What Meaningful Information Are the Instruments Mechanical Testing Giving Us? A Comprehensive Review

Abstract

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. Cyclic fatigue testing allows the identification of conditions that may affect the fatigue strength outcomes, such as the canal radius and degree of curvature, handpiece static versus dynamic motions, test temperature, kinematics, instrument previously wear and sterilization cycles, or instrument's size and metal alloy features. Because of 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, instruments' 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 the understanding of them and molded the way the technical procedures are performed clinically. Even though the artificiality and simplicity of these tests will hardly mimic real working situations, and independent of being capable of producing cornerstone knowledge, these tests are also associated with inconsistency, a 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 behavior. A finite element method and/or a multimethod research approach may lead to superior data collection, analysis, and interpretation of results, which when associated with a reliable confounding factor control and proper study designs may be helpful tools and strategies in order to increase the reliability of the outcomes.

Keywords: Bending test; cyclic fatigue test; endodontics; multimethod; nickel-titanium instruments; torsional test; validity.