Characterisation of deformed or separated nickel-titanium retreatment instruments after clinical use - A multicentre experience: Defect profiles of clinically-used retreatment instruments.

OBJECTIVES: The present study examined the defect characteristics of clinically-used, discarded nickel titanium rotary retreatment instruments and analysed the impact of clinical use on their metallurgical properties. METHODS: 92 XP-endo Shaper (XPS; FKG Dentaire) and 20 XP-endo Finisher (XPFR; FKG Dentaire) instruments with structural deformation or separation were collected after retreatment from four endodontic clinics over a 20-month period. The types of defects and their relative locations were recorded. The lateral and fractured surfaces of the separated instruments were examined with scanning electron microscopy. Differential scanning calorimetry was used to investigate the thermal behaviour of new, deformed and fractured instruments. RESULTS: 77 (84%) XPS and 4 (20%) XPFR had an area with structural change while 15 (16%) XPS and 16 (80%) XPFR were fractured. All unfractured, deformed XPFR showed unwinding close to the coronal end of the flute. Fractures in XPS and XPFR were often close to the coronal end of the flutes or the expanding segments of the insruments. Most of the XPS fractures were torsional failure (67%) while XPFR failed predominantly by cyclic fatigue (81%). The austenite-finishing temperature of XPFR (40 °C) was higher than that of XPS (35 °C). Both XPS and XPFR exhibited 2-stage phase transformations. CONCLUSIONS: Torsional failure was more prevalent in XPS instruments and fatigue failure was more prevalent in XPFR instruments. amongst the investigated instruments, XPFR were more likely to separate without warning whereas XPS frequently exhibited plastic deformation. The latter may be used as a pre-separation forewarning sign during clinical retreatment. CLINICAL SIGNIFICANCE: The failure mode of XP-endo Shaper and XP-endo Finisher used clinically for retreatment appeared to be different. Plastic deformation, the forewarning sign of instrument seaparation, occurs when XP-endo instruments are used for retreatment.

Effect of canal curvature location on the cyclic fatigue resistance of reciprocating files.

OBJECTIVES: To determine the effect of the location of the canal curvature on the fatigue resistance of WaveOne (WO), WaveOne Gold (WOG), Reciproc (Rec), and Reciproc Blue (RecB) files, and to examine the phase transformation behaviors of the reciprocating file systems. MATERIAL AND METHODS: The instruments were subjected to fatigue testing in five artificial canals with a curvature of 60° angle and a 3-mm radius. The location of the curvature was unique for each canal. Each file was inserted 16 mm into the canal and operated until fracture occurred. The time to fracture was recorded and the length of the fragment was measured. Differential scanning calorimetry was used to characterize the thermal behavior of the files. The number of cycles to failure was analyzed using two-way analysis of variance and the post hoc Tukey test. The Kruskal-Wallis test was used to compare the mean fragment lengths between groups. RESULTS: The instruments had significantly lower fatigue resistance in canals with curvatures in the middle and coronal canals compared with those with apical curvatures (p < 0.05). At all tested curvature locations, RecB had superior fatigue resistance compared with WO and Rec (p < 0.05). There were no significant differences between WOG and Rec in canals with curvatures in the middle and coronal canals. The DSC thermograms for RecB exhibit a single exothermic peak during cooling but double endothermic peaks during heating indicating that a two-step phase transformation from martensite to R-phase to austenite takes place. CONCLUSIONS: The reciprocating instruments experience decreased cyclic fatigue resistance when operated in canals with coronal- and middle-third curvatures when compared with curvatures in the apical-third. Instrumenting coronally positioned curvatures with reciprocating files needs to be performed with caution. CLINICAL RELEVANCE: The location of the root canal's curvature influences the fracture resistance of rotary files that are used with reciprocating movements. Therefore, caution needs to be exercised when using reciprocating instruments in canals with coronal or middle curvatures.

Effect of Curvature Location on Fatigue Resistance of Five Nickel-titanium Files Determined at Body Temperature.

INTRODUCTION: The purpose of this study was to evaluate the effect of 5 different curvature locations on the fatigue resistance of thermomechanically treated nickel-titanium (NiTi) files and superelastic NiTi files at body temperature and to document the corresponding phase transformations. METHODS: EndoSequence (ES; Brasseler USA, Savannah, GA), EndoSequence CM (ESCM, Brasseler USA), K3 (SybronEndo, Orange, CA), K3XF (SybronEndo), and Vortex Blue (VB; Dentsply Tulsa Dental Specialties, Tulsa, OK) NiTi files (size 25/.04) were subjected to fatigue tests inside customized artificial canals containing a curvature of 60° and a 3-mm radius. There were 5 different canals based on the location of initial curvature; these included groups in which the distance between the canal orifice and the location of the curvature (DOC) was 5, 6, 8, 10, and 11 mm. The model canal was immersed in water at 37°C ± 1°C. The number of cycles to failure (Nf) was recorded, and the fracture surface of the fragments was examined by scanning electron microscopy. Two unused files of each brand were examined by differential scanning calorimetry. RESULTS: The Nf was highest in the 10-mm and 11-mm DOC groups and lowest with the 5-mm and 6-mm DOC groups (P < .05). ESCM files had the highest fatigue resistance followed by the VB, K3XF, K3, and ES files (P < .05). ESCM files had the highest Nf (P < .05), and ES and K3 files had the lowest Nf (P < .05) depending on the curvature location. Two endothermic peaks were observed on the heating curve of the heat-treated files (ESCM, K3XF, and VB). CONCLUSIONS: The location of the canal curvature had a significant effect on the fatigue resistance of both heat-treated and superelastic NiTi files. The fatigue life of files in the coronal curve was quite short.

Amino acid derivatives as bitter taste receptor (T2R) blockers.

In humans, the 25 bitter taste receptors (T2Rs) are activated by hundreds of structurally diverse bitter compounds. However, only five antagonists or bitter blockers are known. In this study, using molecular modeling guided site-directed mutagenesis, we elucidated the ligand-binding pocket of T2R4. We found seven amino acids located in the extracellular side of transmembrane 3 (TM3), TM4, extracellular loop 2 (ECL2), and ECL3 to be involved in T2R4 binding to its agonist quinine. ECL2 residues Asn-173 and Thr-174 are essential for quinine binding. Guided by a molecular model of T2R4, a number of amino acid derivatives were screened for their ability to bind to T2R4. These predictions were tested by calcium imaging assays that led to identification of γ-aminobutryic acid (GABA) and Nα,Nα-bis(carboxymethyl)-L-lysine (BCML) as competitive inhibitors of quinine-activated T2R4 with an IC50 of 3.2 ± 0.3 µM and 59 ± 18 nM, respectively. Interestingly, pharmacological characterization using a constitutively active mutant of T2R4 reveals that GABA acts as an antagonist, whereas BCML acts as an inverse agonist on T2R4. Site-directed mutagenesis confirms that the two novel bitter blockers share the same orthosteric site as the agonist quinine. The signature residues Ala-90 and Lys-270 play important roles in interacting with BCML and GABA, respectively. This is the first report to characterize a T2R endogenous antagonist and an inverse agonist. The novel bitter blockers will facilitate physiological studies focused on understanding the roles of T2Rs in extraoral tissues.