were randomly divided into four subgroups of 12 instruments each. A.1/B.1: Instruments were exposed to five cycles of sterilization. A.2/B.2: Instruments were exposed to 10 cycles of sterilization. A.3/B.3: Instruments were immersed in NaOCl for 30 min, rinsed in water, and exposed to five cycles of sterilization. A.C/B.C: Instruments were not sterilized (controls). The cutting efficiency of all files was determined by means of a computer-driven testing device. Special plastic samples with cylindrical canals were used and the maximum penetration depth of the files into the lumen was assessed.

RESULTS

The TiN-coated instruments of groups B.1, B.2 and B.3 showed no significant difference in comparison with the penetration depths of the controls (P> 0.05). The uncoated files of groups A.1, A.2 and A.3 displayed significantly lower maximum penetration depths (P< 0.05) when compared to the control files.

CONCLUSIONS

Repeated sterilization under autoclave or exposure to sodium hypochlorite (NaOCl) prior to sterilization did not alter the cutting efficiency of PVD-coated NiTi K-files.

A comparison of the shaping characteristics of two nickel-titanium endodontic hand instruments

AIM

The purpose of this study was to compare the shaping characteristics of Ni-Ti K-files and Ni-Ti S-files manipulated by hand.

METHODOLOGY

A total of 60 extracted human roots were embedded in resin blocks. The embedded roots were divided into three groups: (i) roots with straight; (ii) apically curved; and (iii) continuously curved canals. Each of the three groups was randomly divided into two subgroups; one subgroup in each group was prepared with Ni-Ti K-files and the other with Ni-Ti S-files. The files were used with a step-back technique and enlarged so that the master apical file was size 30, and the canals were stepped back to size 40. The performance of the files was assessed by the superimposition of projected radiographs taken in bucco-lingual and mesio-distal directions before and after the preparation. The results were analysed statistically using analysis of covariance and Duncan's multiple range test.

RESULTS

Although canal preparation using Ni-Ti K-files was quicker, there were no statistically significant differences between file types. The Ni-Ti S-file removed significantly more material at the most coronal level (P < 0.05). There were minor differences between instruments at the apical level. Only in the proximal view of apically curved canals prepared with Ni-Ti S-files was significantly more dentine removed from the outer aspect of the curvature (P < 0.05). At the middle level (wide danger zone) the Ni-Ti S-files removed more dentine from the inner aspect of the curvature in those roots with apically curved canals.

CONCLUSIONS

Under the conditions of this study, preparation with Ni-Ti K-files produced more appropriate shapes in roots with apically curved canals than Ni-Ti S-files.

Comparison of laterally condensed .06 and .02 tapered Gutta-Percha and sealer in vitro

The purpose of this in vitro study was to compare the quality of the seal in canals prepared in a standardized manner and obturated with a .06 or a .02 tapered gutta-percha master cone using lateral condensation. Forty-four extracted human anterior teeth with single, straight canals were divided into two experimental groups of 20 teeth each and two control groups of 2 teeth each. The teeth were instrumented with Series 29 Profile .06 tapered rotary nickel-titanium files to a master apical file of 0.46 mm. Teeth in group 1 were obturated with a .02 tapered master gutta-percha cone and Roth 801 sealer using lateral condensation. Teeth in group 2 were obturated similarly, except a .06 tapered master gutta-percha cone was used. The depth of spreader penetration was recorded in millimeters. Positive control teeth were instrumented but not filled. Negative control teeth were instrumented, obturated, and externally sealed. The teeth were placed into a coronal leakage apparatus that contained an upper and lower reservoir of trypticase soy broth separated by the tooth. A 24-h growth of Proteus vulgaris in 0.25 ml of trypticase soy broth was placed in the coronal reservoir every 7 days for 70 days and incubated at 37 degrees C. Student's t test was used to determine whether there was a difference in spreader penetration between the groups, and a Fisher's exact test was used to determine whether there was a difference in bacterial leakage. The positive and negative controls validated the testing model. When a .02 tapered master cone was used, the spreader penetrated significantly closer to working length than when a .06 tapered master cone was used (p < 0.05). The difference between the groups in the number of samples that demonstrated complete bacterial penetration was not significant (p > 0.05).

Relationship between design features of endodontic instruments and their properties. Part 3. Resistance to bending and fracture

Stainless-steel prototypes characterized by five different cross-sectional shapes (square, triangular, rhomboidal, "S"-shaped, and the cross-sectional design of H-type files) and three different number of flutes (16, 24, and 32) were used for investigation of the relationship between design features and the resistance to bending and fracture of root canal instruments. Both resistance to bending (bending moment) and resistance to fracture (angular deflection and torque) were determined in accordance to ISO 3630-1. Numbers 15, 25, and 35 prototypes were tested with a sample size of 10 instruments each. Whereas prototypes with the rhombus-shaped cross-sectional design displayed the lowest resistance to bending, those of the square cross-section showed the greatest. In general, S-shaped prototypes and those with the H-type cross-section achieved lower angular deflection than all other prototypes, whereas those with the triangular cross-sectional shape and 32 flutes reached the greatest angular deflection. Overall, rhombus-shaped prototypes showed lower torque values than all other prototypes. Results indicate that bending and torsional properties of endodontic instruments are mainly influenced by their cross-sectional design. Moreover, these properties are also affected by the number of flutes and the manufacturing process (twisted or milled) of endodontic instruments.

An overview of nickel-titanium alloys used in dentistry

The nickel-titanium alloy Nitinol has been used in the manufacture of endodontic instruments in recent years. Nitinol alloys have greater strength and a lower modulus of elasticity compared with stainless steel alloys. The super-elastic behaviour of Nitinol wires means that on unloading they return to their original shape following deformation. These properties are of interest in endodontology as they allow construction of root canal instruments that utilize these favourable characteristics to provide an advantage when preparing curved canals. This review aims to provide an overview of Nitinol alloys used in dentistry in order for its unique characteristics to be appreciated.

Cyclic fatigue of ProFile rotary instruments after clinical use

AIM

The purpose of this study was to evaluate cyclic fatigue of .06 ProFile Ni-Ti rotary instruments after clinical use in molar teeth.

METHODOLOGY

In group 1, instruments size 40-15 were used in a crown-down technique using 2.5% NaOCl as an irrigant. Fifty-two molars were included and 13 sets of Profile Ni-Ti rotary instruments were used. Each set of instruments was used in four molars, and was steam autoclaved before each use. Group 2 (10 sets of new ProFile Ni-Ti rotary instruments) was the control group. Cyclic fatigue was tested by rotating the instruments in a 90 degrees metallic tube until they broke.

RESULTS

One-way analysis of variance did not show any statistically significant differences amongst the files from both groups regarding cyclic fatigue.

CONCLUSIONS

It was concluded that sterilization and clinical use in the presence of NaOCl did not lead to a decrease in the number of rotations to breakage of the files.

Endodontic hand instruments: cutting efficiency, instrumentation of curved canals, bending and torsional properties

In an assessment of the usefulness of different root canal instruments, two aspects are of particular interest for the endodontist: the cutting efficiency of the instruments and their ability to enlarge curved canals without undesirable changes of the canal shape such as the formation of zips and elbows. This review paper is based on own investigations on the cutting efficiency and instrumentation of curved canals. Additionally, two parameters which are described in ISO 3630-1, resistance to bending and resistance to fracture, are discussed. With regard to cutting efficiency in rotary motion, flexible stainless steel reamers and K-files clearly display the best results and are superior to conventional stainless steel as well as titanium-based reamers and K-files. Regarding cutting efficiency in linear motion, stainless steel Hedström files made by certain manufacturers are significantly superior to stainless steel and titanium-based Hedström files of other brands. Flexible stainless steel instruments with modified noncutting tips clearly produce the best canal shape in curved canals. With only rare exceptions, all the instruments tested fulfilled the requirements of the ISO standard concerning resistance to fracture and resistance to bending.