Physical properties of carbon steel root canal files and reamers☆

Nickel-titanium (NiTi) alloy has been extensively researched in endodontics, particularly in cleaning and shaping the root canal system. Research advances have primarily focused on the design, shape, and geometry of the NiTi files as well as metallurgy and mechanical properties. So far, extensive investigations have been made surrounding surface and thermomechanical treatments, however, limited work has been done in the realm of surface functionalization to augment its performance in endodontics. This review summarizes the unique characteristics, current use, and latest developments in thermomechanically treated NiTi endodontic files. It discusses recent improvements in nano-engineering and the possibility of customizing the NiTi file surface for added functionalization. Whilst clinical translation of this technology has yet to be fully realized, future research direction will lie in the use of nanotechnology.

The aims of the study were to quantify the transverse stiffness of two aesthetic orthodontic archwires (0·018 inch Teflon-coated stainless steel and 0·017 inch Optiflex) in a simulated clinical setting and to assess the influence of deflection direction on the bending stiffness. The aesthetic archwires were randomly divided into three equal groups: group 1, lingual deflection; group 2, labial deflection; and group 3, occlusal deflection. Each group consisted of six archwires of the same type. The control group consisting of eighteen 0014 inch stainless steel archwires were also subjected to the same grouping. A total of 54 archwires were tested in the study. The deflection of the archwires was measured with a travelling microscope and the load measured with a calibrated strain gauge ring transducer. The mean stiffnesses of the archwires in the lingual, labial and occlusal deflection groups were found to be 2·9, 0·8 and 2·5 mN/mm respectively for 0·017 inch Optiflex (r = 0·9, P < 0·001), 13·2, 10·5 and 24·5 mN/mm respectively for 0·018 inch Teflon-coated stainless steel (r = 0·9, P < 0·001) and 26·6, 16·4 and 32·3 mN/mm respectively for the control (r = 0·9, P < 0·001). Springback was found to be poor for Optiflex and the archwire remained bent upon deactivation. ANOVA showed that the influence of arch curvature on the bending stiffness was significantly different for Optiflex (P < 0·05), Teflon-coated stainless steel (P < 0·005) and the control group (P < 0·005). Stiffness for the Teflon-coated archwire was found to be higher and more in line with the stiffness for the control. Both the Teflon-coated and stainless steel archwires displayed good springback property. However, Optiflex was found to have low stiffness and resilience. Owing to the poor springback, the clinical efficacy of Optiflex is probably limited.

A laboratory study was performed on Gates Glidden and Peeso drills to determine the bending characteristics and related metallurgical properties. Sizes #1 to #6 stainless steel Gates Glidden drills, sizes #1 to #6 stainless steel and carbon steel type P Peeso drills, and sizes #009 to #023 carbon steel-type B-1 Peeso drills were evaluated in cantilever bending with a Tinius Olsen stiffness tester. The bending moments were measured for a 12.5-mm test span at deflection angles of 2 to 90 degrees. The mean maximum bending moments in g cm and the corresponding deflection angles in degrees were plotted graphically to ascertain the bending characteristics of the instruments. The stainless steel Gates Glidden and Peeso drills exhibited considerable permanent bending deformation during testing. Some stainless steel drill samples were able to withstand the full bending deflection of 90 degrees without fracturing. In contrast, the carbon steel Peeso drills were able only to undergo bending deflections of less than 20 degrees and exhibited brittle fracture. If the drill fractured during the in vitro testing, the failure site was always located near the handpiece end.

The torsional properties of a new type of endodontic hand instrument, the Canal Master, were compared with conventional machined K-type endodontic files. Sizes 20 through 50 were tested in clockwise (CW) and counterclockwise (CCW) directions. Canal Master instruments exhibited significantly less torque at yield and at failure in both the CW and CCW directions. Rotation at failure was significantly greater for the Canal Master instruments in both CW and CCW directions. There was no significant difference in either torque or the amount of rotation for the Canal Master instruments when comparing CW versus CCW rotation. There was no clinically visible evidence of deformation of the Canal Master Instruments prior to torsional failure.

Tip angulation and four visual characteristics denoting manufacturing quality of endodontic files were studied. One-hundred eighty K-type, Hedstrom, and Flex-type files from two manufacturers (Kerr and Mallefer) were evaluated. Photomicrographs (×115) were made of each file. Two evaluators examined the photomicrographs. Tip angulation was studied to determine if files were compliant with ANSI/ADA Specification Nos. 28 and 58. The four visual characteristics compared in the study were symmetry versus asymmetry, smooth versus irregular, tip configuration, and shoulder configuration. Statistical analysis was made using the chi-square method. Mallefer files appear to be more in conformity with specifications for tip angle and more meticulously manufactured than Kerr files. Kerr Hedstrom files appear to conform less often to tip angle specifications than Kerr K-type or K-Flex files. K-type and Flex-type file tips appear to be better manufactured than Hedstrom file tips. Certification status of a file manufacturer did not correlate with a higher level of compliance with tip angulation specifications or with a better machined file tip.

Endodontic K files from six manufacturers selected on the basis of file cost were compared for surface debris, corrosion, metal defects, flute uniformity, and accuracy of file dimensions relative to ANSI/ADA Specification No. 28 for endodontic files. We found that there was no correlation between file cost and the amount of surface debris or accuracy of file dimensions.Discrepancy in file dimensions for successive files in ascending number order from a given manufacturer did not appear to be sufficiently large to be a problem during progressive instrumentation. Differences in dimensions for files in ascending number order from different manufacturers, however, were often of a magnitude that might cause clinical problems of fit during root canal instrumentation. The noncorrespondence of succeeding file sizes for the products of different manufacturers is the result of inherent dimensional variability in the products and can cause problems for the clinician. It follows from the observations reported here that endodontic files ideally should be obtained from a single manufacturer to ensure proper fit during canal instrumentation.